Genetic Diversity and Adaptation

Genetic Diversity

Genetic diversity arises from the mutations and meiosis covered in the previous topic. Here we look at what it means for populations and how natural selection acts on it.

It is a key factor in enabling natural selection to occur. Without it, every individual would respond identically to every selection pressure, and populations could not adapt.

Natural Selection

Natural selection acts on genetic diversity. It is the mechanism by which advantageous alleles increase in frequency over generations. Here’s the mechanism:

1. A random mutation produces a new allele
2. Environmental factors act as selection pressures that affect the survival chances in a population
3. In certain environments, this allele is beneficial — giving the organism a better chance of survival or reproduction
4. That individual is more likely to reproduce and pass the allele on
5. Over many generations, the advantageous allele increases in frequency in the population

Types of Natural Selection

Different selection pressures affect the type of natural selection. Two important patterns are directional and stabilising selection.

Directional Selection

Directional selection favours one extreme phenotype, shifting the population mean over generations.

Example — antibiotic resistance in bacteria:

• A random mutation gives some bacteria resistance to an antibiotic
• When antibiotics are present, bacteria without the mutation die; resistant bacteria survive and reproduce
• Over generations, the resistance allele increases in frequency — the population shifts towards resistance
• You can see how this is exploited in recombinant DNA technology in Unit 8

Stabilising Selection

Stabilising selection favours intermediate phenotypes and selects against both extremes. The mean stays the same, but variation narrows.

Example — human birth weight:

• Very low birth weight: higher infant mortality
• Very high birth weight: higher risk of complications during birth
• Intermediate birth weight: highest survival
• Individuals with extreme birth weights are less likely to survive and reproduce, so they are less likely to pass on their alleles — over generations, extreme birth weights decrease in frequency in the population

Adaptations

An adaptation is a feature that increases an organism’s chances of survival and reproduction in its environment. Adaptations don’t arise because an organism needs them. A random mutation happens to be beneficial, natural selection favours it, and over many generations it spreads through the population.

Anatomical Adaptations examples

• Large surface area of alveoli for efficient gas exchange
• Streamlined body shape in fish for reducing drag
• Xerophytic adaptations — e.g. sunken stomata, rolled leaves, and thick waxy cuticles in desert plants to reduce water loss

Physiological Adaptations examples

• Fetal haemoglobin has a higher oxygen affinity than adult haemoglobin, allowing efficient oxygen transfer from mother to fetus — see haemoglobin dissociation curves
• Desert mammals have a longer loop of Henle — this creates a steeper sodium ion gradient in the medulla, allowing more water to be reabsorbed and producing more concentrated urine — see the kidney
• Some bacteria produce enzymes that break down antibiotics

Behavioural Adaptations examples

• Nocturnal activity in desert animals to avoid daytime heat
• Migration to warmer climates in winter
• Courtship displays to attract mates

Required Practical 6 — Antimicrobial Substances

This practical links directly to directional selection and antibiotic resistance, the same evolutionary pressure you investigated theoretically above.

Aseptic Technique

Aseptic technique prevents contamination of the culture (by unwanted microorganisms)

Key steps:

• Sterilise all equipment before use — autoclave glassware and agar; flame the neck of the bottle before removing bacteria
• Work near a Bunsen flame — convection currents carry microorganisms away from the work area
• Incubate at 25°C — not 37°C (body temperature), to reduce the risk of culturing human pathogens

Method

1. Prepare a sterile agar plate and inoculate it evenly with a bacterial culture using a sterile spreader
2. Soak small paper discs in different antimicrobial substances (or different concentrations of the same substance)
3. Include a control disc soaked in distilled water (no antimicrobial)
4. Place discs on the agar surface, evenly spaced
5. Seal, invert, and incubate at 25°C for 24–48 hours
6. Measure the diameter (or calculate the area) of the inhibition zone around each disc

Interpreting Results

• Inhibition zone — the clear area around a disc where bacteria have not grown
• Larger zone = more effective antimicrobial substance
• No zone = bacteria are resistant (or the substance has no effect)
• Area of inhibition zone = π r²

Exam Questions