What Darwin Observed
- Darwin sent specimens collected on his voyage back to England to be examined by experts
- Fossils collected were confirmed to be large versions of the present-day sloth and armadillo
- The Galapagos Island’s bird specimens were in fact many different species of birds
- Darwin originally thought they were just variations of the same species
- Evidence now suggested that a single ancestral species gave rise to a number of similar but distinct species
Homologous Features
- Similar in Origin
- Different in Function
- Recent common ancestor
- May appear in embryonic development and disappear before birth.
Analogous Features
- Different in Origin
- Similar in Function
- Lacks recent common ancestor
Vestigial Features
- Structures that serve of no use to the organisms
- Examples:
- Dew claws in dogs and toes elevated off the ground in pigs, cattle, and deer
- The appendix in humans
- Vestigial genes are found in DNA, which serves no function but consists of similar sequences
Artificial Selection
- Artificial Selection – When offspring with desired traits are selected as ‘breeding stock’ for succeeding generations
- Humans have improved domesticated plant and animal species for thousands of years
- By selecting offspring with desirable traits as breeding stock for succeeding generations
- E.g. breeds of dogs, types of corn
Darwin’s Assumptions
- If humans could change the behaviour and appearance of domesticated species, the environment could have similar effects on wild species
- If Lyell was right about the age of the Earth there could be time for small changes in species to accumulate into large changes over many thousands of generations
The Struggle for Survival
- Darwin was convinced that he had evidence that life had evolved
- Artificial selection was a model of how evolution might operate in nature
- But how does nature chose individuals with desirable traits for reproduction – as breeders do with dogs
- Used Malthus’ postulate to come up with an answer
- In nature, both plants and animals produce far more offspring than are able to survive
Survival of the Fittest
- Far more offspring are born than can survive and reproduce – intense competition among individuals of the same species to survive
- Darwin: “Favourable variations would tend to be preserved, and unfavorable ones to be destroyed. The results of this would be the formation of a new species.”
Origin of Species
- In 1860 Darwin published “On the Origin of Species by Means of Natural Selection”
- Darwin: “…can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favorable variations and the rejection of injurious variations I call Natural Selection.”
Natural Selection
- Supportive Evidence
- Many scientists were swayed by Darwin’s theory
- But even with all of the evidence, there were still opponents
- For entirely new species to develop a great length of time was required
- Not everyone believed the Earth was that old (dating not available at the time)
- The fossil record at the time was limited
- No transitional fossil forms from ancient to modern had been found
- Basilosaurus recently found – an ancient relative of the whale
