LIFESCI 1 Lecture 2: Week 2: Speciation

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University of California - Los Angeles
Life Sciences

In the context of evolution, survival means nothing if you can’t reproduce Sexual selection: selection for a trait that increases mating opportunities and give individuals a reproductive advantage over others ● Intrasexual: males compete with each other ● Intersexual: females can choose ● Sexual selection is a positive feedback loop ○ Females preference for long tails → male tail length Expensive Eggs vs. Cheap Sperm ● Females are often the limiting resource when it comes to reproduction ● Eggs provide nutrition for developing embryo ● Sperm are compact, their only purpose is to deliver DNA to zygote ○ Females begin by investing more energy per gamete Sexual Dimorphism ● Males and females of the same species look morphologically different ● Males are typically the more elaborately colored and/or ornamented sex ● Sexual selection is usually what causes this ○ Females often invest more energy in reproduction, so they are the choosier sex (but there are exceptions) The elaborate plumage of male birds of paradise has evolved due to females mating preferentially with males that display the most impressive feathers. ● The is an example of intersexual selection Microevolution: change in allele OR genotype frequencies in a population over one or generations Macroevolution: THE SAME THING, except over a period of time long enough to allow for phenotypic change Mechanisms of Evolution (AKA violations of Hardy-Weinberg Equilibrium) 1. Natural selection 2. Gene flow a. Migration 3. Nutation 4. Genetic drift a. Can’t sample every single allele in the population 5. Assortative mating Meiosis: one parent cells produces 4 daughter cells ● All DNA in parent cell ends up with ½ the number of chromosomes as the parent cell Law of Independent Assortment: gamete diversity Hardy-Weinberg equilibrium= population is NOT evolving, allele AND genotype frequencies remain the same from one generation to the next (doesn’t actually actually happen in real life, but concept is very useful for understanding mechanisms of evolution ● p + q = 1 ● p^2 + 2pq + q^2 = 1 ● Reminder that calculating allele frequencies IGNORE INDIVIDUALS, it’s all about the GENE POOL of the population ● In contrast, genotype frequencies are determined by how alleles are sorted into particular combinations in the individuals across the population ● ALL of the following assumptions must be met for a population to be in HW equilibrium ○ No selection i. No difference in reproductive success of individuals ● BUT variation in population → differential reproductive success ii. Alleles can become fixed or lost in a population ○ No migration/gene flow i. Individuals cannot add alleles to or subtract alleles from the population via migration ○ No mutation i. Genes must not change due to mutation in heritable cells ii. Mutation: any change in genetic materials ● Deleterious (harmful) ● Advantageous (beneficial) ● Neutral (not functionally important) ● Advantageous mutations increase in frequency via positive selection Deleterious mutations decrease in frequency via negative selection (also called purifying selection) iii. Recombination shuffles mutations to produce new sequences ● Mutation and recombination: the two sources of NEW genetic variation ○ Random ○ Relatively slow process ○ No genetic drift (infinite population size) i. Random changes in allele frequencies from one generation to the next UNLESS populations are very large ii. Genetic drift: some alleles get passed on more than others just by random chance, just like rolling dice ● Non-adaptive mechanism of evolution ● Frequencies change simply by chance, because all alleles can’t be inherited in a population of finite size ● Drift is more pronounced in small populations--alleles are more readily fixed or lost ○ Decrease in size leaves a genetic signature on a population ○ Founder effect can result in loss of alleles ○ Population bottlenecks: catastrophic events ■ Genetic variation is greatly reduced because mutations accumulate slowly despite rapid population growth ○ Random mating i. Individuals have no mate preference (at least in regard to loci of interest) ● Corals are broadcast spawners (release sperm and egg in the water at random) ● Human blood type ● Why exactly do violations of HW equilibrium result in evolution? ○ They always change allele and/or genotype frequencies How do deleterious alleles remain in a population? 1. Mendel’s Law of Dominance a. Heterozygous mask deleterious recessive alleles 2. Heterozygote advantage a. Example: sickle cell, SA (some sickled cells) have an advantage, gives host tolerance to malaria parasite b. Balancing selection: two or more alleles are maintained in the population (this is NOT the same thing as stabilizing selection, whi
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