What is a fish species?

The formal processes of alpha-taxonomy ensure that species have uniquenames and can be identified. No similar process is mandatory forinfraspecific variation, so the species is a uniquely importantpractical term. At present, there is little agreement of the definitionof a species. In the last 30 years, numerous concepts have beenproposed. The nature of fish species is reviewed. Clonal inheritance ofnuclear genes occurs in several lineages. Hybridization is frequent,often leading to introgression, which may lead to extinction of species.Species may have hybrid origins. There is good evidence for parallelspeciation in similar habitats. There are clearly exceptions to thecladistic assumption of dichotomous branching during speciation. Siblingspecies may exist with no discernible niche differentiation.Basic assumptions are violated for the recognition, phylogenetic,ecological and some formulations of the evolutionary species concepts.The most satisfactory definitions are two of the earliest proposed inthe light of evolutionary theory. The Darwinian view is that species arerecognizable entities which are not qualitatively distinct fromvarieties. A restatement of this concept in genetic terms provides ameans of dealing with all forms of species known in present-day fishes.This modified Darwinian concept is operated through the application offuzzy logic rather than rigid definition. This involves a search fordiscontinuities between species, rather than an a priori definition ofhow boundaries are to be determined. A subset of Darwinian species areMayrian or biological species, which are characterized by theirdemonstrable reproductive isolation from other species. The status of apopulation as a Mayrian species is a testable hypothesis. Moleculartechniques allow this hypothesis to be tested more easily thanpreviously, at least when dealing with sympatric populations.     

Divergence population genetic analysis of hybridization between rhesus and cynomolgus macaques

The geographic ranges of rhesus ( Macaca mulatta ) and cynomolgus ( M. fascicularis ) macaques adjoin in Indochina where they appear to hybridize. We used published and newly generated DNA sequences from 19 loci spanning ~20 kb to test whether introgression has occurred between these macaque species. We studied introgression at the level of nuclear DNA and distinguished between incomplete lineage sorting of ancestral polymorphisms or interspecific gene flow. We implemented a divergence population genetics approach by fitting our data to an isolation model implemented in the software IMa. The model that posits no gene flow from the rhesus into the cynomolgus macaque was rejected ( P  = 1.99 × 10⁻⁸). Gene flow in this direction was estimated as 2 Nm ~1.2, while gene flow in the reverse direction was nonsignificantly different from zero ( P  = 0.16). The divergence time between species was estimated as ~1.3 million years. Balancing selection, a special case of incomplete sorting, was taken into consideration, as well as potential crossbreeding in captivity. Parameter estimates varied between analyses of subsets of data, although we still rejected isolation models. Geographic sampling of the data, where samples of cynomolgus macaques derived from Indochina were excluded, revealed a lost signature of gene flow, indicating that interspecific gene flow is restricted to mainland Indochina. Our results, in conjunction with those by others, justify future detailed analyses into the genetics of reproductive barriers and reticulate evolution in these two genome-enabled primates. Future studies of the natural hybridization between rhesus and cynomolgus macaques would expand the repertoire of systems available for speciation studies in primates.     

Go east: phylogeographies of Mauremys caspica and M. rivulata- discordance of morphology, mitochondrial and nuclear genomic markers and rare hybridization

In recent years many cases of hybridization and introgression became known for chelonians, requiring a better understanding of their speciation mechanisms. Phylogeographic investigations offer basic data for this challenge. We use the sister species Mauremys caspica and M. rivulata, the most abundant terrapins in the Near and Middle East and South-east Europe, as model. Their phylogeographies provide evidence that speciation of chelonians fits the allopatric speciation model, with both species being in the parapatric phase of speciation, and that intrinsic isolation mechanisms are developed during speciation. Hybridization between M. caspica and M. rivulata is very rare, suggesting that the increasing numbers of hybrids in other species are caused by human impact on environment (breakdown of ecological isolation). Genetic differentiation within M. caspica and M. rivulata resembles the paradigm of southern genetic richness and northern purity of European biota. However, in west Asia this pattern is likely to reflect dispersal and vicariance events older than the Holocene. For M. caspica three distinct Pleistocene refuges are postulated (Central Anatolia, south coast of Caspian Sea, Gulf of Persia). Morphologically defined subspecies within M. caspica are not supported by genetic data. This is one of the few studies available about the phylogeography of west and central Asian species.     

Hybridization, ecological races and the nature of species: empirical evidence for the ease of speciation

Species are generally viewed by evolutionists as 'real' distinct entities in nature, making speciation appear difficult. Charles Darwin had originally promoted a very different uniformitarian view that biological species were continuous with 'varieties' below the level of species and became distinguishable from them only when divergent natural selection led to gaps in the distribution of morphology. This Darwinian view on species came under immediate attack, and the consensus among evolutionary biologists today appears to side more with the ideas of Ernst Mayr and Theodosius Dobzhansky, who argued 70 years ago that Darwin was wrong about species. Here, I show how recent genetic studies of supposedly well-behaved animals, such as insects and vertebrates, including our own species, have supported the existence of the Darwinian continuum between varieties and species. Below the level of species, there are well-defined ecological races, while above the level of species, hybridization still occurs, and may often lead to introgression and, sometimes, hybrid speciation. This continuum is evident, not only across vast geographical regions, but also locally in sympatry. The existence of this continuum provides good evidence for gradual evolution of species from ecological races and biotypes, to hybridizing species and, ultimately, to species that no longer cross. Continuity between varieties and species not only provides an excellent argument against creationism, but also gives insight into the process of speciation. The lack of a hiatus between species and ecological races suggests that speciation may occur, perhaps frequently, in sympatry, and the abundant intermediate stages suggest that it is happening all around us. Speciation is easy!