F215 Variation and Population Genetics By Ms Cullen

Examples of Variation Sex Blood group Shoe size Tongue rolling Height Plant mass Finger length

Continuous Variation

Discontinuous Variation

Polygenic Inheritance Not all variation caused by genes is discontinuous. Sometimes variation can be caused by many genes at different loci, or many different alleles of the same gene. This means there are many different possibilities, so the variation is continuous. When characteristics are influenced by the combined effect of many genes it is known as polygenic inheritance. Polygenic characteristics tend to show continuous variation. Inherited characteristics that show continuous variation are usually influenced by many genes. For example, human skin colour comes in lots of different shades of colour. Inherited characteristics that show discontinuous variation tend to be influenced by only one gene. For example, violet flower colour (either coloured or white) is controlled by only one gene. We call this monogenic inheritance.

Environment Variation is also affected by the environment. Examples can be climate, food, lifestyle etc Environmental factors can change over an organisms lifetime and therefore so can their characteristics. Can you think of any examples of environmental variation?

Both? Some variation is caused as a result of both genes and environment. Can you think of any examples?

VP = VG + VE Both genotype and environment can contribute to phenotypic variation.

Gene pools and populations A population is the number of individuals of the same species within a particular area. A species is a group of similar individuals which can breed with each other and produce fertile offspring. The gene pool for that population will be the complete range of alleles available within that population. New alleles can be created as a result of mutations. The allele frequency is how often an allele occurs within a population (usually expressed as a percentage). The frequency of an allele will change over time this is evolution.

The Hardy-Weinberg Principle This predicts the frequency of alleles in a population won’t change from generation to generation. But this will only occur in certain conditions; a large population, with no immigration, emigration, mutations or natural selection. If allele frequencies do change then it will be as a result of one or more of the above.

The Hardy-Weinberg Principle – allele frequency p = the frequency of the dominant allele. q = the frequency of the recessive allele. The total frequency for all possible alleles within a population is 1.0. So the frequency of dominant and recessive alleles must add up to 1.0. Q: In a population of plants the allele R (red) is dominant and has a frequency of 0.4. Allele r (white) is recessive. What is its frequency? p + q = 1

The Hardy-Weinberg Principle – genotype frequency p 2 = the frequency of the homozygous dominant genotype. 2pq = the frequency of the heterozygous dominant genotype. q 2 = the frequency of the homozygous recessive genotype. The total frequency for all possible genotypes within a population is 1.0. So the frequency of all the genotypes must add up to 1.0. p 2 + 2pq + q 2 = 1

The Hardy-Weinberg Principle – genotype frequency Example: If there are two alleles for flower colour (R and r) then there are three possible genotypes. RR, Rr and rr. Q: If the frequency of RR (p 2 ) is 0.34 and the frequency of Rr is What is the frequency of rr (q 2 )?

Remember Evolution occurs via Natural selection: Variation with individuals. Selection Pressures eg predation, competition, disease. Best adapted individuals survive, breed and pass on beneficial alleles. This results in a greater proportion of the next generation having the beneficial allele. They in turn survive, breed and pass on the beneficial allele. The frequency of the beneficial allele within the gene pool will increase. This process is called natural selection.

Stabilising Selection Most of the time organisms are well adapted to their surroundings. The alleles present in the species gene pool are advantageous for survival. If the environment remains stable the same alleles will be selected in successive generations. As a result, nothing changes.

Directional Selection This occurs when there is a change in the environment. This can cause a change in the selection pressures on a population. A variation that may not have been previously advantageous, may become so. Or a new variation may arise due to a mutation. These will result in directional (or evolutionary) selection.

Genetic Drift Evolution is not always a direct result of natural selection, sometimes it happens purely by chance! It usually happens in smaller populations, with few selection pressures. Particularly in island populations. If one or two individuals have a better success at breeding then their alleles will become more popular within the population. While other alleles may be lost if some individuals do not have offspring. This causes a change in the gene pool and the population’s characteristics. This has occurred by chance rather than natural selection and is known as Genetic Drift. Genetic drift is also common when a population becomes suddenly smaller, for example after a natural disaster.

Example of Genetic Drift Native American tribes show different blood group frequencies. Blackfoot Indians are mostly blood group A. Navajos are mainly blood group O. Blood group does not affect survival or reproduction, therefore it is not evolution by natural selection. In the past human populations were much smaller and often isolated The blood group differences are due to genetic drift. By chance the allele for blood group A was passed on more frequently by Blackfoot Indians. Over time the allele and blood group A became more common.

Speciation This is the formation of a new species. It usually occurs as a result of either: individuals becoming physiologically or morphologically different to members of the original species. individuals no longer being able to breed and produce fertile offspring with members of the original species. When a new group splits from the original species it is known as isolation.

Isolation Geographical isolation – species can be separated by a physical barrier, a river, mountain etc. When the two groups become so different that they can no longer breed and produce fertile offspring it is called reproductive isolation. They have now become different species and there is no gene flow between them.

Geographical Isolation White-tailed antelope squirrel Harris’ antelope squirrel

Reproductive Isolation can be caused by one or more of these: Temporal (seasonal) isolation – different breeding seasons. Gamete isolation – sex cells from different species are incompatible. Behavioural isolation – different courtship rituals are not attractive to other species. Mechanical isolation – structural differences in the anatomy of reproductive organs prevents sperm transfer between individuals of different species.

The Species Concept The classic definition of a species can cause problems if you can’t see an organism reproduce! For example a species may reproduce asexually, or be extinct, or just might not be practical to watch them reproduce in the wild. Scientists sometimes use the phylogenetic species concept to help classify organisms. Phylogenetics is the study of evolutionary history of groups of organisms (it is also known as the cladistic or evolutionary species concept) All species have evolved from common ancestors.

A cladogram showing the evolutionary relationship between seven insect groups

Artificial and Natural Selection Task: Explain how artificial selection is used to produce varieties of bread wheat (triticum aestivum). Explain how artificial selection has been used to produce the modern dairy cattle. Compare the similarities and differences between natural selection and artificial selection.