Variation and Natural Selection

What you'll learn

This revision guide covers variation within and between species, the mechanisms that produce it, and how natural selection drives evolutionary change. You'll learn to distinguish between genetic and environmental variation, explain how selection pressures lead to adaptation, and understand the evidence for evolution including antibiotic resistance in bacteria.

Key terms and definitions

Variation — differences between individuals of the same species, caused by genetic factors, environmental factors, or both

Mutation — a random change in the DNA sequence that can produce new alleles and is the ultimate source of genetic variation

Natural selection — the process by which individuals with characteristics most suited to their environment are more likely to survive and reproduce, passing on their advantageous alleles

Adaptation — an inherited characteristic that makes an organism well-suited to survival in its environment

Evolution — the change in the inherited characteristics of a population over successive generations through natural selection

Gene pool — all the alleles of all the genes present in a population at any one time

Selective breeding — the process by which humans choose organisms with desirable characteristics and breed them to produce offspring with those characteristics (also called artificial selection)

Species — a group of organisms that can interbreed to produce fertile offspring

Core concepts

Types of variation

Variation exists within species (between individuals of the same species) and between species. Understanding the causes of variation is essential for explaining natural selection.

Genetic variation arises from:

• Different alleles inherited from parents during sexual reproduction
• Mutations creating new alleles
• Random fertilisation of gametes
• Independent assortment of chromosomes during meiosis

Examples of characteristics showing primarily genetic variation include:

• Blood group in humans
• Eye colour in humans
• Flower colour in many plant species

Environmental variation arises from differences in the conditions organisms experience during their lifetime. The environment can modify how organisms develop and function.

Examples of characteristics showing primarily environmental variation include:

• Accent and spoken language in humans
• Scars and other injuries
• Changes in muscle mass from exercise

Continuous variation produces a range of phenotypes with no distinct categories (e.g., height, mass, leaf surface area). These characteristics are usually controlled by many genes (polygenic inheritance) and are significantly influenced by environmental factors.

Discontinuous variation produces distinct categories with no intermediates (e.g., ABO blood groups, ability to roll tongue). These characteristics are typically controlled by a single gene or a small number of genes, with little environmental influence.

Mutations as the source of variation

Mutations are random changes in DNA sequences. They occur spontaneously during DNA replication but can be increased by:

• Ionising radiation (X-rays, gamma rays, ultraviolet light)
• Chemical mutagens (certain chemicals in tobacco smoke, some pesticides)

Most mutations have no effect on the organism because:

• They occur in non-coding regions of DNA
• The mutation does not change the amino acid sequence due to the degenerate nature of the genetic code
• The change in protein structure does not significantly affect function

Some mutations are harmful, reducing the organism's chance of survival. Very rarely, mutations are beneficial and increase survival chances. Beneficial mutations are the raw material for natural selection.

Mutations in body cells (somatic mutations) are not passed to offspring. Only mutations in gametes (sex cells) can be inherited and contribute to variation in future generations.

Natural selection and adaptation

Natural selection acts on variation within populations. The process follows these steps:

1. Variation exists within a population due to different alleles
2. Selection pressure occurs — environmental factors (predators, disease, competition for food/water/mates, climate) create a struggle for survival
3. Differential survival — individuals with advantageous characteristics are more likely to survive
4. Differential reproduction — survivors are more likely to reproduce and pass on the alleles for advantageous characteristics
5. Allele frequency changes — over many generations, the proportion of individuals with advantageous alleles increases in the population

This process leads to adaptation — the accumulation of characteristics that improve an organism's chances of survival and reproduction.

Examples of adaptation include:

• Structural adaptations: polar bear's white fur for camouflage, cactus spines to reduce water loss
• Behavioural adaptations: migration in birds, nocturnal hunting in desert animals
• Functional/physiological adaptations: production of venom in snakes, antifreeze proteins in Arctic fish

Evolution through natural selection

Evolution is the gradual change in the characteristics of a species over time. The mechanism was explained by Charles Darwin in his theory of evolution by natural selection.

Darwin's theory can be summarised:

• Organisms produce more offspring than can survive
• There is variation within populations
• There is competition for limited resources
• Individuals with characteristics best suited to the environment survive (survival of the fittest)
• Survivors reproduce and pass advantageous characteristics to offspring
• Over many generations, the characteristics of the population change

Evolution requires long time periods (thousands to millions of years) for significant changes to accumulate. Fossil records provide evidence of how species have changed over geological time.

Evidence for evolution

Antibiotic resistance in bacteria provides observable evidence of evolution by natural selection occurring over short time periods:

1. Random mutation in bacterial DNA produces an allele conferring antibiotic resistance
2. When antibiotics are present (selection pressure), non-resistant bacteria die
3. Resistant bacteria survive and reproduce rapidly by binary fission
4. The gene for resistance spreads through the bacterial population
5. The population becomes predominantly resistant

This evolution happens quickly because bacteria:

• Reproduce very rapidly (every 20-30 minutes in ideal conditions)
• Produce millions of offspring
• Have higher mutation rates in large populations

MRSA (methicillin-resistant Staphylococcus aureus) is a significant example. These bacteria have evolved resistance to multiple antibiotics through natural selection, making infections difficult to treat.

To slow the development of antibiotic resistance:

• Doctors should only prescribe antibiotics for bacterial infections (not viral)
• Patients must complete the full course of antibiotics to kill all bacteria
• Avoid using antibiotics in animal farming unnecessarily
• Develop new antibiotics to replace those that become ineffective

Other evidence for evolution includes:

• Fossil record — showing gradual changes in organisms over millions of years
• Comparative anatomy — similar bone structures in vertebrate limbs suggest common ancestry
• DNA evidence — organisms with similar DNA sequences are more closely related

Selective breeding vs natural selection

Selective breeding (artificial selection) is controlled by humans, whereas natural selection occurs naturally in response to environmental pressures.

Process of selective breeding:

1. Identify organisms with desirable characteristics
2. Breed these organisms together
3. Select offspring with the best characteristics
4. Repeat over many generations

Examples include:

• Cattle bred for high milk yield or meat production
• Crop plants bred for disease resistance or high yield
• Dogs bred for specific characteristics (size, temperament, appearance)

Selective breeding reduces genetic variation in a population because only a limited number of alleles are selected. This can lead to:

• Increased risk of genetic diseases
• Reduced ability to adapt to environmental changes
• Inbreeding depression

Speciation

A species is defined as a group of organisms capable of interbreeding to produce fertile offspring. Members of different species cannot produce fertile offspring together.

Speciation is the formation of new species. It typically requires:

1. Isolation of populations (geographical barriers like mountains, rivers, or oceans)
2. Different selection pressures acting on separated populations
3. Genetic divergence over many generations
4. Reproductive isolation — populations become so genetically different they can no longer interbreed to produce fertile offspring

The Galápagos finches studied by Darwin demonstrate speciation. Different islands had different food sources (selection pressures), leading to evolution of different beak shapes suited to available food. Over time, distinct species evolved from a common ancestor.

Worked examples

Example 1: Explaining natural selection

Question: The peppered moth exists in two forms: light-coloured and dark-coloured. Before industrial pollution, most moths were light-coloured. During the Industrial Revolution, tree bark became darkened by soot and the proportion of dark moths increased significantly. Explain how natural selection led to this change. (6 marks)

Model answer:

• Mutation produced the allele for dark colouring / variation existed in the population (1)
• Light moths were easily seen on dark bark / dark moths were camouflaged on dark bark (1)
• Light moths were more likely to be eaten by predators / birds (1)
• Dark moths were more likely to survive (1)
• Dark moths reproduced and passed on the allele for dark colour (1)
• Over many generations, the proportion of dark moths in the population increased (1)

Mark scheme notes: Award marks for explaining variation, selection pressure, differential survival, reproduction, and change over time.

Example 2: Continuous vs discontinuous variation

Question: A student measured the height of 30 students in her class. State whether height shows continuous or discontinuous variation and explain your answer. (2 marks)

Model answer:

• Continuous variation (1)
• Because there is a range of heights with no distinct categories / controlled by many genes / influenced by environmental factors such as diet (1)

Example 3: Antibiotic resistance

Question: Explain why doctors should only prescribe antibiotics when necessary and why patients should complete the full course of treatment. (4 marks)

Model answer:

• Overuse of antibiotics increases the chance of resistant bacteria developing / provides more selection pressure (1)
• Bacteria with resistance alleles survive and reproduce (1)
• Not completing the course means some bacteria survive (1)
• Surviving bacteria may include those with some resistance, which can then multiply and spread (1)

Common mistakes and how to avoid them

• Confusing evolution with individual organisms changing: Evolution describes changes in populations over generations, not individuals adapting during their lifetime. A giraffe doesn't stretch its neck to reach leaves — the population evolves over many generations as longer-necked individuals survive better.

• Stating that organisms "need" or "want" to evolve: Evolution has no purpose or direction. Don't write "bacteria became resistant because they needed to survive." Instead write "random mutation produced resistance; resistant bacteria survived when antibiotics were present."

• Forgetting that mutations are random: Mutations occur randomly, not in response to environmental need. Environmental pressures (selection pressures) determine which existing mutations are advantageous.

• Not explaining the inheritance step: Natural selection requires that advantageous characteristics are inherited. Always mention that survivors reproduce and pass on alleles to offspring.

• Confusing characteristics caused by genes vs environment: Muscle mass from weight training is environmental; it won't be inherited by offspring. Only genetic characteristics can be passed on.

• Describing evolution as "survival of the strongest": It's survival of the fittest (best adapted), which may involve many characteristics beyond strength — camouflage, disease resistance, efficiency, reproductive success.

Exam technique for "Variation and Natural Selection"

• Command words matter: "Describe" requires you to state features or how something changes. "Explain" requires reasons using because/therefore. "Suggest" means apply knowledge to unfamiliar situations — there may be multiple acceptable answers.

• Structure natural selection answers logically: Follow the sequence — variation exists, selection pressure identified, differential survival explained, reproduction mentioned, change in population over time stated. This ensures you don't miss steps worth marks.

• Use precise terminology: Write "allele" not just "gene," "mutation" not "change," "selection pressure" not "problem." Examiners reward biological vocabulary used accurately.

• Link structure to function in adaptation questions: When describing adaptations, always explain how the feature helps survival (e.g., "thick fur provides insulation, reducing heat loss in cold climates").

Quick revision summary

Variation within species arises from genetic factors (different alleles, mutations) and environmental factors. Natural selection occurs when selection pressures cause differential survival — individuals with advantageous characteristics survive, reproduce, and pass on beneficial alleles. Over many generations, populations evolve and become adapted to their environment. Antibiotic resistance in bacteria demonstrates rapid evolution. Selective breeding is artificial selection controlled by humans. Speciation occurs when isolated populations diverge genetically and can no longer interbreed to produce fertile offspring.