Tag: adaptive radiation

20-Million-Year-Old Fossils Reveal Ecomorph Diversity in Hispaniola

 

Twenty exquisitely preserved anole fossils in 20 My old Dominican Amber have been reported on in a paper out in Proceedings of the National Academy of Sciences (PNAS) this week.

Previously on AA, I reported that the search was on to find anole fossils in order to piece together the anole family tree. We were extremely fortunate to find in the end 38 amber fossils with anole inclusions, sourced from museums such as the Staatliches Museum für Naturkunde Stuttgart, Germany, American Museum of Natural History, and Naturhistorisches Museum, Basel Switzerland, as well as from generous private collectors.

All of the fossils were exquisite, stunningly-preserved anoles in Dominican Amber. Sometimes just a foot or tail was preserved, sometimes a whole limb or two, or an isolated head, but occasionally a whole lizard was preserved laid out as if it has been pressed into resin just moments before.

Modified from Figure 1 of Sherratt et al. 2015 PNAS.

Modified from Figure 1 of Sherratt et al. 2015 PNAS.

Using micro-CT scanning to peer inside the fossils, we were delighted to find well-preserved skulls and skeletons. We were surprised to find that many of the amber pieces had air-filled pockets representing where the lizard body had once been (but subsequently mostly rotted away), and the scales had left their impression on the amber. This allowed us to view the scales of the limbs and toepads in the greatest of detail.

The forelimb lying atop belly scales of a trunk-ground fossil, specimen M of Sherratt et al. 2015.

The forelimb lying atop belly scales of a trunk-ground fossil, specimen M of Sherratt et al. 2015.

Twenty of these fossils were complete enough, or preserved with the right body parts (limbs with a pelvis, or toepads with countable lamellar scales) to study qualitatively. I micro-CT scanned 100 modern specimens from the Harvard MCZ collection, representing adults and juveniles of all the ecomorphs in Hispaniola. With these data, I build up a dataset of measurements of the limbs, skulls and pelvic girdles that could be used to compare with the fossils. Working fossil by fossil, I used discriminant function analysis to assess the probability that the fossil matched each of the modern ecomorphs.

The fossil twig anole, from Jose Calbeto of Puerto Rico.

The fossil twig anole, from Jose Calbeto of Puerto Rico.

The results were very exciting. We found evidence for four of the six ecomorphs in the amber. Trunk-crown were the most abundant, but there was also one that fell within the twig anoles, two that fell with trunk and two with trunk-ground anoles. Not all the fossils could be assigned to an ecomorph with high probability. Though, my gut feeling is that there is a second twig anole (specimen P) based on the distinct few lamellar scales on its widely-expanded toepads, but sadly it didn’t have enough skeleton and no hind limbs preserved to add to the analysis.

We didn’t find any fossils that resembled crown-giants or grass-bush anoles. Why?

Evolution Meeting 2011: Evolution of Genetic Architecture During Adaptive Radiation

Joel McGlothlin received the 2011 Dobzhansky Prize, given to an outstanding young scientist in the field of evolutionary biology. Successfully fending off technical AV difficulties, Joel gave a fascinating talk in which he examined the idea that evolution should occur along lines of least genetic resistance, which are determined by the genetic correlations among traits. This hypothesis predicts that as traits diverge, they should diverge in ways that mirror the trait genetic correlations. Moreover, one would expect that this effect would attenuate over time, so that more distantly related species would show less evidence of diverging along these lines.

Evolution Meeting 2011: Ecological Opportunity and Adaptive Radiation

Luke Mahler received this year’s Fisher Award, given by the Society for the Study of Evolution for the best paper emanating from a Ph.D. thesis published in Evolution in the preceding year. Mahler asked “what is the trigger for adaptive radiation?” and answered, “ecological opportunity”: a wealth of evolutionarily accessible resources (from Schluter). This leads to the prediction that the pace of adaptive radiation is regulated by competition, and is thus diversity-dependent. This hypothesis has been tested greatly in recent years by seeing if the rate of species diversification decreases through time as a clade radiates. But, Mahler argued, counting species is not enough—rather, we need to focus on patterns of diversification in adaptive traits.

Focusing on Greater Antillean anoles, Mahler asked whether rates of ecomorphological diversification decline as the number of species on an island increases. Testing this hypothesis requires estimating historical ecological opportunity (EO) by inferring ancestral species richness on an island and estimating rates of morphological evolution at ancestral nodes and how they changed as a function EO. To do this, Mahler and co-author Liam Revell developed at maximum likelihood approach to infer ancestral states incorporating uncertainty on the reconstruction of ancestral biogeographic locations.

The result is strong support for the ecological opportunity model. Rates of ecomorphological evolution are high early in anole radiation, but decline with increasing species richness.

Powered by WordPress & Theme by Anders Norén