A species includes all the individuals that are capable of exchanging genes with one another. Some species are composed of very few individuals located in a restricted area, and others have millions of members spread out over large areas of the world. Some plant species are restricted to small areas of rain-forest habitat, while rats and humans live on literally every continent. It is more likely that an individual will mate with another individual that lives close by than farther away, and as a result, most species can be divided into smaller populations. Sometimes geographical factors, such as rivers or mountains or temperature gradients in different depths of water, naturally carve species into populations.

Because of geographical differences among populations, natural selection tends to result in populations varying from one another. A typical widespread species may be divided into many different populations. As long as they exchange genes at least at intervals, populations are likely to remain part of the same species. But how do new species form? New species form when members of a population or subdivision of a species no longer are able to exchange genes with the rest of the species. This is more likely to happen at the edges of the species range than in the center. We can say that speciation has occurred when a population becomes reproductively isolated from the rest of the species.

If a population at the end of the geographic range of a species is cut off from the rest of the species, through time it may become different from other populations. Perhaps natural selection is operating differently in its environment than it is in the rest of the species range, or perhaps the population has a somewhat different set of genes than other populations of the species. Just by the rules of probability, a small population at one end of the range of a species is not likely to have all the variants of genes that are present in the whole species, which might result in its future evolution taking a different turn.

No longer exchanging genes with other populations of the species, and diverging genetically through time from them, members of a peripheral, isolated population might reach the stage at which, were they to have the opportunity to mate with a member of the parent species, they would not be able to produce offspring. Isolating mechanisms, most of which are genetic but some of which are behavioral, can arise to prevent reproduction between organisms from different populations. Some isolating mechanisms prevent two individuals from mating; in some insects, for example, the sexual parts of males and females of related species are so different in shape or size that copulation cannot take place. other isolating mechanisms come into effect when sperm and egg cannot fuse for biochemical or structural reasons. An isolating mechanism could take the form of the prevention of implantation of the egg or of disruption of the growth of the embryo after a few divisions. Or the isolating mechanism could kick in later: mules, which result from crossing horses and donkeys, are healthy but sterile. Donkey genes thus are inhibited from entering into the horse species, and vice versa. When members of two groups are not able to share genes because of isolating mechanisms, we can say that speciation between them has occurred. (Outside of the laboratory, it may be difficult to determine whether two species that no longer live in the same environment are reproductively isolated.)

The new species would of course be very similar to the old one—in fact, it might not be possible to tell them apart. over time, though, if the new species manages successfully to adapt to its environment, it might also expand and bud off new species, which would be yet more different from the parent—now grandparent—species. This branching and splitting has, through time, given us the variety of species that we see today.

We can see this process of speciation operating today. Speciation in the wild usually takes place too slowly to be observed during the lifetime of any single individual, but there have been demonstrations of speciation under laboratory conditions. The geneticist Dobzhansky and his colleagues isolated a strain of Venezuelan fruit fly and bred it for several years. This strain of flies eventually reached a point of differentiation where it was no longer able to reproduce with other Venezuelan strains with which it had formerly been fertile. Speciation had occurred (Dobzhansky and Pavlovsky 1971).

Although not observed directly, good inferential evidence for speciation can be obtained from environments that we know were colonized only recently. The Hawaiian and Galapagos islands have been formed within the last few million years from undersea volcanoes and acquired their plants and animals from elsewhere. The Galapagos flora and fauna derive from South America, whereas the native Hawaiian flora and fauna are more similar to those of the Pacific islands, which in turn derive mostly from Asia. But Hawaiian species are reproductively isolated from their mainland counterparts.

one of the most dramatic examples of speciation took place among cichlid fish in the East African great lakes: Lake Victoria, Lake Malawi, and Lake Tanganyika. Geological evidence indicates that about twenty-five thousand years ago, Lake Tanganyika underwent a drying spell that divided the lake into three separate basins. Perhaps as a result of this and similar episodes, the cichlid fish that had entered the lake from adjacent rivers and streams underwent explosive adaptive radiation. There are at least 175 species of cichlid fish found in Lake Tanganyika and nowhere else. Similar speciation events took place in Lake Victoria and Lake Malawi—only over shorter periods of time (Goldschmidt 1996). Large lakes like these can be watery versions of an island: interesting biological things can go on.

occasionally speciation can take place very quickly. The London subway, known as "the Tube," was built during the 1880s. At that time, some mosquitoes found their way into the miles of tunnels, and they successfully bred in the warm air and intermittent puddles—probably several times per year. Because they were isolated from surface mosquitoes, differences that cropped up among them would not have been shared with their relatives above, and vice versa. In the late 1990s, it was discovered that the Tube mosquitoes were a different species from the surface species. one major, if unfortunate, difference is that the surface mosquitoes, Culex pipiens, bite birds, whereas the related Tube species, Culex molestus, has shifted its predation to people. What is surprising about this discovery is that it shows that at least among rapidly breeding insects like mosquitoes, speciation does not require thousands of years but can occur within a century (Bryne and Nichols 1999). Natural selection, adaptation, adaptive radiation, and speciation—these are the major principles that help us explain the pattern and understand the process of evolution. These principles have resulted in an immense proliferation of living things over time that occupy a mind-boggling array of ecological niches.

A famous anecdote: asked by a member of the clergy what his study of nature had revealed to him about the mind of God, the biologist J. B. S. Haldane is supposed to have answered, "An inordinate fondness for beetles." And in fact one-fifth of the known animal species are species of beetles. Because there are so many different kinds of organisms, and not just beetles, human beings have always sought to make some sense of them by grouping them in various ways. All human cultures attempt to group plants and animals according to various schemes, which often have to do with how they can be used. In the Bible, the dietary laws of the Jews divided animals into clean and unclean, the latter being unsuitable for eating. Plants might be grouped according to whether they are for human consumption, for animal consumption, used for making dyes, or for some other purpose. Students of nature, naturalists, of the 1700s and 1800s sought to group animals and plants according to similarities and differences independent of their utility. The science of systematics, the study of the relationship among organisms, dates to a Swedish scientist known by his Latinized name, Carolus Linnaeus.

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