Types of Selection (AQA A-Level Biology): Revision Notes
Types of Selection
Natural selection operates in different ways depending on environmental conditions and the characteristics being selected. There are three main types of selection that can affect populations: stabilising selection, directional selection, and disruptive selection. Each type produces distinct patterns in population characteristics and drives evolution in different directions.
Understanding these different types of selection is crucial for comprehending how populations adapt to environmental changes and how evolutionary processes shape the diversity of life on Earth.
Overview of selection types
Environmental factors create variation within populations, and these factors can act as agents for evolutionary change. The type of selection pressure exerted determines how the characteristics of a population will change over time.
Critical Concept: The same characteristic can be subject to different types of selection depending on environmental conditions. A trait that is maintained by stabilising selection in one environment may face directional selection when conditions change.
The three main types affect population characteristics as follows:
- Stabilising selection maintains the average phenotype by favouring individuals with characteristics close to the population mean
- Directional selection shifts the population mean by favouring phenotypes at one extreme of the range
- Disruptive selection favours individuals at both extremes of the phenotype range rather than those with intermediate characteristics
Stabilising selection
Stabilising selection works to eliminate extreme variations within a population, preserving characteristics close to the average. This type of selection reduces the phenotype range and limits the capacity for evolutionary change.
When stabilising selection occurs
This form of selection typically operates when environmental conditions remain constant over extended periods. The existing average characteristics represent the optimum for survival and reproduction under these stable conditions.
Mechanism and effects
In stabilising selection, individuals with extreme phenotypes face selection pressure that reduces their survival or reproductive success. For example, consider fur length in mammals living at a constant temperature of 10°C. Animals with very short fur may lose too much body heat, while those with very long fur may overheat during activity.
If environmental conditions fluctuate annually, both extremes may survive during favourable periods - short-furred animals in warmer years and long-furred animals in colder years. However, when the average temperature remains consistently at 10°C, individuals with intermediate fur length have the advantage. Over multiple generations, this eliminates both extremes from the population while maintaining the same mean fur length.
Worked Example: Birth Mass in Humans
Human birth weight demonstrates stabilising selection in action:
Step 1: Identify the optimum range
- Optimum birth weight: approximately 3.2kg
- This weight minimises infant mortality risk
Step 2: Observe selection against extremes
- Babies significantly below 3.2kg: higher mortality due to underdevelopment
- Babies significantly above 3.2kg: higher mortality due to birth complications
Step 3: Result
- Selection pressure maintains birth weights close to 3.2kg
- Population mean remains stable over generations
- Variation around the mean is reduced
Directional selection
Directional selection occurs when environmental changes favour individuals at one end of the phenotype distribution. This creates selection pressure that shifts the population mean towards one extreme over successive generations.
Environmental change and adaptation
Within any population, continuous variation exists for most characteristics, typically forming a normal distribution curve around a mean value. When environmental conditions change, the optimum value for survival also changes, creating new selection pressures.
Mechanism of directional selection
Consider the fur length example again, but with changing environmental conditions. If the average temperature drops from 10°C to 5°C, individuals with longer fur gain a survival advantage due to better insulation. These better-adapted individuals are more likely to survive and reproduce successfully.
The selection pressure causes the mean fur length to shift towards longer values over several generations. This continues until the population reaches a new optimum fur length that matches the changed environmental conditions (2.0cm for 5°C conditions). At this point, directional selection pressure ceases.
Worked Example: Antibiotic Resistance
The development of antibiotic resistance illustrates directional selection:
Initial Population:
- Bacterial population with normal distribution of resistance levels
- Most bacteria susceptible to antibiotics
- Few individuals with genetic variations conferring resistance
Selection Pressure Applied:
- Antibiotics introduced to treat infection
- Susceptible bacteria die, resistant bacteria survive
Evolutionary Outcome:
- Frequency of resistance alleles increases each generation
- Population mean shifts towards greater resistance
- Eventually, most bacteria in population are antibiotic-resistant
Disruptive selection
Disruptive selection represents the opposite of stabilising selection, favouring individuals with extreme phenotypes while selecting against those with intermediate characteristics. Although the least common form of selection, it plays the most important role in driving evolutionary change.
Conditions for disruptive selection
This type of selection occurs when environmental factors take on distinct forms or when different environmental conditions exist simultaneously. These conditions create multiple optimum phenotypes within the same population.
Mechanism and evolutionary significance
In disruptive selection, individuals with intermediate phenotypes face disadvantages compared to those at either extreme. For example, if temperatures alternate between 5°C in winter and 15°C in summer, mammals with very long fur (optimised for winter) and very short fur (optimised for summer) both have advantages over those with intermediate fur length.
Over many generations, this can lead to the formation of two distinct sub-populations with different characteristics. This process represents an important mechanism for speciation - the formation of new species.
Worked Example: Coho Salmon Reproductive Strategy
Male coho salmon demonstrate disruptive selection in reproductive strategies:
Large Males:
- Use direct competition for access to females
- High success rate when they win territorial disputes
- Advantage: physical dominance
Small Males:
- Use stealth tactics to approach females during spawning
- Avoid direct competition with large males
- Advantage: ability to sneak past territorial males
Medium-sized Males:
- Lack competitive ability of large males
- Lack stealth advantages of small males
- Result: lowest reproductive success
Evolutionary Outcome:
- Selection favours both large and small phenotypes
- Medium-sized males are selected against
- Population may split into two distinct size classes
Case study: selection in the peppered moth
The peppered moth (Biston betularia) provides a classic example of directional selection in action. This species demonstrates polymorphism - the existence of two or more genetically distinct forms within the same interbreeding population.
Historical context
Until the mid-nineteenth century, the peppered moth existed almost entirely in its natural light form. The moths rested on lichen-covered tree bark and rocks, where their light colouration provided effective camouflage against predation by insect-eating birds.
A melanic (dark) variety arose through mutation, but these black moths were highly conspicuous against light backgrounds. This made them vulnerable to greater predation, maintaining the dark form at very low frequencies in the population.
Industrial melanism
During the Industrial Revolution, particularly after 1848 when industrial development accelerated in Manchester, environmental conditions changed dramatically. Sulphur dioxide emissions killed the lichens that covered trees and walls, while soot deposits blackened surfaces.
Against this darkened background, the light form became more conspicuous and suffered increased predation. The melanic form, previously disadvantageous, now had superior camouflage. This reversal in selection pressure caused a rapid shift in allele frequencies.
Environmental Change Creates Selection Reversal: This case study demonstrates how the same environmental change can simultaneously create selection pressure against one phenotype while favouring another. The industrial pollution that harmed light moths provided the exact advantage that dark moths needed to thrive.
Evolutionary outcome
By 1895, approximately 98% of Manchester's peppered moth population consisted of the melanic form. This demonstrates evolution in action - a measurable change in allele frequency within populations over time.
Worked Example: Peppered Moth Evolution
Pre-Industrial Conditions (before 1848):
- Light moths: ~99% of population (high survival on lichen-covered surfaces)
- Dark moths: ~1% of population (high predation on light surfaces)
Industrial Revolution Impact:
- Pollution kills lichens and darkens surfaces
- Selection pressure reverses
Post-Industrial Outcome (by 1895):
- Light moths: ~2% of population (high predation on darkened surfaces)
- Dark moths: ~98% of population (high survival on darkened surfaces)
Time Frame: Complete population shift in approximately 47 years (~47 generations)
The example illustrates several key principles:
- How environmental changes can reverse selection pressures
- The speed at which populations can respond to strong directional selection
- The role of predation as a selection agent
- How the same species can be subject to different selection pressures in different environments (melanic forms remain selected for in industrial areas while light forms are selected for in rural areas)
Links to evolution and speciation
Understanding types of selection helps explain patterns of evolutionary change. Stabilising selection maintains existing adaptations in stable environments, while directional selection drives adaptation to environmental change. Disruptive selection, though rare, provides the mechanism for populations to split into distinct groups that may eventually become separate species.
Connection to Speciation: The peppered moth example shows how populations can rapidly adapt to environmental changes, but the moths remain a single species because the different forms can still interbreed. For speciation to occur, populations would need to become reproductively isolated from one another.
Key Points to Remember:
- Stabilising selection maintains the population mean by eliminating extremes - it occurs in stable environments and reduces variation
- Directional selection shifts the population mean towards one extreme - it occurs when environments change and drives adaptation
- Disruptive selection favours both extremes over intermediate forms - it's rare but crucial for evolutionary change and potential speciation
- Environmental conditions determine which type of selection operates, and the same population can experience different selection types as conditions change
- The peppered moth demonstrates how quickly populations can respond to strong directional selection pressure caused by environmental change