What is Natural Selection?
Natural selection is Darwin's mechanism of evolution: individuals with traits better suited to their environment survive and reproduce more successfully, passing those traits on. Over generations, this shifts the traits — and allele frequencies — of an entire population.
Natural selection is the process by which organisms with favorable heritable traits have higher relative fitness, so those traits become more common in a population over successive generations.
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Step-by-step worked examples
In a population, individuals with genotype AA produce an average of 4 offspring, while the fittest genotype BB produces 8 offspring on average. What is the relative fitness of AA?
Relative fitness w = individual's offspring ÷ max offspring w(AA) = 4 / 8 w(AA) = 0.5
A moth form has relative fitness w = 0.7. What is its selection coefficient s?
Selection coefficient s = 1 − w s = 1 − 0.7 s = 0.3 (a 30% reduction in reproductive success versus the fittest form)
Before industrial pollution, the dark allele frequency in a moth population was 0.05. After 50 years of selection favoring dark moths on soot-covered trees, it rose to 0.95. How many percentage points did the frequency change?
Change = final frequency − initial frequency Change = 0.95 − 0.05 Change = 0.90 → a 90 percentage-point increase
Flashcards
Quick quiz
Q1.What does 'relative fitness' measure?
Q2.Which of these is NOT a requirement for natural selection?
Q3.'Survival of the fittest' really means an organism that...
Q4.A genotype has w = 0.4. What is its selection coefficient s?
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Common mistakes
Thinking 'fittest' means physically strongest. — Correct: In evolution, 'fit' means most successful at reproducing and passing on genes, not physical strength.
Believing individuals evolve during their lifetime. — Correct: Individuals don't evolve — populations do, over many generations, as allele frequencies shift.
Assuming natural selection has a goal or direction (e.g., 'toward perfection'). — Correct: Natural selection has no foresight or goal — it simply favors whatever traits currently improve survival/reproduction.
Confusing natural selection with Lamarckism (traits acquired in life are inherited). — Correct: Only heritable genetic variation present at birth can be acted on by natural selection, not traits acquired during life.
FAQ
What is natural selection?
Natural selection is the process by which organisms better adapted to their environment survive and reproduce more successfully, passing their traits to the next generation.
What is the formula for natural selection (fitness)?
Relative fitness is w = Wi/Wmax, comparing an individual's offspring number to that of the fittest genotype in the population.
What are examples of natural selection?
The peppered moth's color shift during industrialization, antibiotic resistance in bacteria, and Darwin's finch beak sizes are classic examples.
How do you calculate natural selection's effect on fitness?
Divide an individual's (or genotype's) average offspring count by the highest offspring count in the population to get relative fitness w.