B18 Populations And Evolution Flashcards
Define species
A group of organisms that can interbreed to produce fertile offspring
Define population
All the organisms of a particular species that live in the same place
Define gene pool
The range of different alleles existing for a particular locus within a population
Define allele frequency
The proportion of a certain allele within a gene pool, expressed as a decimal or percentage
What’s the Hardy-Weinberg principle
Allows us to estimate the frequency of alleles in a population, as well as if allele frequency is changing over time
Give the assumptions made by the Hardy-Weinberg principle
- no mutations occur to create new alleles
- no migration in or out of the population
- no selection, so alleles are all equally passed onto the next gen
- random mating
- large population
Explain the Hardy-Weinberg equation for calculating allele frequency
The frequencies of each allele for a characteristic must add up to to 1.0
The equation is therefore; p + q = 1
p = freq of dominant allele
q = freq of recessive allele
Explain the Hardy-Weinberg equation for calculating genotype frequency
The frequencies of each genotype for a characteristics must add up to 1.0
Equation is therefore; p^2 + 2pq + q^2 = 1
p^2 = freq of homozygous dominant
q^2 = freq of heterozygous recessive
Give factors that cause phenotypic variation within a species
- Mutation of alleles
- Random fertilisation by gametes
- Random assortment of genetic material during meiosis
Other than genetic factors, why else may phenotype vary within a species
Environmental influences
Why does natural selection occur
- Predation
- Disease
- Competition
All resulting in differential survival and reproduction
How does natural selection cause a change in a population’s gene pool over generations
Organisms with advantageous characteristics are more likely to survive and pass their favourable alleles to offspring. Frequency of unfavourable alleles decreases
What is stabilising selection
Occurs when environmental conditions stay the same. Individuals closest to the mean are favoured, and any new characteristics are selected against
Results in low diversity
What is directional selection
Occurs when environmental conditions change. Individuals with phenotypes suited to the new conditions will survive and pass on their genes. Over time the mean of the population will move towards these characteristics
What is disruptive selection
The opposite of stabilising selection, in that both extremes of the normal distribution are favoured over the mean
Over time, the population becomes phenotypically divided
Define speciation
Where a population is split and isolated, there are different selective pressures on the 2 groups. If the genetic makeup changes to the extent the 2 groups can no longer interbreed, they have become separate species
What is meant by allopatric speciation
Speciation resulting from a physical barrier e.g. river, mountain range. The environments occupied by the 2 groups are different, and therefore different alleles are favoured
What is meant by sympatric speciation
Speciation resulting from non-physical barrier e.g. a mutation that no longer allows 2 organisms to produce fertile offspring. Any changes in anatomy or behaviour may also prevent breeding.
Define genetic drift
A change in population’s allele frequencies that occur due to chance rather than selective pressures.
In other words, it is caused by ‘sampling error’ during reproduction
Why does genetic drift affect small populations more than large ones
The gene pool is smaller, so there are less alleles available and any change in frequency becomes pronounced very quickly
