Difficulty: Introductory | Prerequisites: Hardy-Weinberg equilibrium notes, basic understanding of phenotypic variation.
Natural selection is the main mechanism of adaptive evolution, but it does not always push a population in one direction. Depending on which phenotypes have a fitness advantage, selection can shift the population mean, narrow variation around the mean, or split the population into two extremes. This unit looks at the different modes of selection and how each one reshapes the distribution of traits in a population. It builds directly on the Hardy-Weinberg framework: all of these modes represent violations of HWE.
Natural selection acts on phenotypic variation in a population, but the outcome depends on which phenotypes are favoured. Directional selection shifts the mean one way, stabilising selection tightens the distribution around the existing mean, disruptive selection favours both extremes, and frequency-dependent selection gives an advantage to whichever variant is rare.
Natural selection
Differential survival and reproduction among individuals due to differences in heritable traits. In simple terms, individuals with traits better suited to their environment leave more offspring, and those traits become more common.
Variation
Differences in phenotype among individuals in a population. Without variation, selection has nothing to act on.
Directional selection
A mode of selection where individuals at one extreme of the trait distribution have higher fitness. The population mean shifts towards that favoured extreme over time. Think of it as the bell curve sliding left or right.
Stabilising selection
A mode of selection where individuals near the mean of the trait distribution have higher fitness than those at either extreme. The mean stays roughly the same, but the spread (variance) of the distribution shrinks. Think of it as the bell curve getting taller and narrower.
Disruptive selection (diversifying selection)
A mode of selection where individuals at both extremes of the trait distribution have higher fitness than those in the middle. This can increase variance and, over time, may split the population into two distinct groups.
Frequency-dependent selection
A mode of selection where the fitness of a phenotype depends on how common it is in the population. Typically, rare phenotypes have an advantage (negative frequency dependence), which maintains variation in the population.
How selection reshapes trait distributions
All four modes start with a population that has heritable variation in some trait. The trait distribution is usually roughly bell-shaped. What differs is which part of that distribution is favoured.
Directional selection
One tail of the distribution has higher fitness.
The mean shifts toward the favoured extreme over generations.
Variance may decrease slightly as the disfavoured tail is trimmed.
Classic example: beak size in Galapagos finches shifting during drought, when only large, hard seeds are available.
Stabilising selection
Intermediate phenotypes have the highest fitness.
The mean stays roughly the same but the distribution narrows.
Extreme individuals on both sides are selected against.
Classic example: human birth weight. Very small and very large babies have lower survival rates; intermediate weight is optimal.
Disruptive selection
Both extremes have higher fitness than intermediate phenotypes.
The distribution becomes bimodal (two peaks), variance increases.
Can potentially lead to speciation if gene flow between the two groups is reduced.
Classic example: African seedcrackers, where birds with very large or very small beaks do well on the available seeds, but intermediate-beak birds do poorly.
Frequency-dependent selection
Fitness depends on how common a phenotype is, not on where it sits in the distribution.
Negative frequency dependence: rare phenotypes have an advantage. This maintains polymorphism because any variant that becomes rare gains a fitness boost.
Positive frequency dependence: common phenotypes have an advantage. This tends to eliminate rare variants.
Classic example: prey species with multiple colour morphs. Predators form a search image for the most common morph, giving rare morphs a survival advantage.
Antibiotic resistance in bacteria is a textbook case of directional selection: the drug removes susceptible individuals, and resistant ones proliferate. Stabilising selection explains why most physiological traits cluster around a species-typical value. Disruptive selection is invoked in models of ecological speciation, where different niches within the same habitat favour different phenotypes.
Students often assume natural selection always pushes in one direction. Stabilising and disruptive selection are just as important.
A common mistake is thinking disruptive selection immediately creates two species. It increases variance, but speciation requires reproductive isolation as well.
Students sometimes confuse frequency-dependent selection with directional selection. The key difference: in frequency-dependent selection, which phenotype is favoured changes as its frequency changes. In directional selection, the same extreme is always favoured.
Selection acts on phenotypes, not directly on genotypes. Students sometimes forget this distinction.
Be able to identify the mode of selection from a description of how a trait distribution changes (graph or verbal description).
Know the effect of each mode on the mean and the variance of the trait distribution.
Understand why frequency-dependent selection maintains polymorphism in a population.
Expect a question giving you before-and-after distributions and asking you to name the mode of selection.
True or false: Stabilising selection increases the variance of a trait distribution.
Answer: False. It decreases variance by selecting against extremes.
Fill in the blank: In ______ selection, both extremes of the distribution are favoured over the mean.
Answer: Disruptive (diversifying).
True or false: Frequency-dependent selection tends to eliminate rare alleles.
Answer: False (in the negative form). Negative frequency-dependent selection gives rare variants an advantage, maintaining them.
Fill in the blank: Directional selection causes the ______ of the trait distribution to shift.
Answer: Mean.
Q: A population of lizards has variation in body size. After a drought, the mean body size has shifted to be smaller. What mode of selection is most likely acting?
A: Directional selection. The mean has shifted in one direction (toward smaller body size), indicating that individuals at the smaller end of the distribution had higher fitness during the drought.
Q: You observe that a trait distribution has the same mean after selection but the variance has decreased. What mode of selection does this represent?
A: Stabilising selection. The mean is unchanged but the spread has narrowed, meaning intermediate phenotypes were favoured over extremes.
Q: Why does negative frequency-dependent selection maintain genetic variation in a population?
A: Because rare phenotypes have a fitness advantage over common ones. As a variant becomes rare, its fitness rises, preventing it from being lost. As it becomes common, its fitness drops. This balancing effect keeps multiple variants present in the population.
Modes of selection connect directly to Hardy-Weinberg: any form of selection violates HWE and changes genotype frequencies. Disruptive selection links forward to speciation, where divergent phenotypes may eventually become reproductively isolated. Sexual selection (covered in the next set of notes) is a specific form of natural selection that often acts directionally on traits related to mating.
Natural selection, directional selection, stabilising selection, stabilizing selection, disruptive selection, diversifying selection, frequency-dependent selection, trait distribution, bell curve, variance, mean shift, phenotypic variation, modes of selection, BBIO 180, intro biology