Demographic Bottlenecks
In a recent essay, Stoeckle and Thaler (2014) posed the question: “A
universal selection-driven mtDNA clock implies all organisms are
evolving at about the same rate …. What could cause similar rates of
change for diverse organisms in diverse environments?”. Stoeckle and
Thaler (2014) proposed that the external environment of organisms could
cause periodic extreme reductions in the population sizes of essentially
all organisms, perhaps particularly at the point of divergence of
incipient sister species. Severe demographic bottlenecks in population
size would purge populations of genetic diversity in mt genotype and fix
differences between species, potentially creating the pattern of mt DNA
barcode gaps observed in bilaterian animals (Stoeckle and Thaler 2014).
No explanation is given for why such bottlenecks would reduce variation
in mt genotypes but not N genotypes. Moreover, this hypothesis requires
that, at regular intervals that average a few hundred thousand years,
essentially every species is subjected to an extreme population
bottleneck (Stoeckle and Thaler 2014). These authors speculated that
complex “food web, predator-prey, and parasite-host interactions”
might sum to a common selective pressure on animal mitochondrial genomes
“with long-term planetary climate cycles as the ultimate driver of
evolution” (Stoeckle and Thaler 2014). I know of no evidence for such
periodic synchronized collapse of all populations of all bilaterian
organisms to create the pattern of mt DNA barcode gaps observed in
bilaterian animals.