Types of Speciation (College Board AP Biology)

• Allopatric speciation occurs as a result of geographical isolation
• It is the most common type of speciation
• A species population splits into one or more groups which then become separated from each other by geographical barriers
  • The barrier could be natural like a body of water, or a mountain range
  • It can also be man made (like a freeway)
• This separation creates two populations of the same species who are isolated from each other, and as a result, no genetic exchange can occur between them
• If there is sufficient selection pressure or genetic drift acting to change the gene pools within both populations then eventually these populations will diverge and form separate species
  • The changes in the alleles/genes of each population will affect the phenotypes present in both populations
  • Over time, the two populations may begin to differ physiologically, behaviorally and morphologically (structurally)

Example of Allopatric Speciation in Trees

• Imagine there is a population of trees that are all one species
• A new mountain range forms that divides the population into two
• The natural barrier prevents the two groups from interbreeding, so there is no gene flow between them
• The two populations experience different selection pressures and genetic drift
• Over thousands of years the divided populations form two distinct species that can no longer interbreed

Allopatric Speciation Example Diagram

The natural geographical barrier of a mountain range can lead to allopatric speciation in trees

Other Examples of Allopatric Speciation

• Hawaiian Drosophila
  • Two distinct species of fruit fly of the genus Drosophila have emerged by allopatric speciation on different islands of the Hawaiian archipelago
  • Drosophila silvestris inhabits Hawai’i Island and Drosophila planitibia inhabits the island of Maui 
• Caribbean Anolis
  • These are anole lizards of the genus Anolis
  • They have speciated from one colonizing species to around 150 distinct species via reproductive and geographical isolation, over millions of years 

Sympatric Speciation

• Sympatric speciation takes place with no geographical barrier
• A group of the same species could be living in the same place but in order for speciation to take place there must exist two populations within that group and no gene flow occurs between them
• Ecological separation: Populations are separated because they live in different environments within the same area
  • For example, soil pH can differ greatly in different areas. Soil pH has a major effect on plant growth and flowering

• Behavioral separation: Populations are separated because they have different behaviours
  • For example differences in feeding, communication or social behaviour
• Something has to happen that splits or separates the population:

Example of Sympatric Speciation in Fish

• A species of fish lives in a lake
• Some individuals within the population feed on the bottom while others remain higher up in the open water
• The different feeding behaviours separates the population into different environments
  • Behavioural separation leads to ecological separation
• The separated groups experience different selection pressures
  • Long jaws are advantageous for bottom-feeding whereas shorter jaws are advantageous for mid-water feeding
• Over time natural selection causes the populations to diverge and evolve different courtship displays
• They can no longer interbreed; they are separate species

• The apple maggot Rhagoletis is a good example of sympatric speciation
  • Two species within that genus are the hawthorn fly and the apple fly
  • They evolved from the same ancestor species by virtue of preferences for the source of food that they eat
  • The two species maintain their separation by smelling their preferred fruit and being attracted to that, thereby occupying a different area of a habitat

Pre-zygotic Barriers

• Some barriers prevent successful fertilization or the formation of a zygote in populations
• Preventing successful reproduction results in reproductive isolation
  • This may lead to speciation
• Examples of pre-zygotic barriers include:
• • Habitat isolation - Organisms living in two different geographical locations will not interact with each other and will therefore be reproductively isolated
  • Temporal isolation - Organisms can inhabit the same environment, yet if their periods of activity vary throughout the day or if their reproductive seasons occur at different times of the year, they will experience temporal isolation, preventing them from interbreeding
  • Behavioural isolation - Certain species will only mate with others that display compatible mating behaviors, like specific mating calls or dances.
  • Mechanical isolation - If the reproductive organs of the organisms cannot work together and hinder the exchange of gametes, it will result in reproductive isolation for the species.
  • Gametic isolation - Even if two organisms can mate successfully, if their sex cells (gametes) don't match or work together, no fertilized egg (zygote) will form, and the organisms will experience reproductive isolation

Post-zygotic Barriers

• Other barriers prevent the zygote from developing into a viable or fertile adult
• Post-zygotic barriers prevent parental traits from being passed on to the next generation and therefore mixing off alleles between different species is still not possible
  • This means that, even if two individuals from different populations have been able to breed and successfully produce a zygote, they are still reproductively isolated
• Examples of post-zygotic barriers include: 
  • Reduced hybrid viability - the offspring cannot survive to reproductive age, then they will not be able to breed and pass on their alleles to the next generation. This means that the mixing of alleles between species is restricted to one generation
  • Reduced hybrid fertility - Even if the zygote grows into an adult, if that hybrid adult cannot have babies, the two species that made the hybrid will stay reproductively separated
  • Hybrid breakdown - In certain plants, hybrids may be strong and fertile initially, but as each new generation comes along, the hybrid weakens and becomes less robust, eventually disappearing after a few generations.