10.2 Theory of Natural Selection

Short Introduction Natural selection is the process through which organisms who possess traits that are helpful for reproduction in a certain environment produce more children in the following generation. As a result, they gradually expand the population's genetic diversity. This theory was proposed by Charles Darwin after his five-year voyage around the world on the ship HMS Beagle. He published his observations in the book "On the Origin of Species by Natural Selection" in 1859. A classic example of natural selection in action is the case of the peppered moth in Britain.

Main Idea

The theory of natural selection is based on four key principles: overproduction, competition, variations, and natural selection. Individuals with advantageous traits survive and reproduce, passing those traits to the next generation. Over time, this leads to evolutionary change in populations. Charles Darwin developed this theory based on observations made during his voyage on HMS Beagle, particularly the variations in finches on the Galapagos Islands. The peppered moth case study provides strong evidence for natural selection in action.

Core Concept

Definition of Natural Selection Natural selection is the process through which organisms who possess traits that are helpful for reproduction in a certain environment produce more children in the following generation. As a result, they gradually expand the population's genetic diversity. generated by DeepSeek to PDF

Principles of Natural Selection The theory of natural selection is based on four principles: 1. Overproduction: All living creatures are very fertile so that their populations could grow quickly to produce a large number of offspring. 2. Competition: We can observe that population sizes do not rise to a very high level even after overproduction. The numbers largely stay constant. This is because food supplies and other resources are scarce. As a result, there is competition for resources and food. 3. Variations: No two organisms are the same. They will not have the same opportunities to survive and reproduce as others. A large portion of this diversity is heritable. Such qualities are inherited from parents through the genes of the offspring. 4. Natural Selection: Only those organisms who live and reproduce can give rise to the next generation. And only those organisms with the desirable variations will live till reproductive age. The population will have more advantageous genetic variations and fewer harmful ones after many generations. Natural selection is essentially an elimination process. Only the fittest organisms will survive. Charles Darwin and HMS Beagle After a five-year voyage around the world in the ship HMS Beagle, Charles Darwin put forth the Theory of Natural Selection in 1838. In 1859, he released a book titled "On the Origin of Species by Natural Selection" recording his observations. Darwin's Observations at the Galapagos Islands Charles Darwin was able to create his theory of natural selection when he visited the Galapagos Islands. He found a number of finch species that varied from island to island. He noticed that the form of the birds' beaks varied depending on the kind of food they ate. Darwin believed that the processes of speciation and adaptation were linked. The overall accumulation of adaptations in different environments, isolated from the original environment by geographic barriers, would lead to the formation of a new species from an ancestral form. The two groups could differ sufficiently over many generations to be classified as two different species. Darwin identified three basic patterns in biological diversity: 1. Global 2. Local 3. Variation over time in species Case Study: Industrial Melanism of the Peppered Moth (Biston betularia) generated by DeepSeek to PDF

The phrase "Changes in the environment can bring about changes in a population by natural selection" is supported by evidence. Before 1848: All reported variants of the peppered moth in the UK were light-coloured with dark patches. 1848: A dark-coloured moth was discovered. By 1895: Dark-coloured moths made up 95% of the peppered moth population. This variety's dark colour resulted from a mutation. How it works: The moths fly at night and rest on tree trunks throughout the day. The light-coloured lichens growing on the tree trunks blended in with the colour of the light- coloured moth, giving it excellent concealment from predator birds. As the Industrial Revolution progressed, sulphur dioxide pollution from coal burning drove out lichens that were growing on trees in industrial locations, exposing the darker bark. The predator birds could now easily locate and consume the light-coloured moths resting on dark-coloured trunks. Light-coloured moths are "seen and eaten by the birds" when they settle on dark-coloured trunks. The birds are serving as snoopers. The dark colour was the adaptive variable that resulted in the selection advantage. 90% of the peppered moth population in England during the industrial era of pollution was black. This phenomenon is called Industrial Melanism. After the 1950s: England's industrial pollution was reduced, and as a result, the number of light-coloured moths also increased. Analysis of the Case Study: 1. The variation (change in bark colour) is an illustration of variation that is subject to selection (by the birds) in a changing environment. 2. It demonstrates the truth of evolution. 3. It points out that evolution and natural selection operate on the basis of genetic change within a population.

Detailed Explanation

Natural selection is the mechanism by which evolution occurs. It was first proposed by Charles Darwin and Alfred Russel Wallace, but Darwin is more famously associated with it. 1. Overproduction: Organisms produce more offspring than can possibly survive. For example, a single fish can produce thousands of eggs, but only a few survive to adulthood. 2. Competition: Because resources (food, water, space, mates) are limited, individuals must compete for them. Only a fraction of offspring survive. 3. Variations: Individuals within a population vary in their traits. Some variations are beneficial, some are harmful, and some are neutral. Much of this variation is heritable (genetic). 4. Natural Selection: Individuals with beneficial variations are more likely to survive, reproduce, and pass their traits to their offspring. Over many generations, the frequency of beneficial traits increases in the population. Natural selection is an elimination process — only the fittest survive. generated by DeepSeek to PDF

5. Darwin's Finches: Darwin observed that finches on different Galapagos Islands had different beak shapes. He hypothesized that the beaks were adapted to the type of food available on each island. This led him to propose that species can change over time through natural selection. 6. Peppered Moth: Before the Industrial Revolution, light-coloured moths were well-camouflaged on lichen-covered tree trunks. A dark mutation existed but was rare. As pollution killed the lichens and darkened the tree trunks, light-coloured moths became visible to predators, while dark-coloured moths were better camouflaged. The dark moths survived and reproduced, increasing their frequency to 95% by 1895. After pollution controls were introduced in the 1950s, the frequency of light-coloured moths increased again. This is a classic example of natural selection in action.

Intermediate Level

Natural selection is the process by which organisms with traits that improve survival and reproduction in a given environment produce more offspring, gradually increasing the frequency of those traits in the population. It is based on four principles: 1. Overproduction: Organisms produce many offspring. 2. Competition: Limited resources lead to competition. 3. Variations: Individuals vary in heritable traits. 4. Natural Selection: Only the fittest survive and reproduce. Charles Darwin developed the theory after his five-year voyage on HMS Beagle, published in 1859. His observations of finches on the Galapagos Islands (beak variations) helped him formulate the theory. The peppered moth case study demonstrates natural selection: before 1848, light-coloured moths were common. A dark mutation appeared in 1848. During the Industrial Revolution, pollution darkened tree trunks and killed lichens, making light moths visible to predators. Dark moths survived and increased to 95% by 1895. After pollution reduction in the 1950s, light moths increased again. This is called Industrial Melanism.

Important Points

• 1. Natural selection = organisms with helpful traits for reproduction in a certain environment produce
• more offspring, expanding the population's genetic diversity.
• 2. Four principles:
• Overproduction: Living creatures are very fertile and produce many offspring.
• Competition: Populations are limited by scarce resources (food, water); competition occurs.
• Variations: No two organisms are the same; much variation is heritable.
• Natural Selection: Only those with desirable variations survive and reproduce; natural
• selection is an elimination process.
• 3. Charles Darwin: Five-year voyage on HMS Beagle; proposed theory in 1838; published "On the Origin of
• Species" in 1859.
• 4. Galapagos Islands: Darwin observed finches with beak forms varying by food type; linked adaptation and
• speciation.
• 5. Darwin identified three patterns in biological diversity: global, local, and variation over time in species.
• 6. Peppered moth (Biston betularia):
• Before 1848: Light-coloured moths with dark patches.
• 1848: Dark-coloured moth discovered (mutation).
• By 1895: Dark moths made up 95% of the population (Industrial Melanism).
• Cause: Industrial pollution (SO₂) killed lichens, darkened tree bark; dark moths better
• camouflaged; birds ate light moths.
• After 1950s: Pollution reduced; light-coloured moths increased.
• 7. Industrial Melanism = the phenomenon where dark-coloured moths became dominant due to industrial
• pollution.
• 8. The case study shows: (1) variation is subject to selection in a changing environment, (2) it demonstrates
• the truth of evolution, (3) evolution and natural selection operate on genetic change within a population.

Remember

• Natural selection = differential survival and reproduction.
• Principles: Overproduction, Competition, Variations, Natural Selection.
• Darwin: HMS Beagle (5 years), theory in 1838, book in 1859.
• Finches: Galapagos Islands; beak variations by food type; adaptation and speciation linked.
• Peppered moth: Light → dark (95% by 1895) due to pollution; light again after 1950s.
• Industrial Melanism = dark moths dominant due to pollution.
• Natural selection is an elimination process.
• generated by DeepSeek to PDF

Common Mistakes

• 1. Thinking natural selection is "survival of the strongest": It is survival of the fittest, meaning best
• adapted to the environment.
• 2. Forgetting that variations must be heritable: Only heritable traits can be passed to offspring and
• influence evolution.
• 3. Thinking Darwin proposed natural selection in 1859: He proposed it in 1838 and published his book in
• 1859.
• 4. Confusing overproduction and competition: Overproduction produces many offspring; competition
• results from limited resources.
• 5. Thinking the peppered moth changed color individually: The population changed because dark moths
• survived better; individual moths do not change color.
• 6. Forgetting the role of lichens: Lichens on tree trunks camouflaged light moths; pollution killed lichens,
• exposing dark bark.
• 7. Thinking natural selection acts on individuals: Natural selection acts on populations; individuals are
• selected for or against.

Quick Revision

• Natural selection is the process through which organisms with traits helpful for reproduction in a certain
• environment produce more offspring, gradually expanding the population's genetic diversity. It is based on four
• principles: overproduction, competition, variations, and natural selection. Charles Darwin developed the theory after
• his five-year voyage on HMS Beagle, publishing "On the Origin of Species" in 1859. He observed finches on the
• Galapagos Islands with beak variations adapted to different food sources. The peppered moth case study illustrates
• natural selection: before 1848, light-coloured moths were common; a dark mutation appeared; industrial pollution
• darkened tree trunks and killed lichens, making light moths visible to predators; dark moths increased to 95% by
• 1895 (Industrial Melanism); after pollution reduction in the 1950s, light moths increased again.

60-Second Revision

• Natural selection = differential survival and reproduction.
• Principles: Overproduction, Competition, Variations, Natural Selection.
• Darwin: HMS Beagle (5 years), theory in 1838, book in 1859.
• Finches: Galapagos Islands; beak variations by food type.
• Peppered moth: Light → dark (95% by 1895) due to pollution; light again after 1950s.
• Industrial Melanism = dark moths dominant due to pollution.
• Natural selection is an elimination process.
• Frequently Asked Questions (FAQs)
• generated by DeepSeek to PDF

• Q1. What is natural selection? Answer: Natural selection is the process through which organisms who possess
• traits that are helpful for reproduction in a certain environment produce more children in the following generation.
• As a result, they gradually expand the population's genetic diversity.
• Q2. What are the four principles of natural selection? Answer: The four principles are:
• 1. Overproduction: Living creatures produce many offspring.
• 2. Competition: Limited resources lead to competition.
• 3. Variations: No two organisms are the same; much variation is heritable.
• 4. Natural Selection: Only those with desirable variations survive and reproduce.
• Q3. Who proposed the theory of natural selection? Answer: Charles Darwin proposed the theory of natural
• selection after a five-year voyage around the world on the ship HMS Beagle. He put forth the theory in 1838 and
• published it in 1859 in "On the Origin of Species by Natural Selection."
• Q4. What did Darwin observe on the Galapagos Islands? Answer: Darwin found a number of finch species that
• varied from island to island. He noticed that the form of the birds' beaks varied depending on the kind of food they
• ate. He believed that the processes of speciation and adaptation were linked.
• Q5. What is Industrial Melanism? Answer: Industrial Melanism is the phenomenon where dark-coloured moths
• became dominant in the peppered moth population due to industrial pollution. The pollution killed lichens and
• darkened tree trunks, giving dark moths a camouflage advantage.
• Q6. Describe the peppered moth case study. Answer: Before 1848, light-coloured peppered moths were common.
• A dark mutation appeared in 1848. During the Industrial Revolution, sulphur dioxide pollution killed lichens and
• darkened tree bark. Light moths became visible to predators, while dark moths were camouflaged. By 1895, dark
• moths made up 95% of the population. After pollution reduction in the 1950s, light moths increased again.
• Q7. Why is natural selection called an elimination process? Answer: Natural selection is called an elimination
• process because only those organisms with desirable variations survive and reproduce, while those with less
• advantageous traits are eliminated (do not survive or reproduce).
• Q8. What are Darwin's three basic patterns in biological diversity? Answer: Darwin identified three basic
• patterns in biological diversity: (1) global, (2) local, and (3) variation over time in species.
• Q9. What was the adaptive variable in the peppered moth case study? Answer: The dark colour was the
• adaptive variable that resulted in the selection advantage.
• Q10. What does the peppered moth case study demonstrate? Answer: The case study demonstrates that (1)
• variation is subject to selection in a changing environment, (2) it demonstrates the truth of evolution, and (3)
• evolution and natural selection operate on the basis of genetic change within a population.
• Multiple Choice Questions (MCQs)
• Q1. Natural selection is based on how many principles? A. Two B. Three C. Four D. Five Correct Answer: C
• Explanation: The four principles are overproduction, competition, variations, and natural selection.
• Q2. The principle that organisms produce more offspring than can survive is called: A. Competition B.
• Overproduction C. Variation D. Adaptation Correct Answer: B Explanation: Overproduction is the principle that living
• creatures are very fertile and produce many offspring.
• generated by DeepSeek to PDF

• Q3. Charles Darwin made his observations during a voyage on the ship: A. Beagle B. Endeavour C. Victoria D. Santa
• Maria Correct Answer: A Explanation: Darwin voyaged on HMS Beagle.
• Q4. Darwin published "On the Origin of Species" in: A. 1838 B. 1848 C. 1859 D. 1895 Correct Answer: C Explanation:
• Darwin published his book in 1859.
• Q5. Darwin observed finches with different beak shapes on the: A. Hawaiian Islands B. Galapagos Islands C. Maldives
• D. Andaman Islands Correct Answer: B Explanation: Darwin observed finches on the Galapagos Islands.
• Q6. Before the Industrial Revolution, most peppered moths were: A. Dark-coloured B. Light-coloured C. Green-
• coloured D. Red-coloured Correct Answer: B Explanation: Before 1848, all reported variants of the peppered moth
• were light-coloured with dark patches.
• Q7. By 1895, what percentage of the peppered moth population was dark-coloured? A. 50% B. 75% C. 95% D. 100%
• Correct Answer: C Explanation: By 1895, dark-coloured moths made up 95% of the population.
• Q8. Industrial Melanism in peppered moths was caused by: A. Increased light B. Industrial pollution C. Decreased
• predators D. Increased food Correct Answer: B Explanation: Industrial pollution (sulphur dioxide) killed lichens and
• darkened tree bark, giving dark moths a camouflage advantage.
• Q9. The peppered moth case study demonstrates that: A. Evolution and natural selection operate on genetic change
• within a population B. Individual moths can change colour C. Predators do not affect moth populations D. Pollution
• is always beneficial Correct Answer: A Explanation: The case study demonstrates that evolution and natural selection
• operate on the basis of genetic change within a population.
• Q10. In natural selection, which organisms survive? A. The largest B. The strongest C. The fittest (best adapted) D.
• The oldest Correct Answer: C Explanation: Natural selection is essentially an elimination process; only the fittest (best
• adapted) organisms survive.
• Subtopic 10.2 complete. Reply "next" to approve and I will produce Subtopic 3: 10.3 Evidence of Evolution
• (Palaeontology, Comparative Anatomy, Selective Breeding).