There is an interesting article in this month’s edition of Evolution on trilobite diversity. The study, by Melanie Hopkins, a paleontologist at the University of Chicago, looks at the relationship between intraspecific variation, species range, and longevity in the fossil record in a set of late Cambrian trilobites.
This study offers a novel combination of analyses that reveal the quantitative relationships among intraspecific variation, geographic range size and duration in the fossil record using late Cambrian trilobites. Results show that geographic range size and duration are positively correlated. Surprisingly, longer lived species tend to have less intraspecific variation. Phylogenetic effects were also explored and found not to determine the association between these variables. However, the distribution of geographic range sizes shows strong phylogenetic signal. In light of previous work, one possible explanation for these results is that species with shorter durations have comparatively higher rates of morphological evolution, reflected in higher phenotypic variation overall.
I have no particular interest in trilobites, but I am interested in the evolutionary dynamics of polytypic, geographically scattered evolutionary lineages, Homo erectus in particular. The comparison between Cambrian-era marine creatures and recent human ancestors is not an entirely obvious one. Nevertheless, the study points to some issues that have relevance for how we view the hominid record.
Crucial to the interpretation of these results is the recognition that within-species variation as measured herein is the amount of morphospace occupied by a species across time and space, that is, across all sampled populations. For example, among sister species of bivalves within which only one survived the Plio-Pleistocene extinction interval, survivors typically expressed more morphological variation than victims (Kolbe et al. 2011). However, both the sign and magnitude of the difference in variation between survivors and victims depended on whether the variation under consideration was the average population-level variation or the variation pooled across populations. The apparent role of variation in driving evolutionary rates or persistence may often depend on the focal level (population vs. species), particularly as some mechanisms by which variation influences extinction involve one source but not the other.
Thus, although both stress resistance and stabilizing selection influence population-level variation (see Introduction), these factors may affect species-level variation in a number of ways or not at all. In particular, the adequacy of stabilizing selection as a mechanism for long-term stasis requires that selective optima be preserved over a narrow range across traits and over significant amounts of time (Hansen and Houle 2004; Futuyma 2010). Possible mechanisms for providing a stable optimum across environments or as environmental parameters are changing include niche tracking (Eldredge 1999; Gould 2002), differential degrees of plasticity across characters (Wake et al. 1983), averaging across semi-independent populations that track environmental changes separately (Stanley and Yang 1987; Lieberman and Dudgeon 1996), and variation in selection strength across the geographic range of a taxon (Holt and Gaines 1992; Holt 1996). As a general pattern, these mechanisms assume that stabilizing selection is common and strong, which may not be the case (Kingsolver et al. 2001; Hansen and Houle 2004; but see Walsh and Blows 2009).
One of the challenges in studying the Pleistocene human fossil record is that there is considerable variation across time and space, likely spanning a complex interspecific pattern of variation. Understanding the evolutionary dynamics of variation at both the population and species level is no small task. It is a task made more complicated in paleoanthropology by the reality that we are actually dealing with a relatively small amount of evolutionary time, a few million years at most. One of the issues I find myself coming back to again and again is why should we expect a radiation of hominid taxa around the emergence of the genus Homo (a consensus view, I would say) when the fundamental change from Australopithecus to Homo seems to be an increase in plasticity, particularly behavioral, and an associated range expansion. This, coupled with the acceptance of a high degree of variation, including regionally specific variation, within early Homo erectus.
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1. Hopkins, M. J. (2011), HOW SPECIES LONGEVITY, INTRASPECIFIC MORPHOLOGICAL VARIATION, AND GEOGRAPHIC RANGE SIZE ARE RELATED: A COMPARISON USING LATE CAMBRIAN TRILOBITES. Evolution, 65: 3253–3273. doi: 10.1111/j.1558-5646.2011.01379.x
