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Natural Selection Chapter 22 Evolution • Change over time in the genetic composition of a population – NOT individual • Natural Selection – the mechanism by which evolution occurs • Evolutionary adaptation – result of natural selection – increase in frequency of traits in a population Darwin • Published Origin of the Species in 1859 • Common ancestor • modern species descended from ancestral species • Natural selection • populations change over generations • organisms with favorable traits have more offspring than those less fit • “survival of the fittest” + reproduction • More offspring = more genes in the population Precursors to Darwin • Aristotle: species were fixed and unchanging • Linnaeus: classified diversity “taxonomy” • binomial nomenclature: Genus and species • Hutton: gradualism; slow and continuous change • Lamarck: acquired characteristics • characteristics gained during an organism’s lifetime are passed to offspring • Giraffe’s long neck • Cuvier: catastrophism; each boundary in layer represents change • opposed gradualism • Lyell: uniformatarianism; changes in the present occur at the same rate as the past • modern geology • Wallace: proposed a theory similar to Darwin’s Darwin’s Studies • HMS Beagle • South American coastline • noticed organisms resembled European species in similar climates • Galapagos Islands • studied finches; noticed many similarities with a few differences in birds • Common ancestor Origin of the species 1. Descent with Modification • organisms related to ancestor in the remote past • similar to a tree; more similar species have a common ancestor in the nearer past • 99% of species that have ever lived are now extinct not all common ancestors are still alive. Origin of the Species 2. Natural Selection • species would increase exponentially in numbers if all organisms that were born reproduced and all offspring survived to reproduce • populations tend to remain stable in size • resources are limited • competition for resources occurs • organisms within a population demonstrate variation • most variation is heritable • Individuals who inherit favorable traits for their environment are more likely to survive and reproduce • Unequal reproduction leads to a gradual change in a population toward favorable traits Natural Selection • Differential success and reproduction among individuals that vary in their heritable traits based on environment • Creates adaptations of organisms to their environment • If the environment changes new adaptations will arise and potentially create a new species. • Acts on populations NOT individuals • Only effects heritable traits • Environmental factors vary from place to place not all adaptations are favorable in all environments. A flower mantid in Malaysia A stick mantid in Africa Types of Selection 1. Disruptive: extreme phenotypes favored; average is not 2. Directional: one extreme phenotype only is favored 3. Stabilizing: average individuals are selected; extremes are not Types of Selection 4. Sexual Selection: mating selection • Intrasexual:within the same sex • males competing • Intersexual: between the sexes • females choosing the males 5. Artificial Selection: modification of species due to selective breeding to produce organisms with desired traits • pets Evidence for Evolution 1. Paleontology • Fossils: remains or traces of organisms from past • found in sedimentary rock • Igneous and metamorphic involve heat so living organisms don’t remain long enough to preserve • older fossils found in lower layers (strata) • Biogeography: geographic distribution of species • closely related organisms found in the same geographic regions Evidence for Evolution 2. Homologous Structures: comparison of similar structures that have different functions between species. • human arm and bat wing • same bone structure with different functions Human Cat Whale Bat Evidences for Evolution 3. Embryology: comparison of early stages of development • all vertebrate embryos have tail and pharyngeal pouches during embryonic development Pharyngeal pouches Post-anal tail Chick embryo Human embryo Evidence for Evolution 4. Vestigial Organs: structures with little significance in a modern species, yet present and functional in ancestors. • snakes have small leg bones • humans have an appendix Evidence for Evolution 5. Molecular: comparison at the molecular level • DNA, RNA or protein sequences • more similar molecular sequences, more similar the organisms • closer the common ancestor Species Percent of Amino Acids That Are Identical to the Amino Acids in a Human Hemoglobin Polypeptide 100% Human Rhesus monkey 95% Mouse 87% Chicken 69% Frog Lamprey 54% 14%