written by: Ben Burgunder & Eric Hartel
How can modeling and mathematics inform our research of macroevolution? Towson University professor Dr. Daniel Caetano visited the University of Maryland's entomology department to deliver a lecture on his many research interests. Dr. Caetano explores evolution through mathematical modeling and novel approaches to phylogenetic comparative methods. Phylogeny is the scientific approach to understanding the evolutionary relationships between groups of organisms and phylogenomics is the application of genomics to phylogenetics research. Dr. Caetano explained that his research is supported by two pillars: trait evolution and species diversification. He discussed two examples of developing novel methodology from published, publicly available datasets.
Dr. Caetano first introduced the audience to trait evolution through the complicated world of anoles, small, often green or brown, lizards found throughout the Americas. Anoles display ecomorphology, a wide range of morphological variations specialized for their microhabitat. By combining species habitat preference, existing phylogeny, and their geography, researchers have theorized that this group first originated in the equatorial forests of South America before spreading to the Caribbean and then crossing over to Central and North America. Dr. Caetano hypothesized that the historical shift to the West Indies led to a higher rate of trait evolution, as lizards adapted to their new, specialized habitats. He combined simulations and covariance matrices to account for how the evolution of one physical trait might be linked to the evolutionary rate of another. Dr. Caetano determined that the parallel evolution of ecomorphs in different regions could explain why the traits in question did not evolve as predicted by their evolutionary history. He also cautioned against drawing hasty conclusions about this study system. Sampling regimes may be incomplete due to the challenge of locating and obtaining permits to collect these lizards.
Dr. Caetano has also applied novel methods to researching the evolution and diversification of cricket songs in the genus Gryllus. These cricket songs come in a few varieties that all serve different functions of communication. When two male crickets come in close contact with one another, they produce an aggressive song. A male cricket can play a calling song that he uses to call out to potential mates in the area. Finally, a male that has succeeded in getting the attention of a potential mate can use a courtship song that demonstrates his fitness as a mate to the attending female.
Dr. Caetano chose to explore the evolution of calling songs because they vary widely between species but are very consistent within species, making them an excellent target for research into phylogenetic variation, rates of evolution, and species diversification. To analyze the variation between species, Dr. Caetano adapted many molecular tools to tease apart the complex makeup of cricket song by standardizing song data. First, the recordings needed to be coded into regions to establish a song pattern. The three states that were identified were sound, silence, and intersound, when the time interval contained sound and silence. By dividing the recording into three distinct bases the three sound states can be used as a stand-in for nucleotide bases in molecular work allowing the tools used in genetics to be applied to this work. The cricket songs could also be broken into song bouts, portions of the cricket song that are repeated many times, as another level to analyze the divergence and evolution of song. These song bouts are conserved when they are repeated allowing another level of divergence. By using a species tree based on molecular data, Dr. Caetano was able to compare song divergence to the molecular tree to determine differences in song evolution and rates of change. Dr. Caetano has focused his work on the changes in silent portions and changes to the sound portions separately as they appear to have different rates of evolution. This research was published in The American Naturalist in 2020, but Dr. Caetano explained that his model is flexible enough to be applied to other researchers’ work on similar systems. He also noted that he is actively developing more research on this topic
Dr. Caetano has delved into the mathematics and modeling of evolution, exploring a sometimes neglected side of this field. Through innovative methodology, his work can be used to answer questions across a wide range of study systems. His work lays the foundation for phylogeneticists everywhere to investigate trait evolution and species diversification.