Table of Contents
Adaptive selection is the component of evolution by which certain heritable variants become more common because they increase an organism’s chances of surviving and reproducing in a particular environment. Within the synthetic theory of evolution, it is the directed, non-random filter that acts on the random variation created by mutation and recombination.
This chapter focuses on how adaptive selection works, how it can be classified, and what its typical consequences are for populations.
What Natural Selection Acts On
Selection does not act on genes in isolation, but on whole organisms (their phenotypes) in specific environments.
Key points:
- Target of selection:
- Immediate: the phenotype (appearance, physiology, behavior).
- Underlying: the genotype (alleles) that influence that phenotype.
- Prerequisites (already introduced at higher levels, but summarized here for context):
- There is heritable variation in traits.
- More offspring are produced than can survive (competition).
- Individuals differ in fitness (reproductive success).
In population genetics terms, an allele’s success is often described by its relative fitness $w$ and its selection coefficient $s$.
- If $w_A = 1$ for a reference allele $A$, and $w_B = 1 - s$ for allele $B$, then:
- $s > 0$ means $B$ is at a disadvantage (selected against).
- $s < 0$ means $B$ is at an advantage (selected for, relative to $A$).
Adaptive selection thus systematically changes allele frequencies across generations.
Types of Adaptive Selection Based on Direction
Depending on how selection treats individuals with different trait values, we distinguish several main forms.
1. Directional Selection
Definition: Favors individuals at one extreme of the trait distribution, shifting the population mean in that direction.
- Environment consistently rewards higher or lower values of a trait.
- The frequency of alleles associated with the favored extreme increases.
Consequences:
- The population’s average trait value changes generation by generation.
- Can lead to rapid adaptation to changing conditions.
- Genetic variation for that trait may decrease if one set of alleles predominates.
Examples (conceptual):
- Evolution of antibiotic resistance in bacteria when exposed to a drug.
- Increase in melanin pigmentation in regions with strong UV radiation.
- Larger body size in a colder climate (Bergmann-like patterns), when that is advantageous.
Directional selection is central to many cases of rapid, observable evolution.
2. Stabilizing Selection
Definition: Favors individuals with intermediate trait values and selects against extremes.
- The “middle” phenotype has higher fitness.
- The population’s average trait value remains relatively constant.
Consequences:
- Preserves the current adaptation when the environment is stable.
- Reduces phenotypic variation; extreme phenotypes become rarer.
- Maintains traits within a relatively narrow range (canalization).
Examples (conceptual):
- Human birth weight: very low and very high weights are associated with higher mortality, intermediate weights with higher survival.
- Wing size in a flying insect where both too small and too large wings reduce flight efficiency.
Stabilizing selection is probably the most common form in long-established, relatively constant environments.
3. Disruptive (Diversifying) Selection
Definition: Favors individuals at both extremes of the trait distribution and selects against intermediates.
- Two or more distinct phenotypic “peaks” have higher fitness than the average phenotype.
- The population’s mean may stay similar, but the distribution becomes bimodal or multimodal.
Consequences:
- Increases phenotypic variation in the population.
- Can promote polymorphisms (coexistence of distinct forms).
- In combination with isolation, can contribute to speciation (link to “Isolation and Speciation”).
Examples (conceptual):
- A bird population where small-billed individuals specialize on small seeds and large-billed individuals on hard, large seeds, while intermediate bills are inefficient for both.
- Color morphs in a habitat where camouflage is optimal for very light and very dark individuals, but intermediate shades are easily detected by predators.
Disruptive selection is particularly important for understanding how diversity within populations can be maintained or even enhanced.
Types of Adaptive Selection Based on Fitness Relationships of Alleles
Another way to classify adaptive selection is by how alleles relate to each other in terms of fitness and dominance.
1. Positive Selection
Definition: Selection that increases the frequency of an allele because it confers a fitness advantage.
- Often acts on a new beneficial mutation or on an allele that becomes advantageous after an environmental change.
Consequences:
- A selective sweep can occur: the beneficial allele rises rapidly in frequency, often carrying along nearby DNA variants (genetic hitchhiking).
- Genetic diversity in the affected genomic region can become reduced temporarily.
This is the form of selection most often discussed when speaking of an allele being “favored by natural selection.”
2. Purifying (Negative) Selection
Definition: Selection that removes deleterious (harmful) alleles from the population.
- Maintains the functional integrity of proteins and developmental pathways.
- Continuously “cleans up” harmful variants introduced by mutation.
Consequences:
- Most new strongly deleterious mutations remain rare or are eliminated.
- Highly conserved genes and amino acid sites (little variation) often show strong purifying selection.
Purifying selection is a persistent force preventing accumulation of damaging changes.
3. Balancing Selection
Definition: Selection that maintains two or more alleles in a population over long periods.
Balance is achieved because heterozygotes or context-dependent advantages keep multiple alleles circulating.
Main mechanisms:
- Heterozygote advantage (overdominance):
Heterozygotes have higher fitness than either homozygote. - If $w_{AA} < w_{Aa}$ and $w_{aa} < w_{Aa}$, allele $A$ and $a$ can both be maintained.
- This leads to a stable equilibrium frequency of the alleles.
- Frequency-dependent selection:
The fitness of an allele depends on how common it is. - Negative frequency dependence: rare forms are favored (e.g., rare morphs are less recognized by predators).
- Positive frequency dependence: common forms are favored; this tends to reduce variation and is not balancing in the long term.
- Spatial or temporal variation in selection:
Different alleles are favored in different places or at different times, but gene flow or time-averaging maintains both.
Balancing selection is important for explaining long-lasting genetic polymorphisms, such as numerous alleles at immune-system genes.
Levels and Targets of Adaptive Selection
Selection can operate on different biological levels, although the primary level in the synthetic theory is the individual.
1. Individual-Level Selection (Central in the Synthetic Theory)
- Acts on differences in survival and reproduction among individuals.
- Maximizes the number of copies of alleles passed on through offspring.
Fitness includes both:
- Viability (survival to reproductive age).
- Fecundity (number of offspring produced, or their reproductive success).
2. Kin Selection
While still realized through individuals, kin selection emphasizes that genes can increase in frequency by helping related individuals who share those genes.
- Explains traits like altruistic behavior directed toward relatives.
- Uses the concept of inclusive fitness: an individual’s genetic success includes its own offspring plus the additional offspring of relatives gained through its help.
Hamilton’s rule (qualitative here) states that altruism can evolve when:
$$
r \cdot B > C
$$
where:
- $r$ = relatedness between actor and recipient,
- $B$ = benefit to the recipient,
- $C$ = cost to the actor.
3. Group and Species-Level Perspectives
Classic synthetic theory is cautious about group selection, but it is sometimes conceptually useful:
- Group selection: selection among groups, where groups with more cooperative or well-adapted members may leave more descendant groups.
- This is generally weaker and slower than individual-level selection and controversial in its importance for most traits.
Modern treatments often reframe many apparent “group-level” phenomena in terms of kin selection and individual selection in structured populations.
Adaptive Landscapes and Local vs. Global Optima
Adaptive selection can be visualized using the idea of an adaptive landscape:
- Each genotype or phenotype corresponds to a point on a surface.
- The “height” of the surface corresponds to fitness.
Under adaptive selection:
- Populations tend to “climb” up slopes toward local fitness peaks.
- A population can become trapped on a local optimum even if a higher global optimum exists, because moving across a “fitness valley” would involve intermediate states of lower fitness.
This helps explain why:
- Evolution does not produce perfect organisms; it is constrained by existing variation and history.
- Different populations can end up at different adaptive peaks in different environments.
Interaction of Adaptive Selection with Other Evolutionary Factors
Adaptive selection is only one evolutionary factor; its effects must be seen in combination with others considered in the synthetic theory.
- Mutation and recombination: create the variation that selection can act upon.
- Genetic drift: can oppose or override selection when populations are small or when selection coefficients are very small.
- Gene flow (migration) (addressed elsewhere): can introduce alleles that selection might then favor or eliminate.
- Isolation and speciation: patterns of selection in separated populations are key to how new species arise.
The relative strength of adaptive selection vs. these other forces determines the evolutionary trajectory of populations.
Detecting Adaptive Selection
Although the technical methods are not treated in detail here, it is useful to note that biologists infer adaptive selection using:
- Comparative studies: similar adaptations appearing in unrelated lineages (convergent evolution) suggest strong, repeated selection pressures.
- Population genetic signatures:
- Reduced genetic diversity around a locus can indicate a recent selective sweep.
- Excess of intermediate-frequency variants can suggest balancing selection.
- Experimental evolution: populations are observed under controlled, strong selection pressures (e.g., microbes adapting to a new environment).
These approaches help distinguish adaptive selection from random changes such as genetic drift.
Summary
- Adaptive selection is the systematic, non-random change in allele frequencies caused by differences in reproductive success among phenotypes.
- It takes several principal forms: directional, stabilizing, and disruptive selection, each with characteristic effects on trait distributions and genetic variation.
- At the genetic level, positive, purifying, and balancing selection describe how alleles are favored, eliminated, or maintained.
- Selection mostly acts at the individual level, but kin selection and population structure can produce more complex patterns, including altruistic behavior.
- Populations tend to climb toward local fitness optima, and historical constraints mean evolution does not reach a single perfect solution.
- Adaptive selection interacts continually with mutation, recombination, drift, and isolation, shaping the diversity and adaptation of life observed today.