Speciation and Genetic drift

Variation

Differences that exist between individuals which can be caused by environmental and genetic factors

Mutation

Changes in DNA sequence leads to the production of new alleles

Main causes of variation?

Mutation, crossing over of chromatids, independent segregation and random ferilisation

What are selection pressures to for natural selection?

Create a struggle for survival

Why are individuals better adapted to selection pressures?

Variation in alelles

Why does the gene pool of the more advantageous allele increase in natural selection?

They are more likely to survive, reproduce and pass on this gene so a greater proportion of the next generation has this allele.

Stabilising selection

Individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.

When does stabilising selection occur?

Environment isn’t changing

Directional selection

Individuals with alleles for a single extreme phenotype are more likely to survive and reproduce.

When does directional selection occur?

Response to an environmental change

Disruptive selection

Individuals with alleles for extreme phenotypes are more likely to survive and reproduce. Characteristics towards the middle of the range are lost

When does disruptive selection occur?

Environment favours more than one phenotype

Speciation

The development of a new species from an existing species

Why does speciation occur?

Populations of the same species become reproductively isolated

Allopatric speciation

Populations that are geographically separated will experience different conditions

Why does natural selection occur in allopatric speciation?

Different alleles will be more advantageous

Why will allele frequency change in allopatric speciation?

mutations and genetic drift will occur independently in each population

What does change in allele frequencies lead to?

Differences in gene pools, causing changes in phenotypes

Sympatric speciation

Random mutations within a population prevent individuals that carry the mutation from breeding with others that don’t carry the mutations

Why does reproductive isolation occur?

Changes in alleles, genotypes and phenotypes prevent individuals with these changes from breeding with individuals without them

Seasonal changes

Individuals develop different mating seasonal changes which means they can’t breed

Mechanical changes

Changes in size or shape of genetalia can prevent mating

Behavioural changes

A group of individuals may develop courtship rituals that aren’t attractive to the rest of the species

Genetic drift

Chance dictates which alleles are passed on