Hardy-Weinberg p 2 + 2pq +q 2 = 1

1. Hardy-Weinberg p2 + 2pq +q2 = 1 1. A recessive condition occurs in one in every 20000 of a population. What percentage of the population are carriers of the condition? 2. In doves the allele for grey plumage is dominant to the allele for white plumage. In a population of 900 doves 891 are grey and 9 are white. Work out the frequencies of each allele. How many of the grey birds will be heterozygous?

2. The role of genes and environment in evolution

3. Principles of natural selection • There are more organisms born then can survive. • These organisms compete for limited resources. • There is variation amongst the organisms. Some of them are better adapted to their environment than others . • The better adapted organisms out compete the less well adapted ones and so survive and breed, passing on the genes for their adaptations. • over many generations the better advantageous characteristics become more common in the populations, whilst disadvantageous ones become less common.

4. Environmental Factors Population Size Time • Environmental resistance limits the growth of a population • Not all individuals that are born will survive. Selection pressures determine which individuals will survive or not. • The population stays around its carrying capacity, but fluctuates up and down slightly (why?)

5. Selection pressure Stabilising selection

6. Selection pressure Directional selection Climate changes and the ground is covered in snow.

7. Selection pressure Directional selection Climate changes and the ground is covered in snow.

8. Selection pressure Directional selection Climate changes and the ground is covered in snow.

9. Genetic drift • Changes in allele frequencies in a population over time (due to chance) • More likely to cause change in small populations • Eg. Population of 2 heterozygous mice • Aa and Aa • If they only produce two offspring then it is perfectly possible for one of the alleles to be wiped out in the next generation • This is an extreme example but it shows that allele frequencies can change randomly • Genetic often drift leads to less variation in a population, which makes it less likely to survive if the environment changes (why?).

10. The founder effect • The founder effect is a special case of genetic drift, occurring when a small group in a population splinters off from the original population and forms a new one • By chance an individual with an allele, rare in the first population may amongst the individuals that form the new group • As the new group grows the rare alelle will be much more common in the new population than it was in the old one.

11. Example - The Amish migration to Pennsylvania in 1744 • In the early 18th century, many Amish Mennonites emigrated from Switzerland to Pennsylvania for a variety of reasons. • By chance two members of the new colony carried the rare recessive allele for Ellis-van Creveld syndrome. • Members of the colony and their descendants tend to be religious isolates and do not intermarry with people outside of the group • As a result of Ellis-van Creveld syndrome is now much more prevalent among the Amish than in the general population

12. A population bottleneck may also cause a founder effect • Wisent, also called European bison, faced extinction in the early 20th century. The animals living today are all descended from 12 individuals and they have extremely low genetic variation. • Northern Elephant Seals - population fell to about 30 in the 1890s and although it now numbers in the hundreds of thousands there is little variation • Another example are Cheetahs, which are so closely related to each other that skin grafts from one cheetah to another do not provoke immune responses, thus suggesting an extreme population bottleneck in the past.