Author: Jonathan Losos Page 73 of 133

Anolis pulchellus with a cool aggressive display. Photo by Manuel Leal.

A few days back, we reported on a recent paper on hybridization between the Puerto Rican grass-bush anoles, A. krugi and A. pulchellus. But what better way to get the backstory than to hear it straight from the horse’s mouth? So, check out co-author Manuel Leal’s description of how the paper came to be over at Chipojolab.

Yesterday, Matt McElroy presented a phylogenetic analysis of the Puerto Rican radiation of anoles. The work was focused around the “stages of radiation” hypothesis that states that divergence occurs along different niche axes at different points in time. In the case of anoles, it has long been argued that the last stage in radiation is divergence of ecomorphologically similar species into different climatic niches.

McElroy constructed a phylogeny for 180 individuals of eight species, encompassing the geographic distribution of these species (most of which occur island-wide). Ten genes were sequenced, nine nuclear and one mitochondrial. The resulting phylogeny was well-resolved and in agreement with previous phylogenetic hypotheses, indicating that ecomorphs evolved relatively early in the radiation and that closely related sister taxa pairs are usually members of the same ecomorph, but differ in climate–the one exception–which always has struck me as odd, but apparently is correct–is the sister taxon relationship between the deep rainforest trunk ground species A. gundlachi and the xeric grass-bush species, A. poncensis.

The time of divergence was estimated for each of the four sister taxa pairs, indicating that there were three phases of radiation. The deepest split, pegged at 15 mya, was between the two trunk-crown species, A. evermanni and A. stratulus. At 10 mya, two pairs split simultaneously, the aforementioned one above and the two trunk-ground species, A. cooki and A. cristatellus. Both of these pairs include one species that occurs in the xeric southwestern portion of Puerto Rico, perhaps not a coincidence? Finally, 5 mya, the two grass-bush species, A. pulchellus and A. krugi diverged. These latter two species have recently been shown to hybridize, and McElroy’s data confirms that this is the only one of the four pairs in which hybridization occurs, perhaps due to their recency of divergence?

This is a fabulous example of detailed phylogenetic work spanning both interspecific comparisons and including the extensive degree of phylogeographic divergence that occurs within many anole species. More work of this sort is needed on anoles on the other three islands of the Greater Antilles. The monophyletic Jamaican radiation would be a good starting point.

The study of geographic variation has long been a foundation of evolutionary biology. As Martha Muñoz explained, in recent years attention has focused on whether geographic variation is the first stage in the process of speciation resulting from divergent selective pressures. Making the case for such ecological speciation requires demonstrating the occurrence of divergent selective pressures correlated with environmental differences and that the resulting  phenotypic differences lead to a reduction in genetic exchange among populations in different populations.

Martha reported on a study of the highly variable Guadeloupean anole, Anolis marmoratus. Detailed population level sampling confirmed that differences in body color correlate with environmental differences, with bluer populations in wetter habitats. These results strongly suggest the role of divergent natural selection as the cause, particularly because the differences were replicated across two different transects. Moreover, the fact that the color differences occur only in males suggests a role for divergent sexual selection.

Coalescent analyses of molecular genetic differences suggested that these differences occurred in the presence of gene flow, i.e., a parapatric model of divergence, rather than in allopatry. However, estimates of ongoing gene flow find no evidence of reduced gene flow across environmental  borders. In other words, even though selection is driving phenotypic differences, these differences are not leading to a reduction in genetic exchange–selection does not appear to be leading to speciation, contrary to the ecological speciation hypothesis.

These results are in agreement with studies on several other Lesser Antillean anoles. Martha pointed out that in situ cladogenetic speciation only occurs in anoles on the large islands in the Caribbean, suggesting that the lack of opportunity for allopatry on small islands precludes speciation from occurring, even in the presence of strong divergent selection.

These results were recently published in Molecular Ecology and are further discussed in a previous post.

In part II of the day’s Bahamian Anolis sagrei talks, Bob Cox addressed the question: How do males and females evolve different phenotypes despite sharing the same genome? And what better organism with which to study that question than anoles—lots of variation in dimorphism occurs not only among species, but also among populations within species.  This study focused on two populations of Bahamian A. sagrei in which the extent of male-biased dimorphism in body sizes varies—on Exuma, males are 33% bigger than females, whereas on Eleuthera, they are only 22% bigger. The difference is entirely the result of differences in male body size.

Cox asked three questions:

1. Do populations differ in sex-specific natural selection on body size?

2. Are sex-specific growth trajectories that give rise to sexual size dimorphism (SSD)?

3. Are differences indicative of differences in genetic correlations between sexes?

To address the first question, animals were caught at the start of breeding season; measured, marked and released; and then recaptured three months later to test for selection on body size. They found that selection is stabilizing on size in females, whereas there is strong directional  size for large size in males. However, selection doesn’t seem to differ among populations, so differences in selection would not seem to account for the differences in SSD.

However, their recapture studies allowed them to measure growth rates, and they found that males grow significantly faster on the island with higher SSD (Exuma). Animals were then raised in a lab common garden to see if the same differences in growth occur. Preliminary results show that males from Exuma grow faster in the common garden, suggesting either genetic differences or something early in development lead to growth differences (these are wild caught animals).  To further test this hypothesis, lab-raised juveniles were tested and early results indicate no differences in growth rates, which suggests that differences in SSD may not be genetically based, but these results are very preliminary. Despite lack of evidence for sex-specific growth trajectories, there is evidence for sex differences in genetic correlations between the sexes for body size (i.e., are growth rates correlated in opposite sex siblings). These correlations are much weaker in the high SSD population than in the low SSD population—these results, too, are preliminary.