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In the previous lesson, you examined how mutation and sexual reproduction generate genetic diversity, and how reproductive isolation leads to speciation. This lesson explores what happens when humans become a driving force in selection. Sometimes this is unintentional: human activities such as poaching, harvesting and climate change alter the selective pressures acting on wild species, causing them to evolve in new directions. Sometimes it is deliberate: humans choose specific plants and animals for breeding in order to produce desirable traits. Both processes have profound consequences for genetic diversity and, therefore, for the resilience of populations and ecosystems.

Human impacts on natural selection

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3.1.13 Human activities have impacted the selective forces acting on species within ecosystems, resulting in evolutionary change in these species.

Consider human activities that have affected natural selection, for example, climate change due to burning fossil fuels, hunting/poaching/harvesting, or creation of new habitats. Include the example of the tuskless elephants in Gorongosa, Mozambique or a local example.

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Human activities are now so widespread and intense that they are altering the selective forces acting on species across the globe. Crucially, this is not deliberate selection; humans are not intentionally choosing which individuals survive and reproduce. Instead, human actions inadvertently change the environment in ways that favour certain traits over others, shifting the direction of natural selection.

Three categories of human activity are particularly significant:

• Climate change due to burning fossil fuels: rising temperatures, shifting rainfall patterns and changing seasons alter which traits are advantageous. Species that cannot adapt quickly enough face declining populations or local extinction.
• Hunting, poaching and harvesting: these activities selectively remove individuals with particular traits (such as large body size or tusks), leaving individuals without those traits to survive and reproduce. Over time, this shifts the genetic composition of the population.
• Creation of new habitats: urbanisation, agriculture and infrastructure create novel environments. Species that can exploit these new conditions (such as urban-adapted birds or roadside-tolerant plants) gain a selective advantage.

In each case, the key point is the same: human activities change which traits confer a survival advantage, and natural selection responds by shifting the frequency of those traits in the population. The result is evolutionary change driven by human activity.

1. Distinguish between a selective force that is "natural" and one that has been altered by human activity. Use an example to illustrate your answer.

2. Explain why the evolutionary changes described in 3.1.13 are considered examples of natural selection rather than artificial selection, even though humans are involved.

Mandated case study: Tuskless elephants in Gorongosa, Mozambique

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The tuskless elephants of Gorongosa National Park in Mozambique provide one of the most compelling documented examples of human activity driving rapid evolutionary change in a large mammal.

Background: elephant populations naturally contain a mix of tusked and tuskless individuals. Tusks serve as tools for digging, stripping bark and defence, so tusked elephants normally have a selective advantage. Tuskless individuals are usually a small minority.

The Mozambican Civil War (1977 to 1992): during 15 years of conflict, fighters on both sides of the war slaughtered elephants for their ivory, selling tusks to fund the purchase of weapons. Tusked elephants were specifically targeted because their ivory was valuable. Tuskless females, having no ivory, were of no interest to poachers and were more likely to survive.

The population collapse: approximately 90% of Gorongosa's elephant population was killed during the war, with the population falling from around 2,500 to roughly 200 individuals.

The evolutionary shift: before the war, approximately 18.5% of female elephants in Gorongosa were naturally tuskless. Among the females that survived the war, approximately 51% were tuskless. Among the next generation of females born after the war ended, approximately 33% were tuskless. The frequency of the tuskless trait had nearly doubled in a single generation.

The genetic basis: research published in Science in 2021 by Campbell-Staton et al. confirmed that tusklessness in Gorongosa elephants has a genetic basis linked to genes involved in tooth development. Importantly, the tuskless trait is inherited and appears to affect only females: males that carry the tuskless gene variant do not survive to birth. This means that a high frequency of tusklessness in the population could reduce the number of males born, potentially slowing the population's recovery.

Why this matters for ESS: this is an example of human activity (poaching) altering the selective pressure acting on a population. Poachers did not intend to breed tuskless elephants; they simply killed tusked ones for profit. But the consequence was a rapid, measurable evolutionary change. As the park recovers and poaching pressure decreases, the proportion of tuskless females is expected to slowly decline over future generations.

Mozambique: Tuskless elephant evolution linked to ivory hunting

1. Identify the selective pressure acting on the Gorongosa elephant population during the civil war.

2. Explain why tusklessness increased in frequency in the population, using the principles of natural selection (variation, differential survival, heritability).

3. Suggest what effect a high frequency of the tuskless gene variant could have on the elephant population's recovery, given that males carrying this variant do not survive to birth.

4. Explain why the Gorongosa example is classified under 3.1.13 (human-altered natural selection) rather than 3.1.14 (artificial selection).

Local example: Earlier egg-laying in European great tits