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Natural Selection and Drift Drift can affect evolution even when selection is strong Natural selection and drift Drift has a larger effect in smaller populations Selection has a larger effect in larger populations Natural selection in the lab Selection for E. coli to survive better in artificial media Two Views on the Relationship Between Mutation & Selection • Lamarkian: the origin of variation depends on selection. Variation arises (through mutation) in response to challenges. • Darwinian: the origin of variation is independent of selection. Variation exists in a population and some traits are passed down due to their higher relative fitness. Replica Plating as a Test of the Relationship Between Mutation & Selection strpSix bacterial colonies replica strp+ Only one bacterial colony grows in the presence of streptomycin, demonstrating that it has streptomycin-resistance. Did exposure to streptomycin cause the resistance mutation to appear, or did presence of streptomycin resistance mutation allow a resistant colony to grow? Replica Plating as a Test of the Relationship Between Mutation & Selection strpstrp+ replica strp+ streak Streptomycin resistant mutants existed prior to exposure to streptomycin strp+ Based on your understanding of natural selection and of mutation as a mechanism of generating variation, explain the changes that occurred in the tree and dinosaur populations over time. These represent the average heights for an entire population (AAAS 1999) time So... • Mutation doesn’t occur in response to selection. • Selection acts on existing genetic variation. We can use DNA sequences to infer evolutionary history DNA Sequence Analysis 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 A G G C T A C T G C C A T A T T Species 1 ind X . . . . C . . . . T T . A . . . ind Y . . . . C T . . . . T . A . A C ind Z . . A . C T . . . . T . A . A . Species 2 ind X . C . . . . . . . . . G . G . . ind Y T C . . . . . . . . . G . G . . ind Z T C . . . . G . . . . G . G . . Species 3 ind X . C . T . . . . . . . G . . . . ind Y . C . T . . . C . . . G . . . C ind Z . C . T . . . C A . . G . . . . Common Ancestor Map the mutations on the tree Common Ancestor 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 A G G C T A C T G C C A T A T T X Y Z Species one X Y Z Species two X Y Z Species three DNA Sequence Analysis 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 A G G C T A C T G C C A T A T T Species 1 ind X . . . . C . . . . T T . A . . . ind Y . . . . C T . . . . T . A . A C ind Z . . A . C T . . . . T . A . A . Species 2 ind X . C . . . . . . . . . G . G . . ind Y T C . . . . . . . . . G . G . . ind Z T C . . . . G . . . . G . G . . Species 3 ind X . C . T . . . . . . . G . . . . ind Y . C . T . . . C . . . G . . . C ind Z . C . T . . . C A . . G . . . . Common Ancestor Common Ancestor 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 A G G C T A C T G C C A T A T T 1: A->T X Y Z Species one X Y Z Species two X Y Z Species three Variation vs. Substitution • Variation – within-species or withinpopulation diversity at a particular locus. E.g, a gene sequence in different individuals may have a variable base of A or G. • Substitution – loss of an ancestral allele through fixation of a new allele in a population. The sequence has changed and can be observed as a difference in comparison with other species. Is this variation or a substitution? Common Ancestor 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 A G G C T A C T G C C A T A T T 1: A->T X Y Z Species one X Y Z Species two X Y Z Species three The Journey of Man • Spencer Wells • Uses Y chromosome in men as a molecular marker. • All men descended from a single man living in Africa 60,000 years ago Human history, mitochondria, and the Y-chromosome • Y-chromosome only paternally inherited – Mutations passed down, no recombination b/c only ever one copy of Y-chr. in any male • Can use these as markers to track patterns of ancestry INDIA CENTRAL ASIA EUROPE EAST ASIA OUT OF AFRICA AMERICAS OCEANIA EASTERN AFRICA WESTERN/CENTRAL AFRICA SOUTHERN AFRICA