• Created by: portia
  • Created on: 02-08-17 11:26

The general theory of evolution is that organisms have changed over time

Usually, natural selection keeps things the way they are. If a characteristic in a populatiom, such as body mass, shows wide variation, selection pressures often act against the two extremes. Very small or very large individuals are less likely to survive and reproduce than those whose sizes lies nearer the centre of the range. This resuslts in a population with a narrower range of body size. This type of selection, which tends to keep the variation in a characteristic centred around the same mean value, is called stabilising selection. Unless something changes, then natural selection will ensure that this continues to be the case.

If selection would act against smaller individuals but not larger ones, the range of variation shifts towards larger sizes. This type of selection, which results in a change in a characteristic in a particular direction, is called directional selection. It is as a result of a new environmental factor or a new allele appearing, causing allele frequencies to change.

If selection favours both large and small individuals, but acts against those whose size is in the middle of the range would result in disruptive selection. Disruptive selection occurs when conditions favour both extremes of a population. This type of selection maintains different phenotypes (polymorphism) in a population

A new environmental factor

If the climate were to become much colder resulting in snow covering the ground almost throughout the year, white rabbits would now have a selective advantage, as they are now better camouflaged. Rabbits with white fur are more likrly to survive and reproduce, passing on their alleles for white fur to their offspring. The frequency of the allele for white coat increases at the expense of the allele for agouti. Over many generations, almost all rabbits will come to have white coats rather than agouti.

A new allele

Because they are random events, mutations that occur produce features that are harmful; they produce organisms that are less well adapted to their environment than 'normal' organisms. Other mutations may be neutral; conferring neither an advantage or a disadvantage. Occasionally, mutations may prooduce useful features.

If for example, mutation occured in the coat colour gene for rabbits, producing a new allele that gives a better-camauflaged coat colour than agouti, rabbits possessing this new allele will have selective advantage and will be more like to survive and reproduce than agouti, so the new allele becomes more common in the population. Over many generations, almost all rabbits will come to have the new allele.

Such changes in allele frequency in a population are the basis of evolution.

Evolution occurs because natural selection gives some alleles a better chance of survival than others. Over many generations, populations may gradually change, becoming better adapted to their environments.

Examples of such change are the development of antibiotic resistant bacteria and instrial melanism in the peppered moth.

In contrast, the role of malaria in the global distribution…


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