IB ESS HL3.1 Biodiversity & EvolutionHL only~13 min read
How Humans Redirect Evolution
In Gorongosa National Park, decades of ivory poaching produced a population with far more tuskless elephants. Nobody bred them that way. Poachers simply removed every elephant that had tusks, and natural selection did the rest.
📘 What you need to know
Human activities create new selection pressures and so influence evolutionary processes.
Climate change: polar bears face pressure from melting sea ice — smaller body size, shifted hunting behaviour, more land travel.
Hunting and poaching: ivory poaching in Gorongosa, Mozambique increased the frequency of tuskless elephants.
Overfishing of large fish leaves populations dominated by smaller individuals.
Urbanisation favours species that thrive around humans, such as pigeons and rats.
Artificial selection is deliberate human choice of which organisms breed — also called selective breeding.
Artificial selection reduces genetic diversity, leaving populations vulnerable. Named examples: bananas and Panama disease, the Irish potato famine.
Human activity as a selection pressure
Evolution is driven by natural selection: traits that help organisms survive and reproduce get passed on more frequently. Human activities have altered natural environments, creating new pressures on species and so influencing evolutionary processes.
Climate change
Climate change, driven mainly by burning fossil fuels, is altering ecosystems and habitats. Polar bears face evolutionary pressure from melting sea ice: they rely on ice to hunt seals, and as it melts earlier and forms later they have less time to hunt. This has led to:
Smaller body size — some polar bears reach a smaller adult size because of reduced food availability.
Shifts in hunting behaviour — increasingly forced to hunt on land, where food is less abundant, which could eventually change their diet and behaviour long term.
Increased land travel — some range over larger areas to find food, potentially favouring individuals better at long-distance travel.
As environments change rapidly, only individuals with particular traits may survive, causing evolutionary shifts in the population.
Hunting, poaching and harvesting
The trait was already present at low frequency. Poaching simply removed the individuals carrying the alternative, so the tuskless allele became more common.
In Gorongosa National Park, Mozambique, poaching for ivory during the civil war led to an increase in tuskless elephants. Poachers were less likely to kill elephants without tusks, so over time tusklessness became more common.
In some fish populations, overfishing of large fish has produced populations dominated by smaller individuals, because larger fish are more likely to be caught.
New habitats and fragmentation
Urbanisation and agriculture cause habitat fragmentation and create new environments, pressuring native species to adapt or move away. Urban areas favour species that can adapt to human environments — pigeons and rats evolve to thrive in cities, developing traits such as greater tolerance to pollution or better ability to find food in human-dominated areas.
Two more examples worth knowing. Overuse of pesticides in agriculture has driven the evolution of resistant pest species, making pest control harder. Overuse of antibiotics in humans and livestock has driven the evolution of antibiotic-resistant bacteria such as MRSA, which are harder to treat. Both follow the identical five-step sequence from the natural selection note.
Artificial selection
Artificial selection is the process by which humans deliberately choose specific plants or animals to breed based on desirable traits — also known as selective breeding. Humans have been doing it for thousands of years, long before anyone understood the genetics behind it.
That middle case is worth holding on to. Human activity can drive natural selection without any intention to breed anything.
Selective breeding in animals
Selective breeding in plants
Cows, goats and sheep producing higher yields of milk or meat
Disease resistance in food crops
Chickens that lay large eggs
Increased crop yield
Domestic dogs with a gentle nature
Hardiness to weather, such as drought tolerance
Sheep with good quality wool
Better tasting fruits
Horses with fine features and a fast pace
Large or unusual flowers
Example: all breeds of domestic dog descend from wolves
Example: wild brassica gave rise to cauliflower, cabbage, broccoli, Brussels sprouts, kale and kohlrabi
When breeders choose individuals with specific traits to reproduce, only the genes for those traits are passed on. This reduces the overall number of different genes in the population, making the gene pool smaller — which leaves populations more vulnerable to disease and environmental change, because they lack the flexibility to adapt.
Low genetic diversity makes selected breeds vulnerable to sudden environmental change, risking crop failure or large-scale livestock deaths
Example
Dairy cows selectively bred to produce more milk, benefiting the dairy industry
Commercial bananas and potatoes have very low genetic diversity. Panama disease has devastated banana plantations
The longer view
Short-term gains in productivity
The Irish potato famine of the 1840s was made worse because people depended on a single potato variety vulnerable to blight, with significant long-term economic costs
This page closes a loop with the genetic diversity note. There, high genetic diversity was what let a population survive change. Here, artificial selection deliberately narrows the gene pool to maximise one trait. The Irish potato famine is what that trade-off looks like when the environment changes: enormous productivity, and then no fallback at all. If a question asks you to evaluate selective breeding, that is the argument.
Worked examples
WE 1
Human-driven natural selection
Explain how poaching has affected the evolution of elephants in Gorongosa National Park. (4 marks)
Step 1: existing variation
The elephant population already contained variation: most individuals had tusks, a small number were tuskless.
Step 2: the selection pressure
Poaching for ivory during the civil war acted as a selection pressure, because poachers targeted elephants with tusks.
Step 3: differential survival
Tuskless elephants were less likely to be killed, so they were more likely to survive to reproductive age and pass on their genes.
Step 4: the outcome
Over time the frequency of the tuskless trait increased, so tuskless elephants became far more common. Human actions drove natural selection.
Human activity supplied the pressure; natural selection did the workstress this is natural, not artificial, selection — nobody bred these elephants
WE 2
Natural versus artificial
Distinguish between natural selection and artificial selection. (3 marks)
Natural selectionEnvironmental pressures determine which traits become more common in a population over time. It is not directed by humans and is not deliberate.
Artificial selection
Humans deliberately choose specific plants or animals to breed based on desirable traits — selective breeding.
The shared mechanism
Both work by allowing some individuals to reproduce more than others, so the underlying process is the same; only the agent doing the selecting differs.
Same mechanism, different selector: environment or humanthe word “deliberate” is what separates the two
WE 3
The cost of selective breeding
Evaluate the use of artificial selection in agriculture. (4 marks)
Benefit 1: productivity
Artificial selection increases crop yields and livestock productivity, giving short-term economic benefits — dairy cows bred for higher milk yield benefit the dairy industry.
Benefit 2: useful traits
Crops can be bred for disease resistance, drought tolerance and better taste, improving food security in stable conditions.
Cost 1: reduced genetic diversity
Only genes for the selected traits are passed on, so the gene pool becomes smaller and populations lack the flexibility to adapt to change.
Cost 2: vulnerability in practice
Commercial bananas and potatoes have very low genetic diversity, and Panama disease has devastated banana plantations. The Irish potato famine was worsened by dependence on a single blight-vulnerable variety.
Large short-term gains bought at the cost of long-term resiliencefinish with the trade-off rather than listing benefits and costs separately
💡 Exam tips
Keep human-driven natural selection and artificial selection apart. Gorongosa is the former.
Use selection pressure and name what it is in each scenario.
Have examples across all four drivers: climate, hunting, fishing, urbanisation.
Know pesticide resistance and antibiotic resistance (MRSA) as extra examples.
For artificial selection, name both a plant and an animal case: wild brassica, dogs from wolves.
Always link reduced genetic diversity to vulnerability, with bananas or the Irish potato famine.
⚠ Common mistakes
Calling the tuskless elephants artificial selection. Poachers were not breeding them deliberately.
Saying polar bears are “choosing” to adapt. Individuals with certain traits survive; none decides anything.
Forgetting the variation existed first. Selection acts on variation already present.
Presenting selective breeding as purely beneficial. It narrows the gene pool.
Confusing selective breeding with genetic modification. Selective breeding is choosing which organisms mate.
Giving no named example of vulnerability. Bananas or the potato famine are needed.
Up next: Reading the Geological Timescale. Everything so far has happened over human timescales or a few thousand generations. The next three pages zoom out to the whole 4.5 billion years.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
