The Natural History Reader in Evolution

By Niles Eldredge | Go to book overview

6
The Origin of a Species

PETER R. GRANT and NICOLA GRANT

Between 8000 and 9000 species of birds inhabit the world today. Many more species once existed but have since gone extinct. How can we account for the sheer number of species? The theory of natural selection provides a powerful explanation of how organisms change over time, but one hundred years after Darwin, biologists are still struggling to understand the details of the central evolutionary process--the development of two species from one.

The basic unresolved question is whether speciation in birds occurs primarily (or even only) when populations are physically isolated from one another or whether speciation can also take place when populations are adjacent to one another and thus in contact. According to the first view, any of a number of events, ranging from movements of the earth's crust to a few birds blown off course by a storm, may split a population into two or more geographically separate ones. Over time, these isolated populations adapt to their respective environments and become so different--genetically, behaviorally, ecologically--that they never or rarely interbreed even if chance should bring them together again. This kind of speciation is known as allopatric speciation.

The chief alternative to this view is the idea that speciation may result from either the divergence of two adjacent populations, as each adapts to different ecological conditions in its environment, or the divergence of two segments of a single population, as each adapts to different niches. The first of these two possibilities is often referred to as parapatric, or clinal, speciation; the second, sympatric. Still another possibility is that speciation may involve divergence both when the populations are isolated and when they are in contact.

Regardless of the geographical mode of speciation, there are two kinds of

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