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Evolu&on of Popula&ons Ch. 16 p. 392 How common is Gene&c Varia&on? •  Many genes have at least two alleles •  Some you see, many are “invisible” (involves small differences in biochemical processes) Gene pool •  Consists of all genes, including all the different alleles that are present in a popula&on Varia&on and Gene Pools Sample P
Population
•  Gene ool for Fur Color in MFrequency
ice of Alleles
allele for
brown fur
Copyright Pearson Pren&ce Hall allele for
black fur
How is evolu&on defined in gene&c terms? •  The change in the rela&ve frequency of alleles in a popula&on Sources of Gene&c Varia&on 1.  Muta&ons 2.  Gene&c shuffling that results from sexual reproduc&on Muta&ons •  Change in sequence of DNA •  May or may not affect phenotype •  May or may not affect individuals fitness Gene Shuffling (do not write) •  Differences due to gene shuffling during gamete produc&on •  Crossing over increases the number of different genotypes •  Sexual reproduc&on produces different combina&ons of genes, but does not alter the rela&ve frequencies of each type of allele in a popula&on Direc&onal Selec&on •  Occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end Maintenance of Variation
Direc&onal selec&on: acts to eliminate one extreme from an array of phenotypes Natural Selec&on on Polygenic Traits •  In this case, birds with larger beaks have higher fitness. Therefore, the average beak size increases. Copyright Pearson Pren&ce Hall Stabilizing Selec&on •  When individuals near the center of the curve have higher fitness than individuals at either end of the curve •  Favors the middle phenotype Natural Selec&on on Polygenic Traits •  Human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average. Copyright Pearson Pren&ce Hall Disrup&ve Selec&on •  When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. •  Favors individuals of the extreme phenotypes Natural Selec&on on Polygenic Traits •  If average-­‐sized seeds become scarce, a bird popula&on will split into two groups: one that eats small seeds and one that eats large seeds. Copyright Pearson Pren&ce Hall Maintenance of Variation
Disrup&ve selec&on acts to eliminate intermediate types Gene&c Equilibrium •  When allele frequencies remain constant 5 Condi&ons required to maintain gene&c equilibrium 1. 
2. 
3. 
4. 
5. 
Random ma&ng Large popula&on size No movement into or out of the popula&on No muta&ons No natural selec&on Known as the Hardy-­‐Weinberg Principle 
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