4.5. Timing and mode of speciation
In order to estimate time since speciation, data from crabs, shrimps,
and urchins were used as proxy (mutation rate range of 0.016 to 0.026
substitutions per site, per million years) (Nydam & Harrison, 2011).
Using this rates, we obtained the speciation of C. verrucosasp. A and sp. B at 3.58 - 2.20 million years ago (MYA).
Several estimations of whole-genome mutation rates have been calculated
in ascidians pointing out the rapid evolution of this group (Berna &
Alvarez-Valin, 2014; Denoeud et al., 2010), but not specifically for
mitochondrial genome, which typically evolves faster than the nuclear
genome (Havird & Sloan, 2016). On average ascidian evolutionary
whole-genome rate in terms of number of substitutions per million years
is 6.25 times faster than vertebrates, and 2.08 faster than
cephalochordates (Delsuc et al., 2018). This indicates that, even though
we can estimate the speciation time based on other marine invertebrate
taxa data, we should bear in mind that we may be overestimating the time
since divergence and that the range of time since speciation proposed
here would be an older limit with the actual speciation time likely
being more recent. We can hypothesize that speciation took place after
the Miocene or later, when Antarctica already experienced the cooling
process (Zachos, Pagani, Sloan, Thomas & Billups, 2001). A summary of
the evolutionary patterns in Antarctic organisms based on other studies
reported radiation and speciation processes by 8-5 MYA; and cycles of
population concentration, isolation in refugia and expansion, speciation
and transoceanic dispersal by 1 MYA (Rogers, 2007). Some examples of
these processes in Antarctic taxa are arthropods, annelids, echinoderms
and molluscs (Baird, Miller, & Stark, 2011; Hemery et al., 2012; Linse,
Cope, Lörz, & Sands, 2007; Raupach et al., 2010; Riesgo et al., 2015;
Wilson et al., 2007). Then, a similar pattern of allopatric speciation
followed by secondary contact can be attributed to C. verrucosa(Mayr, 1963). On the other hand, speciation in response to ecological
opportunity (Simpson, 1953) can also be hypothesized with our results.
Under this type of speciation, a new trait evolves and affects the
ecological versatility of the specimens (Givnish et al., 2014; Liem,
1973). In this case, the development of a basal disc could represent an
adaptive character for colonizing different substrates.