S-Cool Revision Summary

S-Cool Revision Summary

For each characteristic, the population may show either continuous or discontinuous variation .

In any population, the total variety of genes and alleles present is called the gene pool .

This gene pool can change in content (new alleles arriving, existing alleles being lost) or the ratio of alleles altering due to the following:

  1. mutation,
  2. natural selection,
  3. emigration,
  4. immigration,
  5. mate selection.

The factors favouring stability of the gene pool are:

  1. No mutation,
  2. No natural selection,
  3. The population being large,
  4. No gene flow (due to individuals emigrating or immigrating),
  5. Random mating,

If these factors favouring stability are fulfilled, the ratio of the alleles for a gene can be established using the Hardy-Weinberg Equilibrium .

Using the Hardy-Weinberg Equilibrium , it is possible to establish the ratio of dominant to recessive alleles.

For a species to survive, it must reproduce. However, the population is limited by environmental factors and so remains more or less constant over time.

The change in adaptation that occurs is called evolution .

There are three types of selection that occur in nature:

Stabilizing selection.

Directional selection.

Disruptive selection.

There is much evidence for the process of evolution and that natural selection is the mechanism by which it occurs.

The study of fossils.

Divergent evolution

Also called adaptive radiation .

When a group of organisms all possess a structure that appears to have come from a common ancestor and which has the same microscopic structure and body position, as well as other features, they are said to have homologous structures.

Convergent evolution

This is the opposite of adaptive radiation; it is where structures that on first appearance are similar but are actually found to be unrelated.

Examples include:

Insect wing and bird wing.

The eye of a mollusc (e.g, squid) and the eye of a vertebrate.

These structures are said to be analogous. In these cases, selection pressures from the environment in which each species lives have caused particular structures to be advantageous.

Various chemicals have been studied in order to find evidence of evolution, DNA and proteins such as cytochrome c in the electron transport chain of respiration are often used.

Looking at the order of bases in the lengths of DNA and the order of amino acids in a protein, it is possible to determine how similar they are in different species.

Species that are closely related have the most similar DNA and proteins, those that are distantly related share far fewer similarities. A comparison of DNA sequences show that it is 99.9% certain that chimpanzees are humans' closest relatives.

Evolution occurs when there is a change in the environment or when there is mutation because the allele frequencies will change as a result. This change in allele frequency is called microevolution .

Usually mutations are harmful, occasionally they are neutral and infrequently they are beneficial. In this latter case, the mutation can increase an individual's chances of survival (fitness), reproduction and thus the frequency of that genotype.

Antibiotics are chemicals usually produced by fungi.

They can kill bacteria by preventing cell wall formation and so are given to patients who have a bacterial infection. There may be however, one or more bacteria in the population that are able to inactivate the chemical making them resistant to the antibiotic.

Haemoglobin is the pigment in red blood cells that carries oxygen around the body.

A group of organisms with similar morphological, physiological and behavioural features, which can interbreed to produce fertile offspring, and are reproductively isolated from other species.