What Is Evolution? (OCR A-Level Biology A): Revision Notes

What Is Evolution?

The development of evolutionary theory

The idea that species change over time developed gradually during the eighteenth and nineteenth centuries as scientists made observations about fossils and living organisms. Two distinct theories emerged:

The general theory of evolution states that organisms have changed over time and continue to change. Evidence from fossils, such as those discovered by Mary Anning in Jurassic rocks at Lyme Regis, showed that extinct species once existed, indicating that life forms have altered over millions of years.

The special theory of natural selection explains the mechanism by which evolution occurs. This theory was developed independently by Charles Darwin (1809–1882) and Alfred Russel Wallace (1823–1913).

Darwin spent over $20$ years gathering evidence during and after his voyage on HMS Beagle (1831–1836), collecting specimens and making observations in South America, the Galápagos Islands, Australasia, and Mauritius. His observations convinced him that species were not unchanging – they evolved over time.

Wallace independently arrived at the same conclusion through his travels in South America and South-East Asia. In 1858, Wallace wrote to Darwin describing his ideas about natural selection. This led to a joint presentation at the Linnaean Society in London, followed by Darwin's publication of On the Origin of Species in November 1859.

Variation: the foundation of evolution

Variation refers to differences that exist both between species and within species. No group of organisms is completely identical. Even clones with identical genotypes show slight phenotypic differences due to environmental factors.

Types of variation

Genetic variation describes differences in genotypes within a species. Phenotypic variation encompasses all observable or detectable differences, including physical appearance, biochemistry, and behaviour. For example, your blood group is part of your phenotype, just as much as your height or eye colour.

Interspecific variation

Interspecific variation describes differences between species. These differences help us identify and classify organisms.

Interspecific variation can be even more pronounced:

These two barnacle species show obvious morphological differences and inhabit different environments.

Intraspecific variation

Intraspecific variation describes differences within a single species. This variation provides the raw material for natural selection.

Darwin studied intraspecific variation extensively in domesticated animals, particularly pigeons. Selective breeding by humans has 'released' variation that rarely appears in wild populations:

All domestic pigeon varieties descended from the rock dove (Columba livia), yet selective breeding has produced dramatically different forms such as Pouters, Fantails, and Jacobins.

Other examples of intraspecific variation include:

Morning glory (Ipomoea purpurea) shows genetic variation in flower colour:

Three genes interact to produce this diversity of colours and patterns.

Yeast (Saccharomyces cerevisiae) exhibits metabolic variation. Different strains possess different enzymes and membrane transport proteins, making some better suited for baking, others for brewing, and still others for biofuel production. Researchers have identified over $50$ genes controlling metabolite levels across more than $100$ related strains.

Types of variation

Variation can be classified into two main categories based on how characteristics are distributed within a population.

Discontinuous variation

Discontinuous variation (also called qualitative or discrete variation) produces distinct categories without intermediates.

Examples include:

• Human blood groups (ABO and Rhesus systems)
• Flower colour in snapdragons (red, pink, or white)
• Drug resistance in Mycobacterium tuberculosis (resistant or susceptible)

Human blood groups

The ABO blood group system illustrates discontinuous variation well:

Genetics of the Rhesus system: A gene on chromosome $1$ controls the Rhesus system with two alleles: R and r. The allele R codes for a red blood cell transmembrane protein, while r produces no protein. Individuals who are RR or Rr are Rhesus positive; those who are rr are Rhesus negative.

Continuous variation

Continuous variation (also called quantitative variation) produces a range of phenotypes between two extremes with many intermediates.

Any measurable characteristic shows continuous variation:

• Plant height
• Leaf width
• Animal mass
• Tail length

Polygenic inheritance

Multiple genes typically influence continuously varying characteristics. Consider a simplified example where four genes (A, B, C, D) control plant height. Each gene has two alleles:

• Allele $_1$ adds $10\,\text{mm}$ to height
• Allele $_2$ adds $20\,\text{mm}$ to height

The alleles are co-dominant (both expressed in heterozygotes):

Genes controlling characteristics this way are called polygenes, and the characteristic is described as polygenic.

Analyzing continuous variation

Data on continuous variation requires organization and analysis. Consider leaf length measurements from $50$ leaves:

To identify patterns, raw data must be organized into classes (categories):

This organized data can be presented as a frequency histogram:

Drawing a line through the center of each bar 'smooths' the histogram:

This reveals a normal distribution – a symmetrical, bell-shaped curve where:

• Most individuals cluster around the mean
• $50\%$ of individuals fall below the mean
• $50\%$ fall above the mean
• The mean lies within the modal class

Causes of variation

The relative contributions of genes and environment differ among characteristics.

Genetic control

Characteristics showing discontinuous variation are controlled mainly or entirely by genes. Environmental factors have minimal or no effect. Your blood group, for instance, is determined at conception and cannot change based on diet or lifestyle.

Environmental and genetic interaction

Characteristics showing continuous variation are influenced by both genes and environment.

Purely environmental variation

Some variation results entirely from environmental factors. Examples include:

• Scars from physical damage (e.g., broken tree branches)
• Wounds from fighting or predation
• Accidental injuries

Biochemical variation

Modern techniques such as electrophoresis have revealed variation at the molecular level, extending the concept of 'phenotype' to include biochemical differences.

Allozymes are enzyme variants coded by different alleles of the same gene. Heterozygous individuals produce two slightly different versions of the same enzyme.

Isozymes are enzymes that catalyze the same reaction but are coded by different genes. They may function optimally at different temperatures, potentially allowing organisms to acclimatize to seasonal changes or migration. This represents either biochemical adaptation or simply variation available for selection.