Variation and evolution flashcards

Topic 6: Variation and evolution

Genetic, environmental and combined causes of variation: examples

Genetic: blood group, eye colour. Environmental (acquired): a scar, a suntan, plant height with and without mineral ions. Both: human height and body mass.

Cuttings from one plant grow to 19.0 cm with nitrate and 12.0 cm without. What causes the difference?

The environment. The cuttings are clones with identical genes, so the 7.0 cm difference is environmental.

Mutation

A random change in a gene that makes a new allele. It is the original source of new alleles.

Natural selection vs Lamarck's theory

Natural selection: variation already exists in genes; the best-suited survive and breed, passing on their alleles. Lamarck: changes acquired during a lifetime are inherited. In the vast majority of cases this cannot occur.

The three statements of natural selection

Individuals in a species show a wide range of variation because of differences in their genes. Those most suited to the environment are more likely to survive to breed successfully. The genes that enabled them to survive are passed on to the next generation.

Selection pressures: examples

Factors that affect survival: predators, food supply, disease and climate change.

The four stages in forming a new species

Isolation (populations separated, e.g. geographically); genetic variation within each; natural selection of different alleles in each; speciation, when they can no longer interbreed to produce fertile offspring.

Horses and donkeys can mate. Why are they different species?

Their offspring, the mule, is usually sterile. The test for one species is producing fertile offspring.

Why is evolution counted in generations, not years?

Evolution usually takes many generations. A bacterium dividing every 30 minutes passes 40 generations in 20 hours; a species with a 25-year generation time needs 1000 years. Species that cannot respond to environmental change risk extinction.

Continuous vs discontinuous variation

Continuous: a full range of values (height, leaf length). Discontinuous: separate groups (blood group).