Exploring the Effects of Evolutionary History on Global Patterns of Diversity
Patrick R. Stephens
One of the most important topics in ecology, and biology in general, is the origins of large-scale patterns of species richness. Species richness is the number of species that occur in a given region or habitat. There are often striking differences in the number of species that occur in different regions. For example, a few acres of South American tropical rainforest can contain more than ten times the number of tree species that occur in an entire forest in temperate North America. A pattern that occurs in many groups of organisms is that areas at high latitudes (near the poles) tend to have fewer species than areas that are at lower latitudes (closer to the equator). However in some groups, for example shorebirds, zooplankton, and new world pond turtles, the opposite pattern occurs and more species occur in temperate than equatorial regions. In addition, many groups of organisms occur in more than one continental region at the same latitude, and in such cases one region almost invariably contains many more species than the other. For example, eastern North America contains roughly ten times the number of turtle species that Europe contains. From the time of Darwin until relatively recently it was assumed that one major factor accounted for many, if not most, of these regional patterns of species richness: where a given group originated. If a particular group (or lineage) of organisms has occurred in one region for longer than another, it follows that there has been more time for species to evolve in that region. For example, based on the observation that there are more shore-birds in temperate regions than in tropical (equatorial) regions, it might be assumed that shorebirds first evolved in temperate regions and only recently invaded the tropics. Surprisingly, this simple and obvious hypothesis has almost never been tested. The main reason for this is that until very recently too little was known about the evolutionary history of most groups. However, in recent years advances in computational techniques and molecular biology have created a wealth of information on the history and relationships of numerous organisms. The goal of Dr. Stephens' project is to see what this vast and largely untapped reservoir of information can tell us about the origins of large-scale patterns of species richness in a variety of lineages.