Darwin Presents His Case Chapter 15, Section 3

Lyell’s Influence In attempt to explain the past in terms of present day processes, Darwin went to local farmers and animal breeders and observed… Variation in the organisms that could be inherited Variation in the organisms that could be inherited After many generations, organisms appear very different from ancestors After many generations, organisms appear very different from ancestors Darwin called this process ARTIFICIAL SELECTION Darwin was convinced a similar process occurred in nature:

NATURAL SELECTION 1. Variation in Nature No two organisms are exactly alike! No two organisms are exactly alike! 2. Struggle for Existence (Malthus) High birth rates and limited resources will force organisms to compete High birth rates and limited resources will force organisms to compete Selective pressures (predation, competition, parasitism, disease, pesticides, etc) Selective pressures (predation, competition, parasitism, disease, pesticides, etc) 3. Survival of the Fittest Fitness: an organism’s ability to survive and reproduce Fitness: an organism’s ability to survive and reproduce Adaptation: any inheritable characteristic that increases an organism’s chance of survival Adaptation: any inheritable characteristic that increases an organism’s chance of survival Organisms with the best adaptations survive and reproduce more often Organisms with the best adaptations survive and reproduce more often 4. Reproduction of Viable Offspring Offspring must be fertile and reproduce Offspring must be fertile and reproduce

Descent with Modification Darwin then proposed that over long periods of time, natural selection produces organisms that look different from their ancestors. Darwin then proposed that over long periods of time, natural selection produces organisms that look different from their ancestors. This implies that all living things are related to one another. This implies that all living things are related to one another. Common Descent: all species, living and extinct, were derived from common ancestors.

Darwin’s Evidence of Evolution

The Fossil Record Fossils are buried in sedimentary rock → Relatively, deeper fossils are older Fossils are buried in sedimentary rock → Relatively, deeper fossils are older Fossils provide evidence of changing life Fossils provide evidence of changing life Paleontologists search for transitional fossils Paleontologists search for transitional fossils

Geographic Distribution of Living Species The geographic distribution of animals serves as a clue to how modern species evolved The geographic distribution of animals serves as a clue to how modern species evolved Two islands in different parts of the world (with similar environments) will house species similar to the nearest mainland rather than the other island Two islands in different parts of the world (with similar environments) will house species similar to the nearest mainland rather than the other island Ex: Australia’s unique plants and animals evolved from isolation from other continents Ex: Australia’s unique plants and animals evolved from isolation from other continents

Similarities in Structure The forelimbs of all mammals are made of bones that are similar in origin (skeletal structure), but may vary in function → called homologous structures The forelimbs of all mammals are made of bones that are similar in origin (skeletal structure), but may vary in function → called homologous structures Vestigial structures are remnants of structures in an animal that may have had a function or purpose in ancestral species, but serve no apparent function or purpose in modern species Vestigial structures are remnants of structures in an animal that may have had a function or purpose in ancestral species, but serve no apparent function or purpose in modern species

Similarities in Development Embryology: the study of embryo development Embryology: the study of embryo development Animals that are more closely related will have more similarities in the stage of development Animals that are more closely related will have more similarities in the stage of development