Selection and evolution
Genetic variation
Caused by:
● Independent assortment of chromosomes during meiosis during metaphase
● Crossing over between chromatids of homologous chromosomes during meiosis
● Random mating between organisms with a species
● Random fertilisation of gametes
● Mutation
Genetic variation produces phenotypic variation.
First four reshuffles existing alleles in the population but mutation can produce completely new ones. The new
allele is often recessive so it only shows up after generations. Mutations in somatic cells can’t be passed onto
offspring by sexual reproduction unless it is on the gametes.
Genetic variation provides the raw material on which natural selection can act as some individuals have
advantages over the others.
Environmental variation
❖ Phenotypic variation can also be caused by the environment in which organisms live, for example food
and weather.
Discontinuous variation
❖ Qualitative differences that fall into clearly distinguishable categories with no intermediates e.g blood
groups. Discontinuous variation is always caused by genes and usually just one or two genes involved.
For blood groups, they are controlled by a single gene, with three alleles.
❖ Different alleles on a single gene locus have large effects on the phenotype
❖ Different genes have quite different effects on the phenotype.
Continuous variation
❖ A variation between two extremes with no distinguishable categories such as height. Maybe caused by
genes, or by the environment, or both.
❖ Different alleles at a single gene locus have small effects on the phenotype
❖ Different genes have the same, often additive, effect on the phenotype
❖ A large number of genes may have a combined effect on a particular phenotypic trait ; these genes are
polygenes.
❖ Sickle cell anaemia and haemophilia are examples of discontinuous variation with single genes having a
large effect.
❖ Two typical effects of the inheritance of continuous variation : small effects of the different alleles of one
gene on a phenotype and the additive effect of different genes on the same phenotype.
❖ The variation of two populations can be compared using the t-test.
❖ T-tests are used to assess whether or not the means of two sets of data with roughly normal distributions
are significantly different from one another.
❖ Null hypothesis is when the two samples are from an identical population. If our t-test value represents a
probability of 0.05 or more, then we assume that the differences between the two sets of data are only
due to chance. The probabilities that we look at in the table are probabilities that the null hypothesis is
correct and there is no significant difference in the samples.
Natural selection
❖ Natural selection is a process in which selection pressures act on a population so that individuals
, possessing a particular allele, or a combination of alleles, are more likely to survive and pass on their
alleles than other individuals.
❖ Selection pressures increase the chances of some alleles being passed on to the next generation and
decrease the chance of others.
❖ The effects of such selection pressures on the frequency of alleles in a population.
❖ Biotic factors that control populations can be living organisms such as through predation, competition for
food or infection by pathogens.
❖ Abiotic facts can be caused by non-living components of the environment such as water supply or nutrient
levels in the soil.
❖ Fitness is the capacity of an organism to survive and pass on its genotype to its offspring.
❖ Polymorphism is the co-existence of individuals in a population with different phenotypes caused by
different genotypes.
❖ Some suggestions for the oscillating population size are food supply, food qualities and predators. As the
population size rises, food supplies run out, so the population size decreases. Once the population size
has decreased, food supplies begin to recover, and the population size rises again.
Image credits: https://www.khanacademy.org/science/ap-biology/ecology-ap/population-ecology-ap/a/mechanisms-of-population-regulation
❖ Natural selection occurs as populations have the capacity to produce many offspring that compete for
resources. In the struggle of existence individuals that are best adapted are most likely to survive to breed
and pass on their alleles to the next generation.
● Stabilising selection is when natural selection keeps things the way they are, with little change. Stabilising
selection does not lead to evolution. If the population is already well adapted, then selection will maintain
the favourable characteristics that enable it to be successful. Tends to keep the variation in a characteristic
centred around the same mean.
● Directional selection is when a new environmental factor or a new allele appears (mutation), then allele
frequencies may also change due to the change in the environment. Direction selection produces a
change, or evolution, in a population. Results in a change in a characteristic in a particular direction.
● Disruptive selection can occur when conditions favour both extremes of a population. Maintains
polymorphism in a population.
Genetic variation
Caused by:
● Independent assortment of chromosomes during meiosis during metaphase
● Crossing over between chromatids of homologous chromosomes during meiosis
● Random mating between organisms with a species
● Random fertilisation of gametes
● Mutation
Genetic variation produces phenotypic variation.
First four reshuffles existing alleles in the population but mutation can produce completely new ones. The new
allele is often recessive so it only shows up after generations. Mutations in somatic cells can’t be passed onto
offspring by sexual reproduction unless it is on the gametes.
Genetic variation provides the raw material on which natural selection can act as some individuals have
advantages over the others.
Environmental variation
❖ Phenotypic variation can also be caused by the environment in which organisms live, for example food
and weather.
Discontinuous variation
❖ Qualitative differences that fall into clearly distinguishable categories with no intermediates e.g blood
groups. Discontinuous variation is always caused by genes and usually just one or two genes involved.
For blood groups, they are controlled by a single gene, with three alleles.
❖ Different alleles on a single gene locus have large effects on the phenotype
❖ Different genes have quite different effects on the phenotype.
Continuous variation
❖ A variation between two extremes with no distinguishable categories such as height. Maybe caused by
genes, or by the environment, or both.
❖ Different alleles at a single gene locus have small effects on the phenotype
❖ Different genes have the same, often additive, effect on the phenotype
❖ A large number of genes may have a combined effect on a particular phenotypic trait ; these genes are
polygenes.
❖ Sickle cell anaemia and haemophilia are examples of discontinuous variation with single genes having a
large effect.
❖ Two typical effects of the inheritance of continuous variation : small effects of the different alleles of one
gene on a phenotype and the additive effect of different genes on the same phenotype.
❖ The variation of two populations can be compared using the t-test.
❖ T-tests are used to assess whether or not the means of two sets of data with roughly normal distributions
are significantly different from one another.
❖ Null hypothesis is when the two samples are from an identical population. If our t-test value represents a
probability of 0.05 or more, then we assume that the differences between the two sets of data are only
due to chance. The probabilities that we look at in the table are probabilities that the null hypothesis is
correct and there is no significant difference in the samples.
Natural selection
❖ Natural selection is a process in which selection pressures act on a population so that individuals
, possessing a particular allele, or a combination of alleles, are more likely to survive and pass on their
alleles than other individuals.
❖ Selection pressures increase the chances of some alleles being passed on to the next generation and
decrease the chance of others.
❖ The effects of such selection pressures on the frequency of alleles in a population.
❖ Biotic factors that control populations can be living organisms such as through predation, competition for
food or infection by pathogens.
❖ Abiotic facts can be caused by non-living components of the environment such as water supply or nutrient
levels in the soil.
❖ Fitness is the capacity of an organism to survive and pass on its genotype to its offspring.
❖ Polymorphism is the co-existence of individuals in a population with different phenotypes caused by
different genotypes.
❖ Some suggestions for the oscillating population size are food supply, food qualities and predators. As the
population size rises, food supplies run out, so the population size decreases. Once the population size
has decreased, food supplies begin to recover, and the population size rises again.
Image credits: https://www.khanacademy.org/science/ap-biology/ecology-ap/population-ecology-ap/a/mechanisms-of-population-regulation
❖ Natural selection occurs as populations have the capacity to produce many offspring that compete for
resources. In the struggle of existence individuals that are best adapted are most likely to survive to breed
and pass on their alleles to the next generation.
● Stabilising selection is when natural selection keeps things the way they are, with little change. Stabilising
selection does not lead to evolution. If the population is already well adapted, then selection will maintain
the favourable characteristics that enable it to be successful. Tends to keep the variation in a characteristic
centred around the same mean.
● Directional selection is when a new environmental factor or a new allele appears (mutation), then allele
frequencies may also change due to the change in the environment. Direction selection produces a
change, or evolution, in a population. Results in a change in a characteristic in a particular direction.
● Disruptive selection can occur when conditions favour both extremes of a population. Maintains
polymorphism in a population.
