10.3 gene pools & speciation

gene pools

all the genes & their alleles present in an interbreeding popn (*1 species is made up of many popns)

microevolution

the change in freq of alleles in a gene pool over time — can be due to natural selection

types of natural selection

stabilising selection: favours intermediate phenotypes at the expense of extreme ones → phenotype becomes centrally clustered. operates in stable environmental conditions.

directional selection: favours 1 extreme at the expense of the other extreme. operates in response to gradual environmental changes, typically followed by stabilising selection once an optimal phenotype has been normalised.

disruptive selection: favours both extremes at the expense of intermediates → bimodal spread. operates in fluctuating environmental conditions (eg. seasons) that favour different phenotypes. cont separation may lead to speciation.

calculate allele freq in a gene pool

see the table in notes

criteria for a species

1. able to interbreed

2. offspring must be viable

3. cannot interbreed w other species — reprod isolation

types of reproductive isolation

geographical: physically separated → cannot interbreed → the 2 popns adapt to diff environments → over time natural selection leads to change in allele freq → adaptive radiation.

behavioural: eg. diff mating/courtship behaviour

temporal: diff mating times → prevent organisms from interbreeding

how does reprod isolation lead to speciation?

the 2 groups don’t interbreed → undergo diff selection pressures → allele freq become diff over time (microevolution) → can no longer interbreed → speciation has occurred.

types of speciation

allopatric: due to geographical isolation

sympatric: due to behavioural, temporal isolation or polyploidy

• due to chromosomal mutations involving non-disjunction during meiosis/mitosis:

• eg. non-disjunction: leads to 2n gametes → 4n offspring that cannot interbreed with the original popn (typically n gametes) → fusion leads to non-viable or sterile offspring.

• 4n polyploids may hv selective adv over diploids in some environments → leads to speciation

graduated vs punctuated equilibrium

gradualism: a smooth & continuous process. big changes result from cumulative small changes.

punctuated: species remain stable for long periods before abrupt & rapid change.