🌱 The Four Key Requirements
For natural selection to work, four things must be present: variation in traits, inheritance of traits, selection pressure from the environment and time for changes to accumulate across generations.
Evolution and Selection » Natural Selection Process
What you'll learn this session
Study time: 30 minutes
- Understand what natural selection is and how it works
- Learn about the key factors that drive natural selection
- Explore real-world examples like Darwin's finches and peppered moths
- Discover how variation, inheritance and selection pressure create evolution
- Examine case studies showing natural selection in action
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Unlock This CourseIntroduction to Natural Selection
Natural selection is one of the most important ideas in biology. It explains how species change over time and how new species can form. Think of it as nature's way of choosing which organisms survive and reproduce based on how well they're suited to their environment.
Charles Darwin first described natural selection in 1859, calling it "survival of the fittest." But this doesn't mean the strongest or fastest always win - it means those best adapted to their environment are more likely to survive and pass on their genes.
Key Definitions:
- Natural Selection: The process where organisms with favourable characteristics are more likely to survive and reproduce.
- Variation: Differences between individuals in a population.
- Selection Pressure: Environmental factors that affect an organism's ability to survive and reproduce.
- Adaptation: A characteristic that helps an organism survive in its environment.
- Fitness: How well an organism is suited to its environment and how likely it is to reproduce successfully.
How Natural Selection Works
Natural selection follows a simple but powerful process. It starts with variation - not all individuals in a population are exactly the same. Some might be faster, others might have better camouflage, or some might be more resistant to disease.
The Natural Selection Process
The process happens in stages. First, there's variation within a population. Then, environmental pressures create challenges - like predators, food shortages, or climate changes. Individuals with helpful traits are more likely to survive these challenges. These survivors reproduce more successfully, passing their beneficial traits to their offspring. Over many generations, these helpful traits become more common in the population.
🎲 Variation
Individuals differ in their characteristics due to genetic differences and environmental factors.
⚡ Selection Pressure
Environmental challenges that affect survival, like predators, disease, or climate.
👻 Inheritance
Successful traits are passed from parents to offspring through genes.
Case Study Focus: Darwin's Finches
On the Galápagos Islands, Darwin observed finches with different beak shapes. During droughts, finches with larger, stronger beaks could crack tough seeds and survived better than those with smaller beaks. Over time, the average beak size in the population increased. When rains returned and soft seeds became available, smaller beaks became advantageous again and average beak size decreased.
Types of Selection Pressure
Selection pressures are the environmental factors that influence which organisms survive and reproduce. These pressures can be biotic (living) or abiotic (non-living) factors.
Biotic Selection Pressures
These involve interactions with other living organisms. Predation is a major biotic pressure - organisms that can better avoid or escape predators are more likely to survive. Competition for resources like food, water, or mates also creates selection pressure. Disease resistance is another important factor, as organisms that can fight off infections have better survival chances.
Abiotic Selection Pressures
These are non-living environmental factors. Climate plays a huge role - organisms adapted to their local temperature and rainfall patterns survive better. Geographic features like mountains or rivers can create barriers that lead to different selection pressures in different areas. Chemical factors in soil or water can also influence which organisms thrive.
🌮 Predator-Prey Relationships
As predators evolve better hunting abilities, prey species evolve better defence mechanisms. This creates an "evolutionary arms race" where both species continuously adapt to each other.
Evidence of Natural Selection
We can see natural selection happening all around us, sometimes very quickly. Some of the best examples come from organisms that reproduce rapidly, allowing us to observe changes over just a few generations.
The Peppered Moth Study
This is one of the most famous examples of natural selection observed in real time. Before the Industrial Revolution, light-coloured peppered moths were common in England because they camouflaged well against light tree bark. Dark moths were rare because birds could easily spot them.
During the Industrial Revolution, pollution darkened tree trunks with soot. Suddenly, dark moths had the advantage - they were harder for birds to see against the dark bark. Light moths became easy targets. Within decades, dark moths became much more common in polluted areas.
When pollution controls were introduced in the late 20th century, tree bark became lighter again. The moth populations shifted back, with light moths becoming more common once more. This showed natural selection working in both directions as environmental conditions changed.
Modern Example: Antibiotic Resistance
Bacteria evolve resistance to antibiotics through natural selection. When antibiotics are used, most bacteria die, but a few with resistance genes survive. These resistant bacteria reproduce rapidly, creating populations that are harder to treat. This is why doctors are careful about prescribing antibiotics and why patients must complete their full course of treatment.
Factors Affecting Natural Selection
Several factors influence how quickly and effectively natural selection works in different populations and environments.
Population Size and Genetic Diversity
Larger populations typically have more genetic variation, providing more raw material for natural selection to work with. Small populations may lose beneficial traits by chance, a process called genetic drift. Genetic diversity is crucial because it ensures some individuals will have traits suited to changing conditions.
📈 Generation Time
Species with shorter generation times can evolve more quickly because beneficial traits spread faster through populations.
🌎 Environmental Stability
Stable environments favour well-adapted traits, while changing environments favour flexibility and variation.
🧬 Reproductive Success
Traits that increase the number of surviving offspring will spread more quickly through populations.
Natural Selection and Human Impact
Human activities create new selection pressures that can drive rapid evolutionary changes in other species. Understanding these impacts helps us make better decisions about conservation and environmental management.
Human-Induced Selection Pressures
Pollution, habitat destruction, hunting and climate change all create new challenges for wildlife. Some species adapt quickly to urban environments, developing traits like increased noise tolerance or changed feeding behaviours. Others struggle to adapt fast enough and face extinction.
Overfishing has led to fish evolving smaller body sizes and earlier reproduction, as larger fish are more likely to be caught. Pesticide use has driven the evolution of resistance in many insect species, similar to antibiotic resistance in bacteria.
Conservation Implications
Understanding natural selection helps conservationists protect endangered species. Maintaining genetic diversity in small populations, creating wildlife corridors to connect habitats and reducing human-caused selection pressures all support species' ability to adapt to changing conditions. Captive breeding programmes carefully manage genetics to preserve variation needed for future adaptation.
Summary and Key Points
Natural selection is the mechanism that drives evolutionary change, shaping life on Earth over millions of years. It requires variation, inheritance, selection pressure and time. We can observe it happening today in examples like antibiotic resistance and urban wildlife adaptation.
The process is ongoing and universal - every species faces selection pressures that influence which individuals survive and reproduce. Understanding natural selection helps us appreciate the incredible diversity of life and make informed decisions about conservation, medicine and environmental management.
💡 Remember
Natural selection doesn't create new traits - it selects from existing variation. Mutations and genetic recombination provide the variation, while natural selection determines which variants become more common over time.