The concept of species

Darwin sought to explain the splendid multiformity of the living world—thousands of organisms of the most diverse kinds, from lowly worms to spectacular birds of paradise, from yeasts and molds to oaks and orchids. His On the Origin of Species by Means of Natural Selection (1859) is a sustained argument showing that the diversity of organisms and their characteristics can be explained as the result of natural processes.

Species come about as the result of gradual change prompted by natural selection. Environments are continuously changing in time, and they differ from place to place. Natural selection therefore favours different characteristics in different situations. The accumulation of differences eventually yields different species.

Everyday experience teaches that there are different kinds of organisms and also teaches how to identify them. Everyone knows that people belong to the human species and are different from cats and dogs, which in turn are different from each other. There are differences between people, as well as between cats and dogs, but individuals of the same species are considerably more similar among themselves than they are to individuals of other species.

External similarity is the common basis for identifying individuals as being members of the same species. Nevertheless, there is more to a species than outward appearance. A bulldog, a terrier, and a golden retriever are very different in appearance, but they are all dogs because they can interbreed. People can also interbreed with one another, and so can cats with other cats, but people cannot interbreed with dogs or cats, nor can these with each other. It is clear then that, although species are usually identified by appearance, there is something basic, of great biological significance, behind similarity of appearance—individuals of a species are able to interbreed with one another but not with members of other species. This is expressed in the following definition: Species are groups of interbreeding natural populations that are reproductively isolated from other such groups. (For an explanation and discussion of this concept, see below Reproductive isolation.)

The ability to interbreed is of great evolutionary importance, because it determines that species are independent evolutionary units. Genetic changes originate in single individuals; they can spread by natural selection to all members of the species but not to individuals of other species. Individuals of a species share a common gene pool that is not shared by individuals of other species. Different species have independently evolving gene pools because they are reproductively isolated.

Although the criterion for deciding whether individuals belong to the same species is clear, there may be ambiguity in practice for two reasons. One is lack of knowledge—it may not be known for certain whether individuals living in different sites belong to the same species, because it is not known whether they can naturally interbreed. The other reason for ambiguity is rooted in the nature of evolution as a gradual process. Two geographically separate populations that at one time were members of the same species later may have diverged into two different species. Since the process is gradual, there is no particular point at which it is possible to say that the two populations have become two different species.

A related situation pertains to organisms living at different times. There is no way to test if today’s humans could interbreed with those who lived thousands of years ago. It seems reasonable that living people, or living cats, would be able to interbreed with people, or cats, exactly like those that lived a few generations earlier. But what about ancestors removed by a thousand or a million generations? The ancestors of modern humans that lived 500,000 years ago (about 20,000 generations) are classified as the species Homo erectus. There is no exact time at which H. erectus became H. sapiens, but it would not be appropriate to classify remote human ancestors and modern humans in the same species just because the changes from one generation to the next were small. It is useful to distinguish between the two groups by means of different species names, just as it is useful to give different names to childhood and adulthood even though no single moment can separate one from the other. Biologists distinguish species in organisms that lived at different times by means of a commonsense morphological criterion: If two organisms differ from each other in form and structure about as much as do two living individuals belonging to two different species, they are classified in separate species and given different names.

The definition of species given above applies only to organisms able to interbreed. Bacteria and cyanobacteria (blue-green algae), for example, reproduce not sexually but by fission. Organisms that lack sexual reproduction are classified into different species according to criteria such as external morphology, chemical and physiological properties, and genetic constitution.

The origin of species

Reproductive isolation

Among sexual organisms, individuals that are able to interbreed belong to the same species. The biological properties of organisms that prevent interbreeding are called reproductive isolating mechanisms (RIMs). Oaks on different islands, minnows in different rivers, or squirrels in different mountain ranges cannot interbreed because they are physically separated, not necessarily because they are biologically incompatible. Geographic separation, therefore, is not a RIM.

There are two general categories of reproductive isolating mechanisms: prezygotic, or those that take effect before fertilization, and postzygotic, those that take effect afterward. Prezygotic RIMs prevent the formation of hybrids between members of different populations through ecological, temporal, ethological (behavioral), mechanical, and gametic isolation. Postzygotic RIMs reduce the viability or fertility of hybrids or their progeny.

Ecological isolation

Populations may occupy the same territory but live in different habitats and so not meet. The Anopheles maculipennis group consists of six mosquito species, some of which are involved in the transmission of malaria. Although the species are virtually indistinguishable morphologically, they are isolated reproductively, in part because they breed in different habitats. Some breed in brackish water, others in running fresh water, and still others in stagnant fresh water.

Temporal isolation

Populations may mate or flower at different seasons or different times of day. Three tropical orchid species of the genus Dendrobium each flower for a single day; the flowers open at dawn and wither by nightfall. Flowering occurs in response to certain meteorological stimuli, such as a sudden storm on a hot day. The same stimulus acts on all three species, but the lapse between the stimulus and flowering is 8 days in one species, 9 in another, and 10 or 11 in the third. Interspecific fertilization is impossible because, at the time the flowers of one species open, those of the other species have already withered or have not yet matured.

A peculiar form of temporal isolation exists between pairs of closely related species of cicadas, in which one species of each pair emerges every 13 years, the other every 17 years. The two species of a pair may be sympatric (live in the same territory), but they have an opportunity to form hybrids only once every 221 (or 13 × 17) years.

Ethological (behavioral) isolation

Sexual attraction between males and females of a given species may be weak or absent. In most animal species, members of the two sexes must first search for each other and come together. Complex courtship rituals then take place, with the male often taking the initiative and the female responding. This in turn generates additional actions by the male and responses by the female, and eventually there is copulation, or sexual intercourse (or, in the case of some aquatic organisms, release of the sex cells for fertilization in the water). These elaborate rituals are specific to a species and play a significant part in species recognition. If the sequence of events in the search-courting-mating process is rendered disharmonious by either of the two sexes, then the entire process will be interrupted. Courtship and mating rituals have been extensively analyzed in some mammals, birds, and fishes and in a number of insect species (see reproductive behaviour).

Ethological isolation is often the most potent RIM to keep animal species from interbreeding. It can be remarkably strong even among closely related species. The vinegar flies Drosophila serrata, D. birchii, and D. dominicana are three sibling species (that is, species nearly indistinguishable morphologically) that are endemic in Australia and on the islands of New Guinea and New Britain. In many areas these three species occupy the same territory, but no hybrids are known to occur in nature. The strength of their ethological isolation has been tested in the laboratory by placing together groups of females and males in various combinations for several days. When the flies were all of the same species but the female and male groups each came from different geographic origins, a large majority of the females (usually 90 percent or more) were fertilized. But no inseminations or very few (less than 4 percent) took place when males and females were of different species, whether from the same or different geographic origins.

It should be added that the rare interspecific inseminations that did occur among the vinegar flies produced hybrid adult individuals in very few instances, and the hybrids were always sterile. This illustrates a common pattern—reproductive isolation between species is maintained by several RIMs in succession; if one breaks down, others are still present. In addition to ethological isolation, failure of the hybrids to survive and hybrid sterility (see below Hybrid inviability and Hybrid sterility) prevent successful breeding between members of the three Drosophila species and between many other animal species as well.

Species recognition during courtship involves stimuli that may be chemical (olfactory), visual, auditory, or tactile. Pheromones are specific substances that play a critical role in recognition between members of a species; they have been chemically identified in such insects as ants, moths, butterflies, and beetles and in such vertebrates as fish, reptiles, and mammals. The “songs” of birds, frogs, and insects (the last of which produce these sounds by vibrating or rubbing their wings) are species recognition signals. Some form of physical contact or touching occurs in many mammals but also in Drosophila flies and other insects.