Speciation Parapatric Speciation Sympatric Speciation
Introduction Peripatric Speciation
is the evolutionary process by which new biological species
arise. Four speciation processes are recognized that are differentiated
by the extent to which a population from within a population
becomes geographically isolated from the parent population.
allopatric speciation, a population splits into two geographically
isolated populations due to formation of a barrier between
portions of a population, for example, because of mountain
building as depicted in the animal to the left. The isolated
populations then experience differentiating genotypic and phenotypic
divergence as a result of different selective pressures in
their differing environments. Additionally, genetic drift will
occur differently, eventually differentiating the population’s
genotypes and phenotypes. The separated populations will also
experience different mutations that may persist differently
in their respective environments. Should the populations be
eventually recombine in the same environment, they have each
evolved differently to an extent that they remain isolated
because changes are too great to enable genetic mixing through
sexual reproduction. Allopatric speciation is not necessarily
precluded if some individuals from one group cross the barrier
and mate with members of the other group. In other words, allopatric
speciation can occur if gene flow between groups is greatly
reduced, but not entirely eliminated.
Allopatric Speciation Sequence
A) Original population of brown butterflies
Mountain formation separates the population into two sexually
C) Survival is favored by evolution
of color green in one sexually isolated environment
D) The old brown and new green species
eventually migrate and mix but do not mate
speciation closely resembles allopatric
speciation. New species arise in isolated, smaller peripheral
that are prevented from exchanging genes with the main population.
It is related to the concept of a founder effect, since small
populations often undergo bottlenecks. The essential mechanism
of paripatric speciation is genetic drift. In peripatric speciation,
small population size would makespeciation a more likely consequence
of geographic isolation since genetic drift acts more quickly
in small populations. Genetic drift, and perhaps strong selective
pressures, would cause rapid genetic change in a small population.
This genetic change could be sufficiently substantial to result
in a new species. Because the extent of genetic drift can’t
be measures after the fact, testing the hypothesis of this mode
of speciation is very difficult.
speciation occurs during incomplete, only partial separation
of the geographical environments of two diverging populations.
Individuals within each species may come in contact or cross
habitats at times, but reduced fitness of the heterozygote
leads to selection for behaviors or mechanisms that prevent
their inter-breeding. Thus, in parapatric speciation, selection
acts on existing variation within a single environment, as
opposed to selection proceeded differently in separated environmental
niches, as in the case of peripatric and allopatric speciation.
Ecologists refer to parapatric and peripatric speciation in
terms of ecological niches. In parapatric speciation there
is no physical barrier to gene flow. The population members
are share the same environment, but fail to mate randomly.
Instead, individuals are more likely to mate with their geographic
neighbors than with individuals in a more distant part of the
environment. In this mode, divergence may happen because of
reduced gene flow within the population and varying selection
pressures across the population’s physical range.
sympatric speciation, two or more
species arise from a single parent species all within the
same geographic environment, that is, species
diverge while inhabiting the same place. The existence
of the sympatric speciation mechanism remains a scientific
debate since gene flow due to interbreeding would be expected
to overwhelm any emerging genetic differentiation within a