Welcome to the Evolution Chapter!

Hello Biologists! Evolution is one of the most powerful and fascinating theories in science. It explains not only the incredible diversity of life on Earth but also why organisms are so perfectly adapted to their environments.

Don't worry if terms like 'natural selection' or 'gene pool' seem complicated right now. We'll break down this chapter step-by-step, starting with the raw material for evolution: **Variation**!

17.1 Variation

What is Phenotypic Variation?

Phenotypic variation refers to the differences in characteristics (phenotypes) among individuals within a species. This variation is the essential fuel for evolution, as natural selection can only act if there are differences to select from.

Phenotypic variation can arise from three main sources:

  • Genetic factors: Differences in the alleles inherited (e.g., blood group).
  • Environmental factors: The effects of the surroundings (e.g., acquiring a suntan).
  • A combination of both: Most complex traits are influenced by both genes and the environment (e.g., height or mass).

Continuous vs. Discontinuous Variation

We classify variation based on how the characteristics are expressed in the population.

1. Discontinuous Variation
  • Characteristics: Traits fall into clear, distinct, non-overlapping categories.
  • Genetic Basis: Controlled by alleles of one or two genes (monogenic or oligogenic).
  • Environmental Effect: Generally not affected by the environment.
  • Example: Human blood groups (A, B, AB, O), or flower colour (either red or white, nothing in between).
  • Graph Shape: Usually displayed as a bar chart.
2. Continuous Variation
  • Characteristics: Traits show a complete range of values between two extremes—there are no distinct categories.
  • Genetic Basis: Controlled by the alleles of many genes (polygenic inheritance).
  • Environmental Effect: Strongly affected by the environment.
  • Example: Human height, mass, or leaf area in plants.
  • Graph Shape: Typically results in a smooth, bell-shaped curve (Normal Distribution).

Quick Trick: If you can measure it (like height in meters), it's likely Continuous. If you can count distinct options (like eye colour types), it's likely Discontinuous.

Using the t-test (Syllabus Requirement)

The t-test is a statistical tool used to determine if the difference between the means of two separate samples is statistically significant (i.e., whether the difference is real, or just due to chance).

For instance, if you measure the height of students from School A and School B, the t-test tells you if School A students are genuinely taller on average, or if your results are just a fluke of sampling.

Key Takeaway for 17.1: Variation is caused by genes, environment, or both. Discontinuous variation is clear-cut and monogenic; Continuous variation is measured on a scale and is polygenic and often environmental.

17.2 Natural and Artificial Selection

The Mechanism of Natural Selection

In 1858, Darwin and Wallace proposed the theory of evolution by natural selection. This process explains how the frequency of advantageous alleles increases in a population over successive generations.

Here is the step-by-step sequence:

  1. Variation: Individuals within a population show genetic variation (e.g., some are faster, some have better camouflage).
  2. Overproduction and Competition: Populations produce more offspring than the environment can support, leading to a "struggle for existence" (competition for food, shelter, mates, etc.).
  3. Selection: Environmental factors act as selection pressures. Individuals possessing advantageous characteristics (those best adapted) are more likely to survive this competition.
  4. Reproduction: These survivors are more likely to successfully reproduce.
  5. Inheritance: They pass their advantageous alleles to the next generation.
  6. Evolution: Over many generations, the frequency of these advantageous alleles increases in the gene pool, leading to changes in the species' characteristics and ultimately, evolution.

Did you know? The concept of the "struggle for existence" was influenced by the economist Thomas Malthus, who wrote about human population growth outstripping food supply.

Forces of Natural Selection

Environmental factors can shape variation in predictable ways.

Stabilising Selection

This selection pressure favours the average phenotype and selects against the extremes.

  • Effect: Reduces the range of variation (makes the population more uniform).
  • Example: Human birth weight. Babies of intermediate weight have the highest survival rate, while very light or very heavy babies have lower survival rates.

Directional Selection

This selection pressure favours one extreme phenotype over the average and the other extreme.

  • Effect: Shifts the mean phenotype of the population towards the favoured extreme.
  • Example: The evolution of antibiotic resistance in bacteria (see next section) or the increasing height of giraffes' necks over time in an environment where tall leaves are the only food source.

Disruptive Selection

This selection pressure favours both extremes of the phenotype and selects against the average phenotype.

  • Effect: Can split a population into two distinct phenotypic groups, often leading to speciation.
  • Example: Beak size in African finches. Finches with very large beaks can crack large, hard seeds, and those with small beaks are better at processing small, soft seeds. Mid-sized beaks are inefficient for both.

Factors Affecting Allele Frequencies

Changes in the frequency of alleles in the **gene pool** (all the alleles present in a population) drive evolution.

1. Selection

As discussed above, natural selection systematically favours certain alleles, increasing their frequency.

2. Genetic Drift

This is a random change in allele frequency, often seen in small populations. It happens purely by chance, not due to selection pressure.

  • The Bottleneck Effect: Occurs when a large population is dramatically reduced (e.g., by a disaster or environmental change). The resulting population has a random, smaller gene pool that may not represent the original population's diversity.
  • The Founder Effect: Occurs when a small group breaks away from a larger population to colonise a new area. The allele frequencies in this "founder" population are unlikely to match the original population.

Analogy: Imagine you have a jar of 100 marbles (the gene pool). 80 are blue and 20 are red.
A bottleneck means you accidentally spill 95, leaving 3 red and 2 blue. The allele frequencies have changed dramatically, purely by chance!

Case Study: Antibiotic Resistance

Antibiotic resistance in bacteria is a perfect example of natural selection in action (Directional Selection).

  1. Variation: A bacterial population contains millions of cells, some of which naturally possess an allele that gives them a slight resistance to a specific antibiotic (perhaps through a random mutation).
  2. Selection Pressure: When the antibiotic is applied, it kills the non-resistant bacteria.
  3. Survival of the Fittest: The resistant bacteria survive the treatment.
  4. Reproduction: These resistant survivors multiply rapidly, passing on the resistance allele.
  5. Result: The entire population evolves to become resistant to the drug, making the antibiotic ineffective.

The Hardy-Weinberg Principle

This principle provides a mathematical model for a theoretical population where no evolution is occurring. It relates allele frequencies and genotype frequencies.

The principle applies only if specific conditions are met:

  • The population is very large (no genetic drift).
  • Mating is random.
  • There is no natural selection (all genotypes survive equally well).
  • There is no mutation, migration, or gene flow.

The Hardy-Weinberg equations (which you must know how to use):

1. Allele Frequency Equation:
\[p + q = 1\]
Where:
\(p\) = frequency of the **dominant allele**
\(q\) = frequency of the **recessive allele**

2. Genotype Frequency Equation:
\[p^2 + 2pq + q^2 = 1\]
Where:
\(p^2\) = frequency of **homozygous dominant** individuals
\(q^2\) = frequency of **homozygous recessive** individuals
\(2pq\) = frequency of **heterozygous** individuals

You can use these equations to calculate unknown frequencies in a stable population. For example, if you know the percentage of individuals showing the recessive phenotype (which is \(q^2\)), you can calculate \(q\), then \(p\), and finally \(p^2\) and \(2pq\).

Artificial Selection (Selective Breeding)

Artificial selection (or selective breeding) is similar to natural selection, but the selection pressure is imposed by **humans**, aiming to achieve specific desirable traits.

Key Principle: Humans choose individuals with desirable traits to reproduce, preventing others from breeding. This increases the frequency of those desired alleles in the gene pool over generations.

Examples of selective breeding (as required by the syllabus):

  • Disease Resistance: Breeding new varieties of wheat and rice that are resistant to common fungal diseases, increasing crop yields and reliability.
  • Vigorous, Uniform Maize: Using inbreeding (mating closely related individuals to achieve uniform traits) and hybridisation (crossing two different inbred lines to combine desirable traits and produce vigorous F1 hybrids).
  • Improving Milk Yield: Dairy farmers select cows with the highest milk yields and bulls that are offspring of high-yield mothers to breed the next generation, gradually increasing average milk production.

Key Takeaway for 17.2: Natural selection relies on competition filtering variation. Selection can be directional, stabilising, or disruptive. Allele frequencies also change randomly via drift. Artificial selection is goal-driven human intervention.

17.3 Evolution and Speciation

The Theory of Evolution

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

The core idea is that populations change over vast amounts of time, driven primarily by the mechanisms of natural selection acting on random variation.

DNA Sequence Data and Evolutionary Relationships

How do scientists confirm relationships between species? By looking at their DNA!

  • Species that are closely related (e.g., humans and chimpanzees) have more similar sequences of **DNA bases** and **amino acids** in common proteins (like haemoglobin) than species that are distantly related (e.g., humans and yeast).
  • The greater the difference in DNA sequence, the longer the time since their last **common ancestor**.
  • This molecular data provides strong evidence that all life shares common ancestry and confirms the evolutionary paths suggested by fossil records and comparative anatomy.

Speciation: Forming New Species

Speciation is the process by which one original species splits into two or more distinct species. This requires **genetic isolation**, meaning that the populations stop interbreeding, allowing their gene pools to diverge until they can no longer produce fertile offspring.

Speciation can occur due to two main types of genetic isolation:

1. Geographical Separation (Allopatric Speciation)
  • Process: A physical barrier (e.g., a mountain range, river, or ocean) splits a population into two sub-populations.
  • Mechanism: The two isolated populations experience different environmental conditions, leading to different selection pressures. They also experience independent genetic drift and mutations.
  • Result: Over time, their gene pools become so different that even if the barrier is removed, they can no longer interbreed to produce fertile offspring. They are now separate species.
2. Ecological and Behavioural Separation (Sympatric Speciation)
  • Process: Speciation occurs within the same geographical area. Genetic isolation is achieved by factors other than physical barriers.
  • Ecological Isolation: Two sub-populations begin occupying different habitats or niches within the same area (e.g., feeding on different host plants or living in different depths of a lake).
  • Behavioural Isolation: Changes in mating rituals, songs, or courtship displays prevent interbreeding (e.g., one group mates in the spring, the other in summer).
  • Result: Reduced or zero gene flow, allowing selection and drift to cause the gene pools to diverge, leading to new species.

Quick Review Box: Speciation Types
Allopatric: Needs a physical barrier (A = Away/Apart).
Sympatric: Needs a lifestyle/behavioural barrier (S = Same place).

Key Takeaway for 17.3: Evolution is the change in the gene pool over generations, forming new species (speciation). DNA evidence confirms these relationships. Speciation requires genetic isolation, which is achieved either geographically (allopatric) or through lifestyle/behaviour (sympatric).