Chapter 17: Variation and Evolution
Variation : differences amongst members of the same species.
Variation amongst species occurs by two mechanisms:
- Sexual reproduction: produces variation due to meiosis (see Chapter 13: The Cell Cycle)
- Mutation: produces variation due to changes in the DNA
Mutation: changes in the structure or amount of DNA in a cell.
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
- Radon gas
- Gene mutations
- Chromosome mutations
Gene mutation: changes in the structure of a single gene; e.g. sickle cell anaemia.
Chromosome mutation: changes in the structure or number of chromosomes; e.g. Down's syndrome.
Evolution: genetic changes in species, over a long period of time, to produce new species in response to environmental stresses.
Species: group of similar organisms that can interbreed to produce fertile offspring.
Speciation: formation of a new species following many changes in the structure of an organism until the new species cannot interbreed to produce fertile offspring with the original species.
Theory of Natural Selection
Natural selection: process by which particular traits become more common in a population due to that trait being advantageous to the species.
Examples of natural selection:
- Sickle cell anaemia (SCA):
Charles Darwin and Alfred Russell-Wallace
Darwin and Russell-Wallace were two English naturalists who both independently came up with the Theory of Natural Selection. Darwin was first to publish his findings, hence he received the most recognition.
Darwin's theory of natural selection made a number of observations and conclusions:
- Species produce many more offspring than is necessary (overbreeding).
- There is a limited supply of resources to allow survival of the species.
- Numbers of species remain relatively constant over long periods of time.
- All species show variations among their members and these variations are inherited.
- There is a 'struggle for existence' (competition) between species and between members of the same species for food, space, shelter and mates.
- Organisms most suited to their environment, due to favourable characteristics, will survive and reproduce at a faster rate than those less well suited.
- The characteristics that make an organism most suited to its environment will accumulate among the population.
Evidence for evolution
- Paleontology: study of fossils.
Fossil: preserved remains of an organism, part of an organism or an imprint left by that organism.
- Comparative anatomy: study of the similarities and differences in the structure of living organisms.
- Comparative embryology: study if the similarities and differences in the structure of the embryo from different species.
Comparative embryology is a source of evidence for evolution as the structure of the early embryo from many different species is very similar.
- Comparative biochemistry: study of the similarities and differences in the chemistry of living organisms.