Evolution A Scientific Explanation for Similarities and Differences between Organisms
Definitions • Evolution is a change in the characteristics of a population over time • Two characteristics identified by Darwin’s work • Descent with modification: species diverge from a common ancestor • Natural selection: individuals with favorable traits accumulate within a population • Evolution is a change in the genetic composition of a population from generation to generation • Can include mechanisms other than natural selection such as genetic drift and gene flow Populations evolve as natural selection acts on its individuals.
Prior to Darwin • Prevailing view that species were unchanging • Aristotle proposed a “scale of nature”, that organized fixed species from least to most complex • Special Creation as the origin of species was reinforced by religious thought and interpretation of scientific evidence, such as the classification scheme of Linnaeus • Differing views of the history of the earth as seen in the fossil record • Catastrophism: events in past occurred suddenly and are unlike mechanisms of today • Uniformitarianism: mechanisms of change are constant over time
Evolutionary Thought Before Darwin • Lamarck developed ideas for changes among species • Use and disuse – Individuals retain characteristics that are required for their survival, leads to adaptation • Inheritance of acquired traits – Individuals inherit traits that were developed over the lifetimes of their ancestors • Innate drive for species to become more complex
At least some variation is hereditary More offspring are born than survive and reproduce Favorable traits will accumulate in the population over generations Variations exist between members of a population Relatively constantresources andpopulation over time A Flowchart of Evolutionary Reasoning Individuals whose inherited traits give a higher probability of survival and reproduction leave a greater number of offspring 2 Identify the inference that shows1. Evolution2. Natural Selection observations inferences 1
Darwin’s Observations Islands with saddle shell giant tortoise had tall prickly pear cacti. Adaptation: characteristic that favors survival and reproduction
Darwin’s Observations Cactus-eater Galapagos finches had beak shapes suited to food sources on specific islands. Seed-eater Insect-eater
Darwin’s Observations Galapagos finches resembled the grassquit found on the coast of Ecuador. Recent studies suggest that the finch ancestor may have originated from the Caribbean islands.
Giraffes stretched their necks to reach food on tall trees andpassed the longer necks onto their offspring Explaining the long neck of a giraffe Lamarck versus Darwin
Evolution as a Change in Genetic Composition Natural selection led to an increase in dark colored peppered moths in industrial regions of Britain.
How is this trait inherited? • Wing color is the result of a single gene with two alternative forms called alleles • One allele codes for dark color, the other codes for light color • Each moth inherits two alleles for wing color • Only one dark allele is needed for the moth to have dark wings since the dark color allele is dominant over the recessive light color allele • Two light color alleles are required for a moth to have light wings • If birds remove light winged moths from the population, dark winged moths survive and pass the dominant allele for dark wings to their offspring • The increase in frequency of the dark wing allele is evidence of evolutionary change
How is this trait inherited? The homologous chromosomes carrying alleles of the same gene separate during Meiosis I so that each gamete receives only one of the two alleles.
D D d d d d D d Genotype for Dark moth Genotype for Light moth How is this trait inherited? Dd dd Dd Dd 50% dark moths50% light moths dd dd Genotype for Dark moth Genotype for Dark moth Dd Dd DD Dd 75% dark moths25% light moths Dd dd
Birds remove half of light wing moths dd Dd Dd dd dd dd Dd dd Dd dd dd dd dd dd dd dd 20% dark wing moths 33.3% dark wing moths Simulating Natural Selection Reproduction occursDd x dd 50% Dd + 50% ddDd x dd 50% Dd + 50% dd dd x dd 0% D_ + 100% dd Total Dd = 100/300 = 33.3%dark wing moths Natural selection acts on the phenotype.Evolution is the resulting change in genotypes within the population.
Treehoppers Walking stick Evidence for Evolution: AdaptationObject Mimicry provides camouflage for evading predators
Evidence for Evolution: Direct ObservationIncrease in guppy coloration with predator change
Evidence for Evolution: Direct ObservationResistance of HIV virus to drug therapy
Evidence for Evolution: Fossil RecordLiving organisms resemble extinct fossil forms
Evidence for Evolution: Fossil RecordProgressive changes can be seen fromsimpler to more complex organisms
Evidence for Evolution: Comparative Anatomy • Structures that do not show a common origin • Analogous structures: superficially similar • Structures that show a common origin • Homologous Structures: same evolutionary origin despite differences in function • Vestigial Structures: serve no purpose but are homologous to structures in related organisms
Analogous and Homologous Structures Not homologous;analogous Not homologous;not analogous Homologous;not analogous Homologous;analogous
Homologous Structures Flying Swimming Running Grasping
Functional hindlimb in salamander Remnants of hindlimb seen in boa and whale Vestigial Structures
Evidence for Evolution: BiochemistryMolecular similarities between different organisms
Evidence for Evolution: Molecular BiologySimilarities in sequence measured by ease of separating hybrid DNA strands by heat
Evidence for Evolution: Genetics • Mutation generates diversity • Meiosis and Fertilization generate new combinations due to • Crossing Over • Alternate patterns of chromosome segregation • Unique chromosomes of fertilizing sperm combined with unique chromosomes of egg
Evidence for Evolution: BiogeographyPlants and animals of each continent are distinctive
Evidence for Evolution: Convergent Evolution Unrelated organisms in similar environments show some of the same adaptations
Euphorb of Africa Cactus of North America Meadowlark of North America Yellow-ThroatedLongclaw of Africa Evidence for Evolution: Convergent Evolution
Other Patterns: Divergent EvolutionDifferent phenotypes arise as related species encounter environmental differences
Predators and their Prey Flowering plants and their Pollinators Other Patterns: Coevolution Interacting species adjust together to maintain a relationship
Applying Your Knowledge • Adaptation • Homologous Structures • Analogous Structures • Divergent Evolution • Convergent Evolution • Leads to similar traits in unrelated species • Traits that have a common origin • A trait that makes a species survival more likely is called a(n)