AP Environmental Science

## Learning Outcomes

- Define **selective pressure** clearly and link it to **natural selection**
- Distinguish between selective pressure, adaptation, and evolution
- Explain how selective pressures affect endangered species differently from stable populations
- Identify common natural and human-caused selective pressures
- Describe why small populations are more vulnerable to strong selective pressures
- Apply the concept to AP Environmental Science endangered-species scenarios

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand selective pressures in endangered-species contexts, with a focus on the following syllabus points:

- Know that a **selective pressure** is an environmental factor that changes survival or reproductive success
- Identify the link between selective pressure, **differential survival and reproduction**, and **natural selection**
- Explain the difference between a pressure, a trait response, and a long-term population outcome
- Understand that selective pressures can be natural or anthropogenic
- Recognise major human-driven pressures including habitat loss, climate change, invasive species, pollution, and overharvesting
- Explain why selective pressures do not always lead to successful adaptation
- Understand why small, isolated populations and low genetic diversity increase extinction risk
- Apply the idea of selective pressure to endangered-species case studies and AP-style questions

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is a selective pressure?
2. How is selective pressure different from natural selection?
3. Name two anthropogenic selective pressures affecting endangered species.
4. Why might a strong selective pressure increase extinction risk instead of leading to adaptation?
5. In an elephant population affected by ivory poaching, which is the pressure, which is the trait, and which is the long-term outcome?

## Introduction

Selective pressure is one of those terms that looks simple but causes a lot of confusion in exam answers. Students often mix it up with **adaptation**, **evolution**, or even with the harmful event itself. In AP Environmental Science, that confusion matters because Unit 9 often asks you to connect environmental change to biodiversity loss and extinction risk.

In endangered-species questions, selective pressure should be treated as the **environmental factor** that changes which individuals survive and reproduce more successfully. That definition helps you move from ecology to evolution without losing sight of the APES focus: real environmental threats, shrinking populations, and conservation consequences.

> **Key Term: selective pressure**
> An environmental factor that makes some individuals more likely than others to survive and reproduce.

## Introduction to the definition and the core chain

### Selective pressure, natural selection, and fitness

A selective pressure does not act as a magical force that "gives" organisms the traits they need. Instead, it changes the conditions under which organisms live. If individuals in a population already vary in heritable traits, and one version of a trait works better under that pressure, those individuals tend to leave more offspring.

That gives the main chain you need for the exam:

- selective pressure changes conditions
- some individuals survive or reproduce better
- those individuals pass on their traits more often
- trait frequencies in the population change over generations

This is natural selection.

> **Key Term: natural selection**
> The process by which individuals with advantageous inherited traits are more likely to survive and reproduce, causing those traits to become more common over time.

The textbook language also stresses **evolutionary fitness**. In APES, fitness does not mean strongest, biggest, or fastest. It means reproductive success. An organism is "fit" if it survives long enough and successfully produces offspring that also survive to reproduce.

> **Key Term: evolutionary fitness**
> Reproductive success; the ability of an organism to survive and pass its genes to the next generation.

A useful exam distinction is this:

- **Selective pressure** = cause
- **Natural selection** = process
- **Adaptation** = useful inherited trait
- **Evolution** = population change over time

Students lose marks when they say, for example, "evolution is a selective pressure" or "an adaptation causes natural selection." That reverses the logic.

> **Test Tip:** If a prompt says "identify the selective pressure," do not name the trait. Name the environmental condition or threat, such as drought, predation, habitat loss, or disease.

### Why selective pressures matter at the population level

A common mistake is to describe an individual organism "adapting" during its own lifetime and call that natural selection. APES expects you to remember that individuals experience the pressure, but **populations evolve**.

Suppose a drought hits a plant population. Individual plants do not suddenly grow deeper roots because they need them. Instead, if some plants already had heritable traits linked to deeper roots and that trait improves water access, those plants are more likely to survive and produce seeds. Over many generations, the population may shift toward deeper-rooted plants.

This is why selective pressures are tied to the **gene pool** rather than to short-term individual adjustment.

> **Key Term: gene pool**
> The total collection of genes and alleles present in a population.

In AP Environmental Science, you usually do not need deep genetics detail, but you do need the population logic. A pressure matters evolutionarily only if it affects survival or, even more importantly, reproductive success.

## Common selective pressures affecting endangered species

### Natural pressures and anthropogenic pressures

Selective pressures are not automatically human-caused. Predation, competition, disease, drought, salinity, and temperature have always shaped populations. What APES adds is the environmental-science angle: humans often intensify old pressures or create new ones faster than populations can respond.

A useful comparison is below.

Natural selective pressures

- predators removing more visible prey
- drought favoring drought-tolerant plants
- cold temperatures favoring better insulation
- disease favoring resistant individuals
- competition for limited resources

Anthropogenic selective pressures

- habitat loss and fragmentation
- climate change
- invasive species
- pollution
- overharvesting or overexploitation

Anthropogenic pressures are especially important in endangered-species questions because they are major drivers of the current extinction rate, which is far above background levels.

> **Key Term: background extinction rate**
> The normal rate of extinction that occurs over time without major catastrophic disruption.

### Habitat loss and fragmentation as selective pressures

Habitat loss is the largest single driver of biodiversity decline in many systems. It does two things at once.

First, it has an immediate ecological effect: it reduces food, shelter, nesting sites, migration routes, or breeding habitat. Population size may crash quickly.

Second, it can act as a selective pressure by favoring individuals that can survive in smaller, more disturbed, or more isolated patches. Generalist species often do better than specialists here.

For example, a forest specialist that needs a very specific nesting site may decline sharply when roads divide the forest into fragments. A more flexible species with a broader niche may continue surviving in edge habitat. The pressure is not "specialization" itself. The pressure is the changed environment: smaller fragments, more edges, less interior habitat, and reduced movement.

This is why habitat fragmentation is so dangerous for species that already require large territories, specific food sources, or fixed migration routes.

### Climate change as a selective pressure

Climate change changes temperature, precipitation, season length, storm frequency, snow cover, and sea level. Each of these can become a selective pressure if it changes survival and reproduction.

For example:

- higher temperatures may favor heat-tolerant individuals
- drought may favor plants with water-conserving traits
- reduced snow cover may expose cold-adapted animals
- shifting seasons may disrupt breeding and food timing
- ocean warming and acidification may harm marine species with narrow tolerances

Polar bears are a classic APES-style example of a species under climate-related pressure. Loss of sea ice reduces access to prey and changes the habitat itself. Even if some traits might be favored, the environmental shift may be too fast and too large for successful long-term adaptation.

The key exam idea is that climate change often acts on many variables at once. That makes it more severe than a single, isolated pressure.

### Invasive species, disease, and overharvesting

Invasive species can act as selective pressures through competition, predation, habitat alteration, or disease spread. Because invasives are often adaptable generalists with high reproductive rates, they can quickly outcompete native species, especially specialists.

Disease is also a major pressure. If some individuals in a population are more resistant, disease may favor them. But if the population is already small, the disease may simply reduce numbers below recovery level before any adaptive shift occurs.

Overharvesting is especially important because it often targets specific traits. Hunting, fishing, or poaching does not always remove individuals at random.

If hunters target the largest animals, largest horns, or most valuable body parts, then individuals lacking those traits may be more likely to survive and reproduce. That means overharvesting can become a selective pressure on body size, age at reproduction, or morphology.

A good APES example is ivory poaching in African elephants. The pressure is poaching for tusks. A naturally tuskless elephant may be less likely to be killed, so tusklessness can become more common over generations in heavily poached populations.

> **Test Tip:** In application questions, separate your answer into three parts: pressure, favored trait, outcome. This prevents vague writing and earns clearer credit.

## Why selective pressure can lead to extinction instead of adaptation

### The APES caution students often miss

One of the most important distinctions in this subtopic is that selective pressures do **not** guarantee adaptation. In biology summaries, natural selection can sound neat and efficient. In conservation, it often is not.

Why not?

Because adaptation requires:

- heritable variation already present in the population
- enough individuals surviving to reproduce
- enough time across generations
- environmental change slow enough for selection to act

If one or more of these conditions is missing, a strong selective pressure may simply reduce the population and raise extinction risk.

> **Key Term: adaptation**
> A heritable trait that increases survival or reproductive success in a particular environment.
>
> **Key Term: evolution**
> Change in the inherited characteristics of a population over generations.

Imagine a small amphibian population in wetlands. Pollution increases, water temperature changes, and habitat shrinks. In theory, pollution tolerance or heat tolerance could be favored. In practice, if the population is tiny, reproduction is low, and the wetland is disappearing quickly, there may not be enough variation or time for adaptation to spread.

The likely result is decline, not rescue.

### Small populations, low diversity, and bottlenecks

Small populations are more vulnerable for several reasons.

First, they have fewer breeding individuals, so even a moderate extra mortality rate matters more.

Second, they often have lower genetic diversity. That means fewer heritable trait variants are available for natural selection to favor.

Third, they are more affected by random events, including **genetic drift**, disease outbreaks, and local disasters.

> **Key Term: genetic drift**
> Random changes in allele frequencies, especially important in small populations.

A population bottleneck makes this worse by sharply reducing numbers and, therefore, future genetic variation. Endangered species often experience bottlenecks after overhunting, habitat loss, or severe disturbance. Even if numbers recover later, the reduced diversity can leave the population more vulnerable to future pressures.

This is why APES often links endangered species with low genetic diversity, specialist niches, low reproductive rates, and limited ranges. Those characteristics reduce resilience under strong selective pressures.

### Worked example: pressure vs adaptation vs outcome

Consider this AP-style scenario:

A population of elephants is heavily affected by poaching for ivory.

Work through it carefully:

1. **Selective pressure:** poaching for tusks
2. **Favored trait/adaptation:** tusklessness, if naturally heritable and it reduces risk of being killed
3. **Natural selection process:** tuskless elephants survive longer and reproduce more often than tusked elephants in that poaching environment
4. **Evolutionary outcome:** over generations, the proportion of tuskless individuals may increase
5. **Conservation caution:** if poaching is too intense, total population size may still fall sharply despite selection for tusklessness

That final point is where strong answers stand out. You are showing both ecological and evolutionary thinking.

## Applying the definition in AP Environmental Science questions

### How to tell what the prompt is really asking

Many exam questions on this topic are not phrased as "Define selective pressure." Instead, they may ask you to explain why a species becomes endangered, why a trait becomes more common, or why adaptation might fail.

Ask yourself:

- Is the prompt asking for the **pressure**?
- Is it asking for the **trait** favored by the pressure?
- Is it asking for the **process** of natural selection?
- Is it asking for the **outcome** in the population?
- Is it asking for an **environmental management consequence**?

Examples:

- "Identify a selective pressure on coral reefs."  
  Name a pressure such as ocean warming or acidification.

- "Explain how pollution can act as a selective pressure."  
  Explain that pollution changes survival and reproduction, favoring tolerant individuals if heritable variation exists.

- "Why might an endangered species fail to adapt?"  
  Explain small population size, low genetic diversity, rapid environmental change, and low reproductive rate.

### Common mistakes to avoid

Students often:

- define selective pressure as "survival of the fittest" rather than the environmental factor
- say individuals evolve during their lifetime
- assume every pressure creates successful adaptation
- ignore reproduction and focus only on short-term survival
- confuse an adaptation with the threat itself

For example, "camouflage" is not the selective pressure. Predation is the pressure; camouflage is the adaptation.

Or, "climate change caused the species to evolve heat tolerance immediately" is wrong. Climate change is the pressure; if heat-tolerant heritable variation already exists, the population may gradually shift over generations.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- A selective pressure is an environmental factor that changes survival or reproductive success
- Selective pressure is the cause, while natural selection is the process
- Adaptation is a heritable trait that improves success under a given pressure
- Evolution is population-level change over generations, not individual change within a lifetime
- Common selective pressures include predation, competition, disease, drought, and temperature
- Major anthropogenic pressures include habitat loss, fragmentation, climate change, invasive species, pollution, and overharvesting
- Selective pressures matter only when they affect fitness, especially reproductive success
- Endangered species are especially vulnerable because many have small populations, low genetic diversity, specialist needs, or low reproductive rates
- Strong selective pressures do not always produce adaptation; they may instead cause population decline or extinction
- Overharvesting can act as a selective pressure if it removes individuals with particular traits
- In APES answers, it is useful to separate pressure, favored trait, and long-term outcome
- Small populations are more affected by genetic drift and may lack enough variation to adapt

## Key Terms and Concepts

- selective pressure
- natural selection
- evolutionary fitness
- gene pool
- background extinction rate
- adaptation
- evolution
- genetic drift


Endangered Species - Selective pressures definition