18. POPULATIONS & EVOLUTION Flashcards
Define population
All the individuals of one species in one area at a time
Define gene pool
All the alleles of the genes within a population at one time
Define allele frequency
The proportion of an allele within the gene pool
What does the Hardy Weinberg principle predict?
That the frequency of alleles in a population will not change over time
Describe the five assumptions the Hardy Weinberg principle makes
There is no migration to introduce or remove alleles from the population
There are no mutations to create new alleles
There is no selection favouring particular alleles
Mating is random
The population is large
Recall both Hardy Weinberg equations
p + q = 1
p2 + 2pq + q2 = 1
What is p in the Hardy Weinberg equation?
Number of dominant alleles in the population
What does p2 stand for in the Hardy Weinberg equation?
Number of homozygous dominant genotypes in the population
What is q in the Hardy Weinberg equation?
Number of recessive alleles in the population
What does q2 stand for in the Hardy Weinberg equation?
Number of homozygous recessive genotypes in the population
What does 2pq stand for in the Hardy Weinberg equation?
Number of heterozygous genotypes in the population
How do most new alleles arise?
Mutations
What two factors cause phenotypic variation in a population?
Mutations (creating genetic variation), and environmental factors
Define what a selective pressure is
Any reason for individuals to have a survival advantage
Give examples of selective pressures
Competition for food, space, water, mates and territory. Diseases.
Natural selection will only occur if which two pre-requisites occur together
Phenotypic variation and a selection pressure
In natural selection, which individuals are more likely to reproduce more?
The individuals with the selective advantage
In natural selection, which alleles are more likely to be passed onto offspring?
The alleles that code for the phenotype with the selective advantage
What happens to the frequency of alleles in subsequent generations because of natural selection?
The alleles coding for the phenotype with the selective advantage will increase in frequency
Name the three types of natural selection
Directional, stabilising and disruptive
In directional selection, what happens to the frequency of alleles in subsequent generations?
Directional selection will increase the frequency of alleles in one direction
When will directional selection occur?
When there is a change in the environment
Why does directional selection occur?
Because one phenotypic extreme is better adapted / has a selective advantage
What happens to the modal phenotype in directional selection?
It changes in one direction
Give one example of directional selection
Antibiotic resistance
When will stabilising selection occur?
When the environment doesn’t change
Why does stabilising selection occur?
Because the modal phenotype has the selective advantage
What happens to the standard deviation around the modal phenotyoe in stabilising selection?
It decreases over time
Give one example of stabilising selection
Birth weights
When will disruptive selection occur?
(Often) when an environmental factors takes two extreme forms
Why does disruptive selection occur?
It occurs because extreme phenotypes have the selective advantage
What happens to the modal phenotype in disruptive selection?
Initially the modal phenotype will decrease over time, and two new modes will develop at the extremes
Give one example of disruptive selection
Size of male salmon
Define speciation
Whe two populations of the same species become genetically different enough that they can no longer interbreed to produce fertile offspring
Define reproductive isolation
The inability of two populations to breed due to geographical, behavioural, physiological or genetic barriers.
Which type of speciation occurs when reproductive isolation occurs because of geographical isolation?
Allopatric speciation
Describe how allopatric speciation occurs due to geographical isolation
Reproductive isolation means both populations accumulate different beneficial genetic changes over time, which leads to changes in allele frequencies in each population.
(LOOK AT BOOK FOR EXAM ANSWER)
At which point does allopatric speciation occur?
When indviduals in the population can no longer interbreed to produce fertile offspring
Which type of speciation occurs when reproductive isolation occurs because of behavioural differences?
Sympatric speciation
Give examples of how behavioural differences can cause reproductive isolation
Different courtship rituals
Being fertile at different times of the year
Different feeding times or foods
Which type of selection can lead to sympatric speciation?
Disruptive selection
Describe how sympatric speciation occurs due to behavioural differences
This occurs in the same habitat. Reproductive isolation means both populations accumulate different beneficial genetic changes over time, which leads to changes in allele frequencies in each population.
(LOOK AT BOOK FOR EXAM ANSWER)
At which point does sympatric speciation occur?
When indviduals in the population can no longer interbreed to produce fertile offspring
