4. Evolution

• _{Describe how evolution (a change in the allele}

frequency) can come about through gene mutation, natural selection and the founder effect.

• _{Explain how reproductive isolation can lead to}

Definition of a Species

• _{A group of organisms which can interbreed to}

produce fertile offspring. They are

reproductively isolated from other species.

• _{They have similar morphological, physiological}

and behavioural features

• _{Similarity between DNA sequences}

• _{Donkeys, which look and behave like horses,}

Evolution

• _{Evolution is a change in allele frequency}

• _{This can change what a species looks like over}

time or even result in a new species forming.

• _{Evolution occurs due to various factors acting}

Terminology

• _{Gene pool – all the alleles in the population} • _{Genetic diversity – variety of alleles in a gene}

pool

• _{Allele frequency – the proportion of a}

• _{Gene pool}

• _{Genetic diversity}

Mutations

• _{Mutations are changes in the DNA base sequence. They can}

be deleterious (harmful), neutral or beneficial.

• _{Mutations in genes cause change in the codon which may}

result in a change in amino acids (primary structure) which can result in a change in the tertiary structure of the protein and this can produce different phenotypes.

• _{If the mutations causes the same amino acid to appear, the}

effect will be neutral.

• _{The mutation may change the amino acid that is beneficial.}

Natural selection

– survival of the fittest

• _{There is genetic variation in the population}

created

• _{Only those that are best adapted survive and}

breed

• _{Those organisms pass on their alleles to the}

next generation

• _{Over many generations the allele frequency}

Yellow aphids example

• _{There was genetic variation in the population –a yellow}

allele appeared due to mutation

• _{The yellow aphids were able to survive and reproduce}

and pass on their yellow alleles to the next generation.

• _{The yellow aphids were better adapted where they were}

camouflaged against yellow fruit or leaves and therefore were better able to survive in these conditions.

• _{Over several generations this increase the frequency of}

Example – Peppered Moth

• _{The peppered moth exists in two forms – a}

mottled (black and white) form and a melanic (black) form

• _{When the air is free of pollution the barks of} trees are mottled and so the mottled form of the peppered moth is well camouflaged.

• _{If the environment changes and the air becomes}

polluted the bark of trees becomes dark in colour. This means the melanic form of the peppered moth is better adapted because it is better camouflaged

• _{Over generations the melanic allele becomes more}

frequent in the populations as the melanic moths are able to survive and breed.

• _{The increase in the melanic alleles in the population}

Founder effect

• _{If a very small number of individuals start a}

new population, the population may have a limited variety of alleles.

• _{Some alleles might be much more frequent}

Example of the Founder Effect

• _{An example of the founder effect can be found in} the Amish population. The Amish are a group of people that follow a strict traditionalist Christian church fellowships.

• _{The Amish population was started by a small}

group of individuals. Certain alleles are therefore much more frequent in their populations

Speciation:

Forming a new animal species

• _{The two groups of the same species are separated so they}

cannot interbreed – they become reproductively isolated. Gene flow between the populations is prevented.

• _{If a small number of individuals start the new population, the}

allele frequency can be influenced by the founder effect which is the limited variety of alleles that start the new population.

• _{There are different selection pressures on the two}

populations. This causes a change in the frequency of the alleles in the two populations. This leads to different

Speciation always involves reproductive

isolation

Forming a new animal species may involve either:

1. Allopatric speciation – populations are separated by a physical barrier

2. Sympatric speciation – reproductive isolation occurs through seasonal, behavioural of

Allopatric speciation

Allopatric Speciation

• _{A population becomes physically separated by a barrier that}

prevents them from interbreeding. For example; a stretch of water (as has happened in the Galapagos Islands) or a road being cut into a forest. This means that that gene flow between the populations is prevented.

• _{If there is only a small number of individuals in one population, then}

the alleles present may be limited due to the founder effect.

• _{Different mutations may arise in the two different populations}

• _{In the two areas there could be very different selection pressures,}

resulting in different alleles being advantageous and thus increasing in frequency.

• _{Over time, the morphological, physiological and behavioural}

Sympatric Speciation

• _{This is speciation that occurs in the same physical}

space, but some other factor has caused

Sympatric Speciation

• _{Seasonal: A population becomes separated because two groups} breed at different times. For plants this could mean different flowering times.

• _{Behavioural: A population becomes separated because two} groups behave differently. For example; one group of birds may sing one song, another group sings a different song and neither group recognises the other. For frogs it could be different croaks.

How can plants become reproductively

isolated?

• _{One population of flowering plants – some}

individuals could evolve differing flowering times.

• _{Reproductive structures changes such they are}

How can animals become reproductively

isolated?

• _{An animal species can become reproductively}

isolated due to different breeding times, different breeding behaviors – songs/

• _{Describe the role of the scientific community in validating}

new evidence (including molecular biology, eg DNA,