Evidence from molecular systematics for decreased avian diversification in the pleistocene Epoch.

Abstract

Pleistocene glaciations have been suggested as major events influencing speciation rates in vertebrates. Avian paleontological studies suggest that most extant species evolved in the Pleistocene Epoch and that species' durations decreased through the Pleistocene because of heightened speciation rates. Molecular systematic studies provide another data base for testing these predictions. In particular, rates of diversification can be determined from molecular phylogenetic trees. For example, an increasing rate of speciation (but constant extinction) requires shorter intervals between successive speciation events on a phylogenetic tree. Examination of the cumulative distribution of reconstructed speciation events in mtDNA phylogenies of 11 avian genera, however, reveals longer intervals between successive speciation events as the present time is approached, suggesting a decrease in net diversification rate through the Pleistocene Epoch. Thus, molecular systematic studies do not indicate a pulse of Pleistocene diversification in passerine birds but suggest, instead, that diversification rates were lower in the Pleistocene than for the preceding period. Documented habitat shifts likely led to the decreased rate of diversification, although from molecular evidence we cannot discern whether speciation rates decreased or extinction rates increased.Temporal changes in rates of speciation and extinction result in variation in the net rate of organismal diversification through time (1). Documenting and explaining these rate changes represent major challenges in evolutionary biology. The Pleistocene Epoch presents such a challenge because of its marked environmental fluctuations and its recency, which permit detailed study of factors that influenced changes in species diversity. Many evolutionary biologists hypothesize accelerated vertebrate speciation in North America during the Pleistocene Epoch (2-5), owing to glacial advances and retreats that provided geographic barriers necessary for speciation (2, 3). Mayr (6) remarked, "Evolutionists agree on the overwhelming importance of Pleistocene barriers in the speciation of temperate zone animals." However, major Pleistocene extinctions are also known for some groups, including plants (7-9). The marked environmental effects of the Pleistocene Epoch clearly influenced rates of diversification, although the relative roles of speciation and extinction are unclear.The relatively high passerine bird diversity in modern fauna is often attributed to a burst of Pleistocene speciation (2, 3). Selander (2) also predicted that most extant bird species originated in the Pleistocene Epoch. A correlate of accelerated Pleistocene speciation concerns the average duration of bird species in the fossil record. Brodkorb (10) proposed that passerine bird species persisted for an average of three million yr in the Pliocene Epoch but only 500,000-1,000,000 yr in the Pleistocene Epoch. Thus, he predicted that species' durations decreased as the present time was approached. Although theseThe...

The tempo of avian diversification during the Quaternary

“…We therefore set out to begin investigations based on a null model that incorporated constant speciation and extinction rates. In addition, we generated expectations from models that reflect two opposing evolutionary histories, one of a burst of speciation (Mayr 1970) and another including a burst of extinction (Zink & Slowinski 1995), each occurring late in the simulations, to represent differing views of Quaternary events. Thus, the question is not whether any divergences were initiated or completed during the Late Pleistocene, but whether they occurred at a greater-than-expected frequency.…”

“…In the constant model, phylogenies were generated with equal speciation and extinction rates (birth b = 0.2 and death d = 0.2). Under the extinction model we used the constant parameters initially and added a burst of extinction in the last one-tenth of the simulation, which removed 50% of lineages (to represent a pulse of Late Pleistocene extinction envisioned by Zink & Slowinski (1995)). The speciation model was similar to the extinction model but with a pulse of lineage diversification (b = 0.6 or 0.4, d = 0.2), occurring in place of the extinction event (to represent the prevailing model of heightened Late Pleistocene diversification).…”

“…Whether this deceleration was a function of decreased speciation or increased extinction could not be determined with certainty. However, Zink & Slowinski (1995) suggested that the Late Quaternary might have been a time of increased extinction rather than speciation, because habitats were drastically compressed at the height of the last glacial maximum. That is, instead of envisioning recent glacial cycles as pistons in the engine of speciation, it is equally likely that they eliminated species by fragmenting the landscape and isolating small populations.…”

“…That is, instead of envisioning recent glacial cycles as pistons in the engine of speciation, it is equally likely that they eliminated species by fragmenting the landscape and isolating small populations. New phylogenetic analyses of mtDNA sequences permit a more refined examination of the deceleration in net diversification noted by Zink & Slowinski (1995). Klicka & Zink (1997) compiled mtDNA distances between pairs of species thought by previous authors to have originated during the last one or two glacial cycles (ca.…”

“…Zink & Slowinski (1995) evaluated log-lineage plots derived from species-level phylogenies of avian genera that had been constructed using mtDNA restriction site or sequence data. Analyses of these phylogenies indicated an apparent deceleration in the net rate of diversification as the present time was approached (i.e.…”