Selection
Natural selection is the mechanism by which populations evolve over time. It relies on genetic variation within a population, meaning individuals have different alleles (gene versions) and thus different characteristics.
The process follows these logical steps:
- Overproduction: Organisms produce many more offspring than can possibly survive.
- Struggle for Survival: Because resources (food, space, mates) are limited, there is competition between individuals. This creates a 'struggle for survival'.
- Survival of the Fittest: Individuals with characteristics better suited to the environment (adaptations) are more likely to survive and reproduce.
- Inheritance: These surviving individuals pass their advantageous alleles to the next generation.
- Change in Population: Over many generations, the frequency of these beneficial alleles increases, making the population better adapted to its environment.
It is not just a single trait, but the evolutionary change in the population's characteristics. For example, if a population of beetles has green and brown variants, and birds eat the green ones, the process of adaptation results in a population that is predominantly brown.
This is a classic example of natural selection:
- Variation: In a large population of bacteria, random mutations create genetic variation. A few bacteria may have an allele that makes them resistant to a specific antibiotic.
- Selection Pressure: The antibiotic is applied. This kills the non-resistant bacteria.
- Survival: The resistant bacteria survive because they are better adapted to this environment.
- Reproduction: The surviving resistant bacteria reproduce rapidly, passing the resistance allele to their offspring.
- Result: The next generation consists almost entirely of antibiotic-resistant bacteria. The strain has evolved.
Correction: Mutations occur randomly before exposure. The antibiotic acts as a selection pressure that kills non-resistant bacteria, allowing the pre-existing resistant ones to survive. The antibiotic does not cause the mutation; it selects for it.
Mistake: 'Artificial selection is used to develop antibiotic resistance.'
Correction: Antibiotic resistance develops through natural selection. Artificial selection involves humans choosing traits.
Why examiners accept this: Examiners look for the link between variation, survival, and inheritance. You must mention that variation exists before selection occurs.
Correct phrasing example: 'Random mutations create genetic variation. Individuals with alleles conferring resistance survive the antibiotic treatment and reproduce, passing these alleles to the next generation.'
Key markscheme phrase: 'Survival of the fittest' or 'Differential survival and reproduction based on phenotype.'
Selective breeding (or artificial selection) is where humans choose individuals with desirable features to breed together.
The steps are:
- Selection: Humans select individuals with the desired trait (e.g., high milk yield in cows).
- Crossing: These selected individuals are bred together.
- Re-selection: Offspring are examined. Those showing the desirable feature are selected to breed in the next generation.
- Repetition: This process is repeated over many generations until the population consistently shows the trait.
A farmer wants wheat that flowers early to avoid frost.
- Plant many wheat seeds.
- Identify plants that flower earliest.
- Collect pollen/seeds from these specific plants.
- Cross-pollinate them to produce the next generation.
- Repeat: Select the earliest flowering offspring from this new generation and breed them.
- After several generations, a pure-breeding strain of early-flowering wheat is produced.
Correction: In artificial selection, humans choose which individuals breed based on desired traits. In natural selection, the environment (predators, climate, etc.) determines survival.
Mistake: 'Artificial selection increases genetic variation.'
Correction: Artificial selection often decreases genetic variation because only a few specific individuals are used to breed, leading to inbreeding.
Why examiners accept this: You must show the cycle of selection and breeding. Simply saying 'humans pick good animals' is insufficient.
Correct phrasing example: 'Select individuals with the desired trait, breed them together, select offspring with the trait, and repeat over many generations.'
Key markscheme phrase: 'Repeat the process over several generations' or 'Cross-pollinate selected plants.'
| Feature | Artificial Selection |
|---|---|
| Selector | |
| Purpose/Goal | |
| Speed | |
| Genetic Variation |
- Random mutations occur in the bacterial population, creating genetic variation.
- Some bacteria have alleles that make them resistant to the antibiotic.
- When the antibiotic is used, non-resistant bacteria die (struggle for survival).
- Resistant bacteria survive and reproduce.
- They pass the resistance alleles to offspring, so the next generation is mostly resistant.
- Select cows with the highest milk yield.
- Breed these selected cows together.
- Select offspring with high milk yield and repeat the process over many generations.
- In natural selection, the environment selects; in artificial selection, humans select.
- Natural selection favors traits for survival; artificial selection favors traits desirable to humans.
- Artificial selection often results in less genetic variation than natural selection.