Genetic Diversity & Natural Selection (AQA A-Level Biology): Revision Notes
Genetic Diversity & Natural Selection
What is genetic diversity?
Genetic diversity refers to the number of different alleles present within a species or population. Remember that alleles are different versions of the same gene, and having many different alleles creates high genetic diversity.
This diversity forms the foundation for natural selection - without genetic variation, populations cannot adapt to changing environments or evolve over time.
Genetic diversity is essentially the "raw material" that evolution needs to work with. Think of it like having more colours on an artist's palette - the more colours available, the more varied and adaptable the final artwork can be.
Factors that increase genetic diversity
Genetic diversity within a population can be increased through two main processes:
Mutations
Mutations occur randomly in DNA and can create entirely new alleles. When these mutations happen in reproductive cells, they can be passed to offspring, adding fresh genetic variation to the population.
Gene flow
Gene flow occurs when individuals from different populations migrate and interbreed. This introduces different alleles from other populations, increasing the overall genetic diversity of the receiving population.
Gene flow acts like a genetic "mixing pot" - when individuals from different populations breed, they combine their genetic material, creating offspring with new combinations of alleles that weren't present in either original population.
Factors that reduce genetic diversity
Genetic bottlenecks
A genetic bottleneck happens when a population experiences a dramatic reduction in size, causing many individuals to die before they can reproduce. This severely reduces the gene pool - the total collection of alleles available in the population.
Key consequences of genetic bottlenecks:
- Fewer different alleles remain in the population
- Genetic diversity becomes much lower
- Survivors reproduce to rebuild the population, but from a limited genetic base
- Recovery in population size does NOT mean recovery in genetic diversity
Real-World Example: Northern Elephant Seals
Northern elephant seals were hunted extensively in the 1800s, reducing their population to around 50 individuals. Although they have since recovered to approximately 170,000 seals, their genetic diversity remains much lower than southern elephant seals that never experienced such severe population reduction.
This shows that even when populations recover in numbers, the genetic diversity lost during a bottleneck may never be fully restored.
The founder effect
The founder effect is a specific type of genetic bottleneck that occurs when a small group of individuals establishes a new colony in a different location. This new population starts with only a limited selection of alleles from the original population.
Key characteristics:
- Allele frequencies in the new colony may differ significantly from the original population
- Rare alleles from the original population might become common in the new colony
- Higher risk of genetic diseases due to limited genetic diversity
Real-World Example: The Amish Population
The Amish population in North America descended from a small group of Swiss migrants. Their isolation and limited interbreeding with outside populations has resulted in little genetic diversity and higher rates of certain genetic disorders.
This demonstrates how founder effects can persist for many generations, especially when populations remain relatively isolated.
How natural selection increases advantageous alleles
Natural selection is the process through which beneficial alleles become more common in populations over time. Here's how it works:
- Random mutations occasionally produce new alleles that provide survival or reproductive advantages
- Individuals carrying these beneficial alleles are more likely to survive in their environment
- These successful individuals have greater reproductive success - they produce more offspring
- The advantageous alleles are passed on to the next generation more frequently
- Over multiple generations, the frequency of beneficial alleles increases throughout the population
This process drives evolution - the gradual change in species characteristics over time, leading to the enormous diversity of life on Earth.
Natural selection doesn't create new alleles - mutations do that. Natural selection simply determines which existing alleles become more or less common in the population based on how well they help organisms survive and reproduce.
Types of adaptations
Adaptations are characteristics that help organisms survive and reproduce in their specific environments. There are three main categories:
- Behavioural adaptations
- Physiological adaptations
- Anatomical adaptations
Behavioural adaptations
These involve changes in how organisms act to improve survival chances.
Example: Possum Defence Behavior
Possums may "play dead" when threatened by predators, giving them an opportunity to escape when the predator loses interest. This behavioural adaptation increases their chances of survival without requiring any physical changes to their body.
Physiological adaptations
These are internal processes that enhance survival.
Example: Bear Hibernation
Brown bears provide an excellent example - they hibernate during winter months, dramatically slowing their metabolism to conserve energy when food is scarce and temperatures are harsh. This internal process allows them to survive months without eating.
Anatomical adaptations
These are structural features of an organism's body that improve survival.
Example: Whale Insulation
Whales have evolved a thick layer of blubber (fat) that provides insulation, helping them maintain body temperature in cold ocean waters. This physical structure is essential for their survival in marine environments.
Key Points to Remember:
- Genetic diversity measures the number of different alleles in a population - more alleles means higher diversity
- Mutations and gene flow increase genetic diversity, while genetic bottlenecks and the founder effect reduce it
- Natural selection works because individuals with advantageous traits survive and reproduce more successfully
- Adaptations can be behavioural (how organisms act), physiological (internal processes), or anatomical (body structures)
- Genetic diversity provides the raw material for natural selection and evolution to occur