Darwin: Natural Selection & Evolution
Charles Darwin
 1831 – set sail for __5__ years on HMS _Beagle as a Naturalist
 22 year old English Medical student
 Questioned the belief that organisms never Change
Natural Selection
 In 1859, Darwin offered an explanation – evolution occurs by Natural
 He published his idea in The Origin of Species.
Darwin’s Observations
 Diversity – each area of the world has very different organisms.
 Fossils - preserved remains of an organism that lived in the past. The
fossils Darwin saw looked different than organisms living at that time.
 Characteristics of Galapagos Islands organisms – organisms were
different from organisms elsewhere and from organisms on other
Galapagos Islands
 Adaptation - trait that helps an organism survive & reproduce
 Examples: the finches’ beaks; the iguanas’ claws; poison & bad taste;
bright colors of flowers; animal coloring
 Darwin studied his observations for 20 years after returning to England.
 Reasoned that organisms arriving on Galapagos faced conditions
different from on the mainland of South America
 He hypothesized the species gradually changed over many generations
& became better adapted to the new conditions
 This gradual change is called Evolution.
 However Darwin could NOT explain how the changes happened
Natural Selection
 Natural selection – process by which individuals that are better adapted
to their environment are more likely to survive & reproduce than other
members of the same species
 Factors that affect natural selection
o Overproduction – produce more offspring than can survive
o Variation – any variation between individuals of the same species
o Competition – members of a species will compete for resources
Organisms with a helpful variation will survive & reproduce. Poor variations
will die out. Over a long time, natural selection will lead to change.
Reasons for Species Variation (speciation):
Climate: example is snowshoe rabbit
Geographic Isolation: example is Kaibab squirrel and Abert squirrel
Interaction Between Species:
 Mutualism – relationship in which Both species Benefit
 Commensalism – relationship in which one species benefits and the
other is neither helped nor harmed
 Parasitism – 1 organism lives in or on another organism and harms it
Genetic Mutation: - chromosomal changes
 Darwin knew that changes happened, but couldn't explain how.
 At the same time, Mendel was explaining how change happens through