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Chapter 12 – Who Am I? Species and Races Biological Species • A species is a group of individuals that, in nature, can interbreed and produce fertile offspring but cannot reproduce with members of other species • How can species be reproductively isolated? Reproductive Isolation by Species 1. Spatial Isolation: species don’t come in contact 2. Behavioral: behaviors that prepare partners for mating are different 3. Mechanical: sex organs are incompatible 4. Temporal: timing of readiness to reproduce is different 5. Gamete Incompatibility: proteins on egg that allow sperm binding do not bind with sperm https://www.youtube.com/watch?v=8yvEDqrc3XE Postfertilization Barriers • Hybrid Inviability: zygot cannot complete development because of incomplete genetic instructions • Hybrid Sterility: hybrid organism cannot produce offspring Steps in the Process of Speciation 1. Isolation of the gene pools of subgroups, or populations, of the species 2. Evolutionary changes in the gene pools of one or both of the isolated populations 3. The evolution of reproductive isolation between these populations, preventing any future gene flow Race & the Genealogical Species Concept • Biological race: populations of a single species that have diverged from each other as a result of isolation of their gene pools • Genealogical Species Concept: – Consist of organisms that can interbreed – Are all descendants of a common ancestor – Represent independent evolutionary lineages • Reflects closer shared ancestry-genealogyamong certain individuals within a biological species Humans Originated in Africa • Morphology of Human Ancestors in the fossil record • Human Species is ~200,000 Genetics of Division of Species into Races 1. Alleles that are unique to a particular race 2. Similar allele frequencies for a number of genes among populations within races 3. Differences in allele frequencies among populations in different races Hardy-Weinberg Theorem • Allele frequencies will remain stable in populations that are large in size, randomly mating, and experiencing no migration or natural selection • Describes the relationship between two allele frequency and genotype frequency for a gene with two alleles in a stable population https://www.youtube.com/watch?v=u9EVOzVJeZc Hardy- Weinberg Equations • Groups Quiz Next Week – solving a HardyWeinberg problem (and other questions) • How to calculate: – https://www.youtube.com/watch?v=oG7obMtO8c Why Aren’t Human Races Different Species? • No alleles are found in all members of a race • Populations within a race are often as different as populations compared across races • Human races have never been truly isolated Common Traits & Natural Selection • Sickle-cell allele protects an individual from malaria – In Africa and SW Europe, there is an overlap of sicklecell and malaria distribution • Skin color is dependent on the exposure level of UV light • Nose width is dependent on the moisture of climate – More surface area and moisture in a narrow nose Evolution & Genetic Drift • Genetic Drift: changes in allele frequency due to chance events • Types of chance events: – Founder effect – small sample of a larger population establishes a new population – Bottleneck – a small subset of a population https://www.youtube.com/watch?v=NJQ-i1EgGps Selecting a Mate • Sexual Selection: a trait influences the likelihood of mating – Peacocks with large tails must be both physically strong and smart to survive • Assortative mating: individuals choose mates who are like themselves – Ex. Height in humans