Evolutionary biologists are forever asking the questions “Where did this living organism originate, what is this creature’s history, and from what did it descend?” Yet scientists have always had trouble reconstructing ancestral trees from fossils. The fossil record isn’t the most reliable means of deciphering the history of a species; it has holes. Often an organism doesn’t fossilize due to the composition of its body or the environment on which it died and decomposed. Or perhaps the fossils were destroyed or damaged somehow over time. An alternative to the fossil record within taxonomic groups that are still living is molecular reconstruction: using DNA sequences of a species to estimate what the DNA of its ancestors looked like. Researchers from the University of Pennsylvania were examining this method when they found a glaring problem: molecular reconstructions assume that diversity increases over time.
However, this is not always true. In some groups, most of the species go extinct over time, decreasing the diversity in a phylogenetic tree. Joshua Plotkin from U Penn, working with postdoctoral fellows Helene Morlon and Todd Parsons, used the phylogeny of whales to examine this problem. Ten million years ago, there were 60 more species of whale living than there are now. We know this because the fossil record of whales is very good – whales are large animals and the sea floor is an optimal area for leaving fossil evidence – and makes the phylogeny of the whale quite clear. Using whales as an example, Plotkin developed a new method of molecular reconstruction that allows for extinction and doesn’t assume a steady rate of diversification. The researchers tested their method using whale phylogeny, and found that the molecular reconstruction closely fits what the fossil record tells us is true. Scientists hope to use this new method to examine the phylogenies of other (less well-fossilized) groups of organisms.
Read more at EurekAlert!
The research is in the journal Proceedings of the National Academy of Sciences.