Variation
Variation : differences amongst members of the same species. (E.g. hair colour/height)
Variation can be inherited (in genes) or acquired (learning over time E.g. ability to speak French)
Species
Inherited Variation amongst species occurs by two mechanisms:
1)Sexual reproduction: produces variation due to meiosis (see Chapter 13: The Cell Cycle)
2) Mutation: produces variation due to changes in the amount or structure of DNA.
Result: change in sequence of bases - no protein produced. Can affect a somatic cell (anc could cause cancer) or a gamete (and be passed onto offspring)
Mutations can occur spontaneously – for no apparent reason – or by means of an agent, called a mutagen.
Mutagen: agent that increases the rate of mutations.
Examples of mutagens:
- Cigarette smoke
- UV light
- Asbestos
- Radon gas
- X-rays
Mutations can be of two types:
- Gene mutation: changes in the structure of a single gene; e.g. cystic fibrosis
- Chromosome mutation: changes in the structure or number of chromosomes; e.g. Down’s syndrome.
Evolution
Evolution: the way in which living things change genetically to produce new species over long periods of time. Most commonly proposed theory - Charles Darwin and Alfred Russel Wallace.
The Theory of Natural Selection
Example - MRSA stands for multi resistant Staphylococcus aureus. It is a species of bacterium that is resistant to all known antibiotics. Overuse of antibiotics has given a chance to certain strains of bacteria to become resistant to antibiotics. Strains that develop mutations that give them the ability to survive in the presence of antibiotics are naturally selected for. They will reproduce and pass on this trait to their offspring.
Darwin’s theory of natural selection made a number of observations and conclusions:
| Observations | Conclusions |
| Organisms produce large numbers of offspring eg rabbits produce large families (overbreeding) | there is a struggle for existence - competition for space, food, water |
| the environment can only support a limited number of organisms | |
| there are genetic variations among members of the same population. These variations arise from sexual reproduction and mutations |
the organisms best suited (adapted) live on They pass on genes to next generation Those not well adapted die out This is called survival of the fittest. |
Evidence for evolution - fossil evidence
(Fossil is something that lived a long time ago)
How do they support the theory of evolution-
- Can be dated
- Some species found as fossils no longer exists eg dinosaurs
- Some modern species have no fossils - indicates they have recently evolved eg humans
- Modern fossils are more complex
An example of fossil evidence- the evolution of the height of the horse.
(They have become taller over the last 60 million years)
| Time (million years ago) | Height (metres) |
| 60 | 0.4 |
| 30 | 0.6 |
| 15 | 1.0 |
| 1 | 1.6 |