<P> Speciation has been observed multiple times under both controlled laboratory conditions and in nature . In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence . There are four primary geographic modes of speciation . The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration . Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms . As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed . </P> <P> The second mode of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment . This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population . Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change . </P> <P> The third mode is parapatric speciation . This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations . Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations . Generally this occurs when there has been a drastic change in the environment within the parental species' habitat . One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines . Here, plants evolve that have resistance to high levels of metals in the soil . Selection against interbreeding with the metal - sensitive parental population produced a gradual change in the flowering time of the metal - resistant plants, which eventually produced complete reproductive isolation . Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance . </P> <P> Finally, in sympatric speciation species diverge without geographic isolation or changes in habitat . This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population . Generally, sympatric speciation in animals requires the evolution of both genetic differences and non-random mating, to allow reproductive isolation to evolve . </P>

The process that results in changes in a population as the environment changes is called