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Evolution and Populations. Chapter 17. Genetics and Molecular Biology. Darwin had no idea how heredity worked, and he was worried that this lack of knowledge might prove fatal to his theory . - PowerPoint PPT Presentation


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Evolution and PopulationsChapter 17Genetics and Molecular BiologyDarwin had no idea how heredity worked, and he was worried that this lack of knowledge might prove fatal to his theory.

As it happens, some of the strongest evidence supporting evolutionary theory comes from genetics. A long series of discoveries, from Mendel to Watson and Crick to genomics, helps explain how evolution works.

Also, we now understand how mutation and the reshuffling of genes during sexual reproduction produce the heritable variation on which natural selection operates.Lifes Common Genetic Code All living cells use information coded in DNA and RNA to carry information from one generation to the next and to direct protein synthesis. This genetic code is nearly identical in almost all organisms, including bacteria, yeasts, plants, fungi, and animals.

A Testable Hypothesis Darwin hypothesized that the Galpagos finches he observed had descended from a common ancestor.

He noted that several finch species have beaks of very different sizes and shapes. Each species uses its beak like a specialized tool to pick up and handle its food. Different types of foods are most easily handled with beaks of different sizes and shapes.

Darwin proposed that natural selection had shaped the beaks of different bird populations as they became adapted to eat different foods. Genetics Joins Evolutionary TheoryIn genetic terms, evolution is any change in the relative frequency of alleles in the gene pool of a population over time.

Researchers discovered that heritable traits are controlled by genes.

Changes in genes and chromosomes generate variation.

Genotype and Phenotype in Evolution

An organisms genotype is the particular combination of alleles it carries.

An individuals genotype, together with environmental conditions, produces its phenotype.

Phenotype includes all physical, physiological, and behavioral characteristics of an organism.

Genotype and Phenotype in Evolution

Natural selection acts directly on phenotype, not genotype.

Some individuals have phenotypes that are better suited to their environment than others. These individuals produce more offspring and pass on more copies of their genes to the next generation.

Populations and Gene PoolsA population is a group of individuals of the same species that share a geographical area, mate and produce offspring. Smallest unit in which evolution occursA gene pool consists of all the genes, including all the different alleles for each gene that are present in a population.Combined genetic information of all the members of a particular populationB b b b B b b b B bb b b b b B b b b b

Populations and Gene PoolsResearchers study gene pools by examining the relative frequency of an allele. The relative frequency of an allele is the number of times a particular allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Allele Frequency = # of a certain allele total # of alleles of all types in the population

Red Allele - 9/36 = = 0.25White Allele - 27/36 = = 0.75Populations and Gene PoolsFor example, this diagram shows the gene pool for fur color in a population of mice.

10Populations and Gene PoolsEvolution, in genetic terms, involves a change in the frequency of alleles in a population over time.Note: Although natural selection acts on individuals it is the population that evolves, not individuals0

Allele Frequency ExampleCalculate the frequency of the dominant and recessive alleles in the gene pool below.A a a A a A a a A a

A =a =

4/10 = 0.46/10 = 0.6Alleles Frequencies & the Gene Pool:Phenotype Frequency = # of individuals with a particular phenotypetotal #of individuals in population

Determining Phenotype and Allele Frequencies using Japanese four oclock flowers:1st Generation:

RR RR RW RW RR RW RW RRPhenotype Frequency:Allele Frequency:White - R =Pink - W =Red -

04/8 = 0.504/8 = 0.5012/16 = 0.754/16 = 0.25Predicting Genotypes & Phenotypes of Second Generation:According to the laws of probability, the chance of an R gamete meeting with another R gamete is the product of the allele frequencies in the gene pool.Red (RR) = R x R = RR= 0.75 x 0.75= 0.5625

White (WW) = W x W = WW = 0.25 x 0.25 = 0.0625

The frequency of all types expected in the second generation must add up to 1.01.0 - RR - WW = RW1.0 - 0.5625 - 0.0625 = 0.375Single-Gene TraitsA single-gene trait is a trait controlled by only one gene. Single-gene traits may have just two or three distinct phenotypes. Dominance of an allele for a single-gene trait does not necessarily mean that the dominant phenotype will always appear with greater frequency in a given population.

No widows peak is a recessive traitNatural selection on single gene traitsEvolution does not act on genes. Instead it acts on phenotype frequencies by changing allele frequencies!

Evolution = any change in the relative frequencies of alleles in a populations gene pool


Initial PopulationGeneration 10Generation 20Generation 3090%80%70%40%10%20%30%60%Polygenic TraitsPolygenic traits are traits controlled by two or more genes.Each gene of a polygenic trait often has two or more alleles. A single polygenic trait often has many possible genotypes and even more different phenotypes.

How Natural Selection WorksEvolutionary fitness is the success in passing genes to the next generation.Evolutionary adaptation is any genetically controlled trait that increases an individuals ability to pass along its alleles.

Natural Selection on Single-Gene TraitsNatural selection for a single-gene trait can lead to changes in allele frequencies and then to evolution.For example, a mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms.

Natural Selection on Single-Gene TraitsIf red lizards are more visible to predators, they might be less likely to survive and reproduce. Therefore the allele for red coloring might not become common.Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce.

Natural Selection on Polygenic TraitsPolygenic traits have a range of phenotypes that often form a bell curve. The fitness of individuals may vary from one end of the curve to the other. Natural selection can affect the range of phenotypes and hence the shape of the bell curve.

3 Types of SelectionNatural selection on polygenic traitsNatural selection can affect the relative fitness of phenotypes involving polygenic traits in any of 3 ways:

Stabilizing Selection -average form of a trait is favored.Directional Selection -one extreme form of a trait is favored.Disruptive Selection - either/both extremes of a trait are favored over an average form of a trait.


Types of Natural Selection (2:21)Directional selection

0Stabilizing selection

0Disruptive selection

0Directional SelectionDirectional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than others.

Directional Selection:Anteaters feed by breaking open termite nests (extend their sticky tongues into the nests). New species of termites that build very deep nests. Anteaters with long tongues more effective than those with average or short tongues

Directional Selection

Directional SelectionA drought on the Galpagos island of Daphne Major in 1977 reduced the number of small seeds available to finches, causing many of the small-beaked finches to die. This caused an increase in the finches average beak size between 1976 and 1978.

Stabilizing SelectionStabilizing selection occurs when individuals near the center of the curve have higher fitness than individuals at either end. This situation keeps the center of the curve at its current position, but it narrows the overall graph.

Stabilizing Selection:Most common kind of selectionA small lizards may not be able to run fast enough to escapeB large lizards may be more easily spotted, captured, & eaten by predators

Stabilizing Selection

Disruptive SelectionDisruptive selection occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Disruptive selection acts against individuals of an intermediate type and can create two distinct phenotypes.

Disruptive Selection:Limpets are marine organisms that have shells that vary in color from white to dark brown. Limpets live their adult life attached to rocks. On light colored rock, white shelled limpets are at an advantage because the birds that prey upon them have a difficult time locating them. On dark-colored rock, dark-shelled limpets are well camouflaged. On the other hand, tan-colored limpets are easily spotted on either light or dark rocks.

Genetic DriftGenetic drift occurs in small populations when an allele becomes more or less common simply by chance. Genetic drift is a random change in allele frequency.The smaller the population, the more susceptible it is to such random changes.

Evolution vs. Genetic EquilibriumThe Hardy-Weinberg principle states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. The Hardy-Weinbe