13 Speciation Flashcards
What is a biological species?
A group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring - but do not do this with members of other such groups
What is reproductive isolation?
The existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring
Geographic, behavioural, temporal
What type of selection is speciation associated with?
Disruptive
- Drives two or more peaks in fitness
- Might consider that new species have been formed
- Causes divergence in phenotypes
- Disruptive selection can be countered by gene flow
What is gene flow?
Exchange of genes between populations as a result of movement and interbreeding of individuals
What combinations of selection and gene flow hold a species together or cause speciation?
Directional or stabilising + high gene flow = holds species together
Disruptive + low gene flow = speciation
What two changes occur during speciation?
1) Divergence - species adapt to different environments or selection pressures
2) Reproductive isolation - populations cannot interbreed
What are the main theories of speciation?
Allopatric - geographic and reproductive isolation first, then divergence
Sympatric - divergence first, then reproductive isolation
What is allopatric speciation?
1) Starts with a single population that are interbreeding
2) Populations becomes geographically isolated due to barriers
3) Speciation due to long term geographical isolation
4) Can result in phenotypic divergence due to:
- Different climatic or ecological conditions - different selection pressures favouring different phenotypes
- Random genetic drift
What happens if a geographical barrier is removed?
If populations have diverged enough they continue to be reproductively isolated - don’t interbreed
If the barrier is the only thing stopping interbreeding, partially divergent populations collapse back into a single species
What is sympatric speciation?
Splitting of a species into two or more reproductively isolated groups without geographical isolation
- Occurs when the groups are in contact with each other and are potentially able to interbreed and exchange genes
How does sympatric speciation occur?
Polygenic trait - determined by multiple genes
Selection in two different environments determines fitness - AxA fit offspring, AxB unfit, BxB fit
Heterozygote disadvantaged - removed by natural selection
Higher fitness if mate with individuals with similar phenotypes
Evolution f post-zygotic isolation - reduced survival or viability of hybrids
What are prezygotic barriers?
Block fertilisation from occurring
- By impeding members of different species from attempting to mate
- By preventing an attempted mating from being completed successfully
- By hindering fertilisation if mating completed successfully
What evidence is required to show sympatric speciation has occurred?
1) Species must be largely sympatric
2) Species must be reproductively isolated
3) Taxa must be sister species
4) Biogeographic and evolutionary history of the species must make it extremely unlikely that they were ever allopatric
What is an example of sympatric speciation?
North American apple maggot fly
- Mate on or near host plant - Hawthorn
- Apple trees introduced
- Pre-zygotic barrier between apple feeders and hawthorn feeders
- Apples mature faster so timing of feeding and mating differs - second pre-zygotic barrier
- Reduced gene flow
- Considered as subspecies - may be on way to full speciation
Sexual selection in fish
- Closely related species - females are similar but males differ in colour
Does sexual selection drive reproductive isolation? How was this tested?
Do females choose males according to a biased preference for certain colours or patterns?
- Colour perception depends on light environment
- Under monochromatic orange light males appear almost identical
Hypothesis - females only attracted to males of same species under normal light conditions
Researchers manipulated light environments and compared female mate choice
- Females strongly preferred males of same species in normal light but no preference in monochromatic light
- Strongly suggests female colour preferences is a barrier to reproduction in closely related sympatric species
