Natural Selection in Pocket Mice
Natural selection is the process by which individuals with heritable traits that suit their environment survive and reproduce more than others. Populations contain variation, and selection pressures (e.g., predators against a given background) act on that variation. When a trait is advantageous, the allele(s) for that trait tend to increase in frequency. A generation-to-generation change in allele frequency is microevolution.
How the interactive shows it
- Move the slider to set the habitat (light sand ↔ dark basalt; middle = patchy).
- Predators are more likely to pick off mice that stand out; better-camouflaged survivors have pups nearby.
- p = the proportion of the dark-fur allele (A) in the population (0–1); it updates each generation.
- In a uniform habitat, the better-hidden phenotype tends to increase. In a patchy habitat, dark mice build up on dark bands and light mice on light bands, helping maintain variation.
Explore
- What happens to p over time in light, dark, and patchy settings?
- When do both fur types persist the longest, and why?
This interactive focuses on natural selection. Other forces that change allele frequencies are outlined below.
Key Concepts
Causes of Changing Allele Frequencies
- Environmental selection pressures – e.g., climate, predators, competition, disease.
- Genetic drift – random changes in allele frequency, more pronounced in small populations.
- Bottleneck effect – drastic population reduction changes allele frequencies.
- Founder effect – a small group colonises a new area, carrying a limited gene pool.
- Gene flow – movement of alleles between populations via migration.
- Mutations – the source of new alleles, providing variation for selection to act upon:
- Point mutations – single-nucleotide change (insertion, deletion, substitution).
- Block mutations – changes to a chromosome segment (duplication, deletion, inversion, translocation).
- Frameshift mutations – insertions/deletions not in multiples of three, shifting the reading frame.
Biological Consequences
- Increased genetic diversity – greater adaptability, improved survival.
- Decreased genetic diversity – higher disease risk, reduced adaptability.
Manipulation of Gene Pools
- Selective breeding – choosing organisms with desirable traits to reproduce, altering allele frequencies.
- Benefits: improved yield, specific traits.
- Risks: reduced genetic diversity, inbreeding problems.
Pathogen Evolution
- Bacterial resistance – random mutations arise; with antibiotic use, resistant bacteria survive and spread.
- Viral antigenic drift – gradual mutations in genes for surface antigens (e.g., seasonal flu) → vaccines need updating.
- Viral antigenic shift – major change when a host is co-infected by different viruses; genomes mix to produce a new hybrid (e.g., some pandemic influenza strains).
- Consequence: These processes let pathogens evade immune detection, making treatment and vaccination an ongoing challenge.
Misconceptions to avoid
- Individuals don’t evolve; populations do.
- Selection isn’t goal-directed. It favours traits that work now in this environment.
- Selection acts on phenotypes; allele frequencies change as a consequence.
Don’t mix up drift and gene flow.
- Genetic drift = random changes in allele frequency within a population (stronger in small populations).
- Gene flow = alleles moving between populations via migration, which tends to make populations more alike.
- Tip to remember: Drift = Random; Flow = Go between.
