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Lecture 5

BIOL 359 Lecture 5: Natural Selection

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BIOL 359
Kristen Muller

Natural Selection • Darwin is often considered first person to consider evolution • Natural Selection: o Differential survival and reproduction of phenotypes that lead to differences in their contributions to next generation o Survive and Reproduce to pass on genes to next generation o Change in a population of individuals o Has to be inherited changes o No change in allele frequency means no evolution occurred ▪ Natural selection acts on phenotypes, only if there is a change in allele frequency in a population will evolution have occurred. o Components: ▪ Living things produce more offspring than can be supported ▪ Struggle for existence ▪ variation in phenotype ▪ variation is heritable ▪ Best adapted to current conditions, survive and reproduce ▪ Heritable traits are passed onto offspring • Fitness: o Individuals contribution to the next generation o More offspring means more fit o Measure of reproductive success, affected by: ▪ Viability/mortality selection: ability to survive and reach reproductive age ▪ Sexual Selection: ability to mate (mating success) ▪ Fecundity Selection: Family size, measured by number of female gametes produced o Adaptation is a trait that increases fitness, Natural selection results in adaptive evolution o Atlantic Cod Excess fecundity: ▪ ▪ Females can lay about 2 - 5million eggs, but in the end only about 2 eggs are successful in surviving and reproducing. o Organisms produce many offspring (more than they can take care of) and offspring compete to survive. Only successful offspring will survive and can reproduce o If all offspring survive each time mating occurred, there would be an overload of many species (For example: starfish would amount to about the same number of visible electrons in the universe) • Darwin’s Theory of Natural Selection: o Four postulates 1. Individuals within species are variable: Humans are all the same species, but vary in phenotypes. 2. Some of these variations are heritable 3. More offspring produced than survive 4. Survival and reproduction isn’t random. Reproduction occurs with those who are more favourable and naturally selected for ▪ Those who survive and contribute to next generation are most fit ▪ • Inherited traits that are important to survival and reproduction are passed to offspring and thus become common o Beach Mice: ▪ Variation in coat of mice, which resulted in dark and light coats ▪ Variations that survive predation and reproduce are more fit, and causes a shift in mean coat colour ▪ Darker coat color was easier to spot on beach, and were selected for out of population ▪ Population composition was mainly lighter coat colour because they could camouflage into the sand. ▪ Population evolved, light mice are now dominant • Daphne Major Finches to Test Postulates o Good for testing because no migration of birds in/out of island o Beak shape/length/depth directly correlated to diet o Optimal Foraging Theory: ▪ Animals spends a certain amount of energy to find food, and the energy used to find food should not be more than the energy they get from food. ▪ The food must be worth the work o Postulate 1: Populations are variable, every finch was marked and a variance in beak size, length and depth. o Postulate 2: Variation could be seen as environmental or heritable, but it was evident with evidence that offspring inherited beak size that parents had. ▪ Heritability: Proportion of trait variation in a population that is due to genetic factors and not environmental fa
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