Main Points of Darwin’s Theory of Natural Selection Over production. Most organisms produce more offspring than can survive. Competition. Organisms compete for food and resources. Variation. There is variation among individuals of a species. 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 Remember, population, NOT individual evolves… Individual’s genes stay the same but genes w/in a population may change Gene pool: the entire collection of genes among a population Each new offspring in a population draws its genes from the same gene pool. Allele frequencies  % of an allele in a population (determines frequencies of genotypes & phenotypes in populations) 5

Population Genetics Which trait was a better adaptation and selected for? How were frequencies of different alleles affected?

A) Single gene trait: controlled by single gene with two alleles Gene Traits: A) Single gene trait: controlled by single gene with two alleles Examples: widow’s peak, hitchhiker’s thumb, tongue rolling

Most human traits are polygenic. 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.

Stabilizing Selection = maintains an already existing system. 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 won’t change. Ginkgo Tree Horseshoe Crab

Stabilizing Selection Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness Stabilizing Selection

MALARIA 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) Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness Directional Selection

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 other colors are eaten more often resulting in a selection in favor of the extreme colors. The red and blues are foul-tasting to predators.

Disruptive Selection (pg 399) Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Key Low mortality, high fitness High mortality, low fitness

ORGIN OF SPECIES What is a Species? A Group of interbreeding organisms that can produce fertile offspring.

The Origin of Species (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. 19

Macroevolution Microevolution

How Do New Species Form? Speciation- formation of a new species Causes of Speciation: Geographic Isolation Temporal Isolation 3. Behavioral Isolation 4. Ecological Isolation

Physical separation of members of a population 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 2 Species of Squirrels Formed When They Were Separated By The Grand Canyon

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 Natural selection Gene flow (migration of genes from one population to another) Mutation

DNA frequency in a population changes simply by CHANCE not fitness GENETIC DRIFT DNA frequency in a population changes simply by CHANCE not fitness 29

30 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 30

FOUNDER'S EFFECT: some individuals of a population colonize a new habitat ex: Columbus bringing organisms to USA S. America: DNA Frequency 25% pink 75% red Spain: DNA Frequency 66% pink 33% red 31

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? 33

Genetic drift = decrease in DNA diversity 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 34

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 Adaptive Radiation  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