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(a) Acts on heritable variation = TRUE ✓
(b) "Survival of fittest ONLY" = FALSE ✗ (differential reproduction, not just survival)
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(c) Drives evolutionary change = TRUE ✓
(d) Acts on phenotype = TRUE ✓
Answer: (a), (c) and (d) only
Natural selection: acts on HERITABLE VARIATION, acts on PHENOTYPE, DRIVES evolution
NOT "survival only" — it is DIFFERENTIAL REPRODUCTION (reproductive success matters)
1. Darwin's Theory of Evolution
Darwin proposed four postulates: Variation: individuals differ in heritable traits. Heritability: traits pass from parents to offspring. Differential reproduction: some individuals reproduce more. Fitness: traits associated with higher reproduction increase in frequency. Evolution by natural selection = change in allele frequencies through differential reproduction of heritable phenotypic variants.
2. Fitness — True Meaning
Evolutionary fitness = relative reproductive success = contribution to next generation's gene pool. NOT physical fitness. A bacterium resistant to antibiotics reproducing in presence of antibiotics = maximally "fit" in that environment. Modern definition: Fitness (W) = number of offspring surviving to reproductive age. Relative fitness: fitness compared to most successful genotype in population.
3. Evidence for Natural Selection
Industrial melanism (Biston betularia, peppered moth): pre-industrial → light moths common (hidden on lichen-covered bark). Post-industrial → bark darkened by soot → dark moths selected → frequency increased. After clean air acts → light moths recovered. Antibiotic resistance: rapid, observable natural selection over days-weeks. Drug-resistant tuberculosis, MRSA: selection for resistant variants by antibiotic treatment.
4. Limits of Natural Selection
Natural selection cannot: produce variation (it only selects from existing variation). Create perfectly adapted organisms (historical constraints, genetic limitations). Produce purposeful/directed evolution (it has no foresight). Act instantly (needs many generations). For example, humans still have vestigial structures (coccyx, wisdom teeth) because there has not been sufficient selection pressure to eliminate them.
Frequently Asked Questions
1. What is natural selection? ⌄
Natural selection: Darwin's mechanism for evolution. Requirements: Variation exists in the population. Variation is heritable (genetic). Some variants reproduce more successfully (differential reproduction). Result: heritable traits conferring greater reproductive success increase in frequency over generations. Key point: selection acts on PHENOTYPE, not genotype directly. The genotypic component of phenotypic variation that is heritable gets transmitted.
2. Why is "survival of the fittest" misleading? ⌄
Herbert Spencer coined "survival of the fittest" — often misunderstood. "Fitness" in biology means reproductive success (number of viable offspring), NOT physical strength or endurance. A delicate orchid that produces many seeds is more "fit" than a lion that has few cubs. Natural selection = DIFFERENTIAL REPRODUCTION — organisms leaving more descendants are more evolutionarily fit, regardless of how long they individually survive.
3. How does natural selection act on phenotype? ⌄
Natural selection directly acts on the phenotype (visible/measurable traits). Example: dark-coloured moths are more visible to predators on light tree bark → more likely to be eaten → lower survival and reproduction → fewer offspring. Light moths less visible → more reproduce → frequency increases. The phenotype (colour) is directly selected, and since colour is genetically determined (heritable), the underlying alleles change in frequency.
4. What are the different types of natural selection? ⌄
Directional selection: favours one extreme phenotype (allele frequency shifts in one direction). Example: antibiotic resistance. Stabilising selection: favours intermediate phenotype, reduces variation. Example: human birth weight. Disruptive selection: favours both extremes, reduces intermediate. Can lead to speciation. Sexual selection: differential mating success based on mate preference or competition. Example: peacock tail.
5. What are the agents of evolution besides natural selection? ⌄
Other evolutionary forces: Mutation: source of new variation. Genetic drift: random allele frequency changes (more significant in small populations). Gene flow: migration of individuals between populations transfers alleles. Non-random mating: assortative mating changes genotype frequencies. Natural selection is the only DIRECTIONAL force that consistently produces adaptation. Others are mostly random or neutral.