3.7.3 evolution may lead to speciation

What 3 genetic factors cause genetic variation within a population?

• Mutations - sudden changes to genes and chromosomes which may be passed onto the next generation

• Meiosis - produces new combinations of alleles before they are passed into the gametes

• Random gamete fertilisation - in se*ual reproduction this produces new combinations of alleles and the offspring are genetically different from parents, which gamete fuses with which is a random process which increases variety of offspring.

How does the environment influence genetic variation?

The environment (temperature, rainfall, sunlight, soil conditions, pH and food availability) influences the way an organism’s genes are expressed, and influences where within the limits of genes an organism may lie.

Why should conclusions drawn from the causes of variation in organisms be treated with caution?

Because it is usually due to a combination of genetic and environmental influences, so it is difficult to draw conclusions about the causes of variation.

What is selection pressure?

Environmental factors that limit the population of a species, including predation, disease and competition

Gene pool definition

Total number of all the alleles of all the genes of all the individuals within a particular population at a given time

What factors does evolution by natural selection depend on?

• Organisms produce more offspring that can be supported by the available supply of food, light, space etc

• There is genetic variety within the population of all species

• Variety of phenotypes that selection operates against

What is genetic diversity?

The number of different alleles of genes in a population.

Why have high reproductive rates evolved in many species?

To ensure a sufficiently large population survives to breed and produce the next generation, which compensates for high death rates from predation, food competition, natural disasters etc.

Why have some species evolved low reproductive rates with high parental care?

It results in lower death rates, helping maintaining population size

How does over-production of offspring lead to natural selection?

Where there are too many offspring for the available resources, there is intraspecific competition among individuals for the limited resources. Greater numbers mean greater competition and more individual will die in the struggle. Those best suited to prevailing conditions eg better able to catch prey will be more likely to survive and pass on favourable allele combinations to the next generation, meaning there is a different allele frequency from the previous generation. This causes the population to evolve a combination of alleles better adapted to the prevailing conditions. This depends on individuals being genetically different from each other.

Why is it beneficial to have genetic variation in a population?

It means that there is a greater chance that some individuals will have the combination of alleles leading to a phenotype which is advantageous in surviving and reproducing. These individuals are more likely to breed and pass on their allele combinations to future generations, so variation provides the potential for a population to evolve and adapt to new circumstances.

Outline the stages of natural selection by random mutation:

• Random mutation results in new alleles of a gene, meaning there is genetic variation in a population

• In certain environments a particular allele may benefit the possessor, meaning it is more likely to survive and reproduce

• Advantageous allele is inherited by members of the next generation

• Over many generations allelic frequency of it increases

What are the three types of natural selection?

• Stabilising selection - preserves the average phenotype (around the mean) of a population by favouring average individuals as opposed to the extreme phenotypes.

• Directional selection - changes the phenotype of a population by favouring phenotypes that vary in one direction from the mean of the population, so selects one extreme phenotype

• Disruptive selection - favours individuals with extreme phenotypes instead of those with phenotypes around the mean of the population