Invasive Anole Evolution

One of the major challenges of studying evolution is that animals tend to evolve over time periods that extend far beyond the lifetimes of the scientists who study them. Evolutionary biologists generally make do with reconstructing evolutionary histories via clues left over in the present, and with decades of technological innovation and creative science, this approach has led to a robust patchwork of theory. That said, it is always nice when a capable scientist is blessed with a rare opportunity to study evolution in nature in real time. One of the most memorable papers I've come across in recent years happens to be written about exactly this kind of blessing. Stroud et al.'s "Observing character displacement from process to pattern in a novel vertebrate community," from November 2024, is the fascinating result of the kind of crafty and opportunistic science that I find exhilarating. 

The paper rests on a significant body of theoretical work on adaptive radiation, which is a subject that is a little broad to be covered thoroughly in a casual blogpost (and probably beyond my capability in any format). For those unfamiliar with the concept and looking for some resources to check out, I recommend starting with the work of Dolph Schluter, i.e. the adaptive radiation GOAT, and/or Peter and Rosemary Grant's prolific research on the Darwin's finch study system. Darwin's finches are the textbook example of an adaptive radiation, but other famous examples exist, for instance the cichlids of the central African crater lakes, the silverswords of Hawaii, and--most relevant to today's paper--the Anolis lizards of the Caribbean.

Stroud et al.'s 2024 paper builds upon theoretical work done particularly by Jonathan B Losos on the Anolis lizard system, and I would therefore be remiss if I didn't at least mention Losos' classic 1998 paper "Contingency and Determinism in Replicated Adaptive Radiations of Island Lizards." In this paper Losos shows that on each major Caribbean island, several species of anoles exist and fall into consistent "ecomorphs," or categories of behavior, habitat use, and corresponding morphological characteristics. On each island, anoles consistently occupy six distinct microhabitats that Losos defined as "crown-giant, grass-bush, trunk, trunk-ground, and twig," based on which parts of trees these arboreal lizards primarily spend their time. Observing a system with such remarkably consistent ecomorphs across multiple islands and dozens of species, Losos judged only two explanations to be feasible: either anoles evolved once into these distinct niches and then spread from island to island (which would be the parsimonious option), or anoles colonizing each island were repeatedly and separately driven into the same ecomorph categories. If the former hypothesis were true, anoles in each ecomorph would be more related to anoles in the same ecomorph class across islands, whereas if the latter hypothesis were true, this pattern would not be detected. After some genetic tests, Losos showed that anole relatedness was no stronger among ecomorph classes across islands than across species within islands, and the likelihood of these patterns being driven by repeated and regular adaptive radiation skyrocketed. 

Using genetics and carefully collected behavioral and morphological data, Losos provided strong evidence of some degree of determinism in evolution, as well as the power of adaptive radiations in generating diverse natural forms. While a massive contribution to science, Losos' conclusions still relied on a theoretical reconstruction of past events, leaving adaptive radiation-heads such as myself yearning for real-time, observational confirmation. This is where Stroud et al. 2024 (finally) comes into the picture. Stroud and his collaborators were busy studying invasive anoles in South Florida when one of his study sites that initially only had brown anoles (Anolis sagrei) was invaded and eventually dominated by crested anoles (Anolis cristatellus). Stroud jumped on the opportunity to observe the early phases of these novel competitive interactions, as these two species don't overlap in their native ranges. He and his collaborators observed that while in allopatry, both crested and brown anoles were happy to spend their time occupying similar regions of the lower sections of tree trunks, only spending 2% of their time on the ground. In Lososian terms, both species instantiated the same ecomorph class. In sympatry however, Stroud et al. were able to show that cristatellus individuals moved further up the trunk and sagrei further down, to the point where sagrei individuals were observed spending an average of 33% of their time on the ground. Novel competition had driven these anoles into separate ecomorphs! Aside from these behavioral observations, Stroud et al. showed that the newly ground-dwelling sagrei had developed significantly longer fore and hindlimbs, a result that conforms perfectly to the morphological characterizations made by Losos of Caribbean ground-dwelling ecomorphs. 

Fully-fledged adaptive radiations such as those found among anoles in the Caribbean may take thousands of years to develop, and what Stroud and his collaborators observed was character displacement, which is only a first step towards radiation. However, it is a crucial step, and one that is rarely observed so clearly in nature. The way that the results conform so exactly to the theoretical work laid out by Losos and others is also a lovely vindication of theory. Aside from these great qualities, as a biologist currently working in Florida, I also particularly love this paper because it draws a beautiful result from the invasive disaster zone that is South Florida and interrupts the steady stream of doomsday-level invasive news coming from that region with a terrifically exciting bit of science. 

Works Cited:

Stroud, J.T., et al. Observing character displacement from process to pattern in a novel vertebrate community. Nat Commun 15, 9862 (2024). https://doi.org/10.1038/s41467-024-54302-1
   
Losos, J. B., et al. Contingency and determinism in replicated adaptive radiations of Island Lizards. Science, vol. 279, no. 5359 (1998). https://doi.org/10.1126/science.279.5359.2115.