How do species change over time? Mechanisms of Natural Selection. Part 1: Sources of Variation. Causes of Mutations

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1 How do species change over time? Mechanisms Natural Selection Part 1: Variation Part 2: Selection Part 1: Sources Variation Scientists now know that variations result from: Mutations the nucleotides Recombination chromosomes Causes Mutations 1. Spontaneous switching bases during DNA replication 2. Environmental Mutagens: Radiation can break apart the DNA UV, X-Ray, Gamma Rays Chemicals can cause changes in the DNA Dioxins, Asbestos, Benzene, Formaldehyde 1

2 Three Types Mutations 1. Point Mutations: A change in a nucleotide base can radically change the expression a gene: TAC CGC ACG TTG CAT TAC CGC ACG TTG CAG THE DOG BIT THE CAT THE DOG BIT THE CAR Example: Sickle Cell Anemia Sickle Cell Anemia 2. Frameshift Mutations Deletion a nucleotide base from the DNA strand results in different codons, causing nearly every amino acid in a protein to be changed TAC CGC ACG TTG CAT TAC CCA CGT TGC AT THE DOG BIT THE CAT THE DGB ITT HEC AT 2

3 3. Chromosomal Alterations Parts chromosomes can be broken f during mitosis or meiosis, then rejoined incorrectly or lost. Such mutations can cause homologous chromosomes to not pair, since they are different, resulting in gametes with an extra copy a gene (or lack a gene). Example: Down s Syndrome A Normal Set Human Chromomsomes DOWN S SYNDROME Notice the extra #21 chromosome: it results in the overexpression certain proteins. Recombination Example a mutation in Drosophila. Disruption the Hox genes can lead to a phenotype known as Antennapedia in which fly legs develop in place the antennae. Example polydactyly, a condition having more than the normal number fingers or toes. This condition arises from disruption the Hoxd13 gene. Most variations within a population do not arise from mutation, but rather from recombination existing alleles Allele recombination can occur 3 ways during sexual reproduction: 1. Independent assortment chromosomes 2. Crossing over 3. Combining sperm and egg during fertilization 3

4 Independent Assortment Chromosomes Crossing Over Chromosomes Fertilization Part 2: Selection and the Niche A population s niche = its way life and its use the environment. The niche includes: where the organisms in the population live what they eat what eats them 4

5 Selection and the Niche If a population s niche changes, some the individuals may have characteristics that suit the new conditions better than others The ones with favorable characteristics will survive and reproduce Those with unfavorable traits are less likely to survive and reproduce This is the basis Natural Selection Selection and Distribution There are 3 types selection that we can observe in different populations: 1.Directional Selection 2.Disruptive Selection 3.Stabilizing Selection Normal distribution Imagine this graph represents coat color a population In a normal distribution curve there is a greater number individuals that are in the middle the curve medium coat color There are fewer at the extremes light & dark Frequency Normal Distribution Light Medium Dark 1. Directional Selection the environmental conditions favor individuals one extreme he curve shifts to the right (ex. dark colors are favored) or to the left (ex. light colors are favored) 5

6 2. Disruptive Selection 3. Stabilizing Selection selection against the middle the normal distribution curve favors variants opposite extremes Ex. Medium color is no longer favorable both light and dark colors are favored the conditions the environment remain stable for a long period time selection is against the extremes and favors the intermediate Ex. The medium color is favored Review Questions: 1. Define the word niche. 2. How is an organism s niche different from its environment? 6