Allopatric & Sympatric Speciation

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Allopatric Speciation

Allopatric speciation is a type of speciation where two (or more) populations are geographically isolated from each other.

Geographical isolation

Populations of a species can split into two or more populations.
This may be caused by environmental changes (e.g. emergence of a river) or random events (e.g. a storm causing a population of birds to be carried to another island).
When two populations are physically separated, they are geographically isolated.

Selection pressures

Populations in different environments will be exposed to different environmental conditions (e.g. climate, food availability).
The different environments will place different selection pressures on the populations.
The different selection pressures will drive natural selection in different directions.

Allele frequencies

The different selection pressures on the populations cause the allele frequencies in the two populations to change.
If the populations continue to be geographically isolated for a long time, eventually the allele frequencies will change so much that if the populations are brought back together they can no longer interbreed.

Speciation

Species are defined as a group of actually or potentially interbreeding individuals.
If two populations that have been geographically isolated can no longer interbreed when they are brought back together, they are considered to be two species.
This is allopatric speciation.

Sympatric Speciation

Sympatric speciation is a type of speciation where two (or more) populations are not geographically isolated from each other and involves reproductive isolation.

Reproductive isolation

Scientists organize reproductive isolation into two groups: prezygotic and postzygotic.
- Recall that a zygote is a fertilized egg: the first cell of the development of an organism that reproduces sexually.
So, a prezygotic barrier blocks reproduction from taking place such as barriers that prevent fertilization when organisms attempt reproduction.
A postzygotic barrier occurs after zygote formation, such as organisms that don’t survive the embryonic stage and those born sterile.

Reproductive isolation mutations

Sympatric speciation could be caused by mutations that influence many different factors:
- Polyploidy - where a cell or organism has an extra set, or sets, of chromosomes. Polyploidy organisms are reproductively isolated from diploid organisms.
- Mutations may influence the flowering times or mating times of individuals.
- Mutations may alter the reproductive organs.
- Mutations may alter mating behaviors.

Prezygotic barriers

Differences in breeding schedules, called temporal isolation, can act as a form of reproductive isolation.
- For example, two species of frogs inhabit the same area, but one reproduces from January to March, whereas the other reproduces from March to May.
In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the other populations of the same species.
- This situation is called habitat isolation.

Prezygotic barriers - 2

Behavioral isolation occurs when a specific behavior prevents reproduction from taking place.
- For example, male fireflies use specific light patterns to attract females. If a male of one species tried to attract the female of another, she would not recognize the light pattern.
Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from taking place; this is called a gametic barrier.

Postzygotic barriers

When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction.
Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages.
- This is called hybrid inviability because the hybrid organisms simply are not viable.
In another postzygotic situation, reproduction leads to the birth and growth of a hybrid that is sterile and unable to reproduce offspring of their own.
- This is called hybrid sterility.

Speciation

The biological definition of a species is a group of individuals that can actually or potentially interbreed.
The emergence of a mutation that prevents two (or more) populations from interbreeding is called sympatric speciation.