Plants, People and the Environment
Lecture 23. The wheel of fortune: Evolution of higher taxa.
Today's lecture was a synthesis of the information presented in Lectures 21 and 22 with the addition of tying the population-level processes in with what happens above the species level. One student asked about the role of extinction in evolution. First of all, let's define extinction as the end of a particular lineage (population, species, genus, on up to family and above). As lineages become extinct, the continuum between populations, species, and on up into the higher tiers of the classification hierarchy become fragmented. The gaps between taxa widen and it is difficult to see how lineages are similar to one another. The example I used was a diagram of the mammals compared to a diagram of closely related birds. It is easy to see how the bird species are related to one another because they strongly resemble one another (i.e., share a lot of morphological characters). It is less clear to see how a bat is related to a whale because they appear to be so different from each other. However, there are close resemblances in how the bones are connected in the skeleton, how the embryos develop, the fact that both animals are warm blooded, have hair, and produce milk and so on -- characters that are missing in many other groups of animals. Extinctions establish the gaps between lineages, but if you trace the histories of those lineages back through time by looking at the fossil record it is possible to see how distant ancestors were more closely related to one another than the extant species appear to be today.
The links included for today's lecture have been presented for previous lectures: Introduction to Evolutionary Biology, Observed Instances of Speciation, and Transitional vertebrate fossils.
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