Variation, Natural Selection and Adaptation

Spec Mapping — OCR H420 Module 4.2.2 — Classification and evolution, content statements covering continuous and discontinuous variation, the mechanism of natural selection (Darwin and Wallace), the three modes of selection (directional, stabilising, disruptive), and the classification of adaptations into anatomical, behavioural, and physiological categories (refer to the official OCR H420 specification document for exact wording). This lesson is the mechanistic heart of Module 4.2.2 and the bridge to speciation in Lesson 12.

Evolution happens when populations change over time, and the engine of change is natural selection acting on variation. This lesson ties together the origin of variation (genetic and environmental), the mechanism by which favourable variants spread (natural selection in its three modes), and the adaptations that result. OCR A-Level Biology A Module 4.2.2 requires you to describe variation, explain how it relates to natural selection, and recognise the anatomical, behavioural and physiological adaptations of organisms to their environments.

The intellectual history is dominated by Charles Darwin and Alfred Russel Wallace, who independently arrived at the natural-selection mechanism and presented their findings jointly to the Linnean Society of London in July 1858; Darwin's On the Origin of Species followed in November 1859, expanding the argument with extensive evidence including artificial selection (paraphrased here — Darwin's famous chapter on pigeon breeding makes the argument that humans selecting traits is a direct analogue of nature doing the same thing). Thomas Henry Huxley defended the framework against contemporary critics. Gregor Mendel's 1866 work on inheritance was integrated with natural selection in the 1930s modern synthesis to give us the framework in which evolution is defined as a change in allele frequencies over generations. The peppered-moth example used below paraphrases observations from the Kettlewell era and subsequent re-evaluations; the qualitative pattern (industrial melanism shifting with pollution and reversing with clean air) is robust, but specific verbatim percentages from any one study should be avoided in exam answers.

Key Definitions:

• Variation — the differences between individuals of the same species.
• Interspecific variation — variation between different species.
• Intraspecific variation — variation within the same species.
• Continuous variation — traits that vary along a smooth gradient (e.g. height, mass).
• Discontinuous variation — traits falling into distinct categories (e.g. blood group).
• Natural selection — differential survival and reproduction of individuals best adapted to their environment.
• Adaptation — any feature that enhances an organism's survival and reproduction in its environment.

---

Variation

Variation is the raw material of evolution. Without differences between individuals, natural selection has nothing to act on.

Interspecific vs Intraspecific

• Interspecific variation is the differences between species (e.g. a lion vs a tiger).
• Intraspecific variation is the differences within a species (e.g. two lions from the same pride).

Both are real and measurable; the two are connected because intraspecific variation is the starting point from which new species arise.

Causes of Variation

1. Genetic causes:
   • Mutation — random changes in DNA sequence generate new alleles.
   • Meiosis — independent assortment and crossing over shuffle existing alleles.
   • Random fertilisation — any sperm can fertilise any egg.
   • Sexual reproduction in general combines alleles from two parents.
2. Environmental causes:
   • Nutrition affects growth and size.
   • Temperature can alter phenotype (e.g. Himalayan rabbits have dark fur only on cooler body parts).
   • Injury and disease.
   • Light affects plant growth.
3. Gene–environment interactions:
   • The majority of traits result from both genes and environment.
   • Identical twins reared apart can differ in height or health because of environment.

Continuous and Discontinuous Variation

flowchart TD
    A[Variation] --> B[Continuous]
    A --> C[Discontinuous]
    B --> B1[Polygenic traits]
    B --> B2[Normal distribution]
    B --> B3[Strong environmental influence]
    C --> C1[Usually one or few genes]
    C --> C2[Distinct categories]
    C --> C3[Little environmental influence]

Continuous variation — traits that form a smooth gradient:

• Height, mass, skin colour, leaf length, milk yield.
• Usually polygenic (many genes contribute).
• Often strongly influenced by environment.
• Produces a normal (bell-shaped) distribution when plotted.

Discontinuous variation — traits that fall into distinct categories:

• ABO blood groups (A, B, AB, O).
• Sex (male/female in most mammals).
• Tongue rolling (yes/no).
• Flower colour in many plants.
• Usually controlled by one or a few genes with major effects.
• Little or no environmental influence.
• Produces a discrete distribution (e.g. a bar chart with separate categories).

Exam Tip: OCR mark schemes want you to state clearly: continuous = polygenic + environmental influence + normal distribution; discontinuous = one/few genes + little environment + distinct categories. Use these exact phrases.

---

Natural Selection

Darwin and Wallace

Natural selection was proposed in 1858 by Charles Darwin and independently by Alfred Russel Wallace in a joint paper to the Linnean Society of London. Darwin expanded it the following year into On the Origin of Species (1859). The theory has four essential observations and two inferences:

Observations:

1. Organisms produce more offspring than can survive.
2. Populations nevertheless remain roughly constant over time.
3. Individuals show variation, much of it heritable.
4. Resources are limited (food, mates, nesting sites).

Inferences:

1. A struggle for existence takes place.
2. Individuals with favourable variations are more likely to survive and reproduce; over generations, their traits become more common.

The Modern Synthesis

In the 20th century, natural selection was combined with Mendelian genetics (Darwin did not know about Mendel's work) to produce the modern synthesis — the framework of population genetics in which evolution is defined as a change in allele frequencies over generations.

The Process of Natural Selection (Step by Step)

1. Variation exists within a population (from mutation, meiosis, random fertilisation).
2. More offspring are produced than can survive; there is competition for resources.
3. Selection pressure (predator, disease, climate) means some phenotypes are more successful.
4. Favourable alleles (those contributing to successful phenotypes) are passed to the next generation.
5. Over generations, allele frequencies change in the population.
6. The population becomes better adapted.

This sequence is the standard mark-scheme answer for "describe how natural selection works". Memorise it.

---

Modes of Natural Selection

Natural selection can push a population in three different ways:

flowchart TD
    A[Natural Selection] --> B[Directional]
    A --> C[Stabilising]
    A --> D[Disruptive]
    B --> B1[One extreme favoured]
    C --> C1[Mean favoured, extremes selected against]
    D --> D1[Both extremes favoured, mean selected against]

1. Directional Selection

One extreme of the phenotype range is favoured, shifting the mean of the population over generations.

• Example: Antibiotic resistance in bacteria. Sensitive bacteria die; resistant ones survive and reproduce. Over generations, the population becomes increasingly resistant.
• Example: Peppered moth industrial melanism (see also Lesson 12). In polluted areas, dark moths were favoured; dark alleles increased.
• Example: Beak size in Darwin's finches during droughts — only birds with larger beaks could crack the tough seeds still available.

2. Stabilising Selection

The mean phenotype is favoured; extreme variants are selected against. This keeps a population phenotypically constant over time.

• Example: Human birth weight. Babies that are very small or very large have higher mortality; those around 3.5 kg are most likely to survive.
• Example: Clutch size in birds. Too few eggs gives fewer offspring; too many eggs cannot all be fed. An intermediate optimum is favoured.

Stabilising selection is actually the most common mode, because it keeps well-adapted populations close to their current optimum.

3. Disruptive Selection

Both extremes of the phenotype range are favoured, and the mean is selected against. This can lead to the formation of two distinct subpopulations and potentially to speciation.

• Example: Beak size in some African seed-eaters — birds specialise on either very large or very small seeds, with intermediate beaks being inefficient at both.
• Example: Male plumage in some birds — bright males attract mates; drab males avoid predators; intermediate males do neither well.

Exam Tip: When identifying which type of selection is acting, always consider what is happening to the mean of the distribution. Directional = mean shifts; stabilising = mean stays, extremes shrink; disruptive = mean shrinks, extremes grow.

---

Adaptation

Natural selection produces adaptations — features that enhance survival and reproduction. Adaptations come in three broad categories (an OCR-specific classification):

1. Anatomical Adaptations

Physical structures that aid survival.

• Camel — long eyelashes to keep out sand; thick lips to eat thorny plants; wide feet to walk on sand; humps storing fat (not water); nostrils that can close.
• Polar bear — thick fur and blubber insulate; white coat camouflages against snow; small ears and tail reduce heat loss; large paws distribute weight on ice; black skin absorbs radiation.
• Cactus — reduced leaves (spines) minimise water loss; thick waxy cuticle; swollen stem stores water; shallow, widespread roots catch any rain; CAM photosynthesis.
• Stickleback — protective body armour (plates), spines that deter predators.

2. Behavioural Adaptations

Activities or responses that aid survival.

• Migration — swallows migrate from Europe to Africa to escape winter food shortage.
• Hibernation — hedgehogs, bats and bears reduce metabolism in winter.
• Courtship behaviour — peacocks display tails, frogs croak, fireflies flash to attract mates.
• Social behaviour — wolves hunt in packs; meerkats take turns as lookout.
• Foraging strategies — chimpanzees use tools to extract termites.

Behavioural adaptations can be innate (hard-wired; e.g. reflexes) or learned (e.g. cultural transmission in orca pods).

3. Physiological Adaptations

Internal (biochemical) processes that aid survival.

• Venom production in snakes and spiders.
• Torpor — lowering metabolism during food scarcity (e.g. hummingbirds at night).
• Antifreeze proteins in Arctic fish, lowering the freezing point of body fluids.
• Countercurrent heat exchange in the legs of wading birds and limbs of Arctic mammals.
• Metabolic water production in desert kangaroo rats, which can survive without drinking.
• Blood clotting, immune response, and enzyme adjustments — all are physiological adaptations.

OCR examiners love questions that require you to classify adaptations correctly into these three categories, so learn the distinction.

Case Study: The Marram Grass

Marram grass (Ammophila arenaria), a dune plant, shows anatomical (rolled leaves, hairs, sunken stomata), behavioural (N/A for plants) and physiological (high osmotic potential in cells) adaptations to its dry, salty, windy habitat. It is a classic exam example.

---

Case Study: Evolution of Antibiotic Resistance

Let's apply natural selection to a medically important example (more in Lesson 12 case studies).

1. Variation. In any large bacterial population, random mutations produce some cells with slight resistance to an antibiotic.
2. Selection pressure. When antibiotic is applied, sensitive cells die; resistant ones survive.
3. Inheritance. Resistant cells reproduce, passing resistance genes to daughter cells.
4. Change in frequency. Over generations, the frequency of resistance alleles rises; eventually most of the population is resistant.
5. Horizontal gene transfer. Plasmids carrying resistance genes spread between bacteria, even between species, accelerating the spread.

The result: MRSA, VRE, MDR-TB and other resistant pathogens. This is evolution in real time, happening within human lifetimes.

---

Common Exam Mistakes

• Using "need" or "want" language. Populations do not evolve because they need to; they evolve because some individuals happen to do better.
• Saying individuals evolve. Populations evolve, not individuals.
• Confusing adaptation with acclimatisation. Adaptation is genetic/heritable; acclimatisation is a short-term physiological adjustment (e.g. gaining more red cells at altitude).
• Forgetting mutation is random. The occurrence of mutations is random; the survival of individuals with them is not.
• Describing directional, stabilising and disruptive selection without referring to the mean. Always explain what happens to the distribution.
• Missing examples. Name real adaptations (camel, polar bear, marram grass) in exam answers.

---

Quick Recap

• Variation between individuals comes from mutation, meiosis, random fertilisation and environment.
• Continuous variation is polygenic and environmental; discontinuous variation is monogenic and categorical.
• Darwin and Wallace's natural selection: variation → overproduction → struggle → survival of the fittest → heritable change.
• Three modes: directional (shifts mean), stabilising (narrows range), disruptive (splits population).
• Adaptations are anatomical (structure), behavioural (action) or physiological (internal processes).

---

SVG: Continuous vs Discontinuous Variation