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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 leading to reproductive isolation and continued mutation even after the populations have become reproductively isolated for some period of time, as there is not any genetic exchange anymore.[1] In some cases, subpopulations cover 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]
Causes
[edit]Founder effect
[edit]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
[edit]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 is similar to what happens with the founder effect.[4]
Disruptive selection
[edit]
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]
References
[edit]- ^ "Reproductive Isolation". Understanding Evolution. Berkeley. 16 April 2021.
- ^ a b 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.
- ^ Mayr, Ernst (1942). Systematics and the Origin of Species. New York: Columbia University Press.
- ^ 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.
{{cite book}}: CS1 maint: others (link) - ^ Hill, W. G. (2013-01-01), "Disruptive Selection", in Maloy, Stanley; Hughes, Kelly (eds.), 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
- ^ a b c 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. PMC 2871892. PMID 20439284.
- ^ a b c d e 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.
Genetic divergence
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Fundamentals
Definition
Genetic divergence is the evolutionary process by which two or more populations derived from a common ancestral species accumulate independent genetic changes over time, resulting in heritable differences in allele frequencies or DNA sequences between them.[10][1] This accumulation typically follows the initial separation of populations, often due to barriers that reduce or eliminate gene flow, allowing distinct evolutionary trajectories to emerge.[11] Unlike genetic variation within a single population, which arises from polymorphisms among individuals and maintains overall diversity, genetic divergence focuses on systematic differences that develop between populations, potentially culminating in reproductive isolation and speciation.[1][12] Reproductive isolation prevents interbreeding upon secondary contact, reinforcing the genetic distinctions and marking a critical threshold in evolutionary divergence.[12] The term genetic divergence originated in the field of evolutionary biology during the 20th century, as part of the foundational developments in population genetics that integrated Mendelian inheritance with Darwinian evolution.[11] Pioneering work by figures such as Ronald Fisher, J.B.S. Haldane, and Sewall Wright in the 1920s and 1930s established the theoretical framework for understanding how genetic differences arise and persist between populations.[11] A seminal contribution came from Sewall Wright's 1943 paper on isolation by distance, which demonstrated mathematically how limited dispersal and gene flow lead to increasing genetic differentiation across spatial gradients, providing an early model for post-isolation divergence. At its core, the process of genetic divergence initiates with population separation—geographic, ecological, or otherwise—followed by independent evolution in each group, primarily through the actions of mutation introducing new variants, genetic drift randomly altering frequencies in small populations, and natural selection favoring adaptive traits in differing environments.[11]Molecular Basis
Genetic divergence at the molecular level originates from mutations, which introduce variations in the DNA sequence and serve as the ultimate source of new genetic material in evolving populations. These mutations encompass a range of types, including point mutations that substitute a single nucleotide base, insertions and deletions (indels) that add or remove segments of DNA, and chromosomal rearrangements such as inversions, translocations, or duplications that alter genome structure. Point mutations can be transitions (purine to purine or pyrimidine to pyrimidine) or transversions (purine to pyrimidine or vice versa), while indels often cause frameshift mutations in coding regions by disrupting the reading frame of codons, potentially leading to truncated or nonfunctional proteins. Chromosomal rearrangements, though less frequent, contribute significantly to divergence by reshuffling genetic material, which can disrupt gene regulation or create novel gene combinations, thereby facilitating long-term evolutionary changes.[13][14] Restriction of gene flow between populations is essential for mutations to accumulate differently, as it prevents the exchange of alleles that would otherwise homogenize genetic variation. Physical barriers, such as mountains, rivers, or oceanic distances, impede migration and mating, while behavioral isolation—manifested through differences in courtship signals, mating preferences, or habitat choices—further limits interbreeding even in sympatric conditions. Without such restrictions, mutations arising in one population would spread to others via gene flow, reducing opportunities for independent fixation and divergence; instead, isolation allows local mutations to become established through stochastic or selective processes unique to each group.[15] The neutral theory of molecular evolution, proposed by Motoo Kimura in 1968, posits that the majority of genetic divergence results from the accumulation of neutral mutations—those neither advantageous nor deleterious—that fix in populations via random genetic drift at a roughly constant rate proportional to the mutation rate. This theory explains the observed uniformity in molecular evolutionary rates across diverse taxa, suggesting that selective constraints affect only a small fraction of the genome, while most changes occur in nonfunctional or redundant regions without impacting fitness. Empirical support for this framework comes from molecular clock analyses, where neutral substitutions accumulate predictably over time, underpinning much of the neutral divergence between lineages.[16] In protein-coding regions, the distinction between synonymous and nonsynonymous substitutions highlights the selective pressures shaping divergence. Synonymous substitutions alter the DNA sequence but not the encoded amino acid due to the degeneracy of the genetic code, allowing them to accumulate at higher rates as they are typically neutral and evade purifying selection. Nonsynonymous substitutions, by contrast, change the amino acid, potentially altering protein structure and function, and thus evolve more slowly under purifying selection to preserve functional integrity, though positive selection can accelerate them in adaptive contexts. The ratio of nonsynonymous to synonymous substitution rates (dN/dS) serves as a key indicator of these dynamics, with values near or below 1 reflecting predominant neutrality or constraint in coding sequences.[17][18]Causes
Genetic Drift
Genetic drift is a fundamental mechanism driving genetic divergence through random changes in allele frequencies, arising from sampling error in finite populations rather than adaptive pressures. In every generation, the genetic composition of offspring is a random sample of the parental gene pool, leading to stochastic fluctuations that can cause alleles to increase, decrease, or even fix or be lost over time. This process is particularly pronounced in small populations, where chance events have a disproportionately large impact, promoting divergence between populations by eroding genetic variation and altering allele distributions unpredictably.[19] A prominent manifestation of genetic drift is the founder effect, which occurs when a small subset of individuals from a larger population establishes a new colony, resulting in reduced genetic diversity and potential divergence from the source population. The founding group's allele frequencies may deviate substantially from the original due to sampling bias, setting the stage for further drift in the isolated group. A classic example is the colonization of the Galápagos Islands by Darwin's finches, where small founding populations from South America led to distinct genetic profiles across islands, contributing to morphological and genetic divergence among species. Studies estimate the founding population size for these finches at around 30-100 individuals, amplifying drift's role in their early evolution.[20] Another key form is the bottleneck effect, where a drastic reduction in population size—often due to environmental catastrophes, disease, or habitat loss—causes a severe loss of genetic variation as only a random subset of alleles survives. Post-bottleneck populations exhibit heightened drift, accelerating divergence through fixation of rare alleles or loss of others. In cheetahs (Acinonyx jubatus), a bottleneck approximately 10,000–12,000 years ago, near the end of the last Ice Age, reduced effective population size dramatically, leading to extremely low genetic diversity observed today, including monomorphic profiles at many loci and increased susceptibility to diseases. This event underscores how bottlenecks can cause rapid, non-adaptive genetic divergence lasting for millennia.[21] The mathematical foundation of genetic drift is encapsulated in the Wright-Fisher model, a seminal framework assuming a diploid population of fixed size where each generation is formed by random sampling with replacement from the previous one. For a biallelic locus with initial allele frequency , the variance in the change of frequency per generation is given by:
where is the effective population size, highlighting that drift's strength inversely scales with population size—smaller yields larger random changes, fostering divergence. This model, developed independently by Fisher and Wright, underpins much of population genetics and demonstrates drift's neutral, probabilistic nature.
Natural Selection
Natural selection drives genetic divergence by favoring alleles that confer higher fitness in specific environments, thereby altering allele frequencies in a directional manner and promoting adaptive differences between populations. When populations inhabit contrasting habitats, differential survival and reproduction lead to the fixation or increase of locally advantageous variants, reducing genetic similarity over time. This process relies on heritable phenotypic variation, upon which selection acts to optimize adaptation, as originally conceptualized in foundational evolutionary theory.[22] Disruptive selection contributes to divergence by favoring extreme phenotypes at the expense of intermediates, thereby maintaining polymorphism within populations and facilitating the split into distinct genetic clusters adapted to varied resources. A prominent example occurs in Darwin's medium ground finches (Geospiza fortis), where bimodal beak size variation evolves under disruptive pressures: small-beaked individuals efficiently exploit small, soft seeds, while large-beaked ones handle large, hard seeds, with strong selection against intermediate sizes preserving the modes and promoting assortative mating that limits gene flow. This pattern, observed during periods of resource scarcity, underscores how disruptive selection can accelerate divergence even within sympatric populations.[23] Directional and stabilizing selection further promote divergence when populations encounter heterogeneous habitats, shifting or maintaining trait distributions toward local optima. In the peppered moth (Biston betularia), directional selection during Britain's Industrial Revolution (starting around the 1850s) rapidly increased the frequency of the melanic carbonaria allele in polluted regions, as dark individuals gained camouflage against soot-blackened trees and evaded bird predation more effectively than light forms; experiments confirmed a survival advantage of approximately 2:1 for melanics in industrial woods. Meanwhile, stabilizing selection reinforces divergence by conserving intermediate phenotypes suited to stable but distinct environments across populations, preventing reversion and solidifying genetic differences as selective optima vary spatially.[24][22] Gene-environment interactions, mediated by epistasis and pleiotropy, accelerate divergence rates by generating context-specific fitness landscapes that amplify adaptive responses in isolated populations. Epistasis, where the effect of one allele depends on others, can create non-additive interactions that enhance the evolutionary response to selection, such as positive epistasis boosting trait shifts in novel environments and leading to incompatible genetic backgrounds between diverging groups. Pleiotropy, in which genes influence multiple traits, further hastens isolation by linking adaptive changes (e.g., flowering time adjustments) to reproductive barriers, as seen in plants like Arabidopsis thaliana, where such effects promote rapid divergence under divergent selection.[25][26]Measurement
Genetic Distance Metrics
Genetic distance metrics provide quantitative measures of the genetic differences that have accumulated between populations or species over time, enabling researchers to assess the extent of divergence based on allele frequencies or sequence data. These metrics are essential for comparing genetic variation and are grounded in population genetics theory, often assuming neutral evolution or specific substitution models. They facilitate the construction of phylogenetic trees and the estimation of evolutionary relationships by translating observable genetic differences into estimates of divergence. One widely used metric for allele frequency data is Nei's standard genetic distance, introduced by Masatoshi Nei in 1972. This measure quantifies the extent of genetic differentiation based on the identity of genes between populations, defined as the average number of nucleotide substitutions per locus under the infinite alleles model. The genetic identity for a locus is calculated as , where and are the frequencies of the -th allele in populations and , respectively; the overall identity is the average across loci. The distance is then given by:
This formulation assumes a constant rate of gene substitution and provides a linear relationship with divergence time, making it suitable for evolutionary studies. Nei's distance has been extensively applied in population genetics to evaluate differentiation in species ranging from plants to humans, with values typically small (e.g., 0.01–0.1) indicating recent divergence.
Another key metric is Wright's fixation index , developed by Sewall Wright in 1951 as part of his F-statistics framework to describe population structure. represents the proportion of total genetic variation attributable to differences between populations, calculated as:
where is the total heterozygosity across all populations and is the average heterozygosity within populations. Values of range from 0 (no differentiation) to 1 (complete differentiation), with empirical studies often reporting 0.05–0.15 for subdivided natural populations. This index is particularly useful for codominant markers like microsatellites and allozymes, helping to quantify isolation by distance or barriers to gene flow.
For nucleotide sequence data, Kimura's two-parameter (K2P) model, proposed by Motoo Kimura in 1980, accounts for the higher rate of transitions relative to transversions in DNA evolution. The evolutionary distance , or the number of substitutions per site, corrects for multiple hits and is estimated as:
where is the proportion of transitional differences and is the proportion of transversional differences between sequences. This model improves upon simpler ones like Jukes-Cantor by incorporating rate heterogeneity, yielding more accurate divergence estimates for closely related taxa (e.g., for intraspecies comparisons). It is a cornerstone in molecular phylogenetics software for aligning sequences and building trees.
These metrics are routinely applied in phylogenetics to infer divergence times under the molecular clock hypothesis, which posits a constant rate of genetic change across lineages. By calibrating distances with fossil records or known mutation rates, researchers estimate absolute timescales of speciation events, as demonstrated in studies of vertebrate evolution where Nei's or Kimura's correlates with geological divergence.