Image by Matt Mechtley
…if evolutionary biologists are intent on documenting the
history of life, we need methods that can at least approximate patterns of evolution in deep time for clades without fossil information.
A scientists who wants to understand the evolutionary history of a group of organisms has some serious roadblocks. One of the most obvious of these issues: the majority of the evolutionary history is gone. Dead. Extinct! So unless you have a nice fossil record (most clades do not), you are left trying to understand millions of years of evolution by looking only on what is extant today. For the most part, this doesn’t provide the most accurate picture.
Jonathan Mitchell provides another cautionary tale about interpreting evolutionary history using only extant taxa in a new paper in Evolution. He uses birds as a study system, which have some of the most complete fossil records and phylogenies of all vertebrates, to test the two main hypotheses of when avian evolutionary radiations happened: one at the base of the tree during the Cretaceous and again when the Passeriformes first appeared.
Using morphological and phylogenetic data, Mitchell shows (as expected) that the model of diversification best supported by the data greatly depends on which taxa you include. When fossil data isn’t considered, the result tends to underestimate the diversity in the fossil record. However, the ratio of within-to-between clade differences does provide some evidence of early radiations. These combined results cause Mitchell to predict that modern radiations have washed out the signal of older radiations, providing another example of why interpretations of evolutionary history from only extant taxa can be problematic (feathered or otherwise).
The variance and range in morphology observed in the fossil assemblages from the “halfway point” of avian evolution is ~70% of the modern, which is substantially higher than models based solely on extant taxa would predict for ~50Ma. This observation, of crown Aves having achieved such ecological disparity by the Eocene, stands in stark contrast to expectations from modern data alone. None of the models based on extant taxa only consistently predicted this high level of early disparity, and the fossil-informed method was unable to predict both the high level of ancient disparity and the relatively low modern disparity simultaneously. All of these models are known as extreme simplifications, but they are commonly used to at least predict the broad contours of morphological evolution.
Mitchell, J. S. (2015). Extant‐only comparative methods fail to recover the disparity preserved in the bird fossil record. Evolution. DOI: 10.1111/evo.12738