How species evolve

VCE BIO U4 AOS2

Define gene pool

The sum of the alleles of all the members of a population.

Define allele frequency

The proportion of alleles in a particular gene pool

Issues with small gene pools and low variability

No ability to cope with change as advantageous allele less likely to be present, increased chance of inbreeding

Steps for natural selection & give examples

Variation: identify the variation that exists within the existing population.

Selection pressure: identify the environmental selection pressures within the particular environment that are acting on the population.

Selective advantage: explain the effects of the environmental selection pressure in terms of survivability and how it confers a selective advantage for the organisms with a fitter or more advantageous phenotype.

Heritability: state that organisms with the fitter or more advantageous phenotype will reproduce and pass on the advantageous alleles to the next

Compare germline and somatic mutations

Somatic: only in that affected body cell and the mitotic cells it produces

Germ line: have the potential to be inherited and passed on to the next generation.

Define aneuploidy

When an organism possesses an incorrect number of total chromosomes caused by the addition or loss of an individual chromosome.

Define polyploidy

When an organism has an incorrect number of sets of each chromosome

Define genetic drift

A random event that dramatically alters a population’s gene pool due to a chance event, usually a loss of alleles. 

• Does not favor a particular allele.

• In small or isolated populations– genetic drift can lead to decrease or loss of favorable alleles (and loss of genetic variation)

• Genes of the next generation will be the genes of the survivors, not necessarily the fittest or best suited individuals.

Define founder effect

When a population start with a small number of individuals.  This is a reduced gene pool, that is not a true representation of allele frequencies in original population.

Define bottleneck event

Rapid reduction in population size, due to environmental changes, either natural or human-made.

Define gene flow

The introduction or removal of alleles between populations through either migration or interbreeding.

Define a species

Organisms of the same species must be able to interbreed and produce viable, fertile offspring.

Outline steps for allopatric speciation

1. Geographical barrier: state that a geographical barrier has isolated a population (or populations) of the same species from each other, thereby preventing gene flow.

2. Environment: state that the isolated populations are subjected to different selection pressures (food sources), with some individuals at a selective advantage, enabling them to survive and pass on favourable alleles to offspring. Leading to the accumulation of genetic differences (via mutations).

3. Speciation: once sufficient genetic differences accumulate the two populations are a new species that can no longer interbreed and produce viable offspring

Prezygotic & postzygotic barriers of reproductive isolation

Prezygotic:

• Temporal (e.g. different breeding seasons/ feeding times)

• Behavioural (e.g. different courtship displays / different niches /habitats / feeding areas)

• Mechanical (e.g. mismatch in reproductive parts)

Postzygotic

• Hybrid inviability/ infertility