This post was co-authored by Kristina Kangas, a graduate student in the Integrative Biology Department at UC Berkeley.
Joining us on April 17th for the Integrative Biology seminar was Gene Hunt, a curator at the Smithsonian Institute, whose parents must have known when they named him what occupation he would be getting into. Specifically, he “hunts” changes in the “gene” pool of Ostracoda, a bivalved crustacean that, on average, has a length that is smaller than the thickness of a dime. The genus Hunt investigates, Poseidonamicus, is found at a water depth below one kilometer in every part of the world except the Arctic and the Mediterranean and has a fossil record worth bragging about. Additionally, the fossils carry several distinct morphological traits that can be compared over evolutionary time. These factors enable Poseidonamicus to provide immeasurable insight on how evolutionary changes occur.
Charles Darwin’s monumental publication, On the Origin of the Species, provoked great criticism because it proposed a linear evolution of change not supported by fossil records at the time. In 1859, there were only 51 dinosaur fossils recorded in scientific literature, leading Darwin to argue that the intrinsic lack of data made it difficult to draw any tangible conclusions about how species change over time. The worldwide fossil collection has taken an exponential increase since the time of Darwin, finally allowing paleontologists to observe and measure morphological changes in a species over time.
Modern researchers must now address the issue of how these changes are occurring. Some scientists predict that these morphological changes between and within species happen in a gradual, linear manner. Others argue that changes occur in a pulsatile fashion; they are the same for a while and then suddenly become altered. Scientists still debate which model should be deemed conventional.
Hunt described these, “diametrically opposed conclusions,” as phyletic gradualism, and punctuated equilibrium, respectively. His analysis of 51 lineages published 2007 in PNAS shows phyletic gradualism to be rarely observed on paleontological time scales; it was the most fitting model in only 5% of the fossil sequences considered. Punctuated equilibrium, with extended periods of stasis implying a species has reached optimum morphology (as proposed by Niles Eldredge and Stephan Jay Gould in 1972), seems to be a more plausible mode in this data compilation. However, another equally likely and strongly supported mode of evolutionary change from this statistical model is random walks, the accumulation of stochastic changes that are optimum with respect to the environment.
Since different statistical models applied to the same data input can potentially corroborate each of the previously proposed modes of evolution (random walks, stasis, or gradual changes), Hunt designed a new statistical model to help resolve the ambuiguity. He applied his model to Poseidonamicus because it is one of the few organisms in the world with both many distinct morphological traits and an abundance of fossils, two key factors that allow for robust statistical analysis. His investigation drew from data collected from forty population samples from all over the world.
Hunt’s model compared several morphological traits of Poseidonamicus, such as body size and the number of pores for sensory hairs, to determine how it evolved. Since these crustaceans molt, it was possible to control for the various ontological stages of the fossils. Hunt also distinguished male and female, since, according to Hunt, males have sperm that is longer than the length of the shell, curled up inside like angel hair pasta (yes, that is one bizarre visual).
Using his statistical model, Hunt found that the punctuated equilibrium model, proposed over a century after Darwin, was not supported. This implies that the gradualistic model proposed by Darwin may not be as out-dated as our fossils, and it should continue to be considered in modern phylogenetic analyses. While these conclusions pertain most directly to Poseidonamicus, one could speculate how various genera are likely have different modes of evolution at different time periods.