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Main Points of Darwin’s Theory of Natural Selection 1. Over production. Most organisms produce more offspring than can survive. 2. Competition. Organisms compete for food and resources. 3. Variation. There is variation among individuals of a species. 4. Adaptation. Individuals with traits best suited to the environment will survive. How Gene Frequencies Change Sources of Variation • Ways that new adaptations and gene frequencies arise – S – sexual reproduction – C – crossing over during meiosis (recombination of genes) – A – arrangement of chromosomes (alleles) during meiosis – M – mutations of DNA KEY CONCEPT • Evolution by Natural Selection, causes changes in POPULATIONS! Population Evolution Population genetics genetic principles as they apply to entire populations of organisms Population group of organisms of the same species living in the same area Genotype the representation on the gene of an organism Phenotype the physical trait shown by a genotype Allele different form of a gene Gene pool combined genetic info. for all members of a population Population Genetics • Natural selection acts on individual’s phenotypes not genotypes. • Populations evolve – Individual’s genes will stay the same – Population’s gene pool may change over time due to Natural Selection Population Genetics • Which trait was a better adaptation and selected for? • How were frequencies of different alleles affected? Gene Traits: A) Single gene trait: controlled by single gene with two alleles ♦ Examples: widow’s peak, hitchhiker’s thumb, tongue rolling B) Polygenic trait: controlled by 2 or more genes, each with 2 or more alleles ♦ Examples: height, hair color, skin color, eye color Most human traits are polygenic. This type of variation can cause different types of selection of one phenotype over others Directional, Disruptive, and Stabilizing Selection • Three modes of Natural Selection: – Directional selection favors individuals at one end of the phenotypic range – Disruptive selection favors individuals at both extremes of the phenotypic range – Stabilizing selection favors intermediate variants and acts against extreme phenotypes © 2011 Pearson Education, Inc. Three Types of Natural Selection Stabilizing Selection = maintains an already existing system. > eliminates organisms that deviate from the norm. > environment must remain unchanged. > explains why there are “living fossils.” > as long as environment doesn’t change, organisms Ginkgo Tree won’t change. Horseshoe Crab Stabilizing Selection Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Birth Weight Selection against both extremes keep curve narrow and in same place. Directional Selection = favors one extreme or the other. > eliminates organisms that are not in that extreme. > eventually leads to changes in the population. > occurs when organisms must adapt to a change in their environment. > may develop into a RESISTANCE (the ability of an organism to withstand a harmful agent). MALARIA Directional Selection (page 398) Key Directional Selection Low mortality, high fitness Food becomes scarce. High mortality, low fitness Disruptive Selection = favors two extremes at one time. > eliminates organisms that are more common. > eventually leads to changes in the population. African butterflies can range from red to blue. The red and blues are foul-tasting to predators. The other colors are eaten more often resulting in a selection in favor of the extreme colors. Disruptive Selection (pg 399) Disruptive Selection Low mortality, high fitness High mortality, low fitness Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Number of Birds in Population Largest and smallest seeds become more common. Beak Size ORGIN OF SPECIES What is a Species? A Group of interbreeding organisms that can produce fertile offspring. The Origin of Species (Macroevolution) Macroevolution • the formation of new species between organisms (speciation) and accompanying events Microevolution • refers to changes in allele frequencies in a gene pool from generation to generation. Represents a gradual change in a population. • Macroevolution Microevolution How Do New Species Form? Speciation- formation of a new species Causes of Speciation: 1. Geographic Isolation 2. Temporal Isolation 3. Behavioral Isolation 4. Ecological Isolation Geographic Isolation • Physical separation of members of a population (by formation of a canyon, mountain, river, etc.) – Leads to different adaptations on each side of barrier, and eventually new species (allopatric speciation) Geographic Isolation = New Species: An Example Temporal Isolation • Species isolated because they reproduce in different seasons or times of day • Prevents species from ever breeding together – keeps two species separate but usually arises after species have formed. Behavioral Isolation • Species Differ In Their Mating Rituals (e.g. different bird songs, mating colors, dances, pheromones, etc.) Ecological Isolation – Species inhabit the same area, but different habitats, so they don’t encounter each other Other Factors Can Affect Genetic Variation In A Population • Other factors that increase variation in the genetic material (gene pool) of a population on which natural selection acts: – Random/nonrandom mating = sexual selection – Gene traits-single/polygenic – Isolation – Genetic Drift – Fitness is the relative ability of genotypes to survive and reproduce Microevolution refers to changes in allele frequencies in a gene pool from generation to generation. Represents a gradual change in a population. Causes of microevolution: 1) Genetic drift 2) Natural selection 3) Gene flow (migration of genes from one population to another) 2) Mutation GENETIC DRIFT DNA frequency in a population changes simply by CHANCE not fitness Types of Genetic Drift 1. Founder’s Effect some individuals of a population colonize a new habitat 2. Bottleneck Effect few survivors are left to reproduce FOUNDER'S EFFECT: some individuals of a population colonize a new habitat ex: Columbus bringing organisms to USA Spain: DNA Frequency 66% pink 33% red S. America: DNA Frequency 25% pink 75% red Bottleneck Effect: Small representation of the original population after a catastrophic event. Alleles in gene pool are not equally represented and species may evolve. HOW DOES GENETIC DRIFT AFFECT THE DIVERSITY OF A POPULATION OVER TIME? Genetic drift decreases diversity. In this case yellow and pink DNA are gone, and red DNA is now the majority. Genetic drift = decrease in DNA diversity Patterns of Evolution • Convergent Evolution • Divergent Evolution • Coevolution Convergent Evolution • Organisms appear similar, because of similar habitat and selection pressure. These organisms aren’t closely related • Why? – Similar environments select for similar traits Divergent Evolution • accumulation of differences between groups which can lead to the formation of new species; evolving from a common ancestor • Related populations or species become less and less alike A Second Type Of Divergent Evolution • Artificial Selection – Humans, rather than nature, select traits Coevolution • Two or more species in close association w/one another change together – Predator/Prey – Parasite/Host – Plant/Pollinator