Chapter 2: Darwin's Big Idea + How It Changed Biology

Open questions in Darwin’s Time

(biology before Darwin and Wallace)

1. where do species come from?

2. how can we explain complex adaptations?

• traits with clear and elaborate function for the survival and reproduction of organisms?

William Pale’s Argument from Design

(biology before Darwin and Wallace)

highly complex and functional things are designed for a purpose by a Creator (e.g. tree and watch)

Jean- Baptiste de Lamarck (1744-1829)

(biology before Darwin and Wallace)

• first to use the term evolution

• first to provide a hypothesis for the casual mechanism: inheritance of acquired characters (adaptation)

Why was Lamarck wrong? : August Weiman’s Germplasm Theory (1889)

• inheritance only by germ cells (gametes); somatic cells (soma/body) do not function as agents of heredity

• thus, genetic info cannot pass from soma to gametes to next gen

• modern interpretation molecular terms: genetic info flows in one direction only. from DNA to protein, but never the reverse!

Darwin + Wallace

• Darwin developed first comprehensive theory of evolution

• Darwin and Wallace independently discovered the chief mechanism of evolution: natural selection

Darwin’s and Wallace’s Theory of Evolution

1. all organisms have descended with modification from a common ancester

2. the process leading to evolution is natural selection operating on variation among individuals

Development of Darwin’s ideas on evolution

• exploration

• voyage on HMS Beagle around the world (1831-1836) as ship’s naturalist

• made numerous observations and collections of plants, animals, fossils

• returned and spent rest of life in seclusion at Down House developing his ideas, conducting experiments and writing 25 books

Development of Darwin’s ideas on evolution

• gradualism

• Darwin read Lyell’s book “Principles of Geology’ (1830)

  • Lyell argued for uniformitarianism

  • the forces and processes that shape the earth’s surface are uniform through time

  • i.e. the forces we see today are the same as previous eons

  • he argued that present day geological processes can explain the history of the earth (gradualism of erosions, earthquakes, volcanoes)

• 2 implications for Darwin

  • notion of dynamic rather than a static world

  • changes build up gradually, by the same mechanisms today as in the past

Development of Darwin’s ideas on evolution

• species vary

• variation patterns of Galapagos mockingbirds

• there are four similar species endemic to the islands descended from South American mainland ancestor

• darwin doubt fixity of species (mar. 1837)

Development of Darwin’s ideas on evolution

• struggle for existence

• Darwin reads Thomas Malthus’ ‘An essay on the principle of population’ (1798) in Sept. 1838

  • Thomas talks about struggle for existence

  • favorable variations tend to be preserved and unfavourable ones would be destroyed

  • since resource grow geometrically and human population grow exponentially, resource would run out → competition

events leading up to the publication of the Origin of Species

• 1831-1836: voyage of Beagle

• +20 years : evidence acumulation

• 1844: wrote essay on natural selection - not publish

• 1856: start natural selection book

• June 1858: receive Wallace’s article “On the tendency of varieties to depart indefinitely from the original type”

• July 1858: Linnean Society presentation of Darwin-Wallace paper

• Nov. 1859: publication 490-page book “On the origin of species by means of natural selection or the preservation of favoured races in the struggle for life” by Darwin

Darwin’s mechanism of Natural selection

• variation: individual variation in a population

• heredity: progeny resemble their parents more than unrelated individuals

• differential fitness: some forms are more successful at surviving and reproducing than others in a given environment

*evolution = natural selection is heritable variation in fitness

important elements of Darwin’s Theory

1. evolution occurs primarily at the level of populations (individuals don’t exist)

2. variation is not directed by environment (individuals don’t induce adaptive variation when needed)

3. most fit type depends on the environment

4. ‘survival of the fittest’: evolution works with available variation, and will not necessarily achieve perfection

implications of Darwin’s Theory of evolution

• concept of a changing universe

  • replaced view of a static world

• a phenomenon with no purpose

  • natural selection revealed how complex adaptations with important ‘functions’ can arise through an unplanned process

examples of natural selection

needs to use more drug over time because bacteria evolves to being more resistant to antibiotics

examples of natural selection (cont.)

• blue ones (antibiotic resistant) reproduce more than white ones (susceptible)