Development of Evolutionary Theory

Essential Question

How did the work of many scientists lead to the theory of evolution?

The theory of evolution did not come from one person in one moment. Geologists showed that Earth is very old, an economist described what happens when populations outgrow their food, and naturalists collected evidence from around the world. In this lesson, you’ll follow those ideas to Darwin and Wallace’s explanation of natural selection, and you’ll see why it replaced earlier ideas about how species change.

Ideas Before Darwin

• Hutton (1788) and Lyell (1830): Earth is very old
• Slow processes, such as erosion, shaped Earth
• Lamarck (1809): species change over time
• Malthus (1798): populations outgrow their food

Several ideas came together before Darwin published his theory. The geologists James Hutton and Charles Lyell argued that Earth is extremely old and that the slow processes we see today, such as erosion and the buildup of sediment, shaped it over a long time. That gave life enough time to change gradually. Jean-Baptiste Lamarck proposed that species change over time, which was an important step, although his explanation for how they change was not supported by evidence. Thomas Malthus, an economist, pointed out that human populations can grow faster than their food supply, and Darwin later applied this idea to all living things.

Darwin’s Voyage and the Finches

• Darwin sailed on HMS Beagle from 1831 to 1836
• He collected plants, animals, and fossils
• Galápagos finches have different beak shapes
• Each beak suits the food the finch eats

As a young naturalist, Charles Darwin spent five years on the ship HMS Beagle, collecting specimens from South America and the Pacific islands. On the Galápagos Islands he noticed that similar birds had very different beaks. Later study showed that the finches shared a common ancestor and that each beak shape works best for a certain food. Observations like these led Darwin to ask how a population of one species could change into forms suited to different environments.

Darwin’s Logic: Natural Selection

• Observation: more offspring are born than can survive
• Observation: individuals vary, and some variation is inherited
• Inference: individuals compete for limited resources
• Inference: helpful inherited traits become more common

Darwin built his explanation from observations and inferences. He observed that parents make more offspring than can survive, and that individuals in a population vary in traits that can be inherited. He inferred that individuals must compete for limited resources, and that those with helpful traits are more likely to survive and have offspring. Over many generations, helpful traits become more common in the population. Darwin called this process natural selection. Alfred Russel Wallace reached the same idea independently, and the two men presented it together in 1858.

More Offspring Than Can Survive

• Populations make many more offspring than survive
• Resources such as food and space are limited
• Competition decides which individuals survive

The graph shows sample data for three species. Every species produces far more offspring than will ever survive to breed. A frog may lay 2,000 eggs, yet on average only 2 of its offspring survive to reproduce. If all of them survived, the population would grow out of control. Because resources are limited, the offspring compete, and the ones with traits that suit their environment are more likely to be among the survivors. The graph uses a log scale so that very different numbers can fit on one graph.

Natural Selection or Lamarck’s Idea?

• Lamarck: traits gained during life are passed on
• Traits gained during life are not inherited
• Natural selection works on inherited variation
• Individuals do not evolve; populations do

Lamarck thought that an organism could change during its life by using or not using a body part, and then pass that change to its offspring. A giraffe stretching its neck is the classic example. We now know that changes an organism picks up during its life, such as bigger muscles, are not inherited. In natural selection, the variation already exists in the population. Individuals with longer necks leave more offspring, so the average neck length of the population changes. The individuals themselves do not change.

Many Scientists, One Theory

• Evolution grew from many contributions
• Darwin and Wallace supplied natural selection
• Mendel later explained how traits are inherited

Scientist	Year	Idea	Why it mattered
Hutton	1788	Earth is very old and changes slowly	Gave life enough time to change
Malthus	1798	Populations can outgrow their food	Showed that competition must follow
Lamarck	1809	Species change over time	Raised the idea of change, but the mechanism was wrong
Lyell	1830	Slow processes shaped Earth	Supported an old Earth
Wallace	1858	Natural selection (independently)	A second scientist reached the same idea
Darwin	1859	On the Origin of Species	Natural selection supported with evidence
Mendel	1866	Traits are inherited as separate units	Later explained how variation is inherited

Each scientist in the table added a piece. Mendel’s experiments with pea plants, published in 1866, showed that traits are passed on as separate units that we now call genes. His work was not noticed until it was rediscovered in 1900. Combining natural selection with genetics gave scientists a complete explanation for how populations change over time, and it is the foundation of modern biology.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Beetles on Bark

Beetles on a tree trunk are green or brown, and color is inherited. Birds find green beetles more easily against the brown bark and eat more of them. Use Darwin’s logic to explain how the beetle population changes. Then explain how Lamarck’s idea would explain the change differently.

Show the solution

1. Variation: the population has both green and brown beetles, and the color is inherited.
2. Selection: birds eat more of the green beetles, so fewer green beetles survive to reproduce.
3. Inheritance: the surviving brown beetles pass on the brown trait to their offspring.
4. Result: over generations, brown beetles become more common in the population.
5. Lamarck: his idea says individual beetles would turn brown during their lives and pass the change on. In fact, individual beetles do not change color. The population changes because brown beetles survive and reproduce more.
6. Answer: Brown beetles survive and reproduce more, so brown becomes more common. Individuals do not change; the population does.

Example 2: Too Many Offspring

Use the graph of offspring produced and offspring that survive to breed. (a) Which species produces the most offspring? (b) What fraction of the frog’s offspring survive to breed? (c) What did Darwin infer from observations like these?

Show the solution

1. (a) Most offspring: the oak tree produces about 5,000, more than the frog (2,000) or the mouse (30).
2. (b) Frog: 2 survive out of 2,000, which is 2 ÷ 2,000 = 1/1,000, or 0.1%. Only 1 offspring in 1,000 survives to breed.
3. (c) Inference: parents produce far more offspring than the environment can support, so the offspring must compete. Those with traits that help them survive and reproduce pass those traits on.
4. Answer: (a) Oak tree. (b) 1/1,000, or 0.1%. (c) Offspring compete, and helpful traits become more common.

Example 3: Reading the Timeline

Use the timeline. (a) Name two scientists whose ideas were published before the Beagle sailed in 1831 and helped Darwin. (b) What was missing from Darwin’s theory that Mendel’s work later supplied?

Show the solution

1. (a) Before 1831: Hutton (1788), Malthus (1798), Lamarck (1809), and Lyell (1830). Any two work. Hutton and Lyell showed that Earth is very old, and Malthus described populations outgrowing their food.
2. (b) What was missing: Darwin could not explain how traits are inherited. Mendel (1866) showed that traits are passed on as separate units, which we now call genes. His work was rediscovered in 1900.
3. Answer: (a) For example, Hutton and Malthus. (b) How traits are inherited, which Mendel’s work on genes explained.

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Practice

Match scientists to their ideas, read a graph of offspring and survivors, complete a model of natural selection, and compare Lamarck’s idea with natural selection. Try it on your own first, then check your work with the answer key.

• Development of Evolutionary Theory Practice Worksheet (PDF)
• Development of Evolutionary Theory Practice Answer Key (PDF)

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Lesson Slides

• 4.1 Development of Evolutionary Theory Slides (PDF)

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Big Ideas

• Hutton and Lyell showed that Earth is very old
• Populations make more offspring than can survive
• Natural selection: helpful inherited traits become more common
• Traits gained during life are not inherited
• Mendel’s work on genes explained inheritance

Back to the essential question: the theory of evolution came from many scientists. Geologists showed that Earth is old enough for slow change, Malthus described competition when populations outgrow their food, and Darwin and Wallace combined these ideas with observations of variation to explain natural selection. Later, Mendel’s work on inheritance filled in how traits are passed on. Next, you’ll look at the evidence that supports evolution, starting with fossils.

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