Genetic divergence

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Genetic divergence is the process in which two or more populations of an ancestral species accumulate independent genetic changes (mutations) through time, often after the populations have become reproductively isolated for some period of time. [1] In some cases, subpopulations living in ecologically distinct peripheral environments can exhibit genetic divergence from the remainder of a population, especially where the range of a population is very large (see parapatric speciation). The genetic differences among divergent populations can involve silent mutations (that have no effect on the phenotype) or give rise to significant morphological and/or physiological changes. Genetic divergence will always accompany reproductive isolation, either due to novel adaptations via selection and/or due to genetic drift, and is the principal mechanism underlying speciation.

On a molecular genetics level, genetic divergence is due to changes in a small number of genes in a species, resulting in speciation. [2] However, researchers argue that it is unlikely that divergence is a result of a significant, single, dominant mutation in a genetic locus because if that were so, the individual with that mutation would have zero fitness. [3] Consequently, they could not reproduce and pass the mutation on to further generations. Hence, it is more likely that divergence, and subsequently reproductive isolation, are the outcomes of multiple small mutations over evolutionary time accumulating in a population isolated from gene flow. [2]

Genetic divergence between related populations sometimes start by genetic bottleneck, and founder effects.

Causes of Genetic Divergence

Founder Effect

One possible cause of genetic divergence is the founder effect, which is when a few individuals become isolated from their original population. Those individuals might overrepresent a certain genetic pattern, which means that certain biological characteristics are overrepresented. These individuals can form a new population with different gene pools from the original population. For example, 10% of the original population has blue eyes and 90% has brown eyes. By chance, 10 individuals are separated from the original population. If this small group has 80% blue eyes and 20% brown eyes, then their offspring would be more likely to have the allele for the blue eyes. As a result, the percentage of the population with blue eyes would be higher than the population with brown eyes, which is different from the original population.

Bottleneck Effect

Another possible cause of genetic divergence is the bottleneck effect. The bottleneck effect is when an event, such as a natural disaster, causes a large portion of the population to die. By chance, certain genetic patterns will be overrepresented in the remaining population, which similar to what happens with the founder effect. [4]

Disruptive Selection

A graph showing selection for the extremes and against the mean. Disruptive selection.png
A graph showing selection for the extremes and against the mean.

Genetic divergence can occur without geographic separation, through Disruptive selection. This occurs when individuals in a population with both high and low phenotypic extremes are fitter than the intermediate phenotype. [5] These individuals occupy two different niches, within each niche there is Gaussian trait distribution. [6] If the genetic variation between niches is high then there will be strong reproductive isolation. [6] If genetic variation is below a certain threshold than introgression will occur but if variation is above a certain threshold the population can split resulting in speciation. [6]

Disruptive selection is seen in the bimodal population of Darwin's finches, Geospiza fortis . [7] The two modes specialize in eating different types of seeds small and soft versus large and hard, this results in beaks of different sizes with different force capacities. [7] Individuals with intermediate beak sizes are selected against. [7] The song structure and response to song also differs between the two modes. [7] There is minimal gene flow between the two modes of G. fortis. [7]

Related Research Articles

Natural selection Mechanism of evolution by differential survival and reproduction of individuals

Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which in his view is intentional, whereas natural selection is not.

Speciation Evolutionary process by which populations evolve to become distinct species

Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book On the Origin of Species. He also identified sexual selection as a likely mechanism, but found it problematic.

Quantum evolution Evolution where transitional forms are particularly unstable and do not last long

Quantum evolution is a component of George Gaylord Simpson's multi-tempoed theory of evolution proposed to explain the rapid emergence of higher taxonomic groups in the fossil record. According to Simpson, evolutionary rates differ from group to group and even among closely related lineages. These different rates of evolutionary change were designated by Simpson as bradytelic, horotelic, and tachytelic.

Genetic drift The change in the frequency of an existing gene variant in a population

Genetic drift is the change in the frequency of an existing gene variant (allele) in a population due to random sampling of organisms. The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces. A population's allele frequency is the fraction of the copies of one gene that share a particular form.

Allopatric speciation Speciation that occurs between geographically isolated populations

Allopatric speciation, also referred to as geographic speciation, vicariant speciation, or its earlier name, the dumbbell model, is a mode of speciation that occurs when biological populations become geographically isolated from each other to an extent that prevents or interferes with gene flow.

Founder effect Loss of genetic variation resulting from a few individuals establishing a new population

In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, using existing theoretical work by those such as Sewall Wright. As a result of the loss of genetic variation, the new population may be distinctively different, both genotypically and phenotypically, from the parent population from which it is derived. In extreme cases, the founder effect is thought to lead to the speciation and subsequent evolution of new species.

Sympatric speciation A process through which new species evolve from a single ancestral species while inhabiting the same geographic region

Sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap so that they occur together at least in some places. If these organisms are closely related, such a distribution may be the result of sympatric speciation. Etymologically, sympatry is derived from the Greek roots συν ("together") and πατρίς ("homeland"). The term was coined by Edward Bagnall Poulton in 1904, who explains the derivation.

Cladogenesis

Cladogenesis is an evolutionary splitting of a parent species into two distinct species, forming a clade.

Peripatric speciation Speciation in which a new species is formed from an isolated smaller peripheral population

Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population. Since peripatric speciation resembles allopatric speciation, in that populations are isolated and prevented from exchanging genes, it can often be difficult to distinguish between them. Nevertheless, the primary characteristic of peripatric speciation proposes that one of the populations is much smaller than the other. The terms peripatric and peripatry are often used in biogeography, referring to organisms whose ranges are closely adjacent but do not overlap, being separated where these organisms do not occur—for example on an oceanic island compared to the mainland. Such organisms are usually closely related ; their distribution being the result of peripatric speciation.

Sympatry

In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sharing a common range exemplifies sympatric speciation. Such speciation may be a product of reproductive isolation – which prevents hybrid offspring from being viable or able to reproduce, thereby reducing gene flow – that results in genetic divergence. Sympatric speciation does not imply secondary contact, which is speciation or divergence in allopatry followed by range expansions leading to an area of sympatry. Sympatric species or taxa in secondary contact may or may not interbreed.

Disruptive selection

Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups. In this more individuals acquire peripheral character value at both ends of the distribution curve.

Parapatric speciation Speciation within a population where subpopulations are reproductively isolated

In parapatric speciation, two subpopulations of a species evolve reproductive isolation from one another while continuing to exchange genes. This mode of speciation has three distinguishing characteristics: 1) mating occurs non-randomly, 2) gene flow occurs unequally, and 3) populations exist in either continuous or discontinuous geographic ranges. This distribution pattern may be the result of unequal dispersal, incomplete geographical barriers, or divergent expressions of behavior, among other things. Parapatric speciation predicts that hybrid zones will often exist at the junction between the two populations.

The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring are sterile. These barriers maintain the integrity of a species by reducing gene flow between related species.

Introduction to evolution non-technical overview of the subject of biological evolution

Evolution is the process of change in all forms of life over generations, and evolutionary biology is the study of how evolution occurs. Biological populations evolve through genetic changes that correspond to changes in the organisms' observable traits. Genetic changes include mutations, which are caused by damage or replication errors in organisms' DNA. As the genetic variation of a population drifts randomly over generations, natural selection gradually leads traits to become more or less common based on the relative reproductive success of organisms with those traits.

Bateson–Dobzhansky–Muller model

The Bateson–Dobzhansky–Muller model, also known as Dobzhansky–Muller model, is a model of the evolution of genetic incompatibility, important in understanding the evolution of reproductive isolation during speciation and the role of natural selection in bringing it about. The theory was first described by William Bateson in 1909, then independently described by Theodosius Dobzhansky in 1934, and later elaborated in different forms by Herman Muller, H. Allen Orr and Sergey Gavrilets.

Ecological speciation

Ecological speciation is the process by which ecologically based divergent selection between different environments leads to the creation of reproductive barriers between populations. This is often the result of selection over traits which are genetically correlated to reproductive isolation, thus speciation occurs as a by-product of adaptive divergence.

Reinforcement (speciation)

Reinforcement is a process of speciation where natural selection increases the reproductive isolation between two populations of species. This occurs as a result of selection acting against the production of hybrid individuals of low fitness. The idea was originally developed by Alfred Russel Wallace and is sometimes referred to as the Wallace effect. The modern concept of reinforcement originates from Theodosius Dobzhansky. He envisioned a species separated allopatrically, where during secondary contact the two populations mate, producing hybrids with lower fitness. Natural selection results from the hybrid's inability to produce viable offspring; thus members of one species who do not mate with members of the other have greater reproductive success. This favors the evolution of greater prezygotic isolation. Reinforcement is one of the few cases in which selection can favor an increase in prezygotic isolation, influencing the process of speciation directly. This aspect has been particularly appealing among evolutionary biologists.

History of speciation

The scientific study of speciation — how species evolve to become new species — began around the time of Charles Darwin in the middle of the 19th century. Many naturalists at the time recognized the relationship between biogeography and the evolution of species. The 20th century saw the growth of the field of speciation, with major contributors such as Ernst Mayr researching and documenting species' geographic patterns and relationships. The field grew in prominence with the modern evolutionary synthesis in the early part of that century. Since then, research on speciation has expanded immensely.

Laboratory experiments of speciation

Laboratory experiments of speciation have been conducted for all four modes of speciation: allopatric, peripatric, parapatric, and sympatric; and various other processes involving speciation: hybridization, reinforcement, founder effects, among others. Most of the experiments have been done on flies, in particular Drosophila fruit flies. However, more recent studies have tested yeasts, fungi, and even viruses.

References

  1. "Reproductive Isolation". Understanding Evolution. Berkeley.
  2. 1 2 Palumbi, Stephen R. (1994). "Genetic Divergence, Reproductive Isolation, and Marine Speciation". Annual Review of Ecology and Systematics . 25: 547–572. doi:10.1146/annurev.ecolsys.25.1.547. JSTOR   2097324.
  3. Mayr, Ernst (1942). Systematics and the Origin of Species . New York: Columbia University Press.
  4. Campbell biology. Reece, Jane B., Campbell, Neil A., 1946-2004. (9th ed.). Boston: Benjamin Cummings / Pearson. 2011. pp. 476–480. ISBN   978-0-321-55823-7. OCLC   624556031.CS1 maint: others (link)
  5. Hill, W. G. (2013-01-01), Maloy, Stanley; Hughes, Kelly (eds.), "Disruptive Selection", Brenner's Encyclopedia of Genetics (Second Edition), San Diego: Academic Press, pp. 333–334, doi:10.1016/b978-0-12-374984-0.00411-3, ISBN   978-0-08-096156-9 , retrieved 2020-11-16
  6. 1 2 3 Barton, N. H. (2010-06-12). "What role does natural selection play in speciation?". Philosophical Transactions of the Royal Society B: Biological Sciences. 365 (1547): 1825–1840. doi:10.1098/rstb.2010.0001. ISSN   0962-8436.
  7. 1 2 3 4 5 Hendry, Andrew P; Huber, Sarah K; De León, Luis F; Herrel, Anthony; Podos, Jeffrey (2009-02-22). "Disruptive selection in a bimodal population of Darwin's finches". Proceedings of the Royal Society B: Biological Sciences. 276 (1657): 753–759. doi:10.1098/rspb.2008.1321. ISSN   0962-8452. PMC   2660944 . PMID   18986971.