3.4.4 Genetic diversity and adaptation

Natural selection

-An allele is a variant form of a gene. Different alleles can result in different observable traits.

-Genetic diversity is the total number of different alleles in a population. This leads to variation as different combinations of alleles mean different proteins are produced, resulting in different characteristics.

-Genetic diversity influences natural selection as organisms with advantageous traits are more likely to survive, reproduce, and pass on their alleles, influencing the gene pool. 

-The more successful an organism is at reproducing, the more likely its alleles will be passed on to the next generation, increasing the allele frequency.

Process of natural selection:

1. There is variation in characteristics which results in new alleles within a species due a selection pressure

2. More genetic variation emerges within a population due to random mutations

3. Individuals with alleles that code for traits that are advantageous for survival are more likely to reproduce.

4. These advantageous alleles are passed down to offspring.

5. Over time, the frequency of this allele in a population increases

There are two types of natural selection:

• Directional- favours extreme phenotype, eg. antibiotic resistance 

• Stabilising selection- favours the average phenotypes 

• disruptive selection: favours both extreme phenotypes simultaneously while selecting against the intermediate (average) phenotype.

Directional selection- antibiotic resistance:

• Some bacteria develop random mutations that provide resistance to antibiotics.

• When antibiotics (a selection pressure) are used, only the resistant with an advantageous allele bacteria survive, while the others die off.

• The resistant bacteria reproduce, passing on resistant alleles to their offspring.

• Over time, the frequency of resistant alleles increases, leading to mostly resistant bacteria.

-This resistance complicates treatment options, increases healthcare costs, and raises mortality rates in humans.

Classic AQA Example: Antibiotic resistance in bacteria. When an antibiotic is introduced (environmental change), the rare bacteria carrying a random mutation for resistance survive and reproduce. Over generations, the allele frequency shifts entirely toward resistance. Another common example is the Peppered Moth changing from light to dark during the Industrial Revolution.

Stabilising and disruptive selection

Stabilising

• What it is: This type of selection occurs in a stable, unchanging environment. It favours the average (intermediate) phenotype and selects against both extreme phenotypes.

• Effect on the graph: The mean/modal phenotype remains in the exact same position, but the normal distribution curve becomes narrower and taller. The standard deviation and overall genetic variation decrease.

• Classic AQA Example: Human birth weight. Babies born too small have a lower survival rate due to health complications, and babies born too large face delivery complications. Therefore, babies of an intermediate/average weight (around 3–4 kg) have the highest survival advantage.

Disruptive

• What it is: This occurs in environments with fluctuating or diverse conditions. It favours both extreme phenotypes simultaneously while selecting against the intermediate (average) phenotype.

• Effect on the graph: The single bell curve splits into a bimodal curve (a curve with two peaks at either end). This introduces polymorphism (the stable coexistence of distinct phenotypes) and is crucial because it can eventually lead to speciation (the evolution of new species).

• Classic AQA Example: Galapagos finch beak sizes. In environments with only very large, hard seeds or very small, soft seeds, birds with large beaks or small beaks excel. Birds with medium-sized beaks are inefficient at eating both types of food and are selected against.

Adaptations

-Adaptations are inherited characteristics that enhance an organism's ability to survive and reproduce in its specific environment.

The three adaptation categories are:

• Anatomical - Physical structures, both internal and external.

• Behavioural - Activities and responses, whether inherited or learned.

• Physiological - Internal biological functions.

Anatomical adaptations

Anatomical adaptations are the physical attributes that aid an organism's survival.

• Body coverings - Fur, feathers, scales protect organisms and support their survival.

• Camouflage - Organisms with colouration that blends into their environment are less likely to be detected.

• Mimicry - Some species imitate the appearance of more dangerous ones for protection.

• Teeth - The shape of an animal's teeth are adapted to its diet.

Behavioural adaptations:

Behavioural adaptations are actions that increase an organism's chances of survival and reproduction.

• Defensive responses - For example, opossums play dead and rabbits freeze to avoid detection by predators.

• Courtship displays - For example, male scorpions dance to attract mates.

• Seasonal actions - For example, migration helps birds access resources year-round and hibernation helps bears conserve energy when food is scarce.

Physiological adaptations:

Physiological adaptations are internal processes that give organisms an edge in survival.

• Venom - Used by snakes to immobilise prey and by plants to deter herbivores.

• Antibiotics - Produced by bacteria to outcompete rival species.

• Water storage - Desert frogs can survive for over a year without a water source by storing water in their bodies.