18 Variation & Natural selection

Variation due to genetic factors

Within a population, all members have the same genes
Genetic differences is differnt alleles of the same gene
Change from generation to generation
Mutations - sudden changes that may or may not be passed on, main source of variation
Meiosis - special nuclear division, new combinations of alleles
Random fertilisation of gametes - Which gamete fuses is random adding the variety that two parents can produce

1 of 5

Genes set the limits but the environment determines where on that scale the organism is
Environmental conditions could be temperature, rainfall, sunlight, soil conditions, pH and food availability
e.g. height is mainly determined through multiple genes, however diet can influence how tall or short a person can become
Variation is a combination of genes possessed and environmental factors
It can be hard for scientists to distinguish between these two factors

2 of 5

The process of selection is based on the suitability for survival under the existing conditions
Environmental factors are called selection pressures e.g. predation, disease and competition
Selection pressures determine the allele frequency in a gene pool
Evolution depends on many factors including
- Organisms produce more offspring than can be supported by the resources e.g. light, food, space
- There is genetic variety between the offspring
- There are a variety of phenotypes that selection operates against

3 of 5

Darwin realised that in real life, populations didnt grow exponentially
High birth rates compensates for high death rates caused by predation, competition and extreme conditions
Where there are too many offspring for the available resources, there is intraspecific competition for the limited resources
More individuals will die in the struggle to survive
Those that are better suited to prevailing conditions e.g. better at hiding or running away from predators are morelikely to survive
They are more likely to breed and pass on their favourable alleles
This causes the next generation to have a different allele frequency
The process depends on organisms haveing genetic diversity

4 of 5

Variation may produce offspring that are unsuited to the environment that the parents live in
Conditions change over time and so having a wide range of phenotypes is advantageous to survive in multiple new circumstances
Populations with little genetic variation are susceptible to diseases and climate change
The larger a population, the more likely some of the individuals will be more suited to the new environment
This causes a population to evolve

5 of 5