Changes in allele frequencies
Changes in allele frequency can be caused by:
It is important to realise that a variation must already be present in the population. An adaptive characteristic will not suddenly be generated in response to an environmental change. If the environment changes or the population moves into a new environment, the population will need to adapt to the changes in order to survive. An existing variation that offers a survival advantage will be selected for. This will result in changes in allele frequency.
If the allele frequency changes with time, we know that evolution is taking place.
Within a gene pool we can consider how common or how rare some of the genes are. The number of times a certain gene occurs in a particular population is known as the allele frequency. For example, in a study of the frequency of the alleles for the ABO blood group system, a given population may have approximately:
- 42% group A
- 9% group B
- 3% group AB
- 46% group O.
For that population, we can say that the frequency of the A allele and the O allele is high, while the frequency of allele B is low. The ABO frequencies for other populations may be quite different.
The activity below will help you to understand that, over several generations, natural selection may act to bring a change in the allele frequencies of a gene pool.
Sexual selection is a form of natural selection that affects an individual's ability to mate. In some species, males compete for mates and the winners tend to do most of the mating. An existing variation that offers a competition advantage will be selected for. This will result in changes in allele frequency. For example, antler size in male deer and antelopes.
Genetic drift is the effect of chance events on allele frequency.
We have determined that change in the allele frequencies of a gene pool may occur as a result of natural selection acting on a population.
If a population is large enough, the allele frequency will remain constant from one generation to another. For example, predation would not alter the allele frequency of a population of a million snails as much as a small population of snails, as modelled in the changes in gene pool activity.
In a small population, such as an isolated population, there is often a change in allele frequency from one generation to another due to chance fluctuations.
Two examples of genetic drift in a small population are the founder effect and bottlenecks.
The founder effect
Occasionally a smaller population may branch off from a larger one and become reproductively isolated. The smaller population may or may not be genetically representative of the original population. Some rare alleles may be over-represented or lost completely while previously common alleles may be under-represented in the new population.
As a result of the founder effect, even if the new population increases in size, it will have a different gene pool (due to a different allele frequency) from the original population.
For example, macaroni penguins (Eudyptes chrysolophus) populate many Antarctic islands. Most of these birds are
black-faced; however, a few white-faced varieties are found in some populations. In fact, on one island the population consists of only white-faced varieties. The 'founding' population for this island contains either all or mostly white-faced varieties. It is assumed that the frequency of white and black faces in the founder population for that island was unrepresentative of the original population, which consists mostly of the black-faced variety.
Bottlenecks
Population bottlenecks occur when only a small sample of the total gene pool of a species survives. These situations may arise if a population is drastically reduced by non-selective events such as rising sea levels or habitat destruction 
caused by human activity.
Endangered species are often subject to population bottlenecks. For example, the lack of genetic diversity in the small number of Wollemi Pine (Wollemia nobilis) left in the wild indicates that this plant went through a population bottleneck.
Click on the headings below and answer the questions.
Natural selection – snails example
A population of a snail species has three varieties of shell colour as shown below.
The gene pool for the snail population at a given time is shown below.
For this population, calculate the frequencies of the allele types (B or b) and allele combinations (BB, Bb, bb) by counting the actual numbers, then converting them into a percentage.
For example, 10 pale (Bb) shelled snails in a population of 20 would have an allele combination percentage frequency of 
x 100 = 50%.
- Count the numbers of allele types B and b, and the number of allele combinations BB, Bb, bb.
- Calculate the percentage frequency for allele types (B or b) and allele combinations (BB, Bb, bb).
Record your figures in the table below then check your answers.
Open this link to view answers.
| Gene pool before predation | count | % frequency | |
|---|---|---|---|
| Allele types | B | 20 | 50 |
| b | 20 | 50 | |
| Allele combinations | BB | 5 | 25 |
| Bb | 10 | 50 | |
| bb | 5 | 25 |
Predatory birds find the dark-shelled snails easy pickings.
Some time later, in the same gene pool of the snail population, there was a change in allele frequencies due to predation. For the model below, assume that the population numbers are unchanged as births have replaced deaths.
- Count the numbers of allele types B and b and the number of allele combinations BB, Bb, bb.
- Calculate the percentage frequency for allele types (B or b) and allele combinations(BB, Bb, bb).
Record your figures in the table below then check your answers.
Open this link to view answers.
| Gene pool after predation | count | % frequency | |
|---|---|---|---|
| Allele types | B | 12 | 30 |
| b | 28 | 70 | |
| Allele combinations | BB | 2 | 10 |
| Bb | 8 | 40 | |
| bb | 10 | 50 |
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Natural selection – cockatoo example
There are two subspecies of the white-tailed black cockatoo: the short-billed variety or Carnaby's cockatoo (Calyptorhynchus latirostris) and the long-billed variety or Baudin's cockatoo (Calyptorhynchus baudinii).
The short-billed cockatoos, unlike the long-billed variety, usually feed on 'softer' seeds such as those from banksias, grevilleas, hakeas and dryandras. The fine-pointed bill of the long-billed cockatoo enables it to prise seeds from hard, wooden fruits such as marri. The map below shows the distribution of the long and short-billed cockatoos in the South-West of Western Australia.
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