Allopatric & Sympatric Speciation
Biology: IB Diploma Higher Level, topic 4.3.9
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.
Test yourself on Allopatric & Sympatric Speciation
Which of the following statements are true for allopatric speciation?
Which of these is NOT essential for allopatric speciation?
What is the definition of a species?
Which of the following statements about sympatric speciation are true?
Which of these does NOT cause sympatric speciation?