evolution may lead to speciation

Giv genetic factors that cause phenotypic variation within a species

• mutation of alleles

• Random fertilisation of gametes

• Random assortment of chromosone during meiosis

What else may cause a phenotype to vary other than Ethan genetic factors

Environmental influences

• communicable disease caused by a pathogen

• Change in temperature - if its too cold lakes will freeze, so less run off, less tributriers so less growth of plants and so less food for rabbits and increased competition

• Increased predators

Why does natural selection occur

• predation

• Disease

• Competition

• All leading to different survival and reproduction

How does natural selection cause a change in gene pool

Organisms with advantageous characteristics are more likely to survive and pass o their favourable allele to their offspring. The frequency of unfavourable alleles decreases

What is stabalizing selection

Occurs when environmental conditions stay the same. `individuals who are closest to the mean are favoured and any new characteristics are selected against. This results in a low diversity.

What is directional selection

Occurs when ev=nvirnometla conditions change. Individual with phenotypes suited to the new conditions will survive and pass on their alleles. Over time the mean population will move towards these characteristics

What is disruptive selection

Both extremes of normal distribution are favoured over the mean. Over time the populations become phenotypically divided

Define speciation

Where a population is split and geographically isolated, there is differnt selection pressures on the tow groups, if the genetic make up changes to the extent the two groups can no longer interbreed then speciation has occured

Allopatric speciation

• Speciation resulting form a physical barrier making The species geographically isolated,

• Genetic variation exists in the two isolated groups and these ares exposed to differnt selection pressures

• Natural selection occusr in a population ( usually leading to directional selection)

• Allele frequency changes

• Over time the stow populations are sos different that they can no longer reproduce to produce fertile offspring

Sympatric speciation

When a population becomes reproductively isolated

E.g a amputation that causes a organisms to no longer produce fertile offspring

E.g behavioural differences; mating preferences, courtship, feeding preferences

Genetic drift

A change in a populations allele frequencies that occurs due to chance rather than selective pressures,

(Caused by a sampling error)

Why does genetic drift affect small populations more than large ones

They gene pool is smaller so less allee available and any change in frequency become pronounced very quickly

Why is a mutation more likely in a smaller population

• Genetic drift

• Frequency is higer by chance

• Only interbreeding within a population

• Low genetic diversity/ mall gene pool

Describe the process of secondary succession

• colonisation by pioneer species

• Pioneer species change the environemtla factors

• So environment is less hostile for new species

• Biodiversity increases

• Ends in climax community