Natural Selection & Variation
Biology: IB Diploma Higher Level, topic 3.3.3
Natural Selection & Variation
Natural selection only takes place if there is variation among individuals in a population. These differences must be genetic or selection will not lead to change in the next generation.
Genetic diversity
- Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction.
- Mutation, a change in DNA, is the ultimate source of new alleles, or new genetic variation in any population.
- The genetic changes caused by mutation can have one of three outcomes on the phenotype:
- Reduced fitness and a lower likelihood of survival.
- Increased fitness and a greater likelihood of survival.
- No effect on fitness (neutral).
Changing environments
- Whether a phenotype is favorable depends on the environmental conditions at the time.
- The same traits are not always selected because environmental conditions can change.
- Changes in environmental conditions create selection pressures.
- A selection pressure is a factor which affects the ability of of some members of a population to survive and pass on their genes.
Example
- Consider a species of plant that grew in a moist climate and did not need to conserve water.
- Large leaves were selected because they allowed the plant to obtain more energy from the sun.
- Large leaves require more water to maintain than small leaves, and the moist environment provided favorable conditions to support large leaves.
Example - continued
- After thousands of years, the climate changed, and the area no longer had excess water.
- The direction of natural selection shifted so that plants with small leaves were selected.
- This is because those populations with small leaves were able to conserve water to survive the new environmental conditions.
Test yourself on Natural Selection & Variation
Which of the following is NOT true about natural selection?