Darwin's Theory
The Big Idea🔬 The Most Powerful Idea in Biology
Charles Darwin proposed natural selection in 1859 in "On the Origin of Species," based on observations from his voyage on HMS Beagle (1831–1836). The core insight: individuals in a population vary, some variations improve survival and reproduction, and these variations are inherited. Over generations, beneficial traits accumulate — populations change. This is evolution by natural selection.
Darwin did not know about genes or DNA — the mechanism of inheritance was only understood after the Modern Evolutionary Synthesis (1930s–1940s) combined Darwinian selection with Mendelian genetics.
| Darwin's Observation | His Inference |
|---|---|
| Observation 1: Populations can produce far more offspring than the environment can support | There must be a struggle for survival — not all individuals will survive to reproduce |
| Observation 2: Individuals in a population show variation — no two individuals are exactly alike | Some variations must confer advantages in the struggle for survival |
| Observation 3: Traits are heritable — offspring tend to resemble their parents | Individuals with advantageous heritable traits will leave more offspring → those traits will become more common over generations |
Common Confusion
Darwin vs Lamarck — These Are NOT the Same
Lamarck thought acquired characteristics are inherited. Darwin said variation already exists and is selected. Both are about evolution — but the mechanism is completely different.
🦒 Lamarck (Wrong)
- Organisms change during their lifetime through use/disuse
- These acquired changes are passed to offspring
- Example: giraffe stretches neck during its life → offspring born with longer necks
- Problem: changes to body during lifetime do NOT alter the DNA in gametes
- Proved incorrect by genetics — somatic changes are not heritable
🧬 Darwin (Correct)
- Variation already exists in the population (some giraffes are born with longer necks)
- Longer-necked giraffes can reach more food → better survival → more offspring
- Long-neck alleles become more frequent in gene pool over generations
- No individual changes — the population changes over many generations
- Mechanism: variation + selection + inheritance + time
⚠️ Exam Watch — The Language of Natural Selection
Never say organisms "want to" or "try to" adapt, or that they "develop" a trait because they need it. Natural selection has no foresight or direction. Correct language: "individuals with longer necks were MORE LIKELY TO SURVIVE and reproduce, passing the long-neck allele on to more offspring. Over generations, the frequency of the long-neck allele INCREASED in the population."
Conditions for Natural Selection
The Four Requirements📋 Natural Selection Requires Four Things
Natural selection cannot occur unless all four conditions are present simultaneously. Remove any one condition and the process breaks down. These four requirements explain both why selection occurs and why it produces the outcomes it does.
1
Variation must exist within the population
Individuals must differ from each other in measurable traits. If all individuals were identical, there would be nothing for selection to act on. Sources of variation include mutations, genetic recombination during meiosis (crossing over, independent assortment), and random fertilisation.
Individuals must differ from each other in measurable traits. If all individuals were identical, there would be nothing for selection to act on. Sources of variation include mutations, genetic recombination during meiosis (crossing over, independent assortment), and random fertilisation.
2
Variation must be heritable (inherited)
Traits must be passed from parents to offspring via genes. If variation was purely environmental (e.g. a scar from an injury), selection could not change the gene pool. Only genetic variation can be selected for or against across generations.
Traits must be passed from parents to offspring via genes. If variation was purely environmental (e.g. a scar from an injury), selection could not change the gene pool. Only genetic variation can be selected for or against across generations.
3
More offspring must be produced than the environment can support (overproduction)
Populations have the capacity to grow exponentially, but environmental resources (food, space, mates) are limited. This creates competition — a struggle for survival. Not all individuals will survive to reproduce.
Populations have the capacity to grow exponentially, but environmental resources (food, space, mates) are limited. This creates competition — a struggle for survival. Not all individuals will survive to reproduce.
4
Some variants must have a survival/reproductive advantage (differential reproductive success)
Individuals with certain heritable traits are more likely to survive, find mates, and reproduce than others. These individuals contribute more offspring (and their alleles) to the next generation. Over time, advantageous alleles increase in frequency — the population evolves.
Individuals with certain heritable traits are more likely to survive, find mates, and reproduce than others. These individuals contribute more offspring (and their alleles) to the next generation. Over time, advantageous alleles increase in frequency — the population evolves.
📌 Fitness in Biology ≠ Physical Fitness
In natural selection, fitness means reproductive success — the number of viable offspring an individual produces relative to others. A small, slow animal that produces 100 surviving offspring is more "fit" than a large, fast one that produces 2. Fitness is always relative to the current environment — a trait that is advantageous in one environment may be disadvantageous in another.
⚠️ Exam Watch — Selection Acts on Phenotype, Not Genotype
Natural selection "sees" the phenotype — the physical expression of traits. But what gets passed on is the genotype (alleles). This is why heterozygous carriers of recessive alleles can be "invisible" to selection. Example: a recessive lethal allele (aa = dies) in heterozygous form (Aa) is not selected against — the individual survives. Only when two carriers mate and produce aa offspring is the allele exposed to selection.
Types of Natural Selection
Three Ways the Bell Curve Shifts📊 How Selection Changes the Distribution of Traits
When we plot a phenotypic trait (e.g. body size) across a population, we often get a bell curve. Natural selection can change this distribution in three distinct ways, depending on which part of the range is favoured. Understanding which type of selection is occurring — and predicting the result — is a core exam skill.
Type 1
Directional Selection
One extreme of the trait range is favoured. The bell curve shifts in one direction.
⚙️ How It Works
- One end of the phenotypic range is favoured over the other
- Average of the trait shifts in that direction over generations
- Occurs when the environment changes — or when a new environment is colonised
- Most common in changing or novel environments
- Can eventually lead to a new species if continued long enough
🐘 Classic Examples
- Antibiotic resistance — bacteria with resistance alleles survive; the population shifts toward resistance
- Peppered moths — industrial melanism; dark moths favoured after tree bark darkened; light moths selected against
- Elephant tusk size — heavy poaching of large-tusked elephants; population shifting toward smaller tusks or no tusks
- Darwin's finches — during drought, large hard seeds only; birds with larger beaks survived
Type 2
Stabilising Selection
The middle (average) value is favoured. Both extremes are selected against. The bell curve gets narrower.
⚙️ How It Works
- Intermediate phenotypes have highest fitness
- Both extremes are disadvantaged
- Reduces variation in the population — narrows the bell curve
- Maintains the average — population doesn't change much over time
- Most common in stable, established environments
- The most common type of selection overall
👶 Classic Examples
- Human birth weight — babies too small have high mortality; babies too large have difficult births; intermediate weight is optimal
- Sparrow wingspan — very short wings can't fly efficiently; very long wings are unwieldy in wind; intermediate is optimal
- Explains why well-adapted organisms (sharks, crocodiles) remain relatively unchanged for millions of years
Type 3
Disruptive Selection
Both extremes are favoured. The middle is selected against. The bell curve splits into two peaks.
⚙️ How It Works
- Both extreme phenotypes have higher fitness than the intermediate
- The average (intermediate) value is selected against
- Creates a bimodal distribution — two peaks in the bell curve
- Rarest of the three types in nature
- Can lead to sympatric speciation if the two groups stop interbreeding
🦀 Classic Examples
- African seed-cracker finch — large beaks crack hard seeds; small beaks eat soft seeds; medium beaks handle neither well
- Shore crabs — very dark or very pale survive predation better on mottled rock surfaces; intermediate colouration is most visible
- Can set the stage for speciation if the two extreme groups diverge far enough
| Type | Which phenotypes favoured? | Effect on variation | Effect on average |
|---|---|---|---|
| Directional | One extreme | Shifts in one direction | Average moves toward favoured extreme |
| Stabilising | Intermediate (average) | Decreases variation — narrows curve | Average stays the same |
| Disruptive | Both extremes | Increases variation — splits curve | Average is selected against |
Adaptation
The Result of Selection🌿 What Is an Adaptation?
An adaptation is any heritable trait that increases an organism's fitness (reproductive success) in its particular environment. Adaptations are the product of natural selection acting over many generations — they arise because individuals with those traits left more offspring. Adaptations can be structural (body form), physiological (body function), or behavioural (actions).
🦴 Structural Adaptations
- Physical features of the body
- Camel hump — stores fat, insulates from heat
- Penguin blubber — insulation in cold water
- Shark streamlined body — reduces drag for fast swimming
- Cactus spines — modified leaves to reduce water loss and deter herbivores
- Webbed feet in ducks — increase thrust in water
⚗️ Physiological Adaptations
- Internal functions and biochemical processes
- Desert animals produce very concentrated urine — water conservation
- Hibernation — lowered metabolic rate during food scarcity
- High red blood cell count in high-altitude animals — more oxygen carried
- Antifreeze proteins in Arctic fish — prevent ice crystal formation
- Venom in snakes — prey immobilisation
🐝 Behavioural Adaptations
- Migration — moving seasonally to better resources
- Nocturnal behaviour — avoid daytime predators/heat
- Courtship displays — attract highest-quality mates
- Alarm calls — warn group members of predators
- Tool use — extend foraging ability (crows, chimps)
- Hibernation behaviour — conserve energy in winter
📌 Adaptations Are Environment-Specific
An adaptation is only beneficial in the environment where it was selected. A polar bear's thick white fur is an adaptation in the Arctic — but the same animal in the tropics would overheat. If the environment changes faster than selection can respond, a previously adaptive trait can become a liability. This is why rapid climate change threatens species — their adaptations may no longer match the new conditions.
Speciation
When Populations Become New Species🌍 How One Species Becomes Two
Natural selection acting on separate populations over time can lead to speciation — the formation of new species. A species is defined as a group of organisms that can interbreed and produce fertile offspring. Speciation occurs when gene flow between populations is cut off (by geography or other barriers), allowing the populations to diverge genetically until they can no longer interbreed.
Most Common Type
Allopatric Speciation — Separated by Geography
A physical barrier divides a population. Separate selection pressures drive the two groups apart until they can no longer interbreed.
1
One population exists — gene flow occurs between all individuals; the population is genetically unified.
2
A geographical barrier forms — a mountain range rises, a river changes course, a forest is fragmented, or a group colonises an island. Gene flow between the two separated groups stops.
3
Natural selection acts independently — each population faces different conditions. Different alleles are favoured. Mutations accumulate differently. Genetic drift may also cause random changes. The two populations diverge.
4
Reproductive isolation evolves — after enough divergence, even if the barrier is removed, the two populations can no longer produce fertile offspring. They are now two separate species.
🦜 Classic Example: Darwin's Finches
A small flock of finches from mainland South America colonised the Galápagos Islands. Different islands had different food sources. Over millions of years, isolated populations evolved different beak shapes — each optimised for their island's food. Today there are 15 species, all descended from one ancestor.
🐿️ Kaibab vs Abert's Squirrels
When the Grand Canyon formed, it divided one squirrel population into two. The north rim (Kaibab squirrel) and south rim (Abert's squirrel) populations evolved in isolation. They are now considered separate species — different enough that even if they met, they would not readily interbreed.
Without Geographic Separation
Sympatric Speciation — Same Place, Different Niche
Speciation within the same geographic area — usually via polyploidy in plants or niche divergence.
🌾 Polyploidy (Plants)
- Error in meiosis produces gametes with double chromosomes
- Fertilisation produces offspring with 4 sets of chromosomes (tetraploid)
- Tetraploid cannot interbreed with diploid parents — instant reproductive isolation
- Very common in plants — most crop plants are polyploids (wheat = hexaploid)
- Rare but possible in animals
🐟 Niche Divergence
- Within one population, some individuals begin exploiting a different resource
- Disruptive selection may favour two extreme types
- Over generations, the two groups may diverge in mating preferences too
- Eventually reproductive isolation develops without geographic separation
- Documented in some insect species and fish populations in lakes
⚠️ Exam Watch — The Biological Species Concept
The most commonly used species definition in IEB/CAPS is the Biological Species Concept: a species is a group of organisms that can interbreed and produce fertile offspring under natural conditions. Key word: FERTILE. A horse and donkey can breed and produce a mule — but mules are sterile. Therefore horses and donkeys are different species. Speciation is complete when reproductive isolation is established — not just when populations look different.
🎯 Survival Audition Test
Eight questions on natural selection.
Question 1 of 8
A population of bacteria is exposed to an antibiotic. Most die, but a few survive and reproduce. After many generations, most bacteria in the population are antibiotic-resistant. Which statement best explains this?
Question 2 of 8
Which of the four conditions for natural selection is described here: "In a lizard population, tail length varies from very short to very long, with most individuals having intermediate tails"?
Question 3 of 8
Human birth weight data shows that babies born very small AND babies born very large have higher mortality than babies of average weight. Which type of natural selection does this represent?
Question 4 of 8
In biology, an organism's "fitness" refers to:
Question 5 of 8
A river changes course, permanently dividing a population of salamanders into two isolated groups. After millions of years, the two groups can no longer interbreed. What type of speciation has occurred?
Question 6 of 8
Lamarck proposed that a giraffe's neck grew longer during its lifetime because it stretched to reach higher leaves — and that this longer neck was passed on to its offspring. What is the main reason this explanation is incorrect?
Question 7 of 8
In a population of beetles, those with very light AND very dark colouration survive better on a mottled rock surface, while medium-coloured beetles are most visible to predators. Which type of selection is this?
Question 8 of 8
Why can a horse and a donkey NOT be classified as the same species, even though they can mate and produce offspring (mules)?