Development of Evolutionary Theory

Indigenous stories (evolutionary thought)

Traditions proposing that humans evolved from other species, and that environmental upheaval drove changes in animal morphology and behavior

Chinese Taoist philosophies (evolutionary thought)

Philosophical tradition proposing that species are not fixed and develop different characteristics in different environments

Greek natural philosophies (evolutionary thought)

Traditions featuring disagreements and debates over whether species were fixed/"essential" or changing over time

Medieval Muslim scholarship (evolutionary thought)

Tradition including Al-Jahiz, who proposed that animals adapt to pressures from other organisms and the environment to increase their survival

Al-Jahiz

Medieval Muslim scholar who proposed that animals adapt to environmental and competitive pressures to increase survival

Enlightenment & Scientific Revolution effects on natural science

Overlapping movements in Western Europe that led more scholars to organize formal studies and classify nature (e.g., zoology, botany, geology, Linnean classification); science became more formalized, popularized through publications/lectures/debates, and more men became professional scientists

Gradualism

The idea that large changes in nature are the result of slow, continuous processes in the past (e.g., a valley formed by river erosion); proposed by geologist James Hutton

James Hutton

Geologist who proposed the principle of gradualism

Uniformitarianism

The idea that natural processes which formed patterns of the past are still operating today and across the universe; popularized by geologist Charles Lyell

Charles Lyell

Geologist who popularized uniformitarianism; a friend of Charles Darwin whose book "Principles of Geology" (1830) influenced Darwin

Incognitum

Term used by Thomas Jefferson for the American mastodon; he asked Lewis & Clark to find this creature alive during their westward expedition, since scholars did not yet accept the idea of extinction

Georges Cuvier

Founder of paleontology; his research on vertebrate fossils convinced scholars (including Jefferson) that extinction was real by showing fossil animals were similar to, but distinct from, living ones

Paleontology

The study of fossils; founded by Georges Cuvier

Catastrophism

The idea that sudden events lead to rapid, abrupt changes in nature (e.g., a massive flood causing major extinctions in an area before new species repopulate the site)

Evidence used to support extinction/catastrophism

Older rock layers contain fossils that are more dissimilar to living animals than fossils in more recent rock layers

Thomas Malthus

An economist whose theory of population influenced biological thinking among scientists, including Darwin and Wallace

Malthusian Population Theory

The idea that people (and by extension, other species) overproduce offspring, leading to unsustainable population growth that is corrected by increased mortality from resource scarcity, war, and disease

Malthusian catastrophe

The point at which population growth outpaces available resources, leading to a correction via increased mortality

Jean-Baptiste de Lamarck

Scientist who proposed the first widely known mechanism (causal explanation) for evolutionary change across generations; published his hypothesis of evolution in 1809

Lamarckism

Theory hinging on "use and disuse" — the more an organism used a body part, the stronger/more developed it became; Lamarck believed this strengthening was heritable and passed to offspring, and that organisms had an innate drive to become more complex

Use and disuse (Lamarckism)

The principle that body parts strengthen with use and weaken with disuse during an organism's lifetime, and that these changes are heritable

Classic critique of Lamarckism

The question of whether a blacksmith's child would inherit the large arm muscles built by their parent through physical labor — under Lamarckism, yes, but this is not how heredity actually works

Charles Darwin's voyage

A 5-year world voyage (1831–1836) aboard the HMS Beagle during which Darwin developed his evolutionary ideas from reading and observation, including fossils/earthquakes in South America and unique species (mockingbirds, giant tortoises) in the Galápagos Islands

Galápagos finches

Very different-looking birds Darwin collected from different Galápagos islands and environments that turned out to all be unique species of finches, informing his ideas about descent with modification

Darwin's barnacle studies

Years-long studies of unique reproduction and variation in barnacles in England that contributed to Darwin's developing theory

Descent with modification

Darwin's term for the gradual process by which species change over generations from common ancestors, developed over decades following his Beagle voyage

Why Darwin delayed publishing his ideas

Even though built on extensive evidence and prior ideas, Darwin was afraid to argue his ideas publicly for years because of their potential controversy

Alfred Russel Wallace

Naturalist who independently developed a theory of mechanisms for changes in organisms across generations and formation of new species; based largely on observations from collecting specimens in South Pacific/Malay Archipelago islands; informed by Darwin's Galápagos work and shared reading like Malthus

"On the Tendency of Varieties to Depart Indefinitely from the Original Type"

Title of the paper in which Wallace first presented his theory of natural selection (1858); prompted Darwin to publish his own work

1858 event prompting Darwin to publish

While studying species in the Malay Archipelago, Wallace sent Darwin his hypothesis of natural selection, spurring Darwin to finally go public with his own theory

The Origin of Species

Book published by Charles Darwin in 1859, fully fleshing out his theory of evolution by natural selection with many lines of evidence and argumentation

Darwin's theory of natural selection — Observation 1

Individuals in natural populations often vary in inherited traits

Darwin's theory of natural selection — Observation 2

Organisms produce more offspring than the environment can support, leading to unequal survival and reproduction

Darwin's theory of natural selection — Inference 1

Individuals with inherited traits that improve their chance of survival and reproduction leave more offspring

Darwin's theory of natural selection — Inference 2

Unequal survival and reproduction will lead to a buildup of favorable traits in the whole population over many generations

Natural selection (definition)

A process where individuals with certain heritable traits survive and reproduce at higher rates than others without those traits; over time this can increase the proportion of favorable traits (adaptations) in a population, and favorability may change if the environment changes

Adaptation

A favorable, heritable trait whose proportion increases in a population as a result of natural selection

What Darwin didn't know: Plate tectonics & continental drift

Darwin lacked this knowledge, which helps explain how closely-related organisms end up in very different parts of the world

What Darwin didn't know: Mechanism of heritability

Darwin did not know how traits get passed on from parents to offspring (this was later explained by Mendelian genetics)

What Darwin didn't know: Other mechanisms of modification

Darwin did not know about other drivers of evolutionary change besides natural selection (e.g., genetic drift, gene flow, mutation)

What Darwin didn't know: Rate of change over generations

Darwin did not know whether evolutionary modifications only occur gradually, or whether they could occur at variable rates