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Chapter 1

Lecture 3 (Chapter 1 - Jan21) - PSYCH 2TT3

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McMaster University
Brett Beston

PSYCH 2TT3 2013 Lecture 3 - January 21, 2013 Chapter 1 Evolution  Artificial selection  Natural selection  Genetic variation, heritability and inclusive fitness  Evolution – a change over time of the proportions of individual organisms differing genetically in one or more traits  Humans, not nature, are often the driving force in variation in populations of plants and species Artificial Selection  Dogs and Artificial Selection o All dogs are derived from wolves; dates back to 10,000 (Saluki – Egyptian) o Bred for hunting, herding (sheep dogs) etc o Traits are an example of turning ancestral behaviour of tracking into something useful  “Playing Darwin”(video) – process of artificial selection in dogs; play with gene pool to breed dogs  Artificial selection for small poodles (teacup poodles) o Each generation, the dog breeder allows only the smallest poodle to breed o Smaller poodles are more successful at reproducing o Smaller poodles have higher fitness o Variety of dogs today due to process of artificial selection o We put pressure on dogs that differentially selects fitness  Fitness – the success of an individual in reproducing; number of offspring produced in the lifetime of an organism  Artificial Selection – the process by which humans selectively choose and breed animals with some preferred characteristics o Can and have selected for a large variety of traits in plants and animals we use for food (fruits, vegetables and grains; chickens, cows, pigs etc) o Eg/ Corn vs Teosinte – corn originally an agricultural crop; teosinte (shrub like plant), corn (stalk like plant) o Eg/ Chickens are “treated as fast food on legs, bred to be fried and eaten within seven weeks of emerging from the egg” o Eg/ Turkey “a white-breast-meat robot with short legs and a small breastbone, the new turkey could barely stand or walk, never mind turkey trot or manage the intricate balancing act of mating” o Traits we see determined by artificial selection  Evolution by artificial selection – the process by which humans using selective breeding change, over time, the proportions of individual animals differing genetically in one or more traits  Artificial selection for phototaxis in fruit flies o Maze has 15 levels; flies could score between 16 (all light) to 1 (all dark) – 1=toward the light, 0=toward the dark  Of traits that we see variation in, can we determine the heritability of these traits?  yes  Artificially select for phototaxis behaviour  Allows fruit flies to navigate maze; many corridors with T junction (right toward light, left not toward light)  Had to have many trials (make many decisions)  Took flies that show extremes (only light, only dark) – to breed and see if they can shift population behavioural traits and see the degree of change  Initially – most flies did not have a preference for light or dark  Removed selective pressure (any could breed with any) – traits turned back to middle ground  Fruit Fly Fight Club – frit flies brawl over mates and territory; now some scientists are betting that these battles can help them unravel the genetic basis of aggression o Why do males fight? Why select for increased aggression? o Frequency of fighting – breed aggression; see fighting frequency  After 10 (white bars) and 20 (grey bars) generations of selection (2 control, 2 selection lines)  Neutral (no artificial selection) – fighting ~20% of time  Selected aggressive trait – after 10 generations, 45-60% of time spent fighting; 20 generations, 80% time spent fighting  Humans have same locus for aggression 1 PSYCH 2TT3 2013  Artificial selection can succeed only with a trait that has heritable variation  Heritability – the contribution of genes to observed variation in a trait Artificial vs. Natural Selection  Artificial Selection – the variation in reproductive success (fitness) of individuals is determined by humans, who decide which individuals reproduce  Natural Selection – the variation in reproductive success (fitness) of individuals is determined by survival, mate choice…  Necessary conditions for evolution by natural selection o Heritable individual variation that corresponds to variation in fitness The Importance of Fitness  Fitness = lifetime reproductive success, which is a product of reproductive rate and the length of reproductive lifespan o Eg/ Female 1 lays 10 eggs per day and lives for 20 days  200 eggs Female 2 lays 15 eggs per day and lives for 10 days  150 eggs Female 1 has a higher fitness Frequency of trait will increase in population from female 1 faster than from female 2  Fitness benefits and frequency of traits  Lifetime reproductive success in female red deer from the Isle of Rum (Scotland) o If elk gets to age 1, likely to survive to adulthood o A lot of variation  Lifetime reproductive success in male red deer o Males fight o 50% of all males don’t produce any offspring 2 PSYCH 2TT3 2013  Fitness consequences o Many animal traits influence survival and reproduction o There is typically large differences in fitness among individuals o Even a small difference in fitness (1%) can have strong effects on evolution  Variation within and heritability of human traits – behavioural and cognitive traits o Theory of mind, Intro/extraversion, Athleticism, Anxiety, Aggression, IQ, Metabolism, Food preference, sexual behaviour  Can measure the proportion of trait that is determined by environmental/experimental vs. heritability Individual Variation  Almost all traits show genetic variation  There is typically large variation among individuals  The variation is determined by genetic and environmental factors  Genetic Variation o Without genetic variation, some of the basic mechanisms of evolutionary change cannot operate o There are three primary sources of genetic variation 1. Mutations – random change in DNA;  Can be beneficial, neutral or harmful  Not all mutations matter to evolution  Somatic Mutations – mutations in non-reproductive cells; never passed down to future generation  Germline Mutations – mutations that are passed down to future generations  The only mutations that matter to large-scale evolution are those that can be passed on to offspring; these occur in reproductive cells like eggs and sperm  Causes of Mutations  DNA fails to copy accurately  External influences can create mutations (eg/ radioactivity, poisons) 2. Migration – any movement of genes form one population to another 3. Sex and Migration – sex can introduce new gene combinations into a population and is an important source of genetic variation  Genetic shuffling – can be good or bad o Polygenic variation is caused by several or many loci  Normal distribution  Sources of Variation o A given genotype has an average phenotypic value but individuals sharing this genotype vary because of environmental effects o Genetic Variance – average amount of variance among genotypes o Environmental Variance – average amount of variance among individuals within the same genotype  Components of Phenotypic Variation o Phenotypic Variance – genetic variance + environmental variance  V P V G V E o Heritability (Broad Sense) = h = V / V + V 2 G G E o Heritability (Narrow Sense) = hN = VA/ VP  During recombination; what is the probability that all genes for a trait are going to be passed down o The proportions
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