3.7.3 evolution may lead to speciation

What may cause changes in phenotype

Genetic and environmental factors

What may cause genetic variation

1. Mutations

2. Crossing over of non-sister chromatids during prophase I

3. Independent segregation during metaphase 1

4. Random fertilisation

Independent segregation and crossing over leads to genetic variation of …

Gametes

Random fertilisation leads to genetic variation of…

Zygotes

Mutations lead to genetic variation of…

Individuals within a species

Explain natural selection

Genetic variation means that some organisms will have an advantageous allele which makes them better adapted to survive.

Therefore they pass these favourable alleles onto their offspring

Allelic frequency of favourable alleles increases

Define evolution

The formation of new species from pre-existing species over time, as a result of changes to gene pools and allele frequencies from generation to generation

What are the three types of selection

Stabilising, directional, disruptive

Directional selection

favours individuals with a phenotype at one extreme, causing a shift in a population’s traits over time

Define disruptive selection

Draw the graph

natural selection that maintains high frequencies of two different sets of alleles

Speciation

The evolution of a new species from an existing one

Reproductive isolation

when changes in alleles or phenotypes prevent certain individuals from breeding successfully with others in the same population

What is allopatric speciation?

• occurs when a population is separated by a geographical barrier

• Barriers can be natural or man made

• Separated groups are reproductively separated, so no gene flow

• Different selection pressures act on each group changing allele frequencies via natural selection

What is sympatric speciation

When population living in the same area are separated by ecological or behavioural factors

What is genetic drift

When chance affects which individuals in a population survive

Usually affects very small populations

Large populations mainly affected by natural selection.

Why does genetic drift mainly affect small populations

Random chance has a much larger impact on smaller populations

What is bottleneck?

Extreme example of genetic drift when size of population is suddenly reduced

Results in quick and drastic loss of genetic variation within a population

Remaining members of population become ancestors of all following generations