Evolution by Natural Selection: Core Concepts
Natural selection is a non-random process where individuals with heritable, advantageous traits experience higher reproductive success, causing allele frequencies in a population to shift over time.
Core Principles
- Natural selection acts on individuals, but only populations evolve.
- Evolution is not goal-directed; mutations are random.
- Evolution is not progressive; complex traits can be lost.
- Not all traits are adaptive (e.g., vestigial traits, silent mutations).
- Adaptations are often compromises (fitness trade-offs).
- Evolution is constrained by history, genetics, and environment.
Action Steps
- Identify variation in traits within a population.
- Determine if these traits are heritable.
- Assess if there is a struggle for existence (limited resources).
- Observe if individuals with specific traits have higher reproductive success.
- Confirm that allele frequencies change in the population over generations.
Key Terms
- Natural Selection (Darwin's Postulates 1-4): 1. Variation, 2. Heritability, 3. Overproduction/Competition, 4. Differential Reproductive Success (Fitness). The logical outcome is evolution (change in allele frequencies).:
- Biological Fitness: The ability of an individual to produce surviving, fertile offspring relative to others in the population.:
- Adaptation: A heritable trait that increases an individual's fitness in a specific environment.:
- Acclimatization: A non-heritable change in an individual's phenotype in response to environmental shifts; does not affect genotype.:
- Fitness Trade-off: A compromise between traits (e.g., rapid growth vs. long lifespan) where optimizing one reduces the performance of another.:
- Polygenic Trait: A characteristic influenced by many genes, each with a small effect (e.g., beak depth).:
Pitfalls to Avoid
- Confusing acclimatization with adaptation.
- Assuming evolution always leads to increased complexity.
- Thinking that natural selection acts on the population directly rather than on individuals.
- Ignoring genetic and historical constraints on trait evolution.
- Assuming all traits are adaptive.
Myth vs Reality
- Individuals evolve during their lifetime.: Individuals are selected; only populations evolve over generations.
- Evolution is goal-directed to create 'better' or more complex organisms.: Evolution is not progressive; it is a response to current environmental pressures.
- Organisms develop adaptations because they need them.: Adaptations arise from random mutations and are sorted by selection.
- Lamarckian inheritance (acquired traits are passed on).: Only heritable traits (genotype) are passed to offspring; acquired changes (acclimatization) are not.
People
- Charles Darwin: Proposed the theory of evolution by natural selection.
- Alfred Russel Wallace: Independently arrived at the concept of natural selection.
- Thomas Robert Malthus: Author of the 'struggle for existence' concept regarding population growth.
- Peter and Rosemary Grant: Conducted long-term research on Galápagos finches documenting natural selection in real-time.
Quiz
- A population of rabbits faces food scarcity. What is the most likely outcome according to natural selection theory? (Refer to Page 24 for context). Answer: C) There will be competition for the food and some rabbits will not survive to reproduce.:
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