Natural Selection and Evolution (Chapter 17) Flashcards Preview

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What is genetic drift?

A change in allele frequency that occurs by chance, because only some of the organisms of each generation reproduce


What is the founder effect?

1) a small number of individuals are separated from the rest of a large population - they form only a small sample of the original populations and ∴ this founder population is unlikely to have the same allele frequencies as the large population
2) further genetic drift in the small population will alter the allele frequencies still more and evolution of this population may take a different direction from that of the larger parent population
3) the isolated populations will likely diverge genetically also due to different selection pressures in the new area and random mutations being different in separate populations


What do selection pressures do?

Increase the chances of some allele being passed onto the next generation and decrease the chances of others e.g. predation of foxes


What is natural selection?

The effects of selection pressures on the frequency of alleles in a population


What does natural selection do?

Raises the frequency of alleles conferring an advantage and reduces the frequency of alleles conferring a disadvantage


What is the general theory of evolution?

Organisms have changed over time


What is stabilising selection?

- When selection pressures act against extremes, keeping the variation in a characteristic centred around the mean
- Keeping it the way it is


What is directional selection?

- If a new environmental factor or new allele appears, then allele frequencies may also change
- e.g. if selection acts against smaller individuals, but not larger ones, the range of variation shifts towards larger size, resulting in a change of characteristic in a particular direction


What is disruptive selection?

- This maintains different phenotypes (polymorphism) in a population
- Occurs when conditions favour both extremes of a population
- e.g. acts against those whose size is in the middle of the range


What happens in the population when a particular phenotypic trait is controlled by two alleles of a single genes, A/a?

The population will be made up of three genotypes: AA, Aa and aa


What do calculations based on the Hardy-Weinberg principle allow?

The proportions of each of these genotypes in a large, randomly mating population to be calculated


State the Hardy-Weinberg principle

The frequency of a genotype is its proportion of the total population


What are the two equations for the Hardy-Weinberg principle?

- p + q = 1
- p2 + 2pq + q2 = 1


What does p stand for?

The frequency of A (in decimals)


What does q stand for?

The frequency of a (in decimals)


Describe how the second equation for the Hardy-Weinberg principle is obtained

1) the chance of offspring inheriting a dominant/recessive allele from both parents = p x p = p2 / q x q = q2
2) the chance of offspring inheriting a dominant allele from the mother/father and a recessive allele for the father/mother = p x q = pq ∴ chance = 2pq


What are the 4 situations in which the Hardy-Weinberg population does not apply?

1) when the population is too small
2) when there is significant pressure against one of the genotypes
3) when there is migration of individuals carrying one of the two alleles into/out of the population
4) when there is non-random mating


What can happen when the ratios of the different genotypes in a population have been determined using the Hardy-Weinberg principle?

1) their predicted ratios in the next generation can be compared with the observed values
2) any differences can be tested for significance using the chi squared test
3) if the differences are significant and migration/non-random mating can be discounted, then there is evidence that directional selection is occurring in the population


What do all populations have?

The reproductive potential to increase their populations (exponentially), however under normal circumstances, this rarely happens


What happens as a population increases?

Various factors (biotic and abiotic) keep numbers down e.g. if there are increasing numbers: food will be in short supply, there may be an increase in the number of predators, or overcrowding and ∴ it is easier for disease to spread


What are biotic factors?

Factors caused by other living organisms e.g. through predation, competition for food or infection by pathogens


What are abiotic factors?

Factors caused by non-living components of the environment e.g. water supply or nutrient levels in soil


What do biotic and abiotic factors do?

They act to reduce the rate of growth of the population - only a small proportion of young will grow to adulthood and reproduce ∴ the population growth slows


What happens if the pressure of the environmental factors is sufficiently great?

- The population will decrease
- Only when numbers have considerably fallen will the population be able to grow again
- ∴ over a period of time, the population will oscillate about a mean level


What does variation within a population lead to?

- Some organisms within a species are born with a better chance of survival than others e.g. agouti colour is better than white colour at camouflaging
- ∴ some organisms will have features which give them an advantage in the 'struggle for existence'


What is fitness?

The capacity of an organism to survive and transmit its genotype to its offspring


Give an example of how a new environmental factor would affect natural selection

If it suddenly got colder and there was more snow, white rabbits are more camouflaged ∴ have a selective advantage ∴ they are more likely to survive and reproduce and pass on the allele ∴ the frequency of white coat allele increases in population


What are mutations?

Random events ∴ they can produce features that are harmful, neutral or useful


What is the basis of evolution?

Changes in allele frequency in a population


Why does evolution occur?

- Because natural selection gives some alleles a better chance of survival than others
- Over many generations, populations may gradually change, becoming better adapted to their environments

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