Variation
Variation – Differences between individuals of the same species
| Continuous variation | Discontinuous variation |
|---|---|
| Results in a range of phenotypes between two extremes. E.g. body length and body mass. | Results in a limited number of phenotypes with no intermediates. |
| Caused by both genes and the environment. | Usually caused by genes. |
| E.g. body length and body mass. | E.g. ABO blood groups, seed shape in peas and seed colour in peas. |
- Mutation – Genetic change. It is the way in which new alleles are formed. ionising radiation and some chemicals increase the rate of mutation.
- Gene mutation – A random change in the base sequence of DNA.
Many mutations can cause adverse effects but some are beneficial. E.g. feather colours in birds (more attractive to other sex), albinism (doesn't produce melanin)
Sources of genetic variation: Mutation, meiosis, random mating and random fertilisation
Adaptive features
- An adaptive feature – An inherited feature that helps an organism to survive and reproduce in its environment.
| Hydrophytes (plants adapted to live around excess water) | Xerophytes (plants adapated to water scarcity) |
|---|---|
| Numerous stomata on the upper side of their leaves | Small or compound leaves |
| Large air sac to float | Deep root systems |
| Large leaves | Thick waxy cuticle |
| Rolling of leaves | Stomata adaptations |
| Densely packed spongy mesophyll | Hairs |
Selection
Natural selection:
- There is genetic variation within populations.
- Production of many offspring.
- This causes a struggle for survival and competition for resources.
- Individuals with better adaptations will reproduce.
- These individuals pass on their alleles to the next generation.
Selective breeding:
- Selection by humans of individuals with desirable features.
- Crossing these individuals to produce the next generation.
- Offsprings show desirable features.
Selective breeding by artificial selection is carried out over many generations to improve crop plants and domesticated animals.
- Adaptation – The process, resulting from natural selection, by which populations become more suited to their environment over many generations.
Examples of natural selection:
- Antibiotic resistant bacteria