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Chapter 24

Chapter 24 Evolution by Natural Selection

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Jon Stone

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Chapter 24 Evolution by Natural Selection I) Populations and species evolve, meaning that their heritable characteristics change through time. More precisely, evolution is defined as changes in allele frequencies over time. Evidence for Evolution: 1) Evidence for Change through time: a) Fossil record: (first measured in relative age through sedimentary rock then through radiometric date) i) Extinction ii) Transitional form – “law of succession” – is a fossil species with traits that are intermediate between those of older and younger species a) Vestigial traits – a reduced or incompletely devolved structure that has no function or reduced function but it is clearly similar to functioning organs or structures in closely related species b) Current EXAMPLES: Evolving Bacteria, insect and plant resistance 2) Evidence that species are related: a) Geographic relationship a. Galapagos Mockingbirds – must have evolved from same ancestor therefore are part of a phylogeny b) Homology – a similarity that exists in species descended from a common ancestor. 3 levels: a. Genetic homology – a similarity in the DNA sequences of different species b. Developmental Homology –a similarity in embryonic traits – observed in the overall morphology (form) of the embryos and in the fate of particular embryonic tissues. c. Structural Homology – similarities in adult morphology (i.e. arms) II) Natural Selection occurs when individuals with certain alleles produce the most surviving offspring in a population. An adaptation is a genetically based trait that increases the ability of an individual to produce offspring in a particular environment. Evolution – a change in the allelic frequencies of a population over time Process of Evolution by Natural Selection: occurs when (1) heritable variation leads to (2) differential success in survival and reproduction Adaptation – A trait that increases the fitness if an individual in a particular environment *Finches: Although beak size showed no net change, beak shape changed dramatically (more pointier) and overall body size got smaller III) Evolution by natural Selection is not progressive, and it does not change the characteristic of the individuals that are selected, it changes only the characteristics of the
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