Discerning evolutionary processes in patterns of tamarin (genusSaguinus) craniofacial variation

Abstract

Quantitative genetic theory specifies evolutionary expectations for morphological diversification by genetic drift in a monophyletic clade. If genetic drift is responsible for the evolutionary morphological diversification of a clade, patterns of within- and between-taxon morphological variance/covariance should be proportional. We tested for proportionality of within- and between-species craniofacial morphological variation in 12 species of tamarins (genus Saguinus). We found that within- and between-taxon morphological variations across the entire genus were not proportional, and hence not likely to be due to genetic drift alone. The primary deviation from proportionality is that size and size-related shape in the cranium is more variable relative to other aspects of cranial morphology than expected under genetic drift, suggesting differential size selection between the two major clades, the small-bodied and large-bodied tamarins. Within each of these major clades, most of the interspecific variation is consistent with the pattern expected under genetic drift, although specific contrasts may indicate the involvement of differential selection. Morphological distances among taxa do not correspond very closely to the phylogeny derived from mtDNA. In particular, S. oedipus and S. geoffroyi are very distinct morphologically from the rest of the tamarins, although they are phylogenetically the sister clade to a clade containing S. midas and S. bicolor. Morphological similarity is not a good guide to phylogenetic affinity in the tamarins, especially with regard to deeper nodes in the phylogenetic tree.

Detecting genetic drift versus selection in human evolution

“…Each analyzed hierarchical level contains at least three populations. Because of the inherently small sample sizes, individual fossil specimens are taken to represent population means; whether these populations represent different species or are time-successive taxa should not affect the analysis, as this approach has been shown to work at different levels in a phylogeny (13,17,18). As the fossil record does not allow for a direct estimate of fossil intraspecific variation, extant variation in humans and our two closest living relatives (chimps and gorillas) is substituted for withinpopulation variation of fossil species (see below).…”

“…However, the assumption of constancy in covariation patterning is problematic. Although a number of studies have shown that there are many common aspects to morphological variation and integration in the primate cranium, and especially in the face (13,(22)(23)(24)(25)(26)(27), it is also true that there are differences. To conservatively account for the possible effects of small differences in covariance (CV) structure (22, 28), we use three extant models rather than just one, as we realize that patterns of variation are not homogenous (although they are similar; see ref.…”

“…Here, we test hypotheses of evolutionary diversification in the human fossil record: specifically, whether the facial diversity seen among late Pliocene and early Pleistocene hominins could be explained by genetic drift alone, or whether nonrandom forces like selection played an important role (see ref. 13).…”

“…If populations have diversified through random evolutionary processes such as genetic drift, evolutionary theory predicts a proportional relationship between within-group and between-group phenotypic variation (14-16); a nonproportional relationship indicates the action of nonrandom processes such as differential selection. Although this methodological approach has been used successfully to look at drift versus selection in living New World monkeys (13,17,18), it has never been applied directly to the fossil record of human evolution. Such questions have, however, been addressed in other fossil primates (19) and modern humans (20,21).…”

The Taxonomic Status of Wied's Black-tufted-ear Marmoset, Callithrix kuhlii (Callitrichidae, Primates)

“…They concluded that they should be classified as separate species rather than subspecies, based on their finding that "morphological distances among marmosets are s m lar to or h gher than d stances found among other related taxa usually accepted as good species, like the tamarins (Moore and Cheverud 992; Ackermann andCheverud 2000, 2002)" (p.17). They also failed to find evidence for intergradat on along contact zones, but nstead "a sharp, steep morphological boundary between taxa with no trend of species being more morpholog cally s m lar at contact zones than at other parts of the r ranges.…”