Site author Richard Steane
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Genetic diversity and adaptation

Genetic diversity in a population

In the previous unit the causes of genetic diversity - mutations and the variations in combinations of alleles resulting from meiosis - were discussed. Within a population there will be a number of genes, each of which exists in different forms: alleles.

Genetic diversity can be defined in terms of the number of different alleles in circulation within a population, but really it is the result of the expression of those genes - the range in phenotypes shown by the individual organisms according to the combination of alleles they possess (their genotype) - that matters.

Natural selection acts upon genetic diversity

If an allele, or a combination of alleles, results in a different phenotype that assists the individual with that genotype, it can be said to provide a genetic advantage. Another way of expressing that is to use the term genetic fitness. Here fitness extends beyond physical exercise and covers all aspects of life.
Survival of the fittest is a concept that is often mentioned, but it is not enough simply to survive.

If an organism is more capable of reproducing (than average members of the population) - this results in increased reproductive success, and so the alleles causing the success are more likely to be passed on to the offspring. If the offspring also show the advantageous feature, they will presumably also show increased success in life and have more (similar) offspring, passing on the alleles in question. If the alleles are recessive, they will probably not be represented in the phenotype of the offspring, but they will still be passed on, and eventually be seen when two heterozygotes reproduce. Eventually, these alleles will be present in greater quantitity or frequency in the population.

The principles of natural selection in the evolution of populations

The production of new alleles of a gene occurs as a result of random mutations. The mutation process is a rare event, and (initially, at least) it is not focussed on providing an advantage. Mutations are not produced in response to other events in the sequence.

Many mutations are harmful and some appear to give neither an advantage nor a disadvantage. These neutral versions are likely to be passed on and retained within the population.
Changing environmental conditions, or migration to a new environment, may result in the new allele being in some way of benefit to individuals carrying that allele. This may lead directly or indirectly to increased or differential reproductive success.

The advantageous allele is therefore likely to be inherited by members of the next generation.

The new allele will increase in frequency in the population. This may take place over many generations, or much less time if it gives a marked advantage, e.g. antibiotic resistance in bacteria.

Species are seen to be adapted to their environment

Natural selection results in species that are well adapted to their environment. In effect they are probably better adapted than previous generations, often forgotten. These adaptations may be anatomical, physiological or behavioural.

Species do not decide to adapt to environments in a straightforward 'logical' way. But the point is that adaptation is the result of a chain of events, not the cause.

There is a tendency to use concepts like adaptation in attempting to explain characteristics and behaviour of organisms, especially in an ecological context, but it is important to avoid a teleological approach - the 'doctine of final ends' or 'goal-directedness'. In explaining evolution, Biologists often find themselves in opposition to those who take the 'creationist' approach to life on Earth.

Changes in allele frequency

Within a population there will be a range of expression of many characteristics: a 'spectrum of variation', often seen as a bell-shaped frequency curve characteristic of a normal distribution.

As a consequence, some individuals will be more 'favoured' than others, and this will lead to selection pressures for or against certain alleles held by these individuals, and this may have the effect of changing some parts of the population in an apparently defined way.

birthweightdata (32K) Data from 1295 births

Human birth weights are a case in point: It is obviously advantageous for a foetus to grow in size inside its mother's uterus, but not to grow so large that it threatens the mother's life. Small babies are less capable of survival due to inadequate development in utero. This results in stabilising selection, and most babies' birth weights are close to 3.5 kg.

Antibiotic resistance in bacteria shows a fairly dramatic and quick change in the mean value - directional selection.
In a population of bacteria there will be a number with alleles that make them potentally resistant to certain antibiotics, even if there is no antibiotic in their general area!

In fact some may be capable of producing an enzyme that inactivates a certain type of antibiotic, or they may make their cell wall in a slightly different way than most of their relatives. Sometimes this puts them under some sort of disadvantage, producing something that is not really needed.

BUT - and this is a matter of life and death for the bacteria - if they find themselves in an environment where antibiotics are in circulation, those bacteria which do not have a mechanism providing protection from that particular antibiotic are likely to die fairly quickly.
antibiotic_resistance (34K)

Those bacteria with a form of resistance to the antibiotic are unaffected and they continue to live and reproduce, so the frequency of antibiotic resistance alleles in the population will rise as the antibiotic-sensitive bacteria are replaced the antibiotic-resistant bacteria.

Other related topics on this site

(also accessible from the drop-down menu above)

This series (genetic information)
DNA, genes and chromosomes
DNA and protein synthesis
Genetic diversity

Species and taxonomy
Biodiversity within a community

Web references

Low Birth Weight, a Risk Factor for Cardiovascular Diseases in Later Life, Is Already Associated With Elevated Fetal Glycosylated Hemoglobin at Birth

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