Speciation

SIGNIFICANCE: Speciation, the biological formation of new species, has produced the wide variety of living things on Earth. Although speciation can be caused by other forces or events, natural selection is considered by many researchers to be one of the primary mechanisms promoting speciation.

Species Concepts

Before the time of Charles Darwin, physical appearance was the only criterion for classifying an organism. This “typological species concept” was associated with the idea that species never change (fixity of species). This way of defining a species causes problems when males and females of the same species look different, as with peacocks and peahens, or when there are several different color patterns among members of a species, as with many insects. Variability within species, whether it is a visible part of their anatomy, an invisible component of their biochemistry, or another characteristic such as behavior, is an important element in understanding how species evolve.

The “biological species concept” uses reproduction to define a species. It states that a species is composed of individuals that can mate and produce fertile offspring in nature. This concept cannot be used to classify organisms such as bacteria, which do not reproduce sexually. It also cannot be used to classify dead specimens or fossils. This definition emphasizes the uniqueness of each individual (variability) in sexually reproducing species. For example, in the human species (Homo sapiens), there are variations in body build, hair color and texture, ability to digest milk sugar (lactose), and many other anatomical, biochemical, and behavioral characteristics. All these variations are the result of genetic mutations, or changes in genes.

According to evolutionary scientist Ernst Mayr, to a “population thinker,” variation is reality and type is an abstraction or average; to a “typological thinker,” variation is an illusion and type is the reality. Typological thinking is similar to typecasting or stereotyping, and it cannot explain the actual variability seen in species, just as stereotyping does not recognize the variability seen in people. Additional definitions, such as the “evolutionary species concept,” include the continuity of a species’ genes through time and other factors not addressed by the biological species concept.

Isolation and Divergence of Populations

Species are composed of unique individuals that are nevertheless similar enough to be able to mate and produce fertile offspring. However, individuals of a species are infrequently in close enough proximity to be able to choose a mate from all opposite-sex members of the same species. Groups of individuals of the same species that are at least potential mates because of proximity are called populations.

The basic type of speciation in most sexually reproducing organisms is believed to be allopatric, in which geographic isolation (separation) of the species into two or more populations is followed by accumulation of differences (divergence) between the populations that eventually prevent them from interbreeding. These differences are caused primarily by natural selection of characteristics advantageous to populations in different environments. If both populations were in identical environments after geographic isolation, they would be much less likely to diverge or evolve into new species.

Subforms of allopatric speciation have been proposed based on the type or degree of separation occurring between two populations. Peripatic speciation involves a minority of a population entering a new environmental niche that then becomes isolated. New genetic traits may then arise in the niche population; many researchers believe that genetic drift likely is an important factor in peripatic speciation. Parapatic speciation occurs when a population reaches a new niche but never becomes totally isolated from the parent population. While a new species forms in the new environment, hybridization may occur as the new and parent species interact.

Another type of speciation is sympatric, in which populations are not separated geographically but reproduction between them cannot occur (reproductive isolation) for some other reason. For example, one population may evolve a mutation that makes the fertilized egg (zygote) that results from interbreeding with the other population incapable of surviving. Another possibility is a mutation that changes where or when individuals are active so that members of the different populations never encounter one another.

Darwin thought that divergence, and, thus, speciation, occurred gradually, by the slow accumulation of many small adaptations “selected” by the environment. Geneticists now recognize that a very small population, or even a “founder” individual, may be the genetic basis of a new species that evolves more rapidly. This process, called genetic drift, is essentially random. For example, which member of an insect species is blown to an island by a storm is determined not by genetic differences from other members of the species but by a random event—in this case, the weather. This individual (or small number of individuals) is highly unlikely to contain all the genetic diversity of the entire species. Thus, the new population begins with genetic differences that may be enhanced by its new environment. Speciation proceeds according to the allopatric model, but faster. However, extinction of the new population may also occur.

Plants are able to form new species by hybridization (crossbreeding) more often than are animals. When plants hybridize, postmating incompatibility between the chromosomes of the parents and the offspring may immediately create a new, fertile species rather than a sterile hybrid as in animals (such as the mule). A frequent method of speciation in plants is polyploidy, in which two or more complete sets of chromosomes end up in the offspring. (Usually, one complete set is made up of half of each parent’s chromosomes.)

Many species reproduce asexually (without the exchange of genes between individuals that defines sexual reproduction), including bacteria and some plants, fish, salamanders, insects, rotifers, worms, and other animals. In spite of the fact that reproductive isolation has no meaning in these organisms, they are species whose chromosomes and genes differ from those of their close relatives.

Speciation may also be created artificially, sometimes with the intention of benefiting humans but also inadvertently due to human impact on ecosystems, such as introducing invasive species that may then hybridize with native ones. The domestication of animals through husbandry practices has created numerous species, as has the agricultural manipulation of plants, though in many cases the details of the divergence are undocumented and subject to much speculation. Humans have also developed new species in laboratory settings, including for the purpose of testing theories of speciation. Researchers have also found that human-made urban environments have had a noticeable effect in driving speciation in the plants and animals that live there.

Impact and Applications

Environmentalists and scientists recognize that the biodiversity created by speciation is essential to the functioning of Earth’s life-support systems for humans as well as other species. Some practical benefits of biodiversity include medicines, natural air and water purification, air conditioning, and food.

The impact of understanding the genetic basis of evolving species cannot be underestimated. Artificial selection (in which humans decide which individuals of a species survive and reproduce) of plants has produced better food crops, such as modern corn from teosinte, and alleviated hunger in developing nations by creating new varieties of existing species such as rice. Hybridization of animals has resulted in mules and beefalo for the farm, both of which are sterile hybrids rather than species. Artificial selection of domesticated animals has produced many different breeds of horses, dogs, and cats, each of which is still technically one species. Genetic engineering promises to create crops that resist pests, withstand frost or drought, and contain more nutrients. Finally, understanding the genetics of the evolving human species has broad implications for curing disease and avoiding birth defects.

Key Terms

• allopatric speciationthe genetic divergence of populations caused by separation from each other by a geographic barrier such as a mountain range or an ocean
• populationa group of organisms of the same species in the same place at the same time with the potential ability to mate; the basic unit of speciation
• reproductive isolating mechanisma characteristic that prevents an individual of one species from interbreeding (hybridizing) with a member of another species
• speciesa class of organisms with common attributes; individuals are usually able to produce fertile offspring only when mating with members of their own species
• sympatric speciationthe genetic divergence of populations that are not separated geographically

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