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15.3 Darwin Presents His Case. Darwin Presents His Case. The specimens Darwin brought back had the scientific community in a buzz Learned that Galapagos species are found nowhere else in the world They looked similar to South American mainland species but were clearly different.

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Darwin presents his case
Darwin Presents His Case

  • The specimens Darwin brought back had the scientific community in a buzz

  • Learned that Galapagos species are found nowhere else in the world

  • They looked similar to South American mainland species but were clearly different

Darwin s book
Darwin’s Book

  • Published his ideas about species diversity and evolution in On the Origin of Species

  • Proposed a mechanism for evolution called natural selection

  • Many agreed with Darwin while others strongly opposed him

Natural variation
Natural Variation

  • Scientists used to believe species were perfect and unchanging

  • Darwin argued natural variation (differences among species) is found in all types of organisms

    • Some cows give more milk

    • Some plants give larger fruit

Artificial selection
Artificial Selection

  • Darwin also studied English farmers and breeders

  • Used selective breeding to breed the “best” individuals

    • Largest hogs, fastest horses

  • Darwin called this artificial selection

  • Still used today!

Evolution by natural selection
Evolution by Natural Selection

  • Darwin believed that evolution occurred through natural selection

  • Natural selection = the process by which only the organisms best adapted to their environment will survive and reproduce

  • 3 main parts to natural selection

1 the struggle for existence
1. The Struggle for Existence

  • There are more living things than resources

  • Results in competition

  • Competition = the struggle among living things to get their needs for life

2 survival of the fittest
2. Survival of the Fittest

  • Individuals that are better suited to their environments survive (high fitness)

  • Individuals not suited to their environment will die (low fitness)

  • Fitness – the ability of an individual to survive and reproduce in its environment

    • Is a result of adaptations

2 survival of the fittest1
2. Survival of the Fittest

  • Adaptation – any inherited characteristic that increases an organism’s chance for survival

    • Long necks in giraffes

    • Long toes / webbed feet in birds

    • Coloring / Camouflage

3 descent with modification
3. Descent with Modification

  • Each living species has descended, with changes, from other species over time

  • Implies all organisms are related to one another

    • Known as common descent

Evidence for evolution
Evidence for Evolution

  • Fossil Evidence

    • When living things from the past are compared to living things today, we can see that change has occurred

Evidence for evolution1
Evidence for Evolution

  • Geographic distribution of living things

    • Species are located in different parts of the world but share similarities

    • Beavers and capybara are similar

    • Muskrat and coypu are similar




Beaver andMuskrat



Coypu andCapybara






Evidence for evolution2
Evidence for Evolution

  • Homologous body structures

    • Limbs (arms, wings, legs, flippers) have different form and function but are constructed from the same bones





Typical primitive fish

Evidence for evolution3
Evidence for Evolution

  • Vestigial structures

    • Body parts that no longer have functions

    • i.e. Pink lump in corner of eye

      • Remnant of a third eyelid

      • Frogs and turtles have third eyelid to see underwater

    • Appendix

      • In many mammals the appendix is used to help digest food

Evidence for evolution4
Evidence for Evolution

  • Embryology

    • Studying embryodevelopment to look for similarities and differences