Terms in this set (33)

Speciation is the evolution of new species, which are groups of individuals that can interbreed freely with each other but not with members of other species. Gene flow is impossible between different species. Different selective pressures act upon the gene pools of each group, causing them to evolve independently. Genetic variation, changes in the environment, migration to new environments, adaptation to new environments, natural selection, genetic drift, and isolation are all factors that can lead to speciation.

Before speciation, small, local populations called demes often form within a species. For example, all the beavers along a specific portion of a river form a deme. There may be many demes belonging to a specific species. Members of a deme resemble one another more closely than they resemble members of other demes. They are closely related genetically since mating between members of the same deme occurs more frequently. They are also influenced by similar environmental factors and thus are subject to the same selection process.

If these demes become isolated, speciation may occur. When groups are isolated from each other, there is no gene flow among them. Any difference arising from mutations or new combinations of genes will be maintained in the isolated population. Over time, these genetic differences may become significant enough to make mating impossible. If the gene pools within a species become sufficiently different so that two individuals cannot mate and produce fertile offspring, two different species have developed and one or more new species have formed. Genetic and eventually reproductive isolation often results from the geographic isolation of a population.
Species multiplication is generally accompanied by migration to lessen intraspecific competition. Separation of a widely distributed population by emerging geographic barriers increases the likelihood of genetic adaptations on either side of the barrier. Each population may evolve specific adaptations to the environment in which it lives in addition to accumulating neutral (random, nonadaptive) changes. These adaptations will remain unique to the population in which they evolve-- provided that interbreeding is prevented by the barrier. In time, genetic differences will reach the point where interbreeding becomes impossible between the populations and reproductive isolation would be maintained even if the barrier were removed. Following are two examples:

-- Marsupials: A lineage of pouched mammals (marsupials) paralleling the development of placental mammals developed on the Australian side of a large water barrier. The geographic barrier protected the pouched mammals from competition and hybridization with modern placental mammals. This barrier resulted in the development of uniquely Australian plants and animals such as the eucalyptus tree and duckbilled platypus.
-- Darwin's finches: Over a comparatively short period of time, a single species of Galapagos finch underwent adaptive radiation to form 13 different species of finches. Slight variations in the beak, for example, favored ground or tree feeding. Such adaptations minimized the competition among the birds, enabling each emerging species to become firmly entrenched in its environmental niche. The evolution of these adaptations was helped by the geographic isolation of some of these species on different islands of the Galapagos island chain.