Abstract
Abstract.– Background: Explanations for asymmetric patterns of diversification continue to challenge paleontologists and neontologists with competing hypotheses within genetic-development and ecological frameworks. In 1988, a hypothesis was proposed that tied a primordial germ cell (PGC) determination mechanism to clade (phyla) diversification. Two general mechanisms for PGC determination are recognized: one is termed induced because induction signals are required for the production of primordial germ cells. The other mechanism is cell-autonomous, i.e. determinative, because the cells that develop in response to specific cytoplasmic determinants in the oocyte are pre-destined to become PGCs. We revisited the hypothesis and analyzed phyla diversity with germ cell determination mechanisms and examined sister clade asymmetry. Results: After 25 years of additional data accumulation, the hypothesis that high levels of species diversification are associated with the induced mode is falsified, with the determinative mode revealed as associated with higher rates of diversification. The greater species numbers are significantly associated (ANOVA p>0.003) with the determinative mode. Analysis with appropriate sister clades is unanimous in showing the clade with the determinative mode has a significantly greater number of species relative to its induced sister clade . Conclusions: The primordial germ cell determination mechanism hypothesis explains asymmetrical species diversity and morphological disparity at the phylum level. We argue that the determinative mode of primordial germ cell determination is a constraint release that has enhanced evolvability and increased rates of speciation and morphological disparity among clades. Knowledge of the mechanism for extant theropods allows speculation that its sister clade, the Sauropodomorpha would have exhibited the induced mode. Results: After 25 years of additional data accumulation, the hypothesis that high levels of species diversification are associated with the induced mode is falsified, with the determinative mode revealed as associated with higher rates of diversification. The greater species numbers are significantly associated (ANOVA p>0.003) with the determinative mode. Analysis with appropriate sister clades is unanimous in showing the clade with the determinative mode has a significantly greater number of species relative to its induced sister clade . Conclusions: The primordial germ cell determination mechanism hypothesis explains asymmetrical species diversity and morphological disparity at
Highlights
Asymmetric patterns of species diversification in myriad forms, have been and remain of interest to paleontologists and neontologists (e.g., Stanley, 1975; Erwin et al, 1987; Gould et al, 1987; Eble, 2000; McPeek and Brown, 2007)
In the course of our work, especially Crother et al (2007), we explicitly revealed the asymmetric patterns of species diversity and demonstrated in vertebrates (Evans et al, 2014) that increased rates of speciation are associated with a determinative mode
While we do not have an exhaustive list of extinct species numbers for all phyla, for the estimates we do have, the asymmetry remains because both sides increase in species richness approximately the same, but with still more species added to the determinative groups
Summary
Asymmetric patterns of species diversification in myriad forms, (e.g., temporal, clade, sister clade, speciation rates) have been and remain of interest to paleontologists and neontologists (e.g., Stanley, 1975; Erwin et al, 1987; Gould et al, 1987; Eble, 2000; McPeek and Brown, 2007). Two broad competing hypotheses have dominated attempts at explaining diversification asymmetries, one based on genetics and development and the other grounded in ecological theory. The former hypothesis essentially claims that rates of diversification are dependent on changes in the molecular developmental process and that these changes allow the production of novel body plans and morphologies (e.g., Cracraft, 1990; Davidson and Erwin, 2006) whereas the latter hypothesis states (at its simplest) that rates of diversification are controlled by available niche space (e.g., Hutchinson, 1959; Erwin et al, 1987). We revisited the hypothesis and analyzed phyla diversity with germ cell determination mechanisms and examined sister clade asymmetry
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