As we have noted, an important observation that any useful biological theory needs to explain is why there are so many (millions) of different types of organisms currently present on Earth. The Theory of Evolution explains this observation through the process of speciation. The basic idea is that populations of organisms can split into distinct groups. Over time evolutionary mechanisms acting on these populations will produce distinct types of organisms, that is, different species. At the same time, we know from the fossil record and from modern experiences that types of organisms can disappear – they can become extinct. What leads to the formation of a new species or the disappearance of existing ones?
The concept of an organism’s ecological niche, which is the result of its past evolutionary history (the past selection pressures acting within a particular environment) and its current behavior, combines all of these factors. In a stable environment, and a large enough population, reproductive success will reflect how organisms survive and exploit their ecological niche. Over time, stabilizing selection will tend to optimize the organism’s adaptation to its niche. At the same time, it is possible that different types of organisms will compete for similar resources. This interspecies competition leads to a new form of selective pressure. If individuals of one population can exploit a different set of resources or the same resources differently, these organisms can minimize competition with other species and become more reproductively successful compared to individuals that continue to compete directly with other species. This can lead to a number of outcomes. In one case, one species becomes much better than others at occupying a particular niche, driving the others to extinction. Alternatively, one species may find a way to occupy a new or related niche, and within that particular niche, it can more effectively compete, so that the two species come to occupy distinct niches. Finally, one of the species may be unable to reproduce successfully in the presence of the other and become (at least) locally extinct. These scenarios are captured in what is known as the competitive exclusion principle or Gause's Law, which states that two species cannot (stably) occupy the same ecological niche - over time either one will leave (or rather be forced out) of the niche, or will evolve to fill a different (often subtly) niche. What is sometimes hard to appreciate is how specific a viable ecological niche can be. For example, consider the situation described by the evolutionary biologist Theodosius Dobzhansky (1900-1975):
Some organisms are amazingly specialized. Perhaps the narrowest ecologic niche of all is that of a species of the fungus family Laboulbeniaceae, which grows exclusively on the rear portion of the elytra (the wing cover) of the beetle Aphenops cronei, which is found only in some limestone caves in southern France. Larvae of the fly Psilopa petrolei develop in seepages of crude oil in California oilfields; as far as is known they occur nowhere else.
While it is tempting to think of ecological niches in broad terms, the fact is that subtle environmental differences can favor specific traits and specific organisms. If an organism’s range is large enough and each individual’s range is limited, distinct traits can be prominent in different regions of the species’ range. These different subpopulations (sometimes termed subspecies or races) reflect local adaptations. For example, it is thought that human populations migrating out of the equatorial regions of Africa were subject to selection based on exposure to sunlight in part through the role of sunlight in the synthesis of vitamin D.97 In their original ecological niche, the ancestors of humans were thought to hunt in the open savannah (rather than within forests), and so developed adaptations to control their body temperature - human nakedness is thought to be one such adaptation (although there may be aspects of sexual selection involved as well, discussed in the next chapter). Yet, the absence of a thick coat of hair also allowed direct exposure to the UV-light from the sun. While UV exposure is critical for the synthesis of vitamin D, too much exposure can lead to skin cancer. Dark skin pigmentation is thought to be an adaptive compromise. As human populations moved away from the equator, the dangers of UV exposure decreased while the need for vitamin D production remained. Under such condition, allelic variation that favored lighter skin pigmentation (but retaining the ability to tan, at least to some extent) appears to have been selected. Genetic analyses of different populations have begun to reveal exactly which mutations, and the alleles they produced, occurred in different human populations as they migrated out of Africa. Of course, with humans the situation has an added level of complexity. For example, the human trait of wearing clothing certainly impacts the pressure of “solar selection.”
A number of variations can occur over the range of a species. Differences in climatic conditions, pathogens, predators, and prey can all lead to local adaptations, like those associated with human skin color. For example, many species are not continuously fertile and only mate at specific times of the day or year. When the range of a species is large, organisms in geographically and climatically distinct regions may mate at somewhat different times. As long as there is sufficient migration of organisms between regions and the organisms continue to be able to interbreed and to produce fertile offspring, the population remains one species.