Evolution & Adaptation

What Is Evolution?

Evolution is the change in the inherited characteristics of a population over successive generations. Over very long periods of time, these changes can produce new species from ancestral ones.

Charles Darwin's theory of evolution by natural selection (published in 1859 in "On the Origin of Species") is the central unifying theory of biology.

Darwin's Theory of Natural Selection

Natural selection operates through four key principles:

1. Variation Individuals within a species show variation — they differ from one another in their characteristics. This variation arises from:

• Genetic mutations — random changes to DNA
• Sexual reproduction — mixing of genes from two parents during fertilisation
• Environmental factors — diet, temperature, and other conditions can influence development

2. Overproduction (Struggle for Existence) Most species produce far more offspring than can possibly survive. A single oak tree produces thousands of acorns; most will never become trees. Resources (food, water, space, mates) are limited, so individuals must compete.

3. Selection (Survival of the Fittest) Individuals with beneficial variations — those that are better adapted to their environment — are more likely to survive to reproductive age and reproduce successfully. "Fittest" means best suited to the environment, not necessarily physically strongest.

4. Inheritance and Change Over Time Survivors pass their advantageous traits to their offspring through genes. Over many generations, beneficial traits become more common in the population, and harmful or neutral traits may become rarer. The population gradually evolves.

Adaptations

An adaptation is a heritable characteristic that increases an organism's chance of survival and reproduction in its environment.

Types of Adaptation

Structural (anatomical) adaptations:

Organism	Adaptation	Benefit
Polar bear	Thick white fur	Insulation + camouflage in snow
Cactus	Thick water-storing stem, waxy coating	Survives drought
Shark	Streamlined body, lateral line system	Efficient swimming, detects prey
Cheetah	Long legs, flexible spine, large heart and lungs	High speed to catch prey
Mole	Large spade-like front paws	Efficient digging
Arctic fox	Small ears, thick fur	Reduces heat loss in cold

Behavioural adaptations:

• Migration — swallows fly south in winter to find food
• Hibernation — hedgehogs reduce metabolism to survive winter
• Nocturnal activity — owls and mice avoid daytime predators/heat
• Shoaling (fish) — safety in numbers

Physiological adaptations:

• Camels produce very concentrated urine; store fat (not water) in hump
• High-altitude populations have more red blood cells (carry more oxygen in thin air)
• Arctic fish produce antifreeze proteins preventing ice crystal formation in blood

Evidence for Evolution

Multiple independent lines of evidence support evolutionary theory:

Evidence	How it supports evolution
Fossil record	Fossils show how species changed over millions of years; transitional fossils show links between groups
DNA comparisons	Closely related species share more similar DNA sequences; all life uses the same genetic code
Comparative anatomy	Homologous structures — similar bone arrangements in human arm, whale flipper, bat wing, horse leg — suggest common ancestry
Vestigial structures	Remnant organs with reduced function (e.g., human appendix, whale hip bones) suggest evolutionary history
Geographical distribution	Related species found on nearby continents or islands (e.g., Darwin's finches in Galapagos)
Direct observation	Antibiotic resistance in bacteria; pesticide resistance in insects — evolution observed in real time
Embryology	Vertebrate embryos look very similar early in development, suggesting common ancestry

Speciation

Speciation is the formation of a new species. It occurs when a population becomes isolated and evolves independently until individuals can no longer interbreed with the original population.

How Speciation Works

1. A population is geographically isolated (e.g., a mountain range, river, or ocean separates two groups)
2. Each isolated group faces different environmental pressures
3. Natural selection favours different adaptations in each group
4. Over many generations, the groups become genetically distinct
5. Eventually, they can no longer interbreed to produce fertile offspring — they are now different species

Example: Darwin's finches on the Galapagos Islands — a common ancestor colonised the islands; different populations on different islands evolved different beak shapes suited to available food sources (seeds, insects, nectar).

Antibiotic Resistance — Evolution in Action

Antibiotic resistance is a powerful real-world example of natural selection:

1. In a bacterial population, random mutations produce rare individuals resistant to an antibiotic
2. When antibiotics are used, sensitive bacteria die but resistant ones survive
3. Resistant bacteria reproduce rapidly, passing on the resistance gene
4. The population becomes dominated by resistant bacteria
5. The antibiotic no longer works

This is why patients must complete the full course of antibiotics (to kill all bacteria, not just the most sensitive ones) and why overuse of antibiotics is a serious health concern.

Lamarck vs Darwin

Before Darwin, Jean-Baptiste Lamarck proposed a different mechanism of evolution:

Theory	Key idea	Problem
Lamarck (1809)	Organisms pass on characteristics acquired during their lifetime (e.g., a giraffe stretches its neck; offspring are born with longer necks)	We now know acquired characteristics are NOT inherited — genes are not changed by what an organism does during its life
Darwin (1859)	Natural selection acts on existing genetic variation; only inherited traits (carried in genes) are passed on	Supported by genetics, molecular biology, and abundant evidence

Key Terms

Term	Definition
Evolution	Change in the inherited characteristics of a population over generations
Natural selection	Process where organisms better adapted to environment survive and reproduce more
Adaptation	Heritable feature that increases survival and reproduction in an environment
Variation	Differences in characteristics between individuals of the same species
Mutation	Random change in DNA sequence
Speciation	Formation of a new species from an isolated population
Fossil	Preserved remains or traces of organisms from the past
Antibiotic resistance	Ability of bacteria to survive exposure to antibiotics; a result of natural selection

Worked Example

Q: Explain how a population of moths became mostly dark-coloured after industrial pollution darkened tree bark in 19th century England (industrial melanism).

Step 1: Originally, most moths were light-coloured and well-camouflaged against pale tree bark. Rare dark moths existed due to genetic mutation but were easily seen and eaten by birds.

Step 2: Industrial pollution covered tree bark with black soot. Now light moths were more visible and dark moths were better camouflaged.

Step 3: Natural selection — birds ate more light moths (easy to see); dark moths survived and reproduced more.

Step 4: Dark moths passed their colour gene to offspring. Over generations, dark moths became more common and light moths became rare.

Result: The population evolved — not because individual moths changed colour, but because the proportion of dark moths in the population increased through differential survival and reproduction.

Common Mistakes

• Saying organisms "decide" or "try" to evolve — evolution is not directed or intentional. Mutations are random; natural selection simply favours those that happen to be better suited.
• Confusing Lamarck and Darwin — Lamarck said acquired characteristics are inherited (WRONG); Darwin said natural selection acts on existing genetic variation (CORRECT).
• Thinking evolution is always slow — it can be fast (bacteria develop antibiotic resistance in weeks) when selection pressure is strong and generations are short.
• Saying only the "strongest" survive — "fittest" in evolution means best suited to the specific environment, not physically strongest (e.g., camouflage, not strength, saved the dark moths).

Tips and Tricks

• Remember the four steps of natural selection: Variation, Overproduction, Selection, Inheritance — VOSI.
• Antibiotic resistance is a perfect exam example of natural selection — learn it thoroughly.
• Evidence for evolution: FDH-GVE — Fossils, DNA, Homologous structures, Geographical distribution, Vestigial structures, Embryology.
• Speciation requires isolation first — without isolation, the gene pools mix and the populations do not diverge.