3.4.4 genetic diversity and adaptation

Genetic similarities and differences between organisms can be defined in terms of variation in DNA. This is because if is differences in DNA that lead to genetic diversity.

It is important to understand that al members of the same species have the same genes, which phenotype they express depends on which alleles they possess. Organisms of the same species differ in alleles (not genes!).

Genetic diversity is the total number of different alleles in a population (a population is a group of individuals of the same species in the same habitat at the same time). A species consists of one or more populations, The greater the number of different alleles that all members of a species possess the greater the genetic diversity of that species. This is important as it increases the chance of survival with environmental change as there is a greater probability that an individual will possess a characteristic that suits it to the new environment.

I must add that it is not equally likely that all alleles of a population are to be passed on. This is because not all individuals are reproductively successful so not all pass on their alleles. Differences between the reproductive success of individuals affects allele frequency within populations because:

• within any population of a species there will be a gene pool containing a wide variety of alleles
• random mutation may result in a new allele
• in certain environments the new allele might give its possessor an advantage over the other individuals
• this individual is better adapted so more likely to survive in their competition with others
• these individuals therefore have a better chance of successfully breeding and producing offspring
• only individuals that reproduce successfully will pass on their alleles to the next generation
• over time the frequency of the 'better' allele will increase whilst the 'worse' allele will decrease

This is the principle of natural selection. Natural selection results in species that are better adapted to the environment they live in. these adaptions may be:

• physiological
• to do with like inside stuff (e.g oxidization of fat rather than carbohydrate produces more water in kangaroo rats)
• behavioural
• to do with behaviour
• anatomical
• to do with anatomy

Types of selection

Directional selection

Process covered in 3.7.3

We need to know an example of this by antibiotic resistance in bacteria:

• A spontaneous mutation occurs in the allele of a bacterium that enables it to make a new protein
• this protein is an enzyme that breaks down an antibiotic before it's able to kill the bacterium
• the antibiotic was used to treat an individual and the bacterium was present
• the mutation gave the bacterium an advantage and it was not killed (unlike the rest of the population
• the bacterium that survived reproduced by binary fission and a built up a population of antibiotic-resistant bacteria
• the populations normal distribution curve shifted in the direction of a population having greater resistance to the antibiotic

Stabilising selection

Process covered in 3.7.3

The example we need to know of this is human birth weights. There is a much greater risk of infant mortality if the baby is born outside the 2.5-4.0kg range