Slide
1
The Prezygotic and postzygotic isolating mechanisms described earlier,
explain how closely related species stay separate. But how did they
become distinct species in the first place?
There are two basic ways for speciation to occur - allopatric and sympatric speciation.
Slide 2
The
prefix allo means other while the prefix sym means same. The suffix
patric means homeland.
Slide 3
Allopatric speciation requires a
physical or geographical barrier between populations
Sympatric speciation requires genetic isolation even when populations ranges overlap
Slide
4
This image illustrates the difference between these two processes.
Allopatric speciation, on the left, requires a physical barrier to form between the two populations and speciation ultimately results.
Sympatric speciation, on the right, occurs when populations diverge due to some biological barrier between them - not a physical barrier. The two populations inhabit the same area.
Slide 5
An example of allopatric speciation is the
divergence of two populations of antelope squirrels into separate species.
The Grand Canyon is a barrier to dispersal by small mammals, and as a consequence the isolated populations can diverge and become distinct species.
Slide
6
Allopatric speciation is frequently seen on islands. This image
shows the speciation of a mainland population of kingfishers into several
different island species of kingfishers. Individuals from the mainland
dispersed to the islands where they were geographically and thus genetically
isolated from each other. Over time genetic differences accumulated to
the point where they are all different species.
Slide 7
This type
of island colonization and subsequent speciation is referred to as adaptive
radiation.
In adaptive radiation a single species colonizes an island and adapts to that island and becomes a distinct species. The individuals from the new species subsequently disperse to another island in the group and then adapts to that new environment and another new species evolves.
There are several classic examples of adaptive radiation including Darwinís Finches on the Galapagos islands and Hawaiian Silverswords on the Hawaiian islands.
Slide 8
Sympatric speciation can be more
difficult to conceptualize because populations are not physically separated
from each other but some factor creates genetic isolation.
This is
commonly observed in plants. Some plants can self-fertilize and, if
there was an error during mitosis or meiosis, they may produce polyploid
offspring. Polyploids have more than the diploid number of chromosomes.
Because the offspring have a different number of chromosomes from the
parents - they are genetically isolated and a speciation event can
occur.