15-3 Darwin Presents His Case

1. 15-3 Darwin Presents His Case • Darwin was deeply troubled by all that he had seen and discovered • Hesitated to publish his findings • In 1858, Alfred Russel Wallace wrote to Darwin and evidently had made the same discoveries that Darwin had on his voyage. • Darwin now needed to publish his work before Wallace. • 1859: Darwin publishes The Origin of Species by Means of Natural Selection

2. What did Darwin state in his book? • Evolution had been taking place over millions of years. • Evolution continues to occur now. • Evolution occurs by a mechanism called natural selection

3. What is natural selection? • All organisms have natural variations or differences between them (We now know these to be genetic differences.) • Farmers often used artificial selection to breed specific crops together to get desirable characteristics. (What did we call this in the last chapter?) • Darwin argued that existing environmental conditions could naturally select some organisms to survive and thrive while others would die off depending on their variations.

4. Evolution by Natural Selection • Struggle for Existence • Members of each species compete for space, resources and sometimes mates. • All members of a species must deal with the same environmental conditions.

5. Survival of the Fittest • Darwin had noted how well-suited organisms were for their particular environment. He called this “fitness”. • Organisms that were the most fit had survived, reproduced and passed on their better traits. Overtime the species grew more and more suited for its environment. • Darwin called this “survival of the fittest”- those with the most desirable traits for survival in that particular environment.

6. Descent with Modification • Over long periods of time, natural selection of the most fit members of a species could result in organisms with different structure, habits, etc. according to two different environments. • Organisms that were once the same species could be so different over time due to natural selection of their particular environment that they are no longer very similar at all. (origin of a new species) • He called this “descent with modification”

7. Common Descent • All species, living and extinct were derived from common ancestors. • A single “tree of life” links all living things.

9. Evidence of Evolution • Fossils • Geographic Distribution of Species • Homologous Body Structures • Similarities in Embryology

10. Fossils • Indicates gradual transitions species have gone through over time (there are occasional gaps) • Show that there are species that no longer exist- extinction. – Not “fit” to survive (Also indicates that existing species could become extinct)

12. Geographic Distribution of Species • Organisms have spread out across all areas of the world over time. • Geographical differences lead to evolution of very different features. • Geographical similarities lead to similar characteristics existing in organisms in very different places in the world.

14. Homologous Body Structures • Striking anatomical similarities among very different animals with backbones. • All develop from the same clump of embryonic tissue • Vestigial organs- organs an organism has that serve little or no purpose. In us for example

15. Similarities in Embryology • Early stages of development of embryos in many animals with backbones are extremely similar • What does this mean? Cells develop in same order and patterns to produce early forms of tissues and organs of all vertebrates.

16. Strength of Theory- advances in genetics, geology and physics support ideas of evolution. • Weakness of Theory- As with all theories in science, theory must constantly change with new ideas and discoveries, still lots of uncertainty, still left with many unanswered questions