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Evolutionary Biology
Genes and behaviour
| Question | Answer |
|---|---|
| Colour vision in primates | Humans, Apes, Howler monkeys, Old World monkeys - trichromatic They have 3 receptors in their retina, coded by 3 different loci, allowing them to see all colours - colourblindness more common in males |
| New world monkeys | dichromats AND trichromats Every animal has the blue opsin, but red and green are alleles of the same X-locus |
| Is there a fitness advantage to being a trichromat? | Hypothesis: trichromats are better able to see fruit because dichromats cannot distinguish between red and green |
| Conclusion | Trichromats are better able to find orange/red food, and it is not just better vision (as shown by the green food control experiment) Red/orange fruit is the ripe fruit, and therefore the ability to better distinguish red is advantageous |
| Heritable variation | When heritable genetic variation results in phenotypic variation, and this phenotypic variation leads to differential fitness, natural selection will increase the proportion of the fitter genotypes in the next generation |
| Heritable variation | When variation in the diversity of opsin genes results in variation in the ability to see colourful (ripe) fruit, and this variation in the ability to see ripe fruit leads to variation in the ability to find food |
| Heritable variation | And thus variation in numbers of viable offspring, natural selection will increase the proportion of individuals with better ability to see ripe fruit in the next generation |
| Parenting behaviour in male sticklebacks | During breeding season, males defend territories, construct nests and attract mates Females spawn in the nests Males are the sole providers of parental care – they defend and tend their fertilised eggs “fanning” – oxygenating eggs |
| Parenting behaviour in male sticklebacks | Males adjust their care in response to many environmental factors – diet, algal blooms, temperature, oxygen availability, clutch size etc. ALSO, MALES SHOW CONSISTENT INDIVIDUAL DIFFERENCES IN FANNING BEHAVIOUR |
| Heritability | How much of the variation in fanning behaviour between individuals can be explained by differences in the genetics of the fish? Just ask whether offspring are similar to their parents |
| Another way to think about it | (1) If parents vary in fanning behaviour, and (2) if offspring are similar to their parents even after being raised separately from their parents, then we conclude that genetic variation is causing (at least some of) the phenotypic variation |
| Another way to think about it | First, RAISE THE OFFSPRING SEPARATELY FROM THE PARENTS, which eliminates the phenotypic influence of the stickleback parent (e.g. sons learning fanning by observing their fathers’ behaviours) |
| So? | When heritable genetic variation results in variation in amount of time spent fanning, and this variation in time spent fanning leads to variation in numbers of viable offspring - |
| So? | Natural selection will increase the proportion of individuals with optimal time spent fanning (i.e more than zero but probably not 24 hrs a day) in the next generation |
| Parent-offspring regressions | We can calculate heritability (selectability) from the slope |
| Slope = 1.0 Heritability = 1.0 | Like father, like son |
| Slope = 0.0 Heritability = 0.0 | Fathers vary, but sons are all the same |
| Slope = 0.5 Heritability = 0.5 | Fathers vary more than their sons |
| How heritable is the fanning trait? | Males were allowed to build nests, and then a gravid female was introduced Female removed after spawning, and male fanning behaviour observed |
| How heritable is the fanning trait? | Fathers were removed before eggs hatched Sons were allowed to grow to maturity, build nests, and mate Sons fanning behaviour |
| Heritability = 45/50 = 0.9 Why did this matter? | There is a genetic component to parental care in this species that can be selected on (the genetic variation is heritable), and selection can thus cause the trait’s mean value to change |
| How do we use the regression to estimate selectability (= response to selection)? | The difference between the mean of the whole population, and the mean of only those with viable offspring is called the Selection Differential (S) |
| If we know heritability, we can predict response to selection R! | Mean phenotype value of the parent population before vs after selection |
| If we know heritability, we can predict response to selection R! | If heritability (the slope) = 1, then the response will be the same as the selection differential High heritability combines with strong selection to produce rapid change R = response to selection |
| If we know heritability, we can predict response to selection R! | The mean of the time spent fanning for the offspring is to the right of the parental mean. The difference between the offspring and parent means is called the response to selection (R) If we know heritability, we can predict response to selection R! |
| Variation amongst the parents (here, how much do fathers vary in their fanning rate?) | In this case, fathers vary (and sons are very similar to their fathers), so it is possible for the trait to be selected |
| Variation amongst the parents (here, how much do fathers vary in their fanning rate?) | In this case, fathers hardly vary at all, which means that even though sons are pretty similar to their fathers, there is no way to select for a subset of fathers that have a higher or lower value of the fanning trait. |
| Variation amongst the parents (here, how much do fathers vary in their fanning rate?) | So for this reason, the trait has 0 heritability (i.e. 0 selectability) |
| Can heritability be misleading? | If there is no parental variation for a trait, there can be no selection on that trait Therefore these traits have basically 0% heritability (low selectability) because parental variation is nearly 0 |
| The intercept of the parent-offspring regression line | Heritability = 100% in both environments (1:1 relationship between parental and offspring fanning rate), but the intercept is higher in Environment B than in Environment A |
| The intercept of the parent-offspring regression line | Env A: If father fans at rate 5, son also fans at rate 5 Env B: If father fans at rate 5, son fans at a higher rate 8 |
| The intercept of the parent-offspring regression line | In fact, all sons fan at a higher rate than their respective fathers, shown by parent-offspring regression that has a higher intercept but the same slope |
| The intercept of the parent-offspring regression line | The environment can have a strong effect on a trait even if the trait is 100% heritable. 100% heritable does not mean 100% genetically hardwired. Heritability does not mean how much of a trait was inherited. It means how much a trait is selectable |
| Heritability and human behaviour | We rely on twin studies: Genetic variance (0 for MZ, 0.5 for DZ) Shared environmental variance (family, parenting, home) Unshared environment (illness, random effects, individual perception) |
| Even if IQ is 100% heritable | It is perfectly possible for offspring IQ to be higher (or lower) than parental IQ when raised in a different environment (what are ways to do this?) |
| Even if IQ is 100% heritable in one environment | (dashed line), it is perfectly possible for heritability to drop to 0% in other environments (What kind of Envs A and B could achieve the observed lines?) |
| Heritability and environment | Human traits are heritable to some extent But this does not mean that human behaviour is genetically “hardwired”, since we create and customise our own environments (which can change the intercept and/or slope of the parent-offspring regression) |
| Foraging in the fruit fly: highly heritable in one environment, not heritable in another environment | Rovers = 70% of population Sitters = 30% of population Due to differences in the gene ‘foraging’ forR is the dominant allele fors is recessive |
| Foraging in the fruit fly: highly heritable in one environment, not heritable in another environment | On Yeast (env A), different foraging alleles lead to different behaviours (heritability is high) On Agar (env B), different foraging alleles have no effect on behaviour - Everybody is a rover. (heritability is low) |
| Foraging in the fruit fly: highly heritable in one environment, not heritable in another environment | So genetically encoded ‘innate’ behaviours can be variable or not variable, depending on the environment |
Created by:
rose.coo
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