The question of the pattern of stasis (no change) versus gradualism (persistent and continuous change) in the fossil record continues. A new paper in the Proceedings of the National Academy of Sciences by Uyeda et al. comprises a huge survey and analysis of morphological change in animals (mammals, birds, squamate reptiles, and primates) over ten million years. The purpose was to determine whether change between species in one trait—body size—accumulates gradually with the passage of time since species diverged, or whether that change is more episodic. What they did was take a tremendous amount of data from three sources: current field studies of the rate of evolutionary change, fossil data showing change (again, this is all body size) through time, and estimates of rate of body-size divergence from living organisms whose divergence times can be estimated from molecular data.
Uyeda et al. then plotted the divergence in body size between related species (measured as proportional change, thus requiring a log scale) versus the divergence time. The graph below tells the tale: they see what they call a “blunderbuss” pattern, with not much change accumulating between species until they’ve diverged for about a million years, and then change occurring more rapidly and cumulatively after a million years. I’ve put the caption at the bottom for those who want more information:
What we see here is that up to about a million years of divergence, there is some change between species, but it’s “bounded,” that is, it doesn’t tend to accumulate over time. Then, after about a million years after divergence, body size starts changing without bounds, accumulating over time. This pattern holds for the reptiles, primates, non-primate mammals, and birds.
There authors do all kinds of analyses, and test other explanations, but the pattern seems robust, and is not predicted by some models of morphological evolution, including models involving simple Brownian motion of body size (probably through genetic drift). The best-fitting models involve a period of bounded evolution, in which body size is allowed to fluctuate, but between narrow limits, for a period, and then after that period multiple bursts of more extensive evolution can occur.
But what evolutionary process would keep a trait like body size fairly constant for a while and then, after a million years, allow rapid divergence? The authors don’t have a firm explanation, but suggest this: for much of a species’ history, populations exchange genes, and each population might be drawn in different directions from others by natural selection. (Some populations may be selected to be large, others small.) This genetic interchange among populations within a species keeps the species as a whole from undergoing wholesale change, although there are period of minor fluctuations such that the optimum body size of a species changes a bit for all populations, explaining the slight divergence over short time scales. This optimum can reverse itself (sometimes it’s good to be big, perhaps when there is more food available; at other times, when there’s scarcity, it might be good to be small), but there’s no large directional change in selection pressures that would make all populations go in a single direction over a long period of time.
Why does big change happen after a million years, then? The authors suggest that those big changes in natural selection that are persistent and affect all populations of a species, simply occur rarely. (Climate change is one factor that might obviously change body size in a directional way, but the author rule that out for several reasons.) As they say, “such significant, range-wide changes in selective optima may be sufficiently rare to explain the observed pattern of bounded evolution on time scales of <1 Myr [million years]”. They also suggest that perhaps the range size of a species may also rarely contract to a small size—perhaps only a single population—that can respond to selective pressures without being held back by contervailing pressures in other populations, and then the (smaller) species can undergo rapid and unimpeded change.
I should add that the time scale for formation of a new species, though it varies greatly among groups, is typically on the order of 1-2 million years as well (Allen Orr and I give a table of this in the last chapter of our book, Speciation.) If speciation (which we consider to be the evolution of reproductive isolating barriers) is often a byproduct of changes in morphological traits like body size (themselves often driven by natural selection), then one might expect a similar “blunderbuss” plot of reproductive isolation versus divergence time.
We don’t have that information, and I’m not convinced that Uyeda et al. have the correct explanation for their data (to be fair, they offer only tentative explanations). The “blunderbuss” pattern also needs to be tested for traits other than body size. But if it holds, then we’ll have a new evolutionary phenomenon to explain. We certainly haven’t run out of evolutionary problems!
Uyeda, J. C., T. F. Hansen, S. J. Arnold, and J. Pienaar 2011. The million-year wait for macroevolutionary bursts. Proc. Nat. Acad. Sci. USA 108:15908-15913.
(Here’s the caption from the figure for you science geeks who want more info:
Fig. 1. The “blunderbuss pattern”, showing the relationship between evolutionary divergence and elapsed time. Divergence is measured as the difference between the means of log-transformed size in two populations (ln za and ln zb) standardized by the dimensionality, k. Intervals represent the total elapsed evolutionary time between samples. Microevolutionary data include longitudinal (allochronic) and cross-sectional (synchronic) field studies from extant populations. Paleontological divergence is measured from time series, including both stratigraphically adjacent (autonomous) populations and averaged longer-term trends (nonautonomous). We supplement these data with node-averaged divergence between species with intervals obtained from time-calibrated phylogenies. Pairwise comparisons between species (small points) are also presented to give a visual sense of the range of divergence values across taxonomic groups. Dotted lines indicate the expected 95% confidence interval for the multiple-burst model fitted tothe microevolutionary, fossil, and node-averaged phylogenetic data.)