OEB 53 SECTION NOTES
3 GREAT FACTS
1. Climate doesn’t explain everything
Similar climates in different parts of the world have vastly different
species...Darwin tries to promote the role of geography over the role of climate
Argument against special creation: why create so many distinct/unrelated
species to occupy similar habitats?
2. Barriers are important.
Darwin notices early on that isolation is important in speciation: organisms
have to be isolated so that they don’t mix with other organisms (equivalent to
today’s ideas about allopatry)
Examples of land barriers and oceanic barriers
3. Groups of related organisms confined to a specific region.
Evidence against special creation: it would be redundant to populate similar
niches around the world with different organisms
DARWIN & DISPERSAL
Darwin agrees with land bridges over shallow bodies of water, but does not agree that the
continents had not been where they were in modern times. Model mostly relies on dispersal.
Molluscs on duck feet
Idea of seeds migrating via birds (even if bird died, fell in water, drifted): stuffing
dead pigeons with seeds, could seeds survive a long time in the water, still viable?
Scattered seeds on the water: how many sunk and how many floated, calculated
Took mud from a pond, how much would germinate from it
Darwin observed that on different mountaintops, there were the same species of plants. He
hypothesizes that during the glacial period, ice sheets pushed seeds from north to the south,
and then cold-adapted species retreated either north or up a mountain.
Darwin notices similarity between island species and mainland species, supports descent
with modification and dispersal ideas.