topic 4 - speciation, hybridisation and balancing selection

Speciation

Distinguish between prezygotic isolation and postzygotic isolation, and list the reproductive barriers can lead to speciation

Allopatric speciation

Ancestral populations is divided by a physical barrier

Through founder events, the colonisation of a new region by a small number of individuals, genetic drift and selection have led to the formation of new species in reproduction isolation

Sympatric speciation

Ancestral populations is divided without geographic barriers

Pre-zygotic barriers

Biological mechanisms that prevent different species from mating successfully fertilising to produce a zygote

Eg. Habitat isolation, behavioural isolation, mechanical isolation

Post-zygotic barriers

Reproductive isolation mechanisms that occur after a hybrid zygote has been formed, preventing it from developing into a healthy, fertile adult

Eg. Hybrid inviability, reduced hybrid fertility, sterility

Example Allopatric Speciation Prezygotic barriers

Separated by a river or something

Example Allopatric Speciation Postzygotic barriers

Reduced viability/fertility in hybrids

Example Sympatric Speciation Prezygotic barriers

Are awake during different times of the day

Example Sympatric Speciation Postzygotic

Infertile offspring

Hybridisation

Recognise hybridisation between different species can occasionally occur, resulting in gene flow and exchange of alleles

Balancing selection

Identify that balancing selection maintains genetic diversity in a population by keeping alleles at frequencies higher than expected by chance

Maintains genetic variation in a population

1. Negative frequency dependent selection

2. Heterozygous advantage

Negative frequency dependent selection

Occurs when rare phenotypes have higher fitness than common phenotypes

Very rare phenotypes do not become extinct as they have a strong selective advantage. This strategy has been likened to the game paper rock scissors, where one form will win out over another form

Fitness

Success of an organism at surviving and reproducing

Relative fitness

Describes the success of a genotype. Relative fitness standardised by the success of other genotypes in the population and ranges from 0 to 1.0