ORIGIN OF SPECIES

CHARLES DARWIN

“At last gleams of light have come, and I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) immutable … I think I have found out… the simple way by which species become exquisitely adapted to various ends.”

CHARLES DARWIN He knew that his theory of evolution would be

controversial and challenged. In June 1858, he received a letter from a young biologist working in Malaysia named Alfred Russell Wallace. Wallace outlined his own theory and happened to match Darwin’s theory.

After much hesitation and encouragement from others, Darwin finally published his book “On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life”, which sold out on the first day.

Natural Selection

Natural Selection

Natural selection: the way in which nature favours the reproductive success of some individuals within a population over others.

Evolution is the result of natural selection

Natural Selection

Over time the population changes as advantageous heritable

characteristics become more common generation after

generation

Individuals within a population compete for

resources

In each generation, populations produce more offspring than

there are adults

Popluations do not continue to grow in size

Food and many other sources are limited

Some individuals will inherit characteristics that give them a better chance or

surviving and reproducing

Individuals withing all populations vary

Many variations are hertiable

Theory of Natural Selection

Inferences

Observations

Natural SelectionThese were Darwin’s key observations: All healthy populations produced far

more offspring than could survive and reproduce

Individuals exhibited heritable variability Ie. a slow cheetah vs a faster cheetah

Natural Selection Survival of the fittest: a phrase that has been

used to describe the process of natural selection

This might be misleading because you might think someone who has lived a long life is fit, but the term is used to describe reproductive success

Adaptation: a characteristic or feature of a species that makes it well suited for survival or reproductive success in its environment

Evaluating the Theory All scientific theories must be

explanatory, have predictive powers and be testable.

Explanatory Powers Darwin used the evolution of large-billed

finches

Evaluating the Theory (Explanatory Powers)

Evaluating the Theory (Explanatory Powers) An ancestral population with medium-sized

bills arrives on the island. They eat medium-sized seeds. With no initial competition, the finches establish a growing population on a small island where there are some medium seeds but mostly plants with larger seeds

The finch population grows until it reaches the limit of its food supply. Each year, many eggs are laid, but the island can’t support all the birds. The birds are not all alike – exhibit variation. Some birds have slightly smaller and larger beaks than average beaks.

Evaluating Theory (Explanatory Powers)

Most of the birds must compete for the medium-sized seeds, but the ones with the larger beaks are also able to feed. They have little or no competition. The environment favours these larger beaks and lays more eggs on average because they are healthier. Their young are more likely to be born with larger beaks.

As this process is repeated generation after generation, the average beak size continues to increase. This is evolution by natural selection.

Evaluating the Theory (Prediction & Testing)

Prediction The theory allows scientists to make predictions

about how species may change over time, which are associated with changing environments Ex. The evolution of antibiotic resistance in bacteria Ex. Darwin made predictions that fossils of the most

primitive human ancestors would be found on the African continent. His predictions were proven correct

Testing and Falsification Science theory must be testable – the possibility

of being proven wrong There are examples that could falsify the theory:

Questions

p. 303 #1, 2, 4, 7, 8p. 307 #1(b), 2, 4, 5(a,b,d,f)