How species form is a central question in evolutionary biology. A major
challenge in contemporary speciation research is linking
microevolutionary processes, such as natural and sexual selection, within
populations to broader macroevolutionary patterns of divergence among
species. Much of my research is focused on the evolution of color, pattern,
behavior, chemical signals, and other traits under both natural and sexual
selection that vary geographically within and among species to understand
how species diverge into distinct lineages. I take an integrative approach
that leverages phylogenetics, morphology, behavior, and both field and
lab work to study species traits in space and through time. My Ph.D.
dissertation is on the evolutionary causes and consequences of color
So what is color polymorphism and why do we care?
Color polymorphism is the presence of two or more discrete color morphs
within a breeding population. Color polymorphism defies evolutionary
expectations – evolutionary processes like natural selection, sexual
selection, and genetic drift usually reduce genetic and phenotypic variation
within populations – so how do multiple color morphs persist?
Studying how multiple genetically-determined color morphs coexist and
diverge in nature can reveal fundamental mechanisms of evolution and
speciation. If we can understand historical and contemporary morph
dynamics, we may be able predict how diversity will persist (or not) in the
Snaggin' Podarcis erhardii color morphs for science with Dr. Donihue (2018)
TRAIT DIFFERENCES AMONG COLOR MORPHS IN PODARCIS ERHARDII
Color polymorphism defies evolutionary expectations as striking phenotypic variation is maintained within a single species. Color and other traits mediate social interactions, and stable polymorphism within a population is hypothesized to be related to correlational selection of other phenotypic traits among color morphs. Here, we report on a previously unknown throat color polymorphism in the Aegean Wall Lizard (Podarcis erhardii) and examine morph-correlated differences in traits important to social behavior and communication: maximum bite force capacity and chemical signal profile. We find that both sexes of P. erhardii have three color morphs: orange, yellow, and white. Moreover, orange males are significantly larger and tend to bite harder than yellow and white males. Although the established color polymorphism only partially matches the observed intraspecific variation in chemical signal signatures, the chemical profile of the secretions of orange males is significantly divergent from that of white males. Our findings suggest that morph colors are related to differences in traits that are crucial for social interactions and competitive ability, illustrating the need to look beyond color when studying polymorphism evolution.
This project was a wonderful international collaboration with Simon Baeckens (Belgium/Australia), Colin Donihue (US), José Martín (Spain), Panayiotis Pafilis (Greece), and Danielle Edwards (US). Thanks to the amazing UC Merced undergraduate field assistants who helped catch and care for lizards: Indiana Madden, Adam Rosso, and Cynthia Ramos!
COLOR MORPH-CORRELATED BEHAVIOR IN PODARCIS ERHARDII
Space is a limited resource that many animals need to access for basic functions such as feeding and reproductive opportunities. Competition over access to space can induce aggressive behavior, which may result in differential access to crucial resources related to survival and fitness. The Aegean wall lizard, Podarcis erhardii, is a tri-color polymorphic lizard that eponymously inhabits dry stone walls where they display to other lizards, access food, safely thermoregulate, and shelter from predators. Adult male color morphs exhibit physical differences in size and chemical signaling profiles, but nothing is known about potential morph-specific differences in behavior. Here, we conduct the first behavioral experiments on P. erhardii color morphs to characterize morph differences in behaviors involved in accessing and occupying a limited space resource. We captured 60 adult male lizards (20 orange, 20 yellow, and 20 white) and constructed contests between individuals of each morph type. Experimental trials conducted in a neutral laboratory arena revealed that male color morphs exhibit different behaviors and abilities to access a small stone wall with a heat source. We think these alternative strategies are involved in the evolutionary maintenance of color polymorphism.
Are you not entertained?! This project was a wild adventure in patience. Thank you to field assistants Indiana Madden & Cynthia Ramos and lab assistant Cole Ayton (co-authors) who watched hours of lizard colosseum trials. No lizards were mortally wounded during our experiments :)
THE EVOLUTION OF COLOR POLYMORPHISM IN THE LIZARD FAMILY LACERTIDAE
Color polymorphism – two or more heritable color phenotypes maintained within a single breeding population – is an extreme type of intra-specific diversity widespread across the tree of life but rarely studied in a comparative framework. Color polymorphism is thought to be an engine for speciation, where morph loss or divergence between distinct color morphs within a species results in the rapid evolution of new lineages, and thus, color polymorphic lineages are expected to display elevated diversification rates.
Lizards of the family Lacertidae have evolved multiple lineages with color polymorphism, but lack of a complete and robust phylogeny for the group has made comparative analysis difficult. Here, we produce a comprehensive species-level phylogeny of the lizard family Lacertidae to reconstruct the evolutionary history of color polymorphism and test if color polymorphism has been a driver of diversification. Accounting for phylogenetic uncertainty, we estimate an ancient macroevolutionary origin of color polymorphism within the Lacertini tribe (subfamily Lacertinae). Color polymorphism most likely evolved several times in the Lacertidae and has been lost at a much faster rate than gained. Evolutionary transitions to color polymorphism are associated with shifts in increased net diversification rate in this family of lizards. Taken together, our empirical results support long-standing theoretical expectations that color polymorphism is a driver of diversification.
Thanks to the Society of Systematic Biologists for funding this work! This project was a collaboration with Emily Jane McTavish, Danielle Edwards, and Indiana Madden.
GEOGRAPHIC VARIATION IN COLOR POLYMORPHISM IN PODARCIS ERHARDII
Evolutionary theory predicts that color polymorphic species are ripe for speciation due to striking variation amongst multiple discrete phenotypes within and among populations. One could imagine, for example, that across an entire species' range, that color morphs might be better suited to certain environments and ecologies or that due to random chance a color morph might be lost from a population. But how does population-level variation in color polymorphism change across an entire species distribution? And what affect does this geographic variation in morph presence have on the mode and tempo of divergence (both morphological and genetic) among populations?
TeamSavres 2017-2019 (Team Lizards in Greek) and I travelled to the very farthest reaches of the Aegean wall lizard's (Podarcis erhardii) distribution, exploring more than 40 Aegean islands by ferry, fishing boats, and kayaks, in search of wall lizards to describe and explain color morph and genetic diversity across the archipelago. So far, we've found that color morphs in P. erhardii have been completely lost from some populations, and the pattern of morph loss seems to be ordered and non-random! We are going to use a population-level phylogeny to date island divergence and estimate the rate of morph loss across populations. We're also using environmental and ecological data collected at each site to try to understand drivers of morph loss.
This work has been the thrill of my life, and none it would have been possible without courage, dedication, and persistence from all of the wonderful TeamSavres field assistants. Ευχαριστώ πολύ Adam Rosso, Indiana Madden, Cynthia Ramos, Chris Vetros, Robert Degen, Shelby Stadler, and Cole Ayton! This work is a long-term collaboration with Professor Panayiotis Pafilis at the National and Kapodistrian University of Athens.