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BIO 1M03.pdf

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McMaster University
Luc Bernier

Chapter 24 Evolution by Natural Selection Harleen Kaur Key ConceptsPopulations and species evolve meaning that their characteristics change through time More precisely evolution is defined as changes in allele frequencies over timeNatural selection occurs when individuals with certain alleles produce the most offspring in a population An adaptation is a genetically based trait that increases an individuals ability to produce offspring in a particular environmentEvolution by natural selection is not progressive and it does not change the characteristics of the individuals that are selectedit changes only the characteristics of the population Animals do not do things for the good of the species and not all traits are adaptive All adaptations are constrained by tradeoffs and genetic and historical factorsSection 241 The Evolution of Evolutionary ThoughtPlato claimed that every organism was an example of a perfect essence or type created by God and that these types were unchanging Aristotle ordered the types of organisms into a linear scheme called the great chain of being in which species were organized into a sequence based on increased size and complexity with humans at the top Lamarck was the first to propose a formal theory of evolutionthe idea that species change through time Lamarck claimed that simple organisms originate at the base of the chain by spontaneous generation and then evolve by moving up the chain over time The process responsible for this progression was the inheritance of acquired characters Darwin and Wallace proposed that evolution does not follow a linear progressive pattern Instead they proposed that the process responsible for change through time is based on variation among individuals in populations Darwin claimed that variation among individuals in a population was the key to understanding the nature of speciesDarwin and Wallace proposed that evolution occurs because traits vary among the individuals in a population and because individuals with certain traits leave more offspring than others do 1Harleen Kaur Section 242 Pattern of Evolution Have Species Changed through TimeEvidence for Change through TimeExtinctionTransitional Forms Vestigial TraitsEvidence That Species Are Related GeographicRelationshipsHomologyEvolution Is Change through TimeDarwin described evolution as descent with modification meaning that change over time produced modern species from ancestral species Evidence for Change through TimeFossils are traces of organisms that lived in the pastFossils that have been found and described in the scientific literature make up the fossil recordMost fossils are found in sedimentary rocksLayers of sedimentary rock are associated with different intervals in the geologic time scalea relative time scale based upon fossil contentGeologic time is divided into eons eras periods and epochsResearchers now use radioactive isotopes to assign absolute ages to the geologic time scaleGeologic data show that Earth is about 46 billion years oldFossil data show the earliest signs of life in rocks about 34 billion years old ExtinctionMany fossils provide evidence for extinct species unlike any known living organismsDarwin interpreted extinction as evidence that species are dynamic and can changeHe reasoned that if species have gone extinct then the array of species living on Earth has changed through time Transitional FormsThe law of succession means that extinct fossil species are typically succeeded in the same region by similar species 2Harleen Kaur Darwin interpreted this pattern as evidence that extinct forms and living forms are related and that they represent ancestors and descendantsMany transitional forms have been discovered with traits that are intermediate between older and younger speciesThese transitional forms provide strong evidence for change through time Vestigial TraitsA vestigial trait is a reduced or incompletely developed structure that has no function or reduced function but is clearly similar to functioning organs or structures in closely related speciesVestigial traits are evidence that the characteristics of species have changed over time Evidence That Species Are RelatedData from the fossil record and contemporary species refute the hypothesis that species are immutable Geographic RelationshipsDarwin collected mockingbirds from the Galpagos islands The mockingbirds were superficially similar but different islands had distinct speciesDarwin proposed that the mockingbirds were similar because they had descended from the same common ancestorThe mockingbird species are part of a phylogeny a family tree of populations or speciesThe relationship between different species can be shown on a phylogenetic tree Homology is a similarity that exists in species descended from a common ancestorGenetic homology is similarity in the DNA sequences of different speciesDevelopmental homology is a similarity in embryonic traitsStructural homology refers to similarities in adult morphologiesIn many cases traits are similar in different species because the species in question were related to each other by common descentIf species were created independently of one another these types of similarities would not occur 3
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