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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Natural Selection Theory
Natural selection is the process that drives evolution and explains how species change over time. First proposed by Charles Darwin in 1859, this theory revolutionised our understanding of life on Earth. Today, we'll explore how natural selection works and why it's so important in biology.
Key Definitions:
Charles Darwin developed his theory after his voyage on HMS Beagle (1831-1836). During this journey, he observed countless species and collected specimens from around the world. The Galápagos Islands were particularly influential, where he noticed that finches on different islands had differently shaped beaks, each suited to the available food sources.
While Darwin is often credited with natural selection, Alfred Russel Wallace independently developed the same theory. Both scientists presented their ideas jointly to the Linnean Society in 1858, before Darwin published his famous book "On the Origin of Species" in 1859.
Darwin's theory of natural selection is based on four key principles that explain how species evolve over time:
Individuals within a species show variation in their traits. These differences can be physical (like height or colour) or physiological (like enzyme efficiency). Some variations are inherited, while others are caused by environmental factors.
Example: In a population of beetles, some might be green, others brown, with many shades in between.
Many traits can be passed from parents to offspring through genes. This genetic inheritance is what allows advantageous traits to be preserved in a population over generations.
Example: If a beetle's green colour is genetic, it can pass this trait to its offspring.
Organisms produce more offspring than can survive with limited resources. This creates competition among individuals for food, space and other necessities.
Example: A single oak tree produces thousands of acorns, but only a few will grow into trees.
Individuals with advantageous traits are more likely to survive and reproduce, passing these traits to the next generation. Over time, beneficial traits become more common in the population.
Example: If green beetles are better camouflaged in their environment, they're less likely to be eaten by predators and more likely to reproduce.
Variation is essential for natural selection to work. Without differences between individuals, there would be nothing for nature to "select." Variation comes from several sources:
Changes in DNA that create new alleles (versions of genes). Mutations are random and can be caused by errors in DNA replication or environmental factors like radiation.
Combines genetic material from two parents, creating unique combinations of genes through processes like crossing over and independent assortment during meiosis.
Non-genetic factors like nutrition, climate and disease can affect an organism's development and appearance (phenotype).
The phrase "survival of the fittest" (coined by Herbert Spencer, not Darwin) describes how natural selection favours organisms that are best adapted to their environment. "Fitness" in biology doesn't mean physical strength it refers to an organism's ability to survive and reproduce.
Adaptations can take many forms, all helping organisms survive in their specific environments:
Physical features like the shape of a bird's beak, a giraffe's long neck, or a cactus's spines.
Actions that help survival, such as migration, hibernation, or complex mating rituals.
Internal processes like producing enzymes that work at different temperatures or developing immunity to diseases.
One of the most famous examples of natural selection in action involves peppered moths (Biston betularia) in England during the Industrial Revolution. Before industrialisation, most peppered moths were light-coloured, which helped them blend in with lichen-covered trees. Dark-coloured moths were rare and easily spotted by birds.
When pollution from factories killed lichens and darkened tree trunks with soot, the situation reversed. Light moths became visible against the dark trees and were eaten by birds, while dark moths were camouflaged and survived better. Within decades, the proportion of dark moths increased dramatically in industrial areas.
When air quality laws reduced pollution, trees lightened again and the light-coloured moths once more became predominant. This case study demonstrates natural selection responding to environmental changes in real-time.
Scientists have gathered extensive evidence supporting natural selection from various fields:
Humans have created thousands of domestic plant and animal varieties by selectively breeding for desired traits. From dogs to cabbage varieties, artificial selection demonstrates how traits can be changed over generations just as natural selection does in the wild, but directed by humans.
Fossils show how species have changed over time. For example, fossil sequences of horses reveal how they evolved from small, multi-toed animals to larger, single-toed modern horses as grasslands expanded and their environment changed.
Bacteria can evolve resistance to antibiotics in just a few years a perfect example of natural selection in action. When exposed to antibiotics, bacteria with random mutations that provide resistance survive and reproduce, while others die off.
Scientists have observed new species forming, particularly in plants and insects. For example, when two populations of the same species become geographically isolated and face different selection pressures, they can evolve into separate species that can no longer interbreed.
Natural selection isn't just something that happened in the distant past it's happening all around us right now:
Many insect pests have evolved resistance to insecticides through natural selection. When an insecticide is applied, insects with natural resistance survive and reproduce, passing on their resistant genes. This is why farmers need to rotate different types of insecticides.
After the introduction of warfarin (a rat poison) in the 1950s, some rat populations evolved resistance within a decade. Rats with mutations that allowed them to process the poison survived and reproduced, while others died.
The Galápagos finches that inspired Darwin continue to show natural selection in action. Scientists Peter and Rosemary Grant have studied these birds for over 40 years, documenting how their beaks change in response to environmental conditions.
During a severe drought in 1977, small seeds became scarce, leaving mainly large, tough seeds. Birds with smaller beaks couldn't crack these seeds and died. The average beak size in the population increased by 10% in just one generation!
When the rains returned and small seeds became plentiful again, the selection pressure reversed. This ongoing research provides direct observation of natural selection responding to changing environmental conditions.
Natural selection is the primary mechanism driving evolution, but it's important to understand how individual changes add up to larger evolutionary patterns:
Small changes within a species, like changes in gene frequencies or traits. Examples include peppered moths changing colour or bacteria developing antibiotic resistance. These changes happen relatively quickly sometimes in just a few generations.
Larger-scale changes that lead to new species, genera, or higher taxonomic groups. This happens when microevolutionary changes accumulate over long periods, eventually resulting in organisms that can no longer interbreed with their ancestors.
Natural selection is an elegant explanation for the diversity of life on Earth. By favouring individuals with traits that help them survive and reproduce in their specific environment, natural selection gradually shapes species to be well-adapted to their surroundings.
Remember these key points:
In your next lesson, you'll learn about how species form and the different types of speciation that can occur through natural selection.
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