1. The dynamic
evolutionary process by which biological organisms develop characteristics that allow them to survive and reproduce within their environments.
2. The state or condition reached by a
population during that process.
3. Any character or
phenotypic trait with a functional role in an individual organism and which has evolved and is maintained through
natural selection.
The
Darwinian view that many or most physiological and behavioral traits of organisms are
adaptations that have evolved for specific functions or for specific reasons (as opposed to being
byproducts of the evolution of other traits, consequences of
biological constraints, or the result of random variation).
The simultaneous or near-simultaneous
evolutionary divergence of multiple members of a single
phylogeneticlineage into a variety of different forms with different
adaptations, especially a diversification in the use of resources or habitats.[1]
agamospecies
A species that does not reproduce
sexually but rather by cloning.[2] Agamospecies are sometimes represented by
species complexes that contain some
diploid individuals and other
apomictic forms—in particular, plant species that can reproduce via
agamospermy.[3]
One of multiple alternative versions of an individual
gene, each of which is a viable
DNA sequence occupying a given position, or
locus, on a
chromosome. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the eye-color gene produces brown eyes.
The relative frequency with which a particular
allele of a given
gene (as opposed to other alleles of the same gene) occurs at a particular
locus in the members of a population; more specifically, it is the proportion of all chromosomes within a population that carry a particular allele, expressed as a fraction or percentage. Allele frequency is distinct from
genotype frequency, although they are related.
The isolation of two populations of a species due to a change in breeding periods. This isolation acts as a precursor to allochronic speciation, a type of
speciation which results when two populations of a species become isolated due to differences in reproductive timing. An example is the periodical 13- and 17-year Magicicada species.[3]
The comparative study of the relationship between the size of an organism's body (or of a specific organ, e.g. the brain) and various other biological characteristics, such as body shape, anatomy, physiology, or behavior.
A set of
morphological structures in different organisms which have similar form or function but were not present in the organisms'
last common ancestor. The
cladistic term for the same phenomenon is
homoplasy.
Also called an ancestral character, primitive character, or primitive trait.
For a given
clade, any trait or feature (e.g. a specific
phenotype) that appears in the clade's
common ancestor; the same trait may also appear in some or all of the lineal descendants included within the clade, indicating that it has undergone little or no significant change during the clade's evolutionary history and thus retained its "primitive" condition. Some but not all subgroups within the clade may contain
derived traits, in which the ancestral trait has changed significantly over evolutionary time such that the original ancestral condition no longer exists. Both terms are relative: an ancestral trait for one clade may be a derived trait for a different clade. The term "ancestral trait" is often used interchangeably with the more technical term
plesiomorphy.
A phenomenon by which the symptoms of a
genetic disorder become apparent (and often more severe) at an earlier age in affected individuals with each generation that inherits the disorder.
A
derived character state; i.e. the state or condition of a particular trait or feature (e.g. a specific
phenotype) that is distinct from and derivative of an
ancestral character by virtue of its modification over time in one or more lineal descendants of a given
clade. Apomorphies are often viewed as evolutionary "innovations" which set the
taxa in which they appear apart from the clade's
common ancestor, as well as from other clades;
shared apomorphies are used to construct and define clades. The term is relative; a trait considered an apomorphy in one clade may not be considered an apomorphy in a different clade. Contrast plesiomorphy.
Any character or
phenotypic trait that is currently subject to
natural selection, whether its origin can be ascribed to selective processes (
adaptation) or to processes other than selection or selection for a function that is different from the current function (
exaptation).[8]
The phenomenon by which the
linkage of a neutral locus to a selectively maintained
polymorphism causes the
heterozygosity of the neutral locus to increase.[8]
Also called positive assortative mating and homogamy.
A
mating system in which individuals with similar
phenotypesmate with each other more frequently than would be expected in a completely random mating system. Assortative mating usually has the effect of increasing genetic relatedness between members of the mating population. Contrast disassortative mating.
A modification of a biological structure whereby an
ancestral trait suddenly reappears after having been lost through evolutionary change in previous generations.[9] Atavisms can occur in a number of different ways, including by the re-expression of latent genes for ancestral phenotypes as a result of mutation, or by the shortening or prolongation of the time allocated for the
ontogenesis of a particular trait during development.
The breeding of a
hybrid organism with one of its parents or an individual genetically similar to one of its parents, often intentionally as a type of
selective breeding, with the aim of producing offspring with a genetic identity which is closer to that of the parent. The reproductive event and the resulting progeny are both referred to as a backcross, often abbreviated in genetics shorthand with the symbol BC.
An evolutionary model of the
genetic incompatibility that occurs as a result of negative
epistatic interactions between two or more
genes or
alleles with different evolutionary histories, which may meet when distinct populations
hybridize. The incompatible genes or alleles themselves, referred to as Dobzhansky–Muller incompatibilities, may be the result of
random or
neutral mutations, or they may be specific
adaptations driven by
natural selection. By preventing populations from successfully interbreeding, these incompatibilities can reinforce
reproductive isolation and thereby increase the chance of
speciation.
The scientific study of the spatial distributions of biological organisms, populations, and species. It includes the study of both
extinct and
extant organisms.[10]
The ability of a population to consistently produce the same
phenotype regardless of the variability of its environment or the
genetic variation within its
genome. The concept is most often used in
developmental biology to interpret the observation that developmental pathways are frequently shaped by
natural selection such that developing cell lineages are "guided" or "canalized" towards a single, definite fate, becoming progressively more resistant to any minor perturbations that may redirect development of the cells away from their initial course.
An individual who has
inherited a
recessiveallele for a genetic trait or mutation but in whom the trait is not usually expressed or observable in the
phenotype. Carriers are usually
heterozygous for the recessive allele and therefore still able to pass the allele onto their offspring, where the associated phenotype may reappear if the offspring inherits another copy of the allele. The term is commonly used in
medical genetics in the context of a disease-causing recessive allele.
A variation of
peripatric speciation in which speciation occurs by geographic isolation, but
reproductive isolation evolves in the larger population instead of the peripherally isolated population.[11]
The phenomenon by which differences between similar species that occupy similar
niches and have partially overlapping geographic distributions are accentuated in regions where the species co-occur but are minimized or lost where the species' distributions do not overlap. This occurs because competition between the similar species for one or more limited resources drives evolutionary change that differentiates the species in the common geographic areas such that they no longer occupy the same niche, thereby allowing them to coexist and avoiding
competitive exclusion.
The presence of two or more populations of cells with distinct
genotypes within the body of an individual organism, known as a chimera, which has developed from the fusion of cells originating from separate
zygotes; each population of cells retains its own genome, such that the organism as a whole is a mixture of genetically non-identical tissues. Genetic chimerism may be inherited (e.g. by the fusion of multiple embryos during pregnancy) or acquired after birth (e.g. by
allogeneic transplantation of cells, tissues, or organs from a genetically non-identical donor); in plants, it can result from
grafting or errors in cell division. It is similar to but distinct from
mosaicism.
An approach to biological classification in which organisms are grouped in
clades defined by
shared ancestry; hypothesized relationships between organisms are typically based on
shared derived characters which can be traced to the
most recent common ancestor and are not present in more distant ancestors or unrelated groups.
The branch of
genetics based solely on observation of the visible results of reproductive acts, as opposed to that made possible by the modern techniques and methodologies of
molecular biology. Contrast molecular genetics.
A measurable spatial gradient in a single biological character or trait of a species or population across its geographic range. The nature of a cline may be genotypic (e.g. variation in
allele frequency) or phenotypic (e.g. variation in body size or pigmentation), and may show smooth, continuous gradation or abrupt changes between different geographic regions.
The process of producing, either naturally or artificially, individual organisms or cells which are genetically identical to each other. Clones are the result of all forms of
asexual reproduction, and cells that undergo
mitosis produce daughter cells that are clones of the parent cell and of each other. Cloning may also refer to biotechnology methods which artificially create copies of organisms or cells, or, in
molecular cloning, copies of DNA fragments or other molecules.
The mutual
adaptation of organisms belonging to different populations or species, of different parts of the same organism, or of
genes at different
loci in the same
genome, especially implying that adaptation in both entities is driven by the same evolutionary force.[12]
The process by which two or more distinct
populations,
species, or other groups of organisms, or two or more distinct traits within a species, reciprocally affect each other's
evolution through
natural selection. Each party in a coevolutionary relationship exerts
selective pressures upon the other, leading to the evolution of separate traits in each party.
An organism or
taxon (e.g. a
species) which is hypothesized to be the lineal progenitor of two or more organisms or taxa which exist at a later point in evolutionary time. The concept of common descent is fundamental to the study of
evolution,
phylogenetics, and
cladistics; for instance, all
clades, by definition, are rooted in a common ancestor. See also most recent common ancestor.
(of two or more individuals) Closely genetically related; sharing a recent
common ancestor (usually no more than three or four
generations distant). The effect of consanguineous mating, also known as
inbreeding, is to increase the probability that the progeny will be
homozygous at any given pair of genetic
loci.[13]
An interdisciplinary branch of
population genetics which applies genetic methods and concepts in an effort to understand the dynamics of genes in populations, with a principal aim of avoiding
extinctions and preserving and restoring
biodiversity.
The independent
evolution of similar traits or
adaptations in two or more different taxa from different periods or epochs in time, creating
analogous structures that have similar form or function but were not present in the
last common ancestor of those taxa; e.g. structures enabling flight evolved independently in at least four distinct lineages: insects, birds, pterosaurs, and bats. In
cladistics, the same phenomenon is termed
homoplasy. Contrast divergent evolution.
The breeding of
purebred parents belonging to two different
breeds, varieties, or populations, often intentionally as a type of
selective breeding, with the aim of producing offspring which share traits of both parent lineages or which show
heterosis. In animal breeding, the progeny of a cross between breeds of the same species is called a crossbreed, whereas the progeny of a cross between different species is called a
hybrid.
The understanding of biological
evolution as developed by the English naturalist
Charles Darwin and others, which states that all biological organisms arise and develop through the
natural selection of small, inherited variations that increase the individual's ability to compete, survive, and reproduce. Colloquially, the term is sometimes used to refer more broadly to
modern evolutionary theory as a whole, though in scientific circles distinctions are usually made between Darwin's ideas and later additions to evolutionary biology.
A spontaneous
mutation in the
genome of an individual organism that is new to that organism's
lineage, having first appeared in a
germ cell of one of the organism's parents or in the fertilized egg that develops into the organism; i.e. a mutation that was not present in either parent's genome.[8]
Also derived character, advanced character, and advanced trait.
For a given
clade, any trait or feature (e.g. a specific
phenotype) that is present within one or more subgroups of the clade but not in the clade's
common ancestor. Derived traits show significant differences from the original "primitive" condition of an
ancestral trait found in the common ancestor, implying that the trait has undergone extensive
adaptation during the clade's evolutionary history to reach its derivative condition. Both terms are relative: a derived trait for one clade may be an ancestral trait for a different clade. The term "derived trait" is often used interchangeably with the more technical term
apomorphy.
A mode of
natural selection in which an extreme
trait or
phenotype is favored over other phenotypes, causing
allele frequencies to shift over time in the direction of that trait or phenotype. This shift can occur whether or not the alleles governing the extreme phenotype are
dominant or recessive.
A
mating system in which individuals with dissimilar
phenotypesmate with each other more frequently than would be expected in a completely random mating system. Disassortative mating usually has the effect of decreasing genetic relatedness between members of the mating population. Contrast assortative mating.
A mode of
natural selection in which the extreme values of a
trait or
phenotype within a breeding population are favored over intermediate values, causing
allele frequencies to shift over time away from the intermediate. This causes the variance in the trait to increase and results in the population dividing into two distinct groups, each with trait values at one end of the trait's distribution curve.
The process by which any
phenotypic or
genotypic distinction emerges between two different
populations or evolutionary
lineages. Divergence may occur by any of a variety of mechanisms but is often especially noticeable after the two lineages have been
reproductively isolated for many
generations.[7]
A relationship between the
alleles of a
gene in which one allele produces an effect on
phenotype that overpowers or "masks" the contribution of another allele at the same
locus; the first allele and its associated phenotypic
trait are said to be dominant, and the second allele and its associated trait are said to be recessive. Often, the dominant allele codes for a functional protein while its recessive counterpart does not. Dominance is not an inherent property of any allele or phenotype, but simply describes its relationship to one or more other alleles or phenotypes; it is possible for one allele to be simultaneously dominant over a second allele, recessive to a third, and
codominant to a fourth. In genetics shorthand, dominant alleles are often represented by a single uppercase letter (e.g. "A", in contrast to the recessive "a").
Any mechanism by which organisms neutralize the large difference in
gene dosage caused by the presence of differing numbers of
sex chromosomes in the different sexes, thereby equalizing the
expression of sex-linked genes so that the members of each sex receive the same or similar amounts of the
products of such genes. An example is
X-inactivation in female mammals.
The ecological state of a species being unique to a single geographic location, such as an island, nation, country, or any other clearly defined area, or to a single
habitat type.
The collective action of multiple genes interacting during
gene expression. A form of gene action, epistasis can be either additive or multiplicative in its effects on specific
phenotypic traits.
The
extinction of a
population of organisms (insofar as the population can be defined by one or more identifiable characteristics) as a result of the excessive accumulation of genetic mutations, such that the population loses self-identity because all of its mutated descendants lack the identifiable characteristics.
The condition of a cell or organism having an abnormal number of complete sets of
chromosomes, possibly excluding the
sex chromosomes. Euploidy differs from
aneuploidy, in which a cell or organism has an abnormal number of one or more specific individual chromosomes.
The phenomenon by which the
heritable characteristics of biological
populations change over successive
generations. Evolution occurs when processes such as
natural selection and
genetic drift act on the
variation in characteristics that exists between members of a population, resulting in certain characteristics becoming more or less common within the population.
The
positive feedback mechanism operating between competing sets of
co-evolvinggenes, traits, species, or other taxa which evolve specific
adaptations and counter-adaptations due to each other's presence, which may be seen as analogous with an "
arms race".
For a given
genotype associated with a variable non-binary
phenotype, the proportion of individuals with that genotype who show or express the phenotype to a specified extent, usually given as a percentage. Because of the many complex interactions that govern
gene expression, the same
allele may produce a wide variety of possible phenotypes of differing qualities or degrees in different individuals; in such cases, both the phenotype and genotype may be said to show variable expressivity. Expressivity attempts to quantify the range of possible levels of phenotypic variation in a population of individuals expressing the phenotype of interest. Compare penetrance.
Currently living or existing; still in existence and not
extinct. The term is generally used to refer to the present-day state of existence of a particular
taxon (such as a family,
genus,
species, etc.).
The process by which a single
allele for a particular
gene with multiple alleles increases in
frequency in a given population such that it becomes permanently established as the only allele at that
locus within the population's
gene pool. How long fixation takes depends on
selection pressures and
chance fluctuations in allele frequencies.
The loss of
genetic variation that occurs when a new, physically isolated
population is established by a very small number of individuals who have migrated from a larger population and are not fully representative of the larger population's genetic diversity. As a result, the new population is often distinctively different, both
genotypically and
phenotypically, from the parent population. Besides migration,
population bottlenecks can also result in a type of founder effect; extreme founder effects can lead to
speciation.
A hypothesis that describes the evolutionary advantages of the first-arriving lineages in a new ecosystem.[16] An example could be when a species becomes
reproductively isolated on an island, as in
peripatric speciation.
fugitive species
A
species that only temporarily occupies environments or habitats (either because its members frequently migrate or because its environments frequently change) and so does not persist for many
generations at any one site.[1]
Any segment or set of segments of a
nucleic acid molecule that contains the information necessary to produce a functional
RNA transcript in a controlled manner. Genes are often considered the fundamental
units of
heredity and are typically encoded in
DNA. A particular gene can have multiple different versions, or
alleles, and a single gene may influence many different
phenotypes.
The number of copies of a particular
gene present in a
genome. Gene dosage directly influences the amount of
gene product a cell is able to express, though a variety of controls have evolved which tightly
regulategene expression. Changes in gene dosage caused by mutations include
copy-number variations.
The set of processes by which the information encoded in a
gene is used in the synthesis of a
gene product, such as a protein or non-coding RNA, or otherwise made available to influence one or more
phenotypes. Expression at the molecular level canonically consists of
transcription and
translation, though the information contained within a DNA sequence need not necessarily be transcribed and translated to exert an influence on molecular events, however; broader definitions encompass a huge variety of other ways in which genetic information can be expressed.
1. In any given organism, a single
reproductive cycle, or the phase between two consecutive reproductive events, i.e. between an individual organism's reproduction and that of the
progeny of that reproduction; or the actual or average length of time required to complete a single reproductive cycle, either for a particular
lineage or for a population or species as a whole.
2. In a given population, those individuals (often but not necessarily living contemporaneously) who are equally removed from a given
common ancestor by virtue of the same number of reproductive events having occurred between them and the ancestor.[13]
The co-occurrence within a population of one or more
alleles or
genotypes with a particular
phenotypic trait more often than might be expected by chance alone; such statistical correlation may be used to infer that the alleles or genotypes are responsible for producing the given phenotype.
The process of advising individuals or families who are affected by or at risk of developing
genetic disorders in order to help them understand and adapt to the physiological, psychological, and familial implications of genetic contributions to disease. Genetic counseling integrates
genetic testing,
genetic genealogy, and
genetic epidemiology.[17]
Any illness, disease, or other health problem directly caused by one or more abnormalities in an organism's
genome which are
congenital (present at birth) and not acquired later in life. Causes may include a
mutation to one or more
genes, or a
chromosomal abnormality such as an
aneuploidy of a particular chromosome. The mutation responsible
may occur spontaneously during embryonic development or may be
inherited from one or both parents, in which case the genetic disorder is also classified as a
hereditary disorder. Though the abnormality itself is present before birth, the actual disease it causes may not develop until much later in life; some genetic disorders do not necessarily guarantee eventual disease but simply
increase the risk of the organism developing it.
A measure of the genetic divergence between species, populations within a species, or individuals, used especially in
phylogenetics to express either the time elapsed since the existence of a
common ancestor or the degree of differentiation in the
DNA sequences comprising the
genomes of each population or individual.
The total number of genetic
traits or characteristics in the genetic make-up of a population, species, or other group of organisms. It is often used as a measure of the adaptability of a group to changing environments. Genetic diversity is similar to, though distinct from,
genetic variability.
Also called allelic drift or the Sewall Wright effect.
A change in the
frequency with which an existing
allele occurs in a population due to random variation in the distribution of alleles from one
generation to the next. It is often interpreted as the role that
random chance plays in determining whether a given allele becomes more or less common with each generation, irrespective of the influence of
natural selection. Genetic drift may cause certain alleles, even otherwise advantageous ones, to disappear completely from the
gene pool, thereby reducing
genetic variation, or it may cause initially rare alleles, even neutral or deleterious ones, to become much more frequent or even
fixed.
The study of the role played by genetic factors in determining health and disease, in particular through the interaction of genetic factors with environmental factors, and typically as observed in genetically related individuals, often families or lineages but also populations and subpopulations.
The use of
genealogical DNA testing in combination with traditional
genealogical methods to infer the level and type of genetic relationships between individuals, to find ancestors, and to construct
family trees,
genograms, or other genealogical charts.
A type of
linked selection by which the positive selection of an
allele undergoing a
selective sweep causes alleles for different genes at nearby
loci to change
frequency as well, allowing them to "hitchhike" to
fixation along with the positively selected allele. If selection at the first locus is strong enough, neutral or even slightly deleterious alleles within the same linkage group may undergo the same positive selection because the physical distance between the nearby loci is small enough that a
recombination event is unlikely to occur between them. Genetic hitchhiking is often considered the opposite of
background selection.
Any reduction in the mean
fitness of a
population owing to the existence of one or more
genotypes with lower fitness than that of the most fit genotype.[1]
A broad class of procedures used to identify features of an individual's particular chromosomes, genes, or proteins in order to determine parentage or
ancestry, diagnose vulnerabilities to heritable diseases, or detect
mutant alleles associated with increased risks of developing
genetic disorders. Genetic testing is widely used in human medicine, agriculture, and biological research.
The genetic differences both within and between
populations,
species, or other groups of organisms. It is often visualized as the variety of different
alleles in the
gene pools of different populations.
The formation or the presence of individuals differing in
genotype within a population or other group of organisms, as opposed to individuals with environmentally induced differences, which cause only temporary, non-heritable changes in
phenotype. Barring other limitations, a population with high genetic variability has a greater potential for successful adaptation to changing environmental conditions than a population with low genetic variability. Genetic variability is similar to, though distinct from,
genetic diversity.
The genetic differences both within and between populations, species, or other groups of organisms. It is often visualized as the variety of different
alleles in the
gene pools of different populations.
A mechanism of
sex determination that depends upon the ratio of the number of
X chromosomes (X) to the number of sets of
autosomes (A). Males develop when the X/A ratio is 0.5 or less, females when it is 1.0 or more, and an
intersex develops when it is between 0.5 and 1.0.[8]
genic selection
A type of
natural selection that occurs at the level of individual
genes or
alleles, in which the
frequency of an allele within a breeding
population is determined by its
fitness averaged over the variety of
genotypes in which it occurs; the differential propagation of different alleles within a population as a consequence of properties borne by the alleles themselves, rather than by the genotypes in which they are found.[1]
The entire complement of genetic material contained within the
chromosomes of an organism, organelle, or
virus. The term is also used to refer to the collective set of genetic
loci shared by every member of a population or species, regardless of the different
alleles that may be present at these loci in different individuals.
Any
cell that gives rise to the
gametes of an organism that reproduces
sexually. Germ cells are the vessels for the genetic material which will ultimately be passed on to the organism's descendants and are usually distinguished from
somatic cells, which are entirely separate from the
germ line.
1. In multicellular organisms, the population of
cells which are capable of passing on their genetic material to the organism's progeny and are therefore (at least theoretically) distinct from
somatic cells, which cannot pass on their genetic material except to their own immediate
mitotic daughter cells. Cells of the germ line are called
germ cells.
2. The
lineage of germ cells, spanning many generations, that contains the genetic material which has been passed on to an individual from its ancestors.
A rule formulated by
J.B.S. Haldane which states that if one sex of the
hybrid offspring resulting from a cross between two
incipient species is inviable or sterile, that sex is more likely to be the
heterogametic sex (i.e. the one with two different sex chromosomes).[18]
A
sex-determination system in which the sex of an individual organism is determined by the
number of sets of
chromosomes it possesses: offspring which develop from fertilized eggs are
diploid and female, while offspring which develop from unfertilized eggs are
haploid and male, with half as many chromosomes as the females. Haplodiploidy is common to all members of the insect order
Hymenoptera and several other insect taxa.
A principle of population genetics which states that
allele and
genotype frequencies of a population will remain constant from
generation to generation in the absence of other evolutionary influences. In the simplest case of a randomly mating population of diploid organisms possessing a single locus with two alleles, A and a, with frequencies f(A) = p and f(a) = q, respectively, the expected genotype frequencies are f(AA) = p2 for AAhomozygotes, f(aa) = q2 for aa homozygotes, and f(Aa) = 2pq for
heterozygotes. In the absence of evolutionary forces such as
natural selection,
mutation,
assortative mating,
gene flow, and
genetic drift, p and q will remain constant between generations, such that the population is said to be in Hardy–Weinberg equilibrium with respect to the locus in question.
In a
diploid organism, having just one
allele at a given
genetic locus (where there would ordinarily be two). Hemizygosity may be observed when only one copy of a
chromosome is present in a normally diploid cell or organism, or when a segment of a chromosome containing one copy of an allele is
deleted, or when a gene is located on a
sex chromosome in the
heterogametic sex (in which the sex chromosomes do not exist in matching pairs); for example, in human males with normal chromosomes, almost all
X-linked genes are said to be hemizygous because there is only one
X chromosome and few of the same genes exist on the
Y chromosome.
The storage, transfer, and expression of genetic information in biological organisms,[13] as manifested by the passing on of
phenotypic traits from parents to their
offspring, either through
sexual or
asexual reproduction. Offspring cells or organisms are said to inherit the genetic information of their parents.
2. A statistic used in
quantitative genetics that estimates the proportion of variation within a given
phenotypic trait that is due to
genetic variation between individuals in a particular population. Heritability is estimated by comparing the individual
phenotypes of closely related individuals in the population.
The improved or increased function or quality of any biological
trait in a
hybridoffspring, with respect to the same trait in its genetically distinct parents. If any one or more of the parents' traits are noticeably enhanced in the offspring as a result of the mixing of the parents' genetic contributions, the offspring is said to be heterotic.
In a
diploid organism, having two different
alleles at a given
genetic locus. In genetics shorthand, heterozygous
genotypes are represented by a pair of non-matching letters or symbols, often an uppercase letter (indicating a
dominant allele) and a lowercase letter (indicating a
recessive allele), such as "Aa" or "Bb". Contrast homozygous.
homoallele
A mutant
allele having a different
mutation at the same site as another allele. Intragenic
recombination between homoalleles cannot produce a functional
cistron.[8]
A set of two matching
chromosomes, one maternal and one paternal, which pair up with each other inside the nucleus during
meiosis. They have the same
genes at the same
loci, but may have different
alleles.
In a
diploid organism, having two identical
alleles at a given
genetic locus. In genetics shorthand, homozygous
genotypes are represented by a pair of matching letters or symbols, such as "AA" or "aa". Contrast heterozygous.
Any process by which genetic material is transferred between unicellular and/or multicellular organisms other than by vertical transmission from parent to offspring, e.g.
bacterial conjugation.
The
offspring that results from combining the qualities of two organisms of different
genera,
species,
breeds, or varieties through
sexual reproduction. Hybrids may occur naturally or artificially, as during
selective breeding of domesticated animals and plants. Reproductive barriers typically prevent hybridization between distantly related organisms, or at least ensure that hybrid offspring are sterile, but fertile hybrids may result in
speciation.
A geographic area in which the ranges of two interbreeding species or populations overlap, allowing them to cross-fertilize and generate
hybrid offspring. The formation of a hybrid zone is one of the four outcomes of
secondary contact between divergent genetic lineages.
hybridization
The process by which a
hybrid organism is produced from two parents of different genera, species, breeds, or varieties.
The exaggeration of one or more
phenotypic features of a descendant organism compared to those of its ancestors due to an increase in the duration of
ontogenetic development over evolutionary history.[1]
(of a gene or allele) Traceable back through an arbitrary number of
generations without mutation to a
common ancestor of the group of descendant organisms that carries the gene or allele.[1] A gene or allele present in a group of descendant organisms is said to be identical by descent to a gene or allele in a common ancestor of the group if both sequences are identical, indicating that the sequence has been passed down unmodified from the common ancestor to its descendants.
Any
lineage of a particular species in which individuals are nearly or completely genetically identical to each other due to a long history of repeated
inbreeding, either by natural or artificial means. Lineages are typically considered inbred after at least 20
generations of inbreeding (e.g. by self-fertilization or sib mating), at which point nearly all
loci across the genome are
homozygous and all individuals can therefore effectively be treated as
clones (despite the fact that individuals are still produced by sexual reproduction).
Sexual reproduction between breeds or individuals that are closely
related genetically. Inbreeding results in
homozygosity, which can increase both the probability of offspring being affected by deleterious
recessive traits and the probability of fixing beneficial traits within the breeding population. The reproductive event and the resulting progeny may both be referred to as an incross, and the progeny is said to be inbred. Contrast outbreeding.
The number of
offspring equivalents that an individual organism rears, rescues, or otherwise supports through its behavior, regardless of whether or not the individual is actually a biological parent of the offspring equivalents. Inclusive fitness is one of two metrics of evolutionary success as defined by
W.D. Hamilton in 1964, the other being
personal fitness.
The frequency of new occurrence of a
genetic disorder (or more broadly any genetic condition or
trait, deleterious or otherwise) among the members of a particular population and within a particular period of time.[13]
The
movement of a
gene from the
gene pool of one population or species into that of another population by the repeated
backcrossing of
hybrids of the two populations with one of the parent populations. Introgression is a ubiquitous and important source of
genetic variation in natural populations, but may also be practiced intentionally in the cultivation of domesticated plants and animals.
Two or more
genes that are equivalent and redundant in the sense that, despite coding for distinct
gene products, they each result in the same
phenotype when set within the same
genetic background. If several isomeric genes are present in a single
genotype they may be either cumulative or non-cumulative in their contributions to the phenotype.[13]
The repeated
evolution of similar
phenotypic characteristics or traits in different organisms at different times during the evolutionary history of a
clade,[1] a phenomenon which can result in the seeming
de-extinction of an organism previously considered
extinct.
A reproductive strategy characterized by multiple reproductive cycles during an individual organism's lifetime. Organisms that use such a strategy are said to be iteroparous. Iteroparity is usually contrasted with
semelparity.
A form of
genic selection by which
alleles differ in their rates of propagation by influencing the survival or reproductive success of individuals who carry the same alleles by
common descent (their
kin).[1]
An evolutionary hypothesis which proposes that during
sexual selection, organisms preferentially seek mates with a minimum of unusual or
mutanttraits, e.g. in terms of functionality, appearance, or behavior. The hypothesis attempts to explain the clustering of sexual organisms into distinct
species and other issues described by
Darwin's dilemma.
Also called the last universal cellular ancestor or simply the last universal ancestor.
The most recent
population of organisms from which all
extant organisms on Earth share a
common descent; i.e. the
most recent common ancestor of all organisms now living. LUCA is not thought to have been the earliest life on Earth, but rather the only organism of its time to still have living descendants. Its existence is not known from any specific fossil record but is inferred from
phylogenetic comparisons of modern organisms, all of which are its descendants.
One of three fundamental principles of
Mendelian inheritance, which states that different
alleles of the same
gene may be
dominant or
recessive relative to others, and that an organism with at least one dominant allele will uniformly display the
phenotype associated with the dominant allele.
One of three fundamental principles of
Mendelian inheritance, which states that genes responsible for different phenotypic traits are segregated independently during
meiosis.
Linked genes are a notable exception to this rule.
The average number of
recessive deleterious genes existing in the
heterozygous condition that is carried by a member of a population of
diploid organisms, multiplied by the average probability that each such gene will cause premature
lethality when
homozygous. For example, an organism carrying eight recessive
semilethal alleles, each of which produces only a 50% probability of premature death when homozygous, is said to carry a genetic burden of four "lethal equivalents".[8]
A linear evolutionary sequence connecting an ancestral cell, organism, or species to a particular descendant cell, organism, or species, including all intermediate organisms and spanning any number of
generations; the direct progression of reproductive events (i.e. the line of descent) between two individuals, including vertically related individuals, e.g. parent(s) and offspring, but usually excluding horizontally related individuals who did not themselves directly contribute genetic material to any of the included individuals, e.g. siblings.
lineage-splitting
Also called lineage-branching.
When
gene flow between two populations is completely eliminated.[7]
The tendency of DNA sequences which are physically near to each other on the same chromosome to be
inherited together during
meiosis. Because the physical distance between them is relatively small, the chance that any two nearby parts of a DNA sequence (often
loci or
genetic markers) will be separated on to different
chromatids during
chromosomal crossover is statistically very low; such loci are then said to be more linked than loci that are farther apart. Loci that exist on entirely different chromosomes are said to be perfectly unlinked. The standard unit for measuring genetic linkage is the
centimorgan (cM).
Evolutionary change as it occurs at a relatively large scale, at or above the level of
species, as opposed to
microevolution, which occurs at a smaller scale. Macroevolution is often thought of as the compounded effects of microevolution.
Any nongenetic effect of the mother on the
phenotype of her
offspring, owing to factors such as cytoplasmic inheritance, transmission of congenital disease, and the sharing of nutritional conditions.[1]
(of an
offspring) Resembling the female parent, genotypically or phenotypically, more closely than the male parent; derived from the mother. Contrast patroclinous.
The branch of medicine and medical science that involves the study, diagnosis, and management of
hereditary disorders, and more broadly the application of knowledge about human
genetics to medical care.
A specialized type of
cell division that occurs exclusively in
sexually reproducingeukaryotes, during which
DNA replication is followed by two consecutive rounds of division to ultimately produce four genetically unique
haploid daughter cells, each with half the number of
chromosomes as the original
diploid parent cell. Meiosis only occurs in cells of the sex organs, and serves the purpose of generating haploid
gametes such as
sperm,
eggs, or
spores, which are later fused during
fertilization. The two meiotic divisions, known as Meiosis I and Meiosis II, may also include various
genetic recombination events between
homologous chromosomes.
A theory of biological
inheritance based on a set of principles originally proposed by
Gregor Mendel in 1865 and 1866. Mendel derived three generalized laws about the genetic basis of inheritance which, together with several theories developed by later scientists, are considered the foundation of
classical genetics. Contrast non-Mendelian inheritance.
Evolutionary change as it occurs at a relatively small scale, typically within a particular
species or
population, as opposed to
macroevolution, which occurs at a larger scale. Because of the convenience of observing and modeling small-scale changes in
allele frequencies within discrete populations, the principles of
population genetics are often conceptualized at microevolutionary scales.
The process by which an organism evolves to resemble another object, often an organism of another species. Mimicry can also occur between individuals of the same species. A type of adaptive signaling, mimicry evolves when a signal-receiver, known as the dupe, perceives the similarity between the mimic and the object or organism it is mimicking, known as the model, and as a result changes its behavior in a way that provides a
selective advantage to the mimic; the model may also benefit from the shared resemblance, in which case there is a
mutualism, or the mimicry may be to the model's detriment, making it
parasitic or competitive. The evolved resemblance may be visual, acoustic, chemical, tactile, or electrical, or any combination of sensory modalities. There are many varieties of mimicry, such as
Batesian,
Müllerian, and
Vavilovian.
In
eukaryoticcells, the part of the
cell cycle during which the division of the
nucleus takes place and
replicatedchromosomes are separated into two distinct nuclei. Mitosis is generally preceded by the S stage of
interphase, when the cell's
DNA is replicated, and either occurs simultaneously with or is followed by
cytokinesis, when the
cytoplasm and
cell membrane are divided into two new daughter cells.
A classification scheme of
speciation processes based on the level of
gene flow between two populations.[21] The traditional terms for the three modes—
allopatric,
parapatric, and
sympatric—are based on the spatial distributions of a species population.[22][21]
A branch of
genetics that employs methods and techniques of [[Glossary of genetics (M–Z)molecular biology#{{{1}}}|{{{1}}}]] to study the structure and function of
genes and
gene products at the
molecular level. Contrast classical genetics.
The evolutionary change of certain adaptive structures, traits, or other components of the phenotype at different times or different rates than others, either within a single species or between different species.[23]
mosaic hybrid zone
A zone in which two speciating lineages occur together in a patchy distribution–either by chance, random colonization, or low hybrid
fitness.[21]
mosaic sympatry
A case of
sympatry in which two populations overlapping in geographic distribution exhibit habitat specializations.[21]
The presence of two or more populations of cells with different
genotypes in an individual organism which has developed from a single fertilized
egg. A mosaic organism can result from many kinds of genetic phenomena, including
nondisjunction of chromosomes,
endoreduplication, or mutations in individual
stem cell lineages during the early development of the embryo. Mosaicism is similar to but distinct from
chimerism.
Any permanent change in the
nucleotide sequence of a strand of
DNA or
RNA, or in the
amino acid sequence of a
peptide. Mutations play a role in both normal and abnormal biological processes; their natural occurrence is integral to the process of
evolution. They can result from errors in
replication, chemical damage, exposure to high-energy radiation, or manipulations by
mobile genetic elements.
Repair mechanisms have evolved in many organisms to correct them. By understanding the effect that a mutation has on
phenotype, it is possible to establish the function of the
gene or sequence in which it occurs.
mutation distance
The smallest number of
mutations required to derive one particular gene, sequence, or phenotype from another;[8] the minimum number of nucleobase insertions, deletions, or substitutions necessary to change one sequence into another.
The actual origin of a particular
mutation in time and space; the instance of its original introduction into a genome, as opposed to that of its phenotypic manifestation, which may only occur
generations after the fact.
The frequency of new
mutations at a particular
locus or in a particular
gene,
sequence,
genome, or organism over a specified period of time, e.g. during a single
generation. Mutation rates may be calculated for a specific class of mutation or for all types collectively; they vary widely by organism and with an organism's environment.
1. Any
mutation of a
nucleic acid sequence that is neither beneficial nor detrimental to the ability of an organism to survive and reproduce.
2. Any mutation for which
natural selection does not affect the spread of the mutation within a population.
nexus hypothesis
The hypothesis that each phenotypic trait is likely to be influenced by more than one gene, and conversely that most genes affect more than one phenotype.[23]
1. The ecological role of a particular
species or other
taxon in a larger
community, generally conceptualized as the multidimensional space, of which the coordinates are the various parameters representing the conditions which are necessary for the existence of the species in every aspect of its present form, to which a species is restricted by the presence of competitor species.[23]
2. A particular environment or environmental condition to which a species is matched; the variety of activities, behaviors, and ecological functions carried out by an organism or population in response to its environmental context, e.g. the distribution of resources and competitors, and the ways in which it in turn alters that same context. The term is sometimes used loosely as an equivalent of
microhabitat, in the sense of the physical space occupied by a species.[23] See also fundamental niche and realized niche.
A type of
point mutation which results in a premature
stop codon in the
transcribedmRNA sequence, thereby causing the premature termination of
translation, which results in a truncated, incomplete, and often non-functional
protein.
Also nonsynonymous substitution or replacement mutation.
A type of
mutation in which the
substitution of one
nucleotide base for another results, after
transcription and
translation, in an amino acid sequence that is different from that produced by the original unmutated gene. Because nonsynonymous mutations always result in a biological change in the organism, they are often subject to strong
selection pressure. Contrast synonymous mutation.
Any
allele made non-functional by way of a genetic
mutation. The mutation may result in the complete failure to produce a
gene product or a gene product that does not function properly; in either case, the allele may be considered non-functional.
The condition of a cell or organism lacking all of the copies of a particular
chromosome that are normal for its
ploidy level; e.g. in a
diploid organism, lacking both members of the normal pair. Nullisomy is frequently lethal early in development.
The origination and
biological development of an organism within its own lifetime, as opposed to
phylogeny, which refers to the evolutionary history of the organism's ancestors. In sexually reproducing organisms, ontogeny is the study of the development of an organism from the time of
fertilization to the organism's reproductively mature form; the term may also be used to refer to the study of an organism's entire lifespan.
Any coding or non-coding DNA sequence which is derived from a
tandemmultigene family or
cluster but is physically isolated from the other genes in the family because it is dispersed to a distant
locus in the genome. Orphons are usually non-functional
pseudogenes with highly
variable copy numbers.[24]
One of a set of
genes (or more generally any DNA sequences showing
homology) which are present in different
genomes but are descended from the same ancestral sequence, i.e. they are directly related to one another by vertical descent from a single gene or sequence in the
most recent common ancestor of those genomes. Such genes or sequences are said to be orthologous. Orthologs can be inferred to be related to each other based on the similarity of their sequences; though they may have evolved independently within separate genomes by mutation and natural selection, their
products may still retain similar structures, functions, or levels of
expression across species and populations. The identification of orthologs has proven important in inferring
phylogenetic relationships between organisms. Contrast paralog.
Sexual reproduction between different breeds or individuals, which has the potential to increase
genetic diversity by introducing unrelated genetic material into a breeding population. The reproductive event and the resulting progeny may both be referred to as an outcross, and the progeny is said to be outbred. Contrast inbreeding.
The independent evolution of similar or identical
derived traits or characters in related
lineages, thought usually to be based on similar modifications of common developmental pathways.[1] Contrast convergent evolution.
One of a set of
genes (or, more generally, any DNA sequences showing
homology) which are directly related to each other via one or more genetic
duplication events; such genes or sequences are said to be paralogous. Paralogs result from the duplication of a single sequence within a single
genome and then the subsequent
divergence of the duplicated sequences by mutation and natural selection (either within the original genome, or, during
speciation, in different genomes). Contrast ortholog.
The principle of accounting for empirical observations by whichever hypothesis requires the fewest or the simplest assumptions for which there is limited or no evidence. In biological systematics, maximum parsimony is an optimality criterion which invokes a minimum of evolutionary changes to infer
phylogenetic relationships; i.e. the
phylogenetic tree that minimizes the total number of character-state changes is to be preferred.[1]
A type of
asexual reproduction in which the growth and development of
embryos occurs without
fertilization. In animals which reproduce by parthenogenesis, an unfertilized
gamete of the female parent is capable of developing into an adult without any contribution from a male parent, resulting in offspring possessing only the mother's genetic material (the exact proportion of which depends on the parthenogenetic mechanism, of which there are numerous varieties). Some species reproduce exclusively by parthenogenesis, while others can switch between
sexual reproduction and parthenogenesis under certain environmental conditions.
(of an
offspring) Resembling the male parent, genotypically or phenotypically, more closely than the female parent;[8] derived from the father. Contrast matroclinous.
The proportion of individuals with a given
genotype who express the associated
phenotype, usually given as a percentage. Because of the many complex interactions that govern
gene expression, the same
allele may produce an observable phenotype in one individual but not in another. If less than 100% of the individuals in a population carrying the genotype of interest also express the associated phenotype, both the genotype and phenotype may be said to show incomplete penetrance. Penetrance quantifies the probability that an allele will result in the expression of its associated phenotype in any form, i.e. to any extent that makes an individual
carrier different from individuals without the allele. Compare expressivity.
pericentric
(of a gene or region of a chromosome) Positioned near the
centromere of the
chromosome.
A variation of allopatric speciation where a new species forms from a small, peripheral isolated population.[25] It is sometimes referred to as centripetal speciation in contrast to
centrifugal speciation.
The complete set of
phenotypes that are or can be expressed by a
genome, cell, tissue, organism, or species; the sum of all of its manifest chemical, morphological, and behavioral characteristics or traits.
phenomic lag
A delay in the
phenotypicexpression of a genetic
mutation owing to the time required for the manifestation of changes in the affected biochemical pathways.[13]
The composite of the observable morphological, physiological, and behavioral
traits of an
organism that result from the
expression of the organism's
genotype as well as the influence of environmental factors and the interactions between the two.
A model of evolution which theorizes that most speciation occurs slowly, uniformly, and gradually, and that there is seldom a clear line of demarcation between ancestral species and descendant species unless there is a sudden split which reproductively isolates members of the same population. The theory is often contrasted with
punctuated equilibrium.
A method used to infer the likelihood of specific traits being present in organisms whose phenotypes are incomplete or unknown based on their positions in a
phylogenetic tree relative to ancestors, descendants, or contemporaneous organisms with more completely understood phenotypes. A major application of this method is in
paleontology, where extinct organisms known only from fossils are compared to their closest known relatives in order to infer the presence or absence of certain traits for which fossils provide limited or no evidence, such as soft tissues, integumentary structures, and physiological and behavioral traits, though the method is extremely sensitive to confounds from
convergent evolution.
A graphical representation of a
phylogeny, consisting of a branching, tree-like diagram showing the evolutionary relationships between biological
species or other
taxa as inferred from similarities and differences in their morphological or genetic characteristics, and how they have all descended from a
common ancestor.
The study of the evolutionary history of and relationships between individuals or groups of organisms, such as species or populations, through methods that evaluate similarities and differences between observed
heritabletraits, including
morphological features and
DNA sequences. The graphical presentation of data from such analyses is known as a
phylogeny or
phylogenetic tree.
The phenomenon by which one
gene influences two or more seemingly unrelated
phenotypic traits, by any of several distinct but potentially overlapping mechanisms.
An
ancestral character state; i.e. the state or condition of a particular trait or feature (e.g. a specific
phenotype) that is present in the
common ancestor of a given
clade. Plesiomorphies may or may not be
shared by some or all descendants within the clade. The term is relative; a trait considered a plesiomorphy in one clade may not be considered a plesiomorphy in a different clade. Contrast apomorphy.
The regular and simultaneous occurrence in the same population of two or more
alleles (or
genotypes) at the same
locus at
frequencies that cannot be accounted for by recurrent mutation alone (generally at least 1%), implying that the multiple alleles are being stably inherited by members of the population.[13]
The grouping of organisms which do not share an immediate
common ancestor; such groups are said to be polyphyletic. The term is often applied to groups of organisms that share characteristics which appear to be similar but are not actually closely related, frequently as a result of
convergent evolution. The avoidance of polyphyletic groupings is often a stimulus for major revisions of biological classification schemes. Contrast monophyly and paraphyly.
(of a
cell or organism) Having more than two
homologous copies of each
chromosome. Polyploidy may occur as a normal condition of chromosomes in certain cells or even entire organisms, or it may result from abnormal cell division or a mutation causing the
duplication of the entire chromosome set. Contrast haploid and diploid; see also ploidy.
A group of organisms of the same
species which occupies a more or less well-defined geographic region and which exhibits reproductive continuity from
generation to generation. It is generally presumed that ecological and reproductive interactions occur more frequently among the members of the group than between them and members of other populations of the same species.[1]
A sharp, often sudden reduction in the size of a biological
population, often due to a major environmental event such as a flood, fire, volcanic eruption, drought, famine, or disease. Because only a small population with a narrower range of
genetic diversity remains afterward to pass on genes to future generations, such events tend to reduce the
genetic variation in the population's
gene pool, and often lead to new and distinct populations through
founder effects. Diversity increases again only when
gene flow from another population occurs, or very slowly over time as random mutations accumulate.
Any effect on the
expression or functionality of a
gene or sequence that is a consequence of its location or position within a
chromosome or other DNA molecule. A sequence's precise location relative to other sequences and structures tends to strongly influence its activity and other properties, because different
loci on the same molecule can have substantially different
genetic backgrounds and physical/chemical environments, which may also change over time. For example, the
transcription of a gene located very close to a
nucleosome,
centromere, or
telomere is often
repressed or entirely prevented because the proteins that make up these structures block access to the DNA by
transcription factors, while the same gene is transcribed at a much higher rate when located in
euchromatin. Proximity to
promoters,
enhancers, and other
regulatory elements, as well as to regions of frequent
transposition by
mobile elements, can also directly affect expression; being located near the end of a chromosomal arm or to common
crossover points may affect when
replication occurs and the likelihood of
recombination. Position effects are a major focus of research in the field of
epigenetic inheritance.
Possession of the necessary properties to permit a shift into a new
niche or habitat. A structure is said to be preadapted if it can
assume a new function before it itself becomes modified by
selection.[1]
Any of two or more different genes or sequences which have the same or similar contributions to
phenotype, and thus appear to be genuine
alleles, but are not actually structurally allelic (i.e. they do not occupy homologous
loci on
homologous chromosomes).[23]
A non-functional sequence of DNA that resembles a functional
gene. Pseudogenes are typically superfluous copies of functional genes which have been
duplicated by natural processes, except that they lack
regulatory sequences necessary for proper
transcription or
translation or contain other defects such as
frameshift mutations, premature
stop codons, or missing
introns.
pseudopolyploidy
1. The condition in which the number of
chromosomes in a chromosome set is
doubled (or tripled, etc.) but without a corresponding increase in the actual amount of genetic material (i.e. the
ploidy level). This occurs when the chromosomes of a normal chromosome complement (e.g.
diploid) become fragmented into smaller pieces, increasing the total number of individual chromosomes but not creating additional homologous copies of those chromosomes (such that the cell remains diploid).[23]
2. Any numerical relationship between chromosome sets in groups of related organisms which suggests that some of those organisms are
polyploids of others when in fact they are not.[23]
A tabular diagram used to predict the possible
genotypes that can be inherited by the
offspring of a particular
cross or breeding experiment by summarizing all of the various combinations of maternal
alleles with paternal alleles. The resulting table can then be used to determine the probabilities that the offspring will have a particular genotype. The usefulness of Punnett squares is limited to discrete
phenotypes inherited according to simple
Mendelian patterns.
A specific
nucleotide sequence suspected to be a functional
gene based on the identification of its
open reading frame. The gene is said to be "putative" in the sense that no function has yet been described for its
products.
A branch of
population genetics which studies
phenotypes that vary continuously (such as height or mass) as opposed to those that fall into discretely identifiable categories (such as eye color or the presence or absence of a particular
trait). Quantitative genetics employs statistical methods and concepts to link continuously distributed phenotypic values to specific
genotypes and
gene products.
A chromosomal model of speciation that occurs rapidly when a cross-fertilizing plant species buds off from a larger population on the periphery, experiencing interbreeding and strong genetic drift that results in a new species.[26][27][28] The model is similar to that of Ernst Mayr's
peripatric speciation.[29]
The
natural selection of combinations of traits in organisms or species which appear to involve a trade-off between quantity and quality of
offspring, whereby an organism or species may evolve to make use of either of two different reproductive strategies:
r-strategists tend to produce many, low-quality offspring, yielding large numbers of progeny during their lifespan but investing little or no energy in nurturing or protecting them, whereas
K-strategists tend to produce few, high-quality offspring, yielding small numbers of progeny but with a corresponding increase in parental investment. Which strategy evolves depends on which one results in greater
reproductive success, which itself often depends on the stability of the organism's environment. In an unstable environment, where the probability that any individual offspring will survive to maturity is low, investment in parental care may not be sensible, and the parent may be more likely to pass on its genetic material if it dedicates its metabolic energy to simply producing as many offspring as possible rather than to parenting. Conversely, in more stable environments where survival to maturity is relatively common, the parent may find greater success if it dedicates more time and energy to parental care, improving each individual offspring's likelihood of reproducing successfully. The different strategies are often accompanied by characteristic anatomical or physiological traits, e.g. r-selected species often have small body size, rapid development, and short lifespans.
A relationship between the
alleles of a
gene in which one allele produces an effect on
phenotype that is overpowered or "masked" by the contribution of another allele at the same
locus; the first allele and its associated phenotypic
trait are said to be recessive, and the second allele and its associated trait are said to be dominant. Often, recessive alleles code for inefficient or dysfunctional proteins. Like dominance, recessiveness is not an inherent property of any allele or phenotype, but simply describes its relationship to one or more other alleles or phenotypes. In genetics shorthand, recessive alleles are often represented by a lowercase letter (e.g. "a", in contrast to the dominant "A").
A
crossbreeding experiment designed to test whether parental sex influences the inheritance of a particular trait. In crosses where the parents differ in
genotype or
phenotype or both, and hence only one of the parents (either the male or female parent) expresses the trait of interest, the reciprocal cross is the inverse, in which the parent of the other sex expresses the trait of interest instead. For example, if in the first cross a male expressing the trait is crossed with a female not expressing it, then in the reciprocal cross a female expressing the trait is crossed with a male not expressing it. By observing the
progeny resulting from each cross, geneticists can make inferences about which
sex chromosome, if either, influences the trait's expression.
A type of
chromosomal translocation by which there is a reciprocal exchange of chromosome segments between two or more non-
homologouschromosomes. When the exchange of material is evenly balanced, reciprocal translocations are usually harmless.
A species or population that is the last surviving representative of an otherwise
extinct group,
taxon,
lineage, or
clade, or which has been left behind in a locality after extinction throughout most of a formerly larger geographic distribution.[1]
A graphical representation of the relationship between the number of individuals at a given stage of one
generation and the number of individuals at the same stage in a previous generation.[23]
The condition in which
interbreeding between two or more
populations of organisms is prevented by intrinsic factors, such that the members of one population cannot
mate with the members of another population and produce fertile
offspring. The evolution of reproductive isolation between members of different populations is usually considered the first step in the process of
speciation, because it effectively prevents
gene flow between the populations and thereby allows each to evolve independently; hence the existence of reproductive barriers is often used as a criterion by which to define species in various
species concepts. Isolation may occur when the populations are physically separated by environmental changes or migration such that members of the other population are simply inaccessible, or it may occur when anatomical or genetic differences make copulation between members of different populations impossible or at least ensure that any offspring that happen to develop are sterile, even though the populations are not physically separated from each other. Isolating mechanisms are typically classified as
prezygotic (isolating barriers occurring before the formation of a
zygote) and
postzygotic (isolating barriers occurring after the formation of a zygote).
The successful production of
offspring by an individual, often quantified as the number of offspring produced by the individual per reproductive event or during the individual's entire lifespan, or as the number of an individual's offspring that survive to reproductive maturity themselves or that are surviving at a given time.[23]
reproductivity effect
The decrease in the rate of reproduction of new individuals per colony member as colony size increases.[23]
An experimental approach in
molecular genetics in which a researcher starts with a known
gene and attempts to determine its function or its effect on phenotype by any of a variety of laboratory techniques, commonly by deliberately mutating the gene's nucleic acid sequence or by repressing or silencing its expression and then
screening the mutated organisms for obvious changes in phenotype. When the gene of interest is the only one in the genome whose expression has been manipulated, any observed phenotypic changes are assumed to be influenced by it. This is the opposite of
forward genetics, in which a known phenotype is linked to one or more unknown genes.
Any
mutation in a gene or DNA sequence which restores or
rescues the original function or phenotype that was altered or destroyed by a previous mutation in the same sequence.[8] Contrast forward mutation; see also suppressor mutation.
Connected
populations of the same
species, each of which can interbreed with closely sited, closely related populations, but for which there exist at least two "end" populations in the series which are too distantly related to interbreed.
A type of
chromosomal translocation by which
double-strand breaks at or near the
centromeres of two
acrocentricchromosomes cause a reciprocal exchange of segments that gives rise to one large
metacentric chromosome (composed of the
long arms) and one extremely small chromosome (composed of the
short arms), the latter of which is often subsequently lost from the cell with little effect because it contains very few genes. The resulting
karyotype shows one fewer than the expected total number of chromosomes, because two previously distinct chromosomes have essentially fused together.
Carriers of Robertsonian translocations are generally not associated with any phenotypic abnormalities, but do have an increased risk of generating meiotically unbalanced
gametes.
The persistence of a certain phenotypic trait or characteristic in a biological system despite perturbations or conditions of uncertainty. Robustness is achieved through the combination of many genetic and molecular mechanisms which effectively preserve the integrity of a particular
adaptation, and can evolve by direct or indirect selection.
A sudden and large mutational change from one
generation to the next which is sufficient to cause rapid or immediate
speciation. Various forms of saltation, such as by
polyploidy in plants, have often historically been interpreted as evidence for certain theories of
mutationism, in contrast to
Darwiniangradualism.
The process by which two
allopatrically distributed populations of a species are geographically reunited. Contact between divergent populations may renew the potential for
gene flow between them, depending upon how
reproductively isolated the populations have become.
The process by which strong
positive selection of a new and beneficial
mutation within a population causes the mutation to reach
fixation so quickly that nearby
linked DNA sequences also become fixed via
genetic hitchhiking, thereby reducing or eliminating the
genetic variation of nearby loci within the population.
A reproductive strategy characterized by a single reproductive episode during an individual organism's lifetime, especially one in which the programmed death of the organism immediately after the reproductive event constitutes part of an overall strategy that includes putting all available resources into maximizing the probability of reproductive success, at the expense of the organism's future life. Organisms that use such a strategy are said to be semelparous. Semelparity is usually contrasted with
iteroparity.
semi-geographic speciation
semipermeable species boundary
The idea that gene flow can occur between two species but that certain
alleles at particular
loci can exchange whereas others cannot.[21] It is often used to describe hybrid zones and has also been referred to as porous.[21]
One of several groups of
populations that are partially but not entirely
reproductively isolated from each other by biological isolating mechanisms,[1] and which are therefore neither easily definable as belonging to the same
species nor to separate species. The taxon of species itself is
not a well-defined concept.
A basic unit of
biological classification, traditionally interpreted according to the
biological species concept as the members in aggregate of a group of
populations of organisms which interbreed or potentially interbreed with each other under natural conditions;[1] a basic
taxonomic rank to which individual specimens are assigned and which often but not always corresponds to the definition of a biological species; and a fundamental unit used to interpret and measure
biodiversity in ecological contexts. The concept of species is notoriously complex and often
problematic to define precisely; many different
conceptualizations of what is or should be meant by the term have been defined in scientific literature.
A named geographic
race, or a set of
populations of the same
species which share one or more distinctive features and occupy an area that is geographically separate from other subspecies.[1] Not all species are formally divided into subspecies, and the taxon of species itself is
not a well-defined concept.
A
biogeographic approach to species distributions that uses their phylogenetic histories—patterns resulting from
allopatric speciation events in the past.[33]
A phenomenon by which a reduction of
heterozygosity at a particular genetic locus within a population as a whole is observed when two or more subpopulations have different
allele frequencies at that locus, even if the subpopulations themselves are each in
Hardy–Weinberg equilibrium.
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^Douglas J. Futuyma (1989), "Speciational trends and the role of species in macroevolution", The American Naturalist, 134 (2): 318–321,
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^Luciano Nicolas Naka, Catherine L. Bechtoldt, L. Magalli Pinto Henriques, and Robb T. Brumfield (2012), "The Role of Physical Barriers in the Location of Avian Suture Zones in the Guiana Shield, Northern Amazonia", The American Naturalist, 179 (4): E115–E132,
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