Evolution

Biologists have estimated that there are as many as 50 million species living on Earth today. Remarkably, it is believed that this figure is only 1% of the species that have ever lived. The great diversity of organisms, living and extinct, is the result of speciation, the splitting and divergence of biological lineages to create new species.

The term "species" is derived from the Latin word for kind, and species are considered to be the fundamental natural units of life which can and will evolve. Species have been defined in various ways over the years, but the most widely used definition at present is the biological species concept. Most often associated with the name of Ernst Mayr (1904-), it defines a species as a group of interbreeding populations that is reproductively separate from other such groups.

The crucial event for the origin of new species is reproductive separation. How do barriers to reproduction emerge? Perhaps a homogeneous population becomes separated into two distinct populations due to environmental factors. For example, when a river divides the species into two geographically distinct populations. Once they are separated, one or more evolutionary mechanisms may act on each in different ways over time, with the result that the two will diverge so they can no longer interbreed. As another example, perhaps a genetic variant arises in a population of interbreeding individuals by mutation or immigration; if the new variant spreads, bearers of the new variant may begin to breed preferentially with other bearers, preferring the new type. Along the way, further ecological, behavioral, and phenotypic changes may occur. Over many years, the organisms can diverge into two distinct species based on a The long neck of the giraffe is an adaptation that allows it to survive competition for food resources. Photograph by Stan Osolinski. Stock Market. Reproduced by permission. dually but mutually exclusive preferential mating selection process.