Transcript Slide 1
[Review] Evolution by Natural Selection tends to make the mean phenotype converge on the optimum. Sexual reproduction continually generates variation around the optimum. Sexual selection builds adaptations (multiple optima) that increase mating success: 1. more mates 2. better mates Sexual selection is driven by sex differences in reproductive rate. Evidence in favor of evolution 1. Existence and pattern of the fossil record. 2. Homology. 3. Universality of genetic code. 4. Analogy with plant and animal breeding. 5. Direct observation. [New topics] Facultative adaptations Heritability Useful to recognize two types of gene action: 1.Obligate: resist environmental interference. 2. Facultative: monitor environmental variation and adjust phenotype accordingly. Suntanning (example of facultative adapatation) UVb is both good and bad: vitamin D synthesis, melanoma. Melanin (skin pigment) block UVb. Produce enough melanin to optimize risks. OK, but UVb is seasonally variable. Produce more melanin when there more UVb. Facultative adaptation! Selection favored genes that produce the right pattern of environmental responsiveness: Melanin synth. right dependent variable. right independent variable. right norm of reaction. Ambient UVb What determines your skin color? The way your genes translate your experience into melanin synthesis. “Natural selection wrote the rules for how nurture works” Facultative adaptations: The effects of experience are pre-programmed by natural selection. Figure 03.14 Figure 03.15 Figure 03.16 So… does the nature/nurture dichotomy have any use? Not in relation to individuals. 1.Why is my skin this color? Genes AND environment: Interaction between genes and environment that has been preprogrammed by natural selection. Other nature/nurture questions? 1.Why is my skin this color? causes of development 2. Why do people differ in skin color? causes of variation Differences between individuals can arise because: A. They have different genes, B. They have different experiences. We can “partition the variance” How much of the phenotypic difference between individuals: Is due to the fact that they have different genes? And how much is due the fact that they have different experiences? Vp is the phenotypic variation in a trait. frequency e.g., How much variation is there in height in some human population? height Vp (phenotypic variance) has two components: Vg and Ve Vg is the fraction of phenotypic variance that is due to individuals having different genes affecting the trait. Ve is the fraction of phenotypic variance that is due to individuals having different experiences affecting the trait. So… Vp = Vg + Ve It’s just a sentence! Vp = Vg + Ve Phenotypic variation is the sum of: underlying genetic variation plus the effects of different environments on the expression of those genes. And so, if we want to know how heritable a trait is… h2 = Vg/Vp, thus h2 = Vg/(Vg+Ve) h2 varies from 0 to 1. Selection is more effective when heritability is significant. The nature/nurture dichotomy is: Meaningless for explaining the traits of individuals, But it makes sense when we want to explain differences, Heritability provides the necessary “purchase” for natural selection; if differences are heritable, selection can spread advantageous traits.