speciation
Speciation is the evolution of two species from one
species because of divergent natural
selection in response to changes in environmental conditions. It usually
takes thousands to millions of years to take place.
The
most common mechanism of speciation takes place in two phases: geographic isolation and reproductive
isolation.
Geographic
Isolation
It
occurs when groups of the same population of a species become physically separated for long periods.
For example, part of a population may migrate
in search of food and then begin living in another area with different
environmental conditions (Figure).
Populations also may become separated by a physical
barrier (such as a mountain range, stream, lake, or road), by a change such
as a volcanic eruption or earthquake, or when a few individuals are carried to a new area by wind or water.
Figure: How
geographic isolation can lead to reproductive isolation, divergence, and speciation.
Reproductive
Isolation
It
occurs when mutation and natural selection operate independently in two
geographically isolated populations with same ancestral genetic heritage and
change the allele frequencies in different ways. If this process, called divergence, continues long enough,
members of the geographically and reproductively isolated populations may
become so different in external forms, structures and genetic makeup that they
cannot interbreed—or if they do, they cannot produce live, fertile offspring.
Then one species has become two, and speciation has occurred through divergent
evolution.
It
is now believed that the ostrich, the rhea, and the emu have a common ancestor
but evolved separately, and believed to be examples of divergent evolution.
For some rapidly reproducing organisms,
this type of speciation may occur within hundreds of years.
However, for most species such
speciation takes from thousands to millions of years. Given this time
scale, it is difficult to observe and document the appearance of a new
species.
Biological Evolution
Biological evolution refers to the
change in inherited characteristics of a population from generation to
generation, i.e., a genetic
change in a population of organisms that occurs over time. It can result in new species—populations
that can no longer reproduce with members of the original species.
It
is a one-way process: when a new species evolves, it cannot evolve backward
into its parents.
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