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Evolution key points
Learn more about the key points surrounding evolution, from survival of the fittest to natural selection and help make your work stand out from the rest.
Defining natural selection
The process of natural selection can usually be easily described, but actually defining natural selection can be difficult. Students often use the terms ‘survival of the fittest’ or ‘strongest’ without explanation. A bacterium cannot be described as strong or fit - unless you define what you mean by fit, in which case you don’t need to use the term. There are some key points to remember about natural selection:
It must involve a difference in survival and reproduction. This means some individuals will leave more offspring than others. Only alleles get selected because it is the alleles that get passed to the next generation not the phenotype. The particular set of alleles is only adapted to a particular environment, which is usually the environment in which it evolved. It is sometimes the case that these alleles may adapt the organism to an entirely new environment. As an example, cockroaches will thrive in flats and houses, which are unnatural environments.
Natural selection may be defined as: the differential survival and reproduction of organisms in a population due to the fact that the alleles they possess affect their phenotypes, and thus their chances of surviving and reproducing in a particular environment. This is a long definition but does include the main concepts.
Describing Natural selection
It is worth considering the common misunderstanding, which many students make when discussing the topic of natural selection. As an example when describing how the population of peppered moths, in the famous study by Kettlewell, changed over time they wrote that “The air become(sic) so polluted that the moth changed decided to become black or became black”.
This statement is untrue because the peppered moth population always contained a few black moths, as a result of mutation, well before the Industrial revolution. The changed environment – the polluted air- and a selective pressure, (predation by birds) turned a selective disadvantage into an advantage. The black moths were now less likely to be seen by predators against the darker polluted tree trunks. The moths cannot choose to change colour as they cannot change the alleles they possess and it is these that determine their colour. Genetic change is due to mutations and is therefore accidental. There is no such thing as ‘intentional adaptation’ or ‘intentional mutation’.
New alleles can appear in a population at any time due to mutation. Over time repeated natural selection may take place to give the population a better adaptation to the environment.
Explaining natural selection
There are many examples of natural selection that can be used to test understanding of the key processes involved in natural selection. Many of these examples may be from modern contexts such as the development of antibiotic resistant strains of tuberculosis, development of antibiotic resistant strains of bacteria, or rats resistant to the poison warfarin. When explaining how natural selection has produced a particular outcome it is helpful to consider a number of key questions and answer these in terms of the new situation.
o What is the selection pressure in this new situation? In the case of warfarin resistant rats this would be repeated use of the rat poison.
o How does the population vary? Some rats have a mutated allele that allows them to survive when warfarin is ingested. They have not become ‘immune’ to the poison. They are resistant to its effects.
o Which individuals in the population will be at an advantage? In this case the poison will not kill the rats with the mutated allele whereas the rats without the allele will die if they ingest the poison.
o What will be the result in the population in terms of the phenotypes and the frequencies of each allele in the population?
This key point needs to be explained in terms of the survival of the organism, reproduction of the organism and the fact that alleles are transferred to the next generation. Using the example of the warfarin resistant rats, then the number of these rats will increase over time in the population, since they are surviving to reproduce. The alleles that give warfarin resistance are passed on in increasing numbers with each generation. This means the frequency of the warfarin resistance allele increases.
Natural selection increases the number of certain phenotypes in the population and this means the alleles that determine the advantage also increase in the population. The Hardy-Weinberg equation is used to calculate allele frequencies, and is covered in the genetics study guide.
Further natural selection essays
Variation, adaptation and selection must occur over several generations if a new species is to evolve and this point is often overlooked. If the life cycle of the organism is very short, then these new generations can arise very quickly. The evolution of antibiotic resistant strains of bacteria has occurred since the initial discovery of the fact that a substance made by the mould Penicillium notatum inhibited the growth of the bacterium Staphylococcus aureus in 1928. Both these organisms had co-existed for several hundreds of millions of years by this time. Unfortunately this compound that was preventing the growth of Staphylococcus aureus in minute concentrations, is now useless against certain strains of the pathogen, because of development of resistance in S. aureus found the world over. In terms of evolution this has happened in a very short space of time so saying that evolution has happened over millions of years is not always the case.