Genetic diversity and adaptation

In a nutshell

Genetic diversity is the number of different alleles of genes in a population. It is the raw material that natural selection acts on: without variation between individuals, there is nothing to select.

This subtopic is about how natural selection uses that diversity to make populations better adapted to their environment, shown through the two selection patterns AQA names: directional selection (antibiotic resistance) and stabilising selection (human birth weight).

Assumed knowledge: How genetic diversity can arise, DNA and protein synthesis.

Core content

Genetic diversity

Genetic diversity is the number of different alleles of genes in a population.

• The more different alleles a population has, the greater its genetic diversity.
• Genetic diversity is a factor enabling natural selection to occur. Selection can only favour a phenotype if a range of alleles is present for it to act on.

Do not confuse this with species diversity (the number of different species in a community, covered in 3.4.6). Genetic diversity is variation within a population.

The principles of natural selection

Natural selection is the mechanism by which populations become better adapted over generations. Learn it as a causal chain, in this order, because an "explain" answer needs every link:

1. Random mutation produces a new allele of a gene. Mutation is spontaneous; the environment only affects the rate at which it happens, never which allele appears.
2. Many mutations are harmful, but in a certain environment the new allele may benefit its possessor, giving that individual increased reproductive success (it is more likely to survive and reproduce).
3. The advantageous allele is inherited by members of the next generation, because the survivors pass it on to their offspring.
4. Over many generations, the new allele increases in frequency in the population.

Two phrases carry marks and are easy to drop:

• It is the individual (organism) that survives and reproduces, not the allele. "The allele survives" is rejected.
• The allele increases in frequency (proportion), not in number.

Still don't get it? · why the mutation comes first, not the antibiotic

Imagine a huge crowd of people and one deadly quiz. A handful of people, by pure luck, already knew the answers before the quiz was ever written. The quiz does not teach anyone the answers. It simply removes everyone who did not already have them. Run the quiz again on the survivors and their children, and knowing the answers becomes normal in the crowd.

The bacteria work the same way. A random mutation gives a few bacteria a resistance allele before they ever meet the antibiotic. The antibiotic does not create resistance and it does not cause the mutation. It is only the selection pressure: it kills the non-resistant bacteria and leaves the resistant ones to survive and reproduce, passing the allele on.

So in the exam: say the mutation happened first and at random, name the antibiotic as the selection pressure, then say resistant bacteria survive, reproduce and pass on the allele, so its frequency rises. Never write "the antibiotic caused a mutation" or "the bacteria became immune".

Adaptations

Natural selection results in species that are better adapted to their environment. An adaptation is a feature that increases an organism's chance of survival and reproduction. AQA groups them into three types:

• Anatomical (structural): a physical feature, e.g. the streamlined body of a fast-swimming fish.
• Physiological (functional): a process inside the body, e.g. an enzyme that works at the organism's body temperature.
• Behavioural: how the organism acts, e.g. an animal migrating to avoid winter cold.

Directional selection

Directional selection favours one extreme phenotype, shifting the whole population towards it.

• It happens when the environment changes or a new selection pressure appears.
• The mean phenotype moves towards the favoured extreme over generations; the other extreme is selected against.

Spec exemplar: antibiotic resistance in bacteria.

1. Within a population of bacteria, a random mutation produces an allele for antibiotic resistance.
2. Using the antibiotic is the selection pressure: non-resistant bacteria are killed, but the resistant bacteria survive.
3. The survivors reproduce and pass on the resistance allele to the next generation.
4. Over many generations, the frequency of the resistance allele increases, so the population becomes resistant.

Stabilising selection

Stabilising selection favours the average (modal) phenotype and selects against both extremes.

• It occurs in an unchanging (stable) environment.
• The mean phenotype stays the same, but the range and standard deviation are reduced, because extreme phenotypes are removed. An increasing proportion of the population is well adapted.

Spec exemplar: human birth weight. Babies of around the mean birth weight have the highest survival rates. Very low birth weight (harder to keep warm and develop) and very high birth weight (birth complications) both lower survival, so alleles for those extremes decrease in frequency while the mean stays roughly constant.

Directional vs stabilising selection

Still don't get it? · reading the two distribution curves

Think of the class heights plotted as a bell: most people are near the middle, few are very short or very tall. Selection is a filter over that bell.

Directional selection is a filter that removes one whole side. If a change (say, a new predator) makes tall a disadvantage, the tall side gets thinned out, so the whole bell slides to the left: a new mean, shifted towards short.

Stabilising selection is a filter that removes both ends. If the environment is stable and average is safest, both the very short and the very tall are removed. The bell keeps the same middle but becomes taller and thinner: same mean, smaller range and standard deviation.

Exam wording: directional "shifts the mean towards one extreme"; stabilising "favours the mean and reduces the range/standard deviation in an unchanging environment".

Required practical 6: aseptic technique and antimicrobials

This practical uses aseptic technique to investigate the effect of antimicrobial substances (antibiotics or antiseptics) on the growth of bacteria on an agar plate.

Why aseptic technique: to prevent contamination of the culture by unwanted microorganisms from the air, equipment or surfaces, and to prevent the cultured bacteria escaping into the environment.

Key aseptic steps and their reasons:

• Flame the inoculating loop until red hot before and after use, to kill any microorganisms on it.
• Flame the neck of the culture bottle as it is opened and closed, to stop airborne microorganisms entering.
• Work near a Bunsen flame (an upward air current carries airborne microorganisms away) and disinfect the work surface.
• Lift the Petri dish lid only at an angle, and tape the lid partially (not sealed) so that no anaerobic pathogens are favoured while airborne contamination is still kept out.
• Incubate at 25 °C, not 37 °C, so that human pathogens (which grow best at body temperature) are not favoured.

Measuring the effect: a clear zone (zone of inhibition) forms around a disc where bacteria cannot grow. A larger clear zone means a more effective antimicrobial. A disc soaked in water or solvent only is the control, showing that any inhibition is due to the antimicrobial and not the disc.

Because bacterial numbers grow very quickly, a logarithmic scale is often used to plot the number of bacteria in a culture over time.

Worked examples

Model answer, directional selection: "Explain how a population of bacteria can become resistant to an antibiotic." (5 linked points)

1. There is genetic variation in the population caused by a random mutation, which produces an allele for antibiotic resistance.
2. Using the antibiotic acts as a selection pressure.
3. Bacteria without the resistance allele are killed; bacteria with it survive.
4. The surviving resistant bacteria reproduce and pass on the resistance allele to their offspring.
5. Over many generations, the frequency of the resistance allele increases in the population.

Notice the answer names the mutation as the source, calls the antibiotic the selection pressure, and ends on allele frequency. Dropping any one of these five points loses a mark.

Model answer, stabilising selection: "Explain how stabilising selection affects human birth weight." (4 linked points)

1. The environment is unchanging (stable), and babies of average birth weight have the highest survival rate.
2. Babies at both extremes (very low and very high birth weight) are less likely to survive and reproduce, so are selected against.
3. Alleles for the average birth weight are passed on, while alleles for the extremes decrease in frequency.
4. The mean birth weight stays the same but the range and standard deviation are reduced over generations.

Data-handling: area of a zone of inhibition. A clear zone has a diameter of 22 mm. The area is:

$A = \pi r^2 = \pi \times (11)^2 = 380 \text{ mm}^2 \ (3\text{ s.f.})$

with radius $r = \frac{22}{2} = 11$ mm. Comparing areas (not diameters) is the fair way to rank two antimicrobials, because area scales with the square of the radius.

Common exam mistakes

• Writing that the antibiotic (or environment) "causes the mutation" or that bacteria "mutate in response to" it. The mutation is random and happens first; the antibiotic only selects. This is rejected by examiners.
• Saying bacteria become "immune". The correct term is resistant. Immunity is an immune-system response and is the wrong idea here.
• Writing that "the allele survives and reproduces". The individual organism survives and reproduces; it passes the allele on.
• Saying the advantageous allele increases in number. The mark needs increase in frequency (proportion).
• Using "gene" where the mark needs "allele", or hedging with "gene/allele". Genetic diversity is the number of different alleles, not genes; "range of alleles" or "number of alleles" (without "different") is not credited.
• In an "explain" question, describing the outcome or the data instead of giving the mechanism. Naming that the mean shifted is not the same as explaining why.
• Forgetting to name the selection pressure, or naming selection in the wrong organism (e.g. saying antibiotics select cattle, when selection acts on the bacteria).
• For stabilising selection, omitting that it occurs in an unchanging environment, or saying the mean changes. The mean stays the same; the range/standard deviation falls.
• Saying "the species survives because of variation". It is the better-adapted individuals that survive and reproduce.

Key definitions

• Genetic diversity: the number of different alleles of genes in a population.
• Natural selection: the process by which organisms with an allele that gives increased reproductive success are more likely to survive, reproduce and pass that allele on, so its frequency in the population increases over generations.
• Selection pressure: an environmental factor (such as an antibiotic, a predator or climate) that affects an organism's chance of survival and reproduction.
• Directional selection: selection that favours one extreme phenotype, shifting the mean of the population towards it (occurs when the environment changes).
• Stabilising selection: selection that favours the average phenotype and selects against the extremes, reducing the range and standard deviation (occurs in an unchanging environment).
• Adaptation: a feature (anatomical, physiological or behavioural) that increases an organism's chance of survival and reproduction in its environment.

Specification

• I can state that genetic diversity is the number of different alleles of genes in a population.
• I can explain that genetic diversity is a factor enabling natural selection to occur.
• I can describe the principles of natural selection: random mutation produces a new allele; in certain environments it benefits its possessor, giving increased reproductive success; the allele is inherited by the next generation; over many generations it increases in frequency.
• I can explain directional selection using antibiotic resistance in bacteria as the example.
• I can explain stabilising selection using human birth weights as the example.
• I can state that natural selection produces species better adapted to their environment, and that adaptations may be anatomical, physiological or behavioural.
• I can use unfamiliar information and interpret data to explain how selection produces change within a population.
• I can carry out Required practical 6: use aseptic techniques to investigate the effect of antimicrobial substances on microbial growth.

Related notes

• How genetic diversity can arise
• Species and taxonomy
• Biodiversity within a community