Isolated Populations
Species is defined as a group of organisms that look similar
and have the ability to interbreed and produce fertile offspring in the
natural environment. For a new species to arise, either interbreeding or
the production of fertile offspring must somehow cease among members of
a formerly successful breeding population. For this to occur, populations
or segments of a population must somehow become isolated. Two forms of
isolation prevent interbreeding or cause infertility among members of the
same species. These forms of isolation are geographic isolation and reproductive
isolation.
Geographic Isolation
Geographic isolation is the physical separation of members of
a population. Populations may be physically separated when their original
habitat becomes divided, as, for example, when new land or water barriers
form. Also, when part of a population colonizes anew, remote area such
as an island, the colonizers geographically isolated from other populations
of the species. For example, when a group of American finches colonized
the Hawaiian islands, the group became geographically isolated from other
populations of the species. these finches eventually gave rise to the 23
species of Hawaiian honeycreepers. Geographic isolation of a population
may occur as a result of physical changes in an environment. When a river
changes course or even when a highway is built across a field, populations
may become geographically isolated. An example in which geographic isolation
may have led to speciation. The desert of Death Valley, California, has
a number of isolated ponds a formed by springs. Each pond contains a species
of fish that lives only in that pond. Scientists suggest that these species
arose through geographic isolation. Geologic evidence from a study of wave
patterns in sedimentary rocks indicates that most of Death Valley was covered
by a huge lake during the last ice age. When the ice age ended, the region
became dry. Only small, spring fed ponds remained. Members of a fish species
that previously formed a single population in the lake may have become
isolated in different ponds. The environments of the isolated ponds differed
enough that natural selection and perhaps genetic drift acted on the separate
populations. Eventually the fish in the different ponds may have diverged
so much genetically that they could no longer interbreed even if brought
together. In this way geographic isolation of fishes in Death Valley probably
led to the formation of new species. Geographic isolation, plus reproductive
isolation, probably is the usual cause of the formation of new species.
Reproductive Isolation
Sometimes groups of organisms within a population become isolated
genetically without prior geographic isolation. when barriers to successful
breeding arise among population groups in the same area, the result is
reproductive isolating. Reproductive isolation is the inability of formerly
interbreeding organisms to produce offspring. Reproductive isolation can
arise through disruptive selection. Remember that in disruptive selection
the two extremes of a specific trait in a given population ate selected
for.The wood frog and the leopard frog have become reproductively isolated
, possibly as a result of disruptive selection. Though the wood frog and
the leopard frog sometimes interbreed in captivity, they do to interbreed
where the ranges overlap in the wild. The wood frog usually breeds in early
April, and the leopard frog usually breeds in mid-April. This reproductive
isolation may have resulted from disruptive selection. In the ancestral
frog species frogs that bred earlier and frogs that bred later may have
both been selected for, while frogs that bred between these times may been
selected against, perhaps because some predator was especially active during
that time. The two groups of frogs may have become reproductively isolated
because of differences in breeding times. Probably it was in part through
such reproductive isolation that speciation occurred in these frogs. Eventually
different selection pressures led to the type of morphological variations.
