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History of Evolutionary Thinking
Evolution: hypothesis, fact, theory, scientific law.
Evolution is a fact in the sense that it is overwhelmingly validated by the evidence.
A scientific theory is "a wellsubstantiated explanation of some aspect of the natural world that
can incorporate facts, laws, inferences, and tested hypotheses".
Theories are formed from hypotheses that have been repeatedly tested.
The theory of evolution is actually a network of theories Darwin proposed five separate theories
which included mechanistic explanations for:
Populations changing over generations
Law: A statement based on repeated experimental observations that describes some
phenomenon of nature. Proof that something happens and how it happens, but not why it
Evidence for Evolution
Look at how animals relate to each other
Universally shared features
Hierarchical classification (phylogeny)
Biological relationships, geological history and geographical distributions
In general, evolution happens over a long timescale so we can't observe it.
But there are places where it is happening much faster, and it can be observed over a single
A famous example is a 30 year study on beak sizes of Geospiza fortis.
Daphne major is a relatively untouched island: no goats and no farming, and it is possible to
measure every type of seed that the finches eat.
Beak size is heritable, and size of beak makes a difference between life and death.
A drought in 1977 meant that the spurge plants all died, so no small seeds were available.
There was natural selection for larger beaks, and size increased by 15%.
Five years later conditions returned to normal, spurge seeded again and beak size decreased
For evolution to occur the feature needs to be heritable.
The Discovery of Evidence for Evolution
Greek philosophers had some ideas which made some sense.
They had no concept of how organs might change, so thought in terms of switching body parts
the combinations that worked were kept (selection).
Anaximander nature is ruled by laws, anything that disturbs the balance of nature
doesn't last long.
Empedocles explained biological diversity by the "body parts theory".
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Xenophanes the first to suggest that fossils in sedimentary rocks have previously been
People were not thinking about evolution so they thought that animal diversity originated from
different combinations of body parts.
Sometimes it went well (harmony) but things can also go wrong.
Illustrated by Bosch's painting The Garden of Earthly Delights.…read more
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He also suggested that humans and apes are related.
He hid his views in a 44 volume book series so avoided broad public criticism.
He asserted that species can change over generations but publically rejected the idea that species
could evolve into other species.
Insisted that natural phenomena must be explained by natural laws rather than theological
Lamarck was the first evolutionist to state his ideas about the processes leading to biological
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He suggested that the earth and life on it must have been evolving for "millions of ages before the
commencement of the history of mankind".
Charles Lyell was the foremost lawyer of his day.
Influenced by James Hutton who believed in gradual landscape transformation through
imperceptibly slow changes.
Champion of uniformitarianism.
A major influence on Darwin but not an evolutionist.
Uniformitarianism vs catastrophism: two opposing schools of geologists.…read more