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Evidence that Earth and the species on it had changed over very long spans of time led to the theory of evolution by natural selection. Natural selection is a process in which individuals of a population survive and reproduce with differing success depending on the details of their shared, heritable traits. Traits favored in a particular environment are adaptive.

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A heritable trait that enhances an individual’s fitness in a particular environment.

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Degree of adaptation to an environment, as measured by an individual’s relative genetic contribution to future generations.

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Economist

Correlated size increases in humans

famine

disease

war

Proposed: humans run out of food, living space, resources because they reproduce beyond environmental capacity

Individuals of population must either compete with one another for the limited resources, or develop technology to increase productivity

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Proponent of what became known as theory of uniformity:

idea that gradual, everyday geological processes such as erosion could have sculpted Earth’s current landscape over great spans of time

challenged the prevailing belief that Earth was 6,000 years old

By Lyell’s calculations, it must have taken millions of years to sculpt Earth’s surface

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Earned theology degree from Cambridge after attempt to study medicine

Through school, had spent most of time with faculty members and other students who embraced natural history

1831 (age 22): joined 5-year survey expedition to South America on Beagle, quickly became enthusiastic naturalist

during voyage, found many unusual fossils, saw diverse species living in environments ranging from the sandy shores of remote islands to plains high in Andes

read the first volume of a new and popular book, Charles Lyell’s Principles of Geology

Exposure to Lyell’s ideas gave insight to history the regions encountered on journey

Among specimens collected during voyage were fossil glyptodons

extinct armored mammals are extinct

many unusual traits in common with modern armadillos

Armadillos also live only in places where glyptodons once lived

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Line of descent.

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Change in a line of descent.

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Thinking about processes that drive evolution

Thought that species gradually improved over generations because of inherent drive toward perfection, up the chain of being

environmental pressures cause internal need for change in body, resulting change inherited by offspring

(Lamarck was correct in thinking that environmental factors affect traits, but his understanding of how traits are passed to offspring was incomplete.)

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Around 1800, naturalists were trying to explain the mounting evidence that life on Earth, and even Earth itself, had changed over time.

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Expert in zoology and paleontology

Proposed idea startling for time:

many species once existed, now extinct

Knew about evidence that Earth’s surface had changed

had seen fossilized seashells on mountainsides far from modern seas

assumed Earth’s age to be in the thousands, not billions, of years

Reasoned that geologic forces unlike any known at time would be necessary to raise sea floors to mountaintops in short time span

Catastrophic geological events would have caused extinctions, after which surviving species repopulated Earth