Populations Flashcards
Population
- group of organisms of the same species occupying a particular area at a particular time, that can potentially interbreed
population genetics
- The study of genetics at the level of a population
gene pool
- all the alleles in a population
allele frequency
- number of times an allele appears in a gene pool
What does the Hardy- Weinberg principle let us calculate?
- allele frequencies in these populations, and predicts that allele frequencies for a particular gene will not change from generation to generation
What assumptions are made when using the Hardy-Weinberg principle?
- there is a large population
- random mating occurs
- no selective advantage for any organism
- no mutations occur
- no immigration/ emigration (populations are genetically isolated)
- no genetic drift (random changes in allele frequencies that occur over time)
Key things to consider when it comes to the assumptions:
- Mutations can lead to new alleles and alter the relative frequency of others for the same gene
- Natural selection can increase or decrease allele frequencies
- Small populations are at greater risk of genetic drift, which involves random changes in allele frequency
- Individuals migrating in or out of a population could alter allele frequencies
- Individuals deliberately choosing mates that have (or don’t have) certain characteristics will alter allele frequencies over the generations.
Formula 1:
- p + q = 1 (same as 100% probability)
Formula 2:
- p^2+ 2pq + q^2= 1
What is p?
- frequency of the dominant allele
What is q?
- frequency of the recessive allele
What is p^2?
- frequency of homozygous dominant
What is q^2?
- frequency of homozygous recessive
What is 2pq?
- frequency of the heterozygous
What units are probabilities given as?
- %
What range will the frequency of the allele be in?
- 0-1
If 98 out of 200 individuals in a population of mice express the recessive phenotype, what percent of the population would you predict would be heterozygotes?
- 42% heterozygotes
Brown fur is dominant to grey fur colouring in mice. There are 168 brown mice in a population of 200 mice. What is the predicted frequency of homozygous dominants, heterozygotes and homozygous recessives?
- 36% homozygous dominants
- 48% heterozygous
-16% homozygous recessives
What are the three types of selection?
- Stabilising selection
- Directional selection
- Disruptive selection
Stabilising selection
- preserves the average phenotype (phenotypes around the mean) of a population by favouring average individuals. In other words, selecting against extreme phenotypes. This tends to occur when environmental conditions are constant over long periods of time
Directional selection
- changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population. In other words, selecting for one extreme phenotype. This tends to occur where there are fluctuating environmental conditions
Disruptive Selection
- Favours individuals with extreme phenotypes, rather than those with phenotypes around the mean of the population. It tends to occurs when an environmental factor takes two or
more distinct forms. It is the opposite to stabilising selection, but the most important in bringing about evolutionary change
What effect does shorter tail in squirrels have?
- avoids predators
What effect does longer tails in squirrels have?
- balance in trees
What are some examples of disruptive selection?
- squirrels short and long tails
- organisms with long fur adapt better and are more active in winter, in hot summers those with short hair adapt better and are more active
- coho salmon, large males are competitors whilst small salmons sneak up on females in spawning grounds
- Small mammals can escape from predators by hiding in small spaces, whilst large ones can resist the attacks
What are some examples of stabilising selection?
- A baby with a birth weight greater than 4kg or less than 2.5kg has an increased risk of dying
- Some species of insects have changed very little over millions of years
What are some examples of directional selection?
- Elephants have developed longer trunks, enabling them to reach leaves higher up in trees
- The mean ear length in arctic foxes has reduced over time
