1. Trait evolution Up until now, we focused on microevolution – the forces that change allele and genotype frequencies in a population This portion of the class will focus on the evolution of traits - morphology (features of the physical body) - behavior (mate preference; habitat choice; nocturnal vs diurnal activity) - life history (age at 1 st reproduction; life span; # of offspring)

2. Quantitative traits and heritable variation Some traits are discrete, meaning they can fall into certain categories but not have in-between values - eye color can be blue, brown, or hazel, but not a blend - you either have cystic fibrosis, or you don’t What about traits controlled by multilocus genotypes?..

3. Quantitative traits and heritable variation Quantitative traits show continuous variation among individuals - height: you can be 4’6” tall, or 6’9” tall, or any height in between (there is a continuum of heights in a population) - quantitative traits are determined by multiple genes, and are also affected by the individual’s growing environment

4. Quantitative traits and heritable variation Most quantitative traits reflect contributions both of genes and the environment you grew up in For instance, tall parents tend to have tall kids However, children of first generation immigrants are usually taller than either parent, because of better childhood nutrition Thus, both genes and environment determine your adult height

5. Quantitative traits and heritable variation Quantitative traits usually have a normal (bell-curve) distribution of values What makes the tallest person so tall? Is it my genes? Or the environment I grew up in? Or how much of each?

6. How heritable is a quantitative trait? Take a pair of identical twins from short parents Raise one under the conditions in which the shortest individual grew up (environment #1) Raise his twin under the conditions in which the tallest individual grew up (environment #2) 4’ 6” If his twin grows up to be just as short, then all the variation in height must be genetic (heritable) 4’ 6”

7. How heritable is a quantitative trait? Take a pair of identical twins from short parents Raise one under the conditions in which the shortest individual grew up (environment #1) Raise his twin under the conditions in which the tallest individual grew up (environment #2) 4’ 6” If his twin grows up to be very tall, then all the variation in height must be due to the environment he grew up in 6’ 9”

8. How heritable is a quantitative trait? Take a pair of identical twins from short parents Raise one under the conditions in which the shortest individual grew up (environment #1) Raise his twin under the conditions in which the tallest individual grew up (environment #2) 4’ 6” If his twin grows up to be in the middle, then the variation in height must be due to a combination of genetics and environment 5’ 11”

9. Parental Generation --- - individuals from extremes of the trait distribution are mated with their own kind - offspring raised in a common, controlled environment - if the means of offspring differ, the trait is heritable mean heritable a population

10. Parental Generation --- - individuals from extremes of the trait distribution are mated with their own kind - offspring raised in a common, controlled environment - if the means of offspring differ, the trait is heritable mean not heritable

11. Sources of phenotypic variation The total variation in a trait is the phenotypic variation, V P - subtract the height of the smallest person from the tallest person; this will give you the range in heights, V P Variation among individuals due to differences in their genes is genetic variation, V G Variation among individuals due to differences in their environment is environmental variation, V E V P = V G + V E

12. Quantitative traits and heritable variation The fraction of the total variation in a trait that is due to variation in the genes is termed the heritability of that trait broadheritability = V G = V G V P V G + V E =genetic variation, V G total variation, V P =genetic variation, V G genetic variation + environmental variation, V E This is termed H 2, the broad-sense heritability

13. If variation is due to genes, offspring will resemble their parents Plot the average value for 2 parents against the average for their offspring, for many sets of parents & kids midparent height (average height of mother & father) average height of offspring How do you measure heritable variation? 1) Scatterplot of values for all families Family #2: tall parents, tall kids Family #1: short parents, short kids

14. If variation is due to genes, offspring will resemble their parents Compute the best-fit line through the points for all families, using least-squares linear regression midparent height (average height of mother & father) average height of offspring How do you measure heritable variation? 2) Best-fit line through all the points The slope of this line estimates how much variance in parents is due to variance in their genes

15. The slope of the best-fit line is used to directly measure the heritability of a trait midparent height (average height of mother & father) average height of offspring How do you measure heritable variation? Slope of the line, h 2, is termed the narrow-sense heritability

16. Sources of genetic variation Total genetic variation, V G, comes from two sources: VG = VA + VD (1) additive genetic variation is variation between individuals due to the combined effects of many genes working together in each individual (2) dominance genetic variation is the variation due to gene interactions like dominance and epistasis, where an allele of one gene can “over-rule”... - another allele of the same gene (dominance) - any allele of a different gene (epistasis)

17. Narrow-sense vs. broad-sense heritability Slope of the line, h 2, is termed the narrow-sense heritability h 2 = V A = V A V A V P V G + V E V A + V D + V E Narrow-sense heritability is the fraction of variation between individuals that is due to additive genetic variation only h 2 = additive genetic variance variance from genes + variance from environment

18. Narrow-sense vs. broad-sense heritability Broad-sense heritability = all variation due to genetic differences total phenotypic variance Narrow-sense heritability = variance due to the effects of many alleles all added together total phenotypic variance My height is due in part to many alleles of different genes each contributing a certain amount; this is additive genetic variation My height may also result from dominance of one allele over a recessive allele at a particular locus; that’s dominance genetic variation, and we aren’t so interested in that

19. Narrow-sense vs. broad-sense heritability Slope of the line, h2, is termed the narrow-sense heritability h 2 = V A = V A V A V P V G + V E V A + V D + V E Why do we care about all this? Understanding narrow-sense heritability lets us: (1) measure the heritability of a trait in a population (2) determine if the mean value of a trait is likely to change in response to selection

20. Genes vs. environment for offspring + parents Parents often raise their offspring under similar environments to those in which the parents themselves grew up - how then do you distinguish whether offspring are like their parents because of shared genes, or shared environments? 1) Reared-apart experiments: offspring of same parents raised under different conditions (esp. useful with identical twins) 2) Common garden experiments: offspring of different parents raised under identical conditions 3) Random distribution of young: scatter offspring so they are not raised by their own parents; environment is a random variable

21. Eggs taken from many nests, placed in other nests with foster parents Result: chicks grew up to resemble their biological parents and not their foster parents Heritability, h 2 = 0.98 Heritability of beak depth in song sparrows

22. Strength of selection To predict if a trait will evolve in response to selection, you need to know two things: (1) the heritability of the trait, since only inherited traits can change in response to selection (2) the strength of selection, or how much of a reproductive advantage a trait confers on parents - if parents with a high mid-parent value for a certain trait are more likely to reproduce, then that trait will respond more strongly to selection 

23. Strength of selection The strength of selection can be calculated as the mean value among parents that successfully reproduce, compared with the mean value of the whole population This value, S, is termed the selection differential mean of successful – mean of all parents parents

24. Predicting the response to selection The response to selection is given as: R = h 2 S Response = (heritability) x (selection differential) Can be expressed as h 2 = R S

25. Summary: heritability and strength of selection Heritability can be estimated by comparing the similarity of relatives The strength of selection can be measured as the relationship between phenotype and fitness-- how a trait plays out as a reproductive advantage Response to selection tells you how much a trait will change over a generation, based on how much of the trait is genetic and how much that trait contributes to fitness