Biology (9700) | Natural and artificial selection

Welcome to Selection and Evolution!

Hi there! This chapter is one of the most exciting in Biology, dealing with how life has changed over billions of years and how humans harness these processes for our own needs.
Don't worry if concepts like genetic drift seem abstract at first. We will break down Darwin's ideas and modern genetics using clear examples and helpful analogies. By the end, you will understand the fundamental mechanisms driving all biological change!

17.1 Variation: The Raw Material of Change

Evolution can only occur if there is variation within a population. Without differences, selection has nothing to 'select' against.

1. Causes of Phenotypic Variation

Phenotypic variation refers to the observable differences in the characteristics of individuals within a species. It can be due to three things:

• Genetic Factors: Differences in the alleles inherited (e.g., blood type). This variation is inherited and cannot be changed by the environment.
• Environmental Factors: Differences caused by the environment (e.g., getting a suntan, or a plant's size being limited by soil nutrients). This variation is not inherited.
• Combination of Both: Most characteristics are influenced by both genes and the environment (e.g., height potential is genetic, but actual height depends on nutrition).

2. Types of Variation

Discontinuous Variation

This type of variation results in distinct, non-overlapping categories.

• Description: Characteristics fall into clear-cut categories (e.g., either you have blood type A or B, never anything in between).
• Genetic Basis: Usually controlled by one or two genes (monogenic inheritance).
• Environmental Influence: Minimal or none.
• Example: Human blood groups (A, B, AB, O), flower colour (Red or White).

Continuous Variation

This variation results in a smooth range of phenotypes between two extremes.

• Description: Characteristics can take any value between the minimum and maximum (often shown by a bell-shaped curve on a graph).
• Genetic Basis: Controlled by many genes (polygenic inheritance).
• Environmental Influence: Significant influence from environmental factors.
• Example: Human height, mass, leaf length, milk yield in cattle.

17.2 Natural and Artificial Selection

1. The Mechanism of Natural Selection

Natural selection is the central process of evolution. It explains how populations adapt to their environment.

Key Explanation:

1. Overproduction & Variation: Populations produce more offspring than the environment can support. Crucially, there is natural variation among these offspring.
2. Struggle for Existence: Due to limited resources (food, water, shelter), there is competition, leading to a "struggle for existence" (or survival).
3. Differential Survival (Selection): Individuals possessing alleles that give them a phenotypic advantage in the current environmental conditions are best adapted and most likely to survive.
4. Differential Reproduction: These surviving, well-adapted individuals are more likely to successfully reproduce and pass on their advantageous alleles to the next generation.
5. Allele Frequency Change: Over many generations, the frequency of the advantageous alleles in the population's gene pool increases, leading to adaptation and evolution.

2. Environmental Factors as Forces of Selection

The environment dictates which phenotypes are successful. Selection pressures can be physical (abiotic, like temperature) or biological (biotic, like predators). These forces act in three main ways:

A. Stabilising Selection

• Effect: Favours the average phenotype and selects against the extremes.
• Outcome: The range of variation decreases, and the mean stays the same. The population becomes more uniform.
• Example: Human birth weight. Babies of intermediate weight have the highest survival rate, while very small or very large babies are selected against.

B. Directional Selection

• Effect: Favours one extreme phenotype and selects against the average and the other extreme.
• Outcome: The mean phenotype shifts over time in the direction of the favoured extreme, often in response to an environmental change.
• Example: If a climate becomes consistently colder, individuals with thicker fur (an extreme trait) are favoured, and the average fur thickness increases over generations.

C. Disruptive Selection

• Effect: Favours both extremes of the phenotype and selects against the average.
• Outcome: This is rare but important, as it can lead to the formation of two distinct sub-populations, potentially resulting in new species.
• Example: In areas where seeds are either very large or very small, birds with medium-sized beaks might struggle, while those with very large or very small beaks thrive.

3. Changes in Allele Frequency: Genetic Drift and Founder Effect

While natural selection acts systematically based on phenotype, random chance events can also change allele frequencies. This is called genetic drift.

A. Genetic Drift

Genetic drift is the random fluctuation of allele frequencies from generation to generation due to chance events, particularly prominent in small populations. It often leads to the loss of some alleles and the fixation (100% frequency) of others, regardless of whether they are advantageous or not.

Analogy: Imagine flipping a coin 1000 times (large population sample, expect 50% heads). Now imagine flipping it only 5 times (small population sample, very easy to get 80% heads purely by chance). Genetic drift is the random deviation from expected ratios in small samples.

B. The Bottleneck Effect

This occurs when a population size is severely reduced (e.g., by a natural disaster or human activity). The surviving population often has a much smaller, non-representative gene pool than the original population. Even after the population recovers, its genetic diversity remains low.

C. The Founder Effect

This is a type of genetic drift that occurs when a small group of individuals breaks away from a larger population and establishes a new colony. The gene pool of this new "founder" population is limited to the alleles carried by the founders, which may not represent the diversity of the original population.

4. The Hardy-Weinberg Principle

The Hardy-Weinberg principle is a model that describes a hypothetical, non-evolving population where allele and genotype frequencies remain constant from generation to generation. It provides a baseline for comparison to real populations, allowing us to detect when evolution (changes in allele frequency) is occurring.

Hardy-Weinberg Conditions (When the principle applies):

• No mutation
• No natural selection (all genotypes survive equally well)
• Random mating
• No gene flow (no migration in or out)
• Extremely large population size (no genetic drift)

The Equations:

If \(p\) is the frequency of the dominant allele (A) and \(q\) is the frequency of the recessive allele (a):

1. Allele Frequency:
\(p + q = 1\)

2. Genotype Frequency:
\((p + q)^2 = p^2 + 2pq + q^2 = 1\)

Where:

• \(p^2\) = Frequency of homozygous dominant genotype (AA)
• \(2pq\) = Frequency of heterozygous genotype (Aa)
• \(q^2\) = Frequency of homozygous recessive genotype (aa)

5. Selective Breeding (Artificial Selection)

In artificial selection, humans act as the selection pressure, intentionally choosing organisms with desirable traits to breed together. This process rapidly changes allele frequencies to produce specific breeds or crop varieties.

Principles of Selective Breeding:

1. Identify individuals in the population with the desired characteristics (e.g., high milk yield, large fruit size).
2. Select these individuals and breed them together.
3. Select the offspring that show the desired trait most strongly.
4. Repeat the process over many generations until the trait is uniform and enhanced.

Examples of Selective Breeding (Syllabus Requirements):

• Disease Resistance in Crops (Wheat and Rice): Breeders select varieties that naturally resist common pathogens (fungi, bacteria). This saves huge amounts of crops that would otherwise be lost and reduces the need for expensive pesticides.
• Vigorous, Uniform Varieties of Maize:
  - Inbreeding: Inbred strains are produced to achieve uniformity (genetically identical homozygous lines). While uniform, inbred strains often suffer from reduced vigour (inbreeding depression).
  - Hybridisation: Crossing two different inbred strains produces hybrids. These hybrids show increased vigour and yield (known as hybrid vigour) because they are highly heterozygous.
• Improving Milk Yield in Dairy Cattle: Farmers use selection criteria like quantity and quality of milk produced, selecting and breeding only the cows (and bulls whose mothers had high yields) that demonstrate the best traits. This has drastically increased average milk production per animal over decades.

17.3 Evolution and Speciation

1. Outline of Evolution

Evolution is defined as the process leading to the formation of new species from pre-existing species over time, resulting from changes to the gene pools of populations from generation to generation.

2. Evidence for Evolutionary Relationships

We can now use molecular data to determine how closely related different species are.

• DNA Sequence Data: Species that have diverged more recently share more similar DNA base sequences.
• By comparing the sequence of bases in a specific gene or even the entire genome, we can estimate how long ago two species shared a common ancestor.
• The fewer the differences in the DNA sequence, the closer the evolutionary relationship.

Did you know? Humans and chimpanzees share approximately 98% of their DNA sequence, reflecting a relatively recent shared ancestry.

3. Speciation: Forming New Species

Speciation is the formation of a new species. It occurs when members of a population become so genetically isolated from the rest of the species that they can no longer interbreed to produce fertile offspring.

A. Allopatric Speciation (Geographical Isolation)

The most common form of speciation:

1. A single population is separated into two groups by a geographical barrier (e.g., a mountain range, sea, or river).
2. The two groups cannot interbreed, leading to genetic isolation.
3. Each population experiences different selection pressures (different environments). Mutations also arise independently.
4. Natural selection acts differently on each population, changing their respective gene pools.
5. Eventually, accumulated genetic differences mean that even if the geographical barrier is removed, the two populations are reproductively isolated and are now two separate species.

Memory Aid: Allopatric = Away (geographical separation).

B. Sympatric Speciation (Ecological and Behavioural Isolation)

Speciation occurring within the same geographical area. Genetic isolation occurs due to ecological or behavioural differences.

• Ecological Isolation: Individuals exploit different resources or habitats within the same area (e.g., some fish in a lake prefer deep water, others prefer shallow water, limiting mating opportunities).
• Behavioural Isolation: Differences in mating rituals, courtship displays, or active times (e.g., nocturnal vs. diurnal) prevent successful interbreeding, even though the populations live side-by-side.