Evolution describes population change across generations
Biological evolution is change in the inherited characteristics of populations over generations. Individuals develop and respond during their lifetimes, but populations evolve because the frequencies of heritable variants change.
Natural selection is one mechanism of evolution. Mutation, genetic drift and gene flow also change allele frequencies, so evolution should not be reduced to the statement that organisms simply become 'better.' Fitness is relative to an environment and measured through reproductive contribution.
Lamarck proposed use, disuse and acquired inheritance
Lamarck argued that structures used frequently became stronger, structures not used deteriorated, and characteristics acquired during life could be inherited. His explanation was historically important because it treated species as changeable rather than fixed.
The central inheritance mechanism is not supported as a general explanation of adaptation. Changes such as enlarged muscles acquired through exercise do not rewrite germ-line alleles in a way that offspring inherit as the same developed trait.
| Question | Lamarckian explanation | Darwinian explanation |
|---|---|---|
| Source of useful trait | Need and use alter the individual | Heritable variation already exists or arises |
| What is inherited | Acquired characteristics | Inherited variants affecting phenotype |
| Where change is observed | Individual during life | Population across generations |
Natural selection requires four connected conditions
Individuals vary, and at least some variation is heritable. More offspring are produced than can all survive and reproduce, creating competition under environmental limits. Individuals with variants that improve reproductive success in those conditions contribute more alleles to the next generation.
Over many generations, advantageous heritable variants become more common. The environment does not create the needed mutation on demand; it filters existing and newly arising variation.
Individuals differ in heritable traits.
The environment affects survival and reproduction.
Some variants leave more offspring.
Associated alleles become more common.
Adaptation is environment-specific
A trait is adaptive when it increases reproductive success in a particular environment. The same trait may be neutral or harmful elsewhere. Antibiotic resistance, for example, can be strongly favoured when an antibiotic is present but may carry a cost when it is absent.
Resistance does not appear because bacteria consciously need it. Resistant variants may already exist through mutation or acquire resistance genes. Antibiotic exposure removes susceptible competitors, allowing resistant cells to contribute disproportionately to later generations.
Multiple evidence lines support common descent
Fossils document organisms from different times and transitional combinations of features. Homologous structures share an underlying plan because of common ancestry even when their current functions differ. Vestigial features are reduced remnants inherited from ancestors.
Biogeography explains why related species cluster geographically, while molecular comparisons reveal nested similarities in DNA and proteins. No single observation stands alone; independent evidence lines converge on branching descent with modification.
Time-ordered records and transitional combinations.
Homology and vestigial structures.
Geographic patterns of related organisms.
Shared DNA and protein sequences.
The ideas to carry forward
- Populations evolve; individuals are selected within a generation.
- Lamarck proposed acquired inheritance, while Darwin emphasised selection of heritable variation.
- Natural selection changes variant frequencies through differential reproduction.
- Fossil, anatomical, geographic and molecular evidence support common descent.
Answer first. Then reveal the marking logic.
01Why is antibiotic resistance not evidence that bacteria mutate because they need to survive?2 marks · show the biological link
Answer: Variation arises independently of need; antibiotic exposure selects resistant variants that already exist or were acquired.
02What is the difference between homologous and analogous structures?2 marks · show the biological link
Answer: Homologous structures share underlying ancestry even if functions differ; analogous structures perform similar functions but evolved independently.
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