Competitive and Facilitative Evolutionary Diversification

C estimates place the number of named species at about 1.4 million (WRI 2001). Scientists continue to discover new species daily, and estimates suggest the total number of species on Earth is somewhere between 2 million and 100 million. More remarkable is that over the past 3 billion to 4 billion years, this enormous diversity of living creatures appears to have evolved from very few ancestral species (Freeman and Herron 2001). This poses a daunting but fundamental question for evolutionary biologists: How has such an astonishing level of evolutionary diversification taken place throughout Earth’s history (Hutchinson 1959)? Why, for example, does a single hectare of tropical rain forest sometimes contain more than 300 species of trees instead of just a few? Unfortunately (or perhaps fortunately, if you are a scientist), such questions do not yet have satisfactory answers. Indeed, the scale of the question is so large that it is difficult to know where or how to begin. Because evolution by natural selection is the most important unifying concept in biology, many scientists begin by looking to The Origin of Species ([1859] 1993) to see what Charles Darwin had to say on the subject. Although Darwin certainly recognized the origin of evolutionary diversification (i.e., species) as a central question in biology, he did not produce a fully compelling answer, despite the title of his most famous work. With the advent of genetics, biologists realized that one fundamental issue involved in the evolutionary diversification of sexual organisms is the evolution of reproductive isolation, the equivalent of speciation according to the concept of biological species (Dobzhansky 1951, Mayr 1963, Kondrashov and Mina 1986, Johnson and Gullberg 1998). This is typically viewed as an essential component of evolutionary diversification, because, even if divergent natural selection favors different kinds of adaptation under different conditions, gene flow must typically be reduced and even halted between areas of divergent selection pressures before morphological, behavioral, and physiological differences, the hallmarks of diversity, can take hold (Dobzhansky 1951, Kondrashov and Mina 1986, Johnson and Gullberg 1998). Much research has been carried out in this area, and scientists’ understanding of the causes and consequences of reproductive isolation has progressed considerably since The Origin of Species. Although reproductive isolation may set the stage for evolutionary diversification, there is another equally important