Natural Selection: Mechanisms and Fitness
How Natural Selection Works
Selection requires variation
Natural selection occurs when individuals differ in heritable traits and those differences cause differential survival or reproduction. Selection is a statistical process acting on phenotype, but its long-term effects are measured as changes in allele frequencies.
Fitness and components of fitness
Fitness is relative reproductive success, not strength, speed, or size in isolation. It may include viability, mating success, fecundity, and offspring survival. A trait can increase one component of fitness while decreasing another.
Fitness depends on context
A trait is only advantageous relative to a particular environment, time, and set of competing genotypes.
Types of selection
Directional selection
- Favors one extreme phenotype
- Shifts the population mean
Stabilizing selection
- Favors intermediate phenotypes
- Reduces variance
If medium-sized beaks are favored over very small and very large beaks, what pattern is this?
Favorable intermediate phenotypes indicate stabilizing selection.
Correct answer: Stabilizing selection
Why can selection be described as acting on phenotypes even though evolution is tracked genetically?
Phenotypes mediate selection; genes record its population-level consequences.
Correct answer: Because differences in survival and reproduction arise from observable traits, but the evolutionary outcome is a change in the frequencies of underlying alleles.
Fitness landscapes
A fitness landscape maps genotype or phenotype to reproductive success. Peaks represent high-fitness combinations, but populations may be constrained by mutation, genetic correlations, and local optima. Adaptive evolution can be imagined as movement across this landscape.
Which statement best captures stabilizing selection?
Stabilizing selection increases the frequency of intermediate phenotypes.
Correct answer: It favors intermediate values and reduces variation
Analyzing a selection scenario
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1
Identify the trait under selection.
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2
Determine which phenotypes leave more descendants.
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3
Check whether the trait is heritable.
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4
Predict the expected change in mean trait value and variance over time.