Explain how natural selction may bring about changes in a population and what conditions may be necessary to bri about speciation A species is a group of organisms that look alike and can reproduce successfully

Although sexual reproduction produces variation it does not actually introduce new alleles. These are brought in by mutations. A mutation is a change in the structure or amount of DNA in the amino acids sequence produced. A mutation in a gamete will be inherited, whereas a mutation in other body cells will not be passed to offspring. Gene mutation is an alteration in the base sequence of one or two base pairs by addition, deletion or substitution. This occurs at a single gene locus on a chromosome altering the DNA sequence of bases and the amino acid sequence being produced. The resulting protein shape may be significantly altered and then does not function properly. This is important if the altered protein is an enzyme which if changed, may no longer match the shape of its substrate, and therefore stop and subsequent enzyme action. Some mutations are harmful, others may have no effect. Chromosome mutations involve changes to either the number of chromosomes of to large sections of the actual single chromosome. Thus meaning many genes are changed. Some mutations are harmful, however the mutations which are beneficial may increase the chance of survival. It is the new rare alleles that determine the path of evolution.

The effect of the environment also plays an important part on variation, as it provides the selection pressure that determines which organisms survive to reproduce and pass on their alleles. For example in a population of trees, if their were to be a reduction in light to due to long term pollution the taller trees would be able to reach sufficient light, which would allow them to survive longer an pass on their alleles for tallness to the next generation. The smaller plants unable to benefit from the light will not survive and therefore not be able to pass on their alleles to the next generation.

Darwin's theory was based on four observations: Individuals within a species differ from each other - there is variation. Offspring resemble their parents - characteristics are inherited. Far more offspring are generally produced than survive to maturity - they suffer from predation, disease and competition. Populations are usually fairly constant is size. Darwin concluded that individuals that were better adapted to their environment compete better than the others, survive longer and reproduce more, so passing on more of their successful characteristics to the next generation. Darwin used the memorable phrases survival of the fittest, struggle for existence and natural selection.

There are three kinds of Natural Selection. Directional Selection, This occurs whenever the environment changes in a particular way. There is therefore selective pressure for species to change in response to the environmental change e.g. The peppered moth.These light coloured moths are well camouflaged from bird predators against the pale bark of birch trees, while rare mutant dark moths are easily picked off. During the industrial revolution in the 19th century, birch woods near industrial centres became black with pollution. In this changed environment the black moths had a selective advantage and became the most common colour, while the pale moths were easily predated and became rare. Populations do not have to decide to adapt, or mutate, after an environmental change. The mutation, or combination of alleles giving resistance, have to already be there by chance, otherwise the population may become extinct. "Environment" includes biotic as well as abiotic, so organisms evolve in response to each other. e.g. if predators run faster there is selective pressure for prey to run faster, or if one tree species grows taller, there is selective pressure for other to grow tall. Most environments do change (e.g. due to migration of new species, or natural catastrophes, or climate change, or to sea level change, or continental drift, etc.), so directional selection is common.

Stabilizing (or Normalizing) Selection. This occurs when the environment doesn't change. Natural selection doesn't have to cause change, and if an environment doesn't change there is no pressure for a well-adapted species to change. Fossils suggest that many species remain unchanged for long periods of geological time. One of the most stable environments on Earth is the deep ocean e.g. The Coelacanth. This fish species was known only from ancient fossils and was assumed to have been extinct for 70 million years until a living specimen was found in a trawler net off South Africa in 1938. So this species has not changed in all that time. Anther example of stabilizing can be seen in the birth weight of humans. The heaviest and lightest babies have the highest mortality and are less likely to survive to reproduce and pass on their alleles.

Disruptive (or Diverging) Selection, this occurs where an environment change mayroduce selection pressures that favour two extremes of a characteristic e.g. Sickle-cell anaemia. People homozygous for this recessive allele usually die before reproducing. Their red blood cells contain abnormal haemoglobin which makes them become sickle-shaped and stick in their capillaries. People heterozygous for the allele should be at a disadvantage, because their red cells can sickle during exercise - the allele should therefore be selected against and rare, however, its frequency is high in parts of the world where malaria is common - in some populations over 20% carry the allele (as heterozygotes). People heterozygous for sickle-cell anaemia are more resistant to malaria than people homozygous for the normal allele. Where malaria is found, people heterozygous for sickle-cell have an advantage and are likely to survive, reproduce and pass on the allele. People without the allele also have an advantage, because their red cells behave normally. This produces populations with an equilibrium for numbers of people heterozygous for sickle-cell and non-carriers (balances polymorphism).

A species is defined as a group of interbreeding populations that are reproductively isolated from other groups. Reproductively isolated can mean that sexual reproduction between different species is impossible for physical, ecological, behavioural, temporal or developmental reasons. For example horses and donkeys can apparently interbreed, but the offspring (mule) doesn't develop properly and is infertile. This definition does not apply to asexually reproducing species, and in some cases it is difficult distinguish between a strain and a species.

New species usually develop due to: geographical isolation (allopatric speciation) andreproductive isolation (sympatric speciation). Allopatric Speciation: It is meaningless to say that one species is absolutely better than another species, only that it is better adapted to that particular environment. A species may be well-adapted to its environment, but if the environment changes, then the species must adapt or die. In either case the original species will become extinct. Since all environments change eventually, it is the fate of all species to become extinct (including our own). Reproductive Isolation (Sympatric Speciation) Reproductive Isolation is a type of genetic isolation. Here the formation of a new species can take place in the same geographical area, e.g. mutations may result in reproductive incompatibility. A new gene producing, say, a hormone, may lead an animal to be rejected from the mainstream group, but breeding may be possible within its own groups of variants. When this mechanism results in the production of a new species it is known as sympatric speciation.

Ben Dreyfuss 6mms