Genetic Diversity & Adaptations
Genetic diversity is the number of different alleles in a population.
A gene pool can be described as all the genes and alleles in a population at a particular time. The allele frequency is the proportion of organisms within the population carrying a particular allele.
The genetic diversity of a population is what enables natural selection to occur.
Natural selection is the process that leads to evolution within a population. Describing the process of natural selection is often a 5 mark question.
- Random mutations occur within the population.
- This introduces genetic variation to the population.
- Whilst some mutations are harmful, sometimes new alleles created by mutations provide an organisms with an advantage to survive in their environment.
- The new allele provides a reproductive selective advantage. Therefore the individual with the allele is more like to reproduce and pass the allele onto their offspring.
- Over many generations, there will be an increase in the frequency of this allele within the population.
This change in the allele frequency in a population is what evolution is. The end result is that species are better adapted to their environment. Adaptions can be classified as either anatomical, physiological or behavioural. Below each of these adaptations are applied to the hedgehog.
Anatomical- spikes on a hedgehog
Physiological- hedgehogs hibernating and slowing their metabolic reactions in winter
Behavioural- Hedgehogs curling up when in danger.
- What is a mutation?
- Your answer should include: DNA / structure / change
Types of Selection
Selection causes some alleles to become more frequent in a population whereas others may be lost. Selection pressues, e.g. the type of food available, determines which alleles are successful.
Two key twos of selection are directional and stabilising.
Stabilising selection is when the middle trait has the selective advantage and therefore continues to be most frequent in the population. This impact can be represented by a normal distribution graph below. The range decreases as the extreme traits are lost over time.
A common example of stabilising selection is human birth weight. A middling birth weight is the selective advantage as extremely low birth weights may be disadvantageous to survival due to the underdevelopment of the baby and an extremely large birth weight may lead to complications in the birth. Therefore babies born with a middling weight are more likely to survive and pass on the allele to their offspring later in life.
Directional selection is when one of the extreme traits has the selective advantage. The result of this is demonstrated on the graph below.
An example of directional selection is antibiotic resistance in bacteria. A random mutation creates an allele that provides bacteria in a population resistance to an antibiotic. If this population of bacteria are then exposed to this antibiotic, only those with the resistance allele survive and all others die. This allele is then passed on and over generations most of the bacteria will carry this allele.
Antibiotic resistance in bacteria can be tested by growing bacteria, using aseptic technique and exposing them to different antibiotics.
Aspetic technique is when microbes are grown in sterile conditions. This includes disinfecting all surfaces you will work on, washing your hands with soap and sterilising all equipment. Bacteria, such as E.coli, are evenly spread over a petri dish filled with agar. Discs of paper soaked in different antibiotics and then placed on the petri dish. This is sealed and left to grow at 25C for a few days. After this incubation period, if there are any clear zones around the paper discs, or inhibition zones, this indicates that the bacteria have been killed by the antibiotic.
Any discs that do not have an inhibition zone around them indicates that the antibiotic did not kill the bacteria. To determine which antibiotic is the most effective, the diameter of the inhibition zone can be measured, and the largest diameter indicates the most bacteria killed.