Figure 2 in Reproductive isolation and the causes of speciation rate variation in nature

Figure 2. Alternative species taxonomies can lead to radically different perspectives on species and speciation. A, time-calibrated phylogeny for stickleback fishes (Gasterosteidae) from Rabosky et al. (2013). Divergence times are broadly congruent with the known fossil history for the genus (Bell, Stewart & Park, 2009). B, hypothetical shape of the stickleback phylogeny if some of the numerous intraspecific morphological variants (open circles) within the threespine stickleback (Gasterosteus aculeatus) are elevated to full species status. Many freshwater stickleback populations are morphologically distinct from the marine ancestral form and potentially warrant recognition as distinct biological species (Moodie & Reimchen, 1976; Mcphail, 1994; Bell, 1995; Nelson, 2006; Reimchen et al., 2013); there are potentially dozens or hundreds of such forms. Bell (1995) suggests that, given the extent of morphological parallelism among divergent forms, '... it is neither practical nor useful to describe the biological species within the G. aculeatus complex as separate taxonomic species'. However, given that many such forms could be recognized as distinct taxonomic species, it is worth considering how their recognition would change our interpretation of speciation rates. Previous studies on speciation rates across fishes (Near et al., 2013; Rabosky et al., 2013) include only information from deep nodes (dark circles). However, recent divergences, almost all of which are no older than the most recent glacial maximum (arrow), are ignored by this approach. For the phylogeny in (A), the maximum likelihood estimate of the speciation rate under a constant-rate birth-death model (Nee et al., 1994) is 0.055 lineages Myr–1. For the phylogeny in (B), which assumes the presence of 30 biological species in 'Gasterosteus aculueatus', all of which have diverged within the past 30 000 years, we infer a speciation rate of 65.4 lineages Myr–1. The number of recently-diverged species spliced into the phylogeny shown in (B) is arbitrary, although it illustrates the profound effects that taxonomic decisions can have on inferences about speciation rate. Understanding the relationship between speciation as studied at population genetic scales (open circles) and macroevolutionary scales (dark circles) is one of the central challenges for the future of speciation research.