Abstract
Why organisms diversify into discrete species instead of showing a continuum of genotypic and phenotypic forms is an important yet rarely studied question in speciation biology. Does species discreteness come from adaptation to fill discrete niches or from interspecific gaps generated by reproductive isolation? We investigate the importance of reproductive isolation by comparing genetic discreteness, in terms of intra- and interspecific variation, between facultatively sexual monogonont rotifers and obligately asexual bdelloid rotifers. We calculated the age (phylogenetic distance) and average pairwise genetic distance (raw distance) within and among evolutionarily significant units of diversity in six bdelloid clades and seven monogonont clades sampled for 4211 individuals in total. We find that monogonont species are more discrete than bdelloid species with respect to divergence between species but exhibit similar levels of intraspecific variation (species cohesiveness). This pattern arises because bdelloids have diversified into discrete genetic clusters at a faster net rate than monogononts. Although sampling biases or differences in ecology that are independent of sexuality might also affect these patterns, the results are consistent with the hypothesis that bdelloids diversified at a faster rate into less discrete species because their diversification does not depend on the evolution of reproductive isolation.
Highlights
Genetic, morphological, and behavioral evidence have accumulated to show that species are real entities maintained by ecological differences and reproductive isolation (Schluter 2001; Coyne and Orr 2004; Puritz et al 2012)
Does species discreteness come from adaptation to fill discrete niches or from interspecific gaps generated by reproductive isolation? We investigate the importance of reproductive isolation by comparing genetic discreteness, in terms of intra- and interspecific variation, between facultatively sexual monogonont rotifers and obligately asexual bdelloid rotifers
Sampling biases or differences in ecology that are independent of sexuality might affect these patterns, the results are consistent with the hypothesis that bdelloids diversified at a faster rate into less discrete species because their diversification does not depend on the evolution of reproductive isolation
Summary
Morphological, and behavioral evidence have accumulated to show that species are real entities maintained by ecological differences and reproductive isolation (Schluter 2001; Coyne and Orr 2004; Puritz et al 2012). Species are a fundamental unit of biology, and the existence of discontinuous groups of organisms, as opposed to a continuum of genotypes and phenotypes, is regarded as a ubiquitous phenomenon of life (Rieseberg et al 2006). Why this discreteness exists at all is a neglected question and “perhaps the most important question about speciation” Reproductive isolation breaks up the continuum of genotypic and phenotypic diversity, and prevents the formation of hybrid forms (i.e., hybrid inferiority—Burke and Arnold 2001), whereas sexual reproduction maintains genetic coherence within species. Comparing diversification patterns between the two reproductive modes will enable us to test the two hypotheses regarding why diversity is discontinuous (Maynard Smith and Szathmary 1995; Barraclough and Nee 2001; Barraclough and Herniou 2003; Coyne and Orr 2004)
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