Although the tree of life occasionally produces withered offshoots, it tends to branch outward at a constant rate, yielding a new species every 2 million years or so. The constancy with which life diversifies indicates that speciation may depend less on adaptation than was once thought. Unexpectedly, speciation may depend more on random genetic events and geographic isolation.

This finding has emerged from a review of genomic data amassed by researchers at Temple University. These researchers say that they have used their data trove to assemble a tree of life timeline—the largest, most comprehensive, and most accurate ever.

The researchers, led by S. Blair Hedges, Ph.D., director of Temples’ Center for Biodiversity, add that because their tree of life timeline is so comprehensive, it has afforded an uncommonly broad view of speciation. From their perspective, say the researchers, “species diversity has been mostly expanding overall and in many smaller groups of species, and that the rate of diversification in eukaryotes has been mostly constant.”

That conclusion appeared March 3 in the journal Molecular Biology and Evolution, in an article entitled, “Tree of life reveals clock-like speciation and diversification.” This article describes how the Temple scientists synthesized information from 2,274 studies representing 50,632 species. It also argues that its authors managed to identify, and avoid, potential biases that may have influenced previous analyses of diversification including low levels of taxon sampling, small clade size, and the inclusion of stem branches in clade analyses.

“The constant rate of diversification that we have found indicates that the ecological niches of life are not being filled up and saturated,” said Dr. Hedges. “This is contrary to the popular alternative model which predicts a slowing down of diversification as niches fill up with species.”

Previous analyses of taxonomically restricted timetrees, the Temple team noted, had found a decline in the rate of diversification in many groups of organisms, often attributed to ecological interactions among species. The new analysis, however, led the Temple team to a different conclusion.

“Speciation and diversification are separate processes from adaptation, responding more to isolation and time,” stated Dr. Hedges. “Adaptation is definitely occurring, so this does not disagree with Darwinism. But it goes against the popular idea that adaptation drives speciation, and against the related concept of punctuated equilibrium which associates adaptive change with speciation.”

Besides the new evolutionary insights gained in this study, the Temple team’s Timetree of Life will provide opportunities for researchers to make other discoveries across disciplines, wherever an evolutionary perspective is needed, including, for example, studies of disease and medicine, and the effect of climate change on future species diversity.

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