The role of natural selection in speciation, first described by Darwin, has finally been widely accepted. Yet, the nature and time course of the genetic changes that result in speciation remain mysterious. To date, genetic analyses of speciation have focused almost exclusively on retrospective analyses of reproductive isolation between species or subspecies and on hybrid sterility or inviability rather than on ecologically based barriers to gene flow. However, if we are to fully understand the origin of species, we must analyze the process from additional vantage points. By studying the genetic causes of partial reproductive isolation between specialized ecological races, early barriers to gene flow can be identified before they become confounded with other species differences. This population-level approach can reveal patterns that become invisible over time, such as the mosaic nature of the genome early in speciation. Under divergent selection in sympatry, the genomes of incipient species become temporary genetic mosaics in which ecologically important genomic regions resist gene exchange, even as gene flow continues over most of the genome. Analysis of such mosaic genomes suggests that surprisingly large genomic regions around divergently selected quantitative trait loci can be protected from interrace recombination by “divergence hitchhiking.” Here, I describe the formation of the genetic mosaic during early ecological speciation, consider the establishment, effects, and transitory nature of divergence hitchhiking around key ecologically important genes, and describe a 2-stage model for genetic divergence during ecological speciation with gene flow.

Abstract Populations of marine fishes are show relatively weak genetic differentiation due to the lack of geographical barriers and the potential for the strong diffusion ability favoring high levels of gene flow. The large yellow croaker (Larimichthys crocea) is a commercially important fish species with high gene flow, and its aquaculture production ranks first among all marine fishes cultured in China. To better understand the population structure of L. crocea, a total of 7161 quality-filtered SNP markers were identified in 120 individuals from five farmed and wild populations by using restriction site-associated DNA sequencing (RAD-seq). Our population genetic inferences suggest the existence of two genetic lineages of this croaker in the wild populations in the coastal waters of China. We suggested that an allopatric speciation mechanism is typically characterized by the formation of the Taiwan Strait between the South and East China Sea. The farmed and wild populations present in Ningde City exhibit similar genetic ancestry, which implies that the latter population may represent escaped farmed fish with a higher discharge frequency, rather than areal wild population.

<h4>Background</h4>Speciation corresponds to the progressive establishment of reproductive barriers between groups of individuals derived from an ancestral stock. Since Darwin did not believe that reproductive barriers could be selected for, he proposed that most events of speciation would occur through a process of separation and divergence, and this point of view is still shared by most evolutionary biologists today.<h4>Results</h4>I do, however, contend that, if so much speciation occurs, the most likely explanation is that there must be conditions where reproductive barriers can be directly selected for. In other words, situations where it is advantageous for individuals to reproduce preferentially within a small group and reduce their breeding with the rest of the ancestral population. This leads me to propose a model whereby new species arise not by populations splitting into separate branches, but by small inbreeding groups "budding" from an ancestral stock. This would be driven by several advantages of inbreeding, and mainly by advantageous recessive phenotypes, which could only be retained in the context of inbreeding. Reproductive barriers would thus not arise as secondary consequences of divergent evolution in populations isolated from one another, but under the direct selective pressure of ancestral stocks. Many documented cases of speciation in natural populations appear to fit the model proposed, with more speciation occurring in populations with high inbreeding coefficients, and many recessive characters identified as central to the phenomenon of speciation, with these recessive mutations expected to be surrounded by patterns of limited genomic diversity.<h4>Conclusions</h4>Whilst adaptive evolution would correspond to gains of function that would, most of the time, be dominant, this type of speciation by budding would thus be driven by mutations resulting in the advantageous loss of certain functions since recessive mutations very often correspond to the inactivation of a gene. A very important further advantage of inbreeding is that it reduces the accumulation of recessive mutations in genomes. A consequence of the model proposed is that the existence of species would correspond to a metastable equilibrium between inbreeding and outbreeding, with excessive inbreeding promoting speciation, and excessive outbreeding resulting in irreversible accumulation of recessive mutations that could ultimately only lead to extinction.

Our understanding of speciation processes is constantly changing. Two important concepts that have influenced thinking over the long term are the isolation and the genic views of speciation. However, neither of these views is fully compatible with our current understanding of speciation processes. The fact that many species with overlapping ranges remain distinct despite hybridisation and introgression suggests that species are not co‐adapted gene pools that have evolved in allopatry and are protected from merging by reproductive isolation, as the isolation view of speciation assumes. Speciation can occur without geographic isolation, and alleles of some genes may be exchanged between species, while others may not, as suggested by the genic view of speciation. However, in contrast to the genic view, the alleles of genes underlying differential adaptation may not have opposite fitness effects. Rather, the traits under selection may be determined by effects of many genes. The facts that individual genes have usually only small effects on adaptive traits, the same trait can be achieved by different combinations of alleles, homologous traits may be determined by different gene regulatory underpinnings, new traits resulting from transgressive segregation may be essential for differential adaptation, and that non‐genic traits can cause speciation, indicate that traits rather than individual genes are the most relevant units of speciation. The development of strong barriers that allow diverging populations to coexist may require a coupling of barrier effects, which can be facilitated by several factors, such as the pleiotropic effects of many genes or structural variants that reduce recombination, which are not considered in the genic view. Considering traits rather than single genes as units of speciation, and considering factors above the level of genes that contribute to the coupling of barrier effects, distinguishes the trait view of speciation from the genic view.

Understanding general principles about the origin of species remains one of the foundational challenges in evolutionary biology. The genomic divergence between groups of individuals can spawn hybrid inviability and hybrid sterility, which presents a tantalizing developmental problem. Divergent developmental programs may yield either conserved or divergent phenotypes relative to ancestral traits, both of which can be responsible for reproductive isolation during the speciation process. The genetic mechanisms of developmental evolution involve cis- and trans-acting gene regulatory change, protein-protein interactions, genetic network structures, dosage, and epigenetic regulation, all of which also have roots in population genetic and molecular evolutionary processes. Toward the goal of demystifying Darwin's "mystery of mysteries," this review integrates microevolutionary concepts of genetic change with principles of organismal development, establishing explicit links between population genetic process and developmental mechanisms in the production of macroevolutionary pattern. This integration aims to establish a more unified view of speciation that binds process and mechanism.

For speciation to occur, two new populations must form from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation (allo- = "other"; -patric = "homeland") involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation (sym- = "same"; -patric = "homeland") involves speciation occurring within a parent species remaining in one location. Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why more than two species might not form at one time except that it is less likely and we can conceptualize multiple events as single splits occurring close in time. A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across a species' range, is relatively free because individuals can move and then mate with individuals in their new location.

Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for 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 identifie In parapatric speciation, there is only partial separation of the zones of two diverging populations afforded by geography; individuals of each species may come in contact or cross habitats from time to time, but reduced fitness of the heterozygote leads to selection for behaviours or mechanisms that prevent their interbreeding. Parapatric speciation is modelled on continuous variation within a "single", connected habitat acting as a source of natural selection rather than the effects of isolation of habitats produced in peripatric and allopatric speciation. Parapatric speciation may be associated with differential landscape-dependent selection. Even if there is a gene flow between two populations, strong differential selection may impede assimilation and different species may eventually develop. Habitat differences may be more important in the development of reproductive isolation than the isolation time. Caucasian rock lizards Darevskia rudis, D. valentini and D. portschinskii all hybridize with each other in their hybrid zone; however, hybridization is stronger between D. portschinskii and D. rudis, which separated earlier but live in similar habitats than between D. valentini and the two other species, which separated later but live in climatically different habitats. Ecologists refer to parapatric and peripatric speciation in terms of ecological niches. A niche must be available in order for a new species to be successful. Ring species such as Larus gulls have been claimed to illustrate speciation in progress, though the situation may be more complex. The grass Anthoxanthum odoratum may be starting parapatric speciation in areas of mine contamination. Hybridization between two different species sometimes leads to a distinct phenotype. This phenotype can also be fitter than the parental lineage and as such natural selection may then favor these individuals. Eventually, if reproductive isolation is achieved, it may lead to a separate species. However, reproductive isolation between hybrids and their parents is particularly difficult to achieve and thus hybrid speciation is considered an extremely rare event. The Mariana mallard is thought to have arisen from…

primary geographic modes of speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically Evolution is the change in the heritable characteristics of biological populations over successive generations. It occurs when evolutionary processes such as genetic drift and natural selection act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. The process of evolution has given rise to biodiversity at every E…

Finally, I analyze a fairly general class of underdominant speciation models involving multigene families, concluding for these models under weak conversion that although the probability of fixation may be relatively high, the expected time to fixation is extremely long, so that speciation by "molecular drive" is unlikely. Furthermore, speciation occurs faster by fixing underdominant alleles of the same effect at single-copy genes than by fixing the same number of loci in a single multigene family under the joint effects of selection, conversion, and drift. Published in Proceedings of the National Academy of Sciences of the United States of America (1985)

More than one hundred years ago, JBS Haldane noted that in a cross between two species, if one of the sexes is absent, rare or sterile, the affected sex is heterogametic. The underlying genetic causes for this phenomenon have been a source of debate ever since. Here, we test how Haldane's rule operates in Caenorhabditis nematodes by studying (1) crosses involving sex-determination mutants of two hybridizing species, and (2) crosses involving tetraploids. Our results indicate that the critical feature underlying Haldane's rule is incompatibility between a sex chromosome derived from only one species, and autosome pairs derived from both. In addition, we show that the mechanisms for evaluating the X:Autosome ratio have diverged during recent nematode evolution. We conclude that the way interactions between sex chromosomes and autosomes are structured causes them to play an important role in establishing genetic barriers between newly separating species.