Evolution by Natural Selection 1 Variation and Adaptation Individuals have differences among their heritable traits but some of those differences are better for life in their environment than others Some of these traits increase an organism s ability to survive and reproduce in an environment adaptation These could involve body parts structures colors or physiological functions 2 Survival of the Fittest Differences in adaptations affect an individual s fitness which describes how well an organism can survive and reproduce in its environment Individuals with adaptations that are better for their environment have a higher fitness can survive and reproduce Individuals with characteristics that are not well suited to their environment die out low fitness The difference in rates of survival is called survival of the fittest 3 Natural Selection The process by which organisms with variations most suited to their local environment survive and leave more off spring The environment influences fitness Well adapted individuals survive and reproduce Some penguins are born brown while others are white The white ones can can camouflage Organisms produce more than can survive A penguin lays 200 eggs and only 30 survive Variation and Adaptation in the snow Survival of the fittest Since they can camouflage the white penguins are more fit and reproduce more White penguins become more common over time because there are more born that can Natural selection survive and have higher fitness Common Descent All living species descended from the same ancestors with modifications So all organisms are related but over time and many generations adaptation caused successful species to evolve into new species Tigers panthers and cheetahs share the same ancestor Recent Fossil Finds Many recently discovered fossils form series that trace evolution of modern species from extinct ancestors Scientists uncover fossils that resemble intermediate changes in the process of evolution Recent fossil finds connect the dots between dinosaurs and birds Comparing Anatomy and Embryology 1 Homologous Structures Animals with similar structures evolved from a common ancestor with the basic version of that structure These structures are shared by similar species Body parts that share common function but not common structure Analogous Structures Vestigial Structures Functionless structures that have been inherited Genetics and Molecular Biology 1 Life s Common Genetic Code common ancestors that shared this code 2 Homologous Molecules Genetic code is nearly identical in most organisms This proves that all organisms evolved from Molecules contain the same structures but are found in different organisms and species Sources of Genetic Variation 1 Mutations Some mutations do not change an organism s phenotype Others might or might not affect fitness 2 Genetic Recombination in Sexual Reproduction Most heritable differences are due not to mutations but genetic recombination in sexual reproduction Crossing over increases the number of new genotypes in every generation 3 Polygenic Traits A single polygenic trait often has many possible genotypes and even different phenotypes Normal distribution Natural Selection 1 Natural Selection on Single Gene Traits Changes in allele frequencies Change in phenotype frequencies 2 Natural Selection on Polygenic Traits Direction Selection One end higher than other end Stabilizing Selection Center higher than sides Disruptive Selection Outer ends higher than middle Genetic Drift frequencies 1 Genetic Bottlenecks Disease kills many individuals in a population Smaller population have different allele 2 The Founder Effect A few individuals colonize a new habitat they carry alleles with different frequencies from the actual population