Evolution of Populations Chapter 16

Gene and Variation Although Mendel and Darwin both worked in the 1800’s, they were not able to share information as scientists do today. By the 1930’s evolutionary biologists started to focus on genes as a way to understand evolutionary change.

When Darwin developed his theory of evolution, he did not understand: how heredity worked. This left him unable to explain two things: a. source of variation b. how inheritable traits pass from one generation to the next

In the 1940’s, Mendel’s work on genetics was “rediscovered” and scientists began to combine the ideas of many branches of biology to develop a modern theory of evolution. When studying evolution today, biologists often focus on a particular population. This evolution of populations is called microevolution.

16-1 Genes and Variation How common is genetic variation?

How Common Is Genetic Variation? Many genes have at least 2 forms or alleles Animals such as horses, dogs, mice, and humans often have several alleles for traits such as body size or coat color

2. allele: one form of a gene

16-1 Genes and Variation Variation and gene pools Gene pool – all genes present in a population Relative Frequency – the number of times an allele occurs in the gene pool, compared to other alleles

In genetic terms, evolution is any change in the relative frequency of alleles in a population.

Genetic Variation Genetic variation is studied in populations. A gene pool consists of all the genes, including the different alleles that are present in a population.

16-1 Genes and Variation Scientists now recognize two main sources of genetic variation Mutations Gene shuffling

16-1 Genes and Variation Mutations Any change in a sequence of DNA

Mutations Can occur because of: mistakes in replication environmental chemicals May or may not affect an organism’s phenotype

3. Sources of Variation b. Gene Shuffling: recombination of genes that occurs during production of gametes Cause most inheritable differences between relatives Occurs during meiosis As a result, sexual reproduction is a major source of variation in organisms. Despite gene shuffling, the frequency of alleles does not change in a population. Explain why this is true. Similar to a deck of cards – no matter how many times you shuffle, same cards (alleles) are always there.

16-1 Genes and Variation Sexual Reproduction Think of a deck of cards Remember…an organism is successful if it reproduces Can produce many phenotypes, BUT the relative frequency will stay the same Think of a deck of cards 52 different cards Chance of an Ace off the top – 1/13 (4/52) Many combinations, BUT the frequency stays the same

16-1 Genes and Variation Single-gene and polygenic traits Widows peak is a single gene trait – a single gene with two alleles Polygenic Traits are controlled by two or more genes This means that a polygenic trait can have many possible genotypes ant thus phenotypes.

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.

why? Only two phenotypes possible type: polygenic Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain. type: single gene why? Only two phenotypes possible Example: tongue roller or non-tongue roller type: polygenic why? Multiple (many) phenotypes possible Example: height range 4feet to 9 feet all

Review Which of the following statements describes what all members of a population share? a.They are temporally isolated from one another. b.They are geographically isolated from one another. c.They are members of the same species. d.They have identical genes.

Review The combined genetic information of all members of a particular population is the population’s a.relative frequency. c.genotype. b.phenotype. d.gene pool.

Review The two maun sources of genetic variation are ___________ and _________________________ Mutations Genetic shuffling from sexual reproduction

Population Genetics Study of how genetics plays a role in evolution **New definition of evolution: it’s a gradual change in the genetic material within a population over time.

Among any population, there are always variations Line up in the front of the class from the shortest person to the tallest person. Let’s graph the frequencies of the heights in this class.

5 4 3 2 1 Frequency 4’ 5” 6” 7” 8” 9” 10” 11” 5” 1” 2” 3” 4’ 5” 6” 7” Height

Gene frequencies within a population follow this curve. Bell Curve Graph showing frequencies of occurrences with the majority in the middle and curving out towards the extremes. Gene frequencies within a population follow this curve.

So what causes these variations in height among your classmates? Variations – caused by: Environmental factors (food availability, amount of sunlight, etc) Mutations – gene and chromosomal Recombinations (crossing over, independent assortment. Random fusion of gametes - each brings their own set of genes

All the members of the same population contribute their genes into a gene pool. If the people on the right side of the classroom during the height lineup were to all throw their genes into a gene pool, what would be the expected outcome of their offspring? The tall trait would show up more often thus increasing the tall gene (allele) frequency.

Amount of times a certain gene (allele) appears in a population. Allele frequency Amount of times a certain gene (allele) appears in a population. If two parents are hybrids for hand preference, it’s 0.5: 0.5 or 50:50 If one is a hybrid and the other is pure recessive, it’s 25:75

Review!!! Section 16-1 Genes and Variation 1. Is the Following sentence true or false? Mendel’s work on inheritance was publish after Darwin’s lifetime. False 2. Which two important factors was Darwin unable to explain without an understanding of heredity? The source of variation and how genes were passed from generation to generation 3. List the three fields that collaborate today to explain evolution a. Genetics b. Molecular Biology c. Theory Gregor Mendel

Gene Pools 4. A collection of individuals of the same species in a given area is a population 5. The combined genetic information of all members of a particular population is a gene pool 6. Is the following statement true or false? A gene pool typically contains just one allele for each inheritable trait False 7. The number of times that an allele occurs in a gene pool compared to the number of times other alleles occur is called the relative frequency

Source of Genetic variation Sources of Genetic Variation 8. Complete the concept map Source of Genetic variation Include Mutations Gene Shuffling 9. What is a mutation? Any Change in the sequence of DNA 10. Why do mutations occur? As a result of mistakes in the replication of the DNA or as a result of radiation or chemicals in the environment

11. Circle the letter that is true about mutations They can be limited to a single base of DNA They always affect lengthy segments of a chromosome They always affect an organism’s phenotype They always affect an organism’s fitness 12. Is the following statement true of false? Most inheritable differences are due to gene shuffling that occurs during the production of gametes 13. Circle the letter of each choice that is true of sexual reproduction It is a major source of variation in many populations It can produce many different phenotypes It can produce many different gene combinations It can change the relative frequencies of a population

Frequency of Phenotype Single- Gene and Polygenetic traits 14. Is the following sentence true or false. ?The number of phenotypes produced for a given trait depends upon how many genes control the trait 15. Is the following sentence true or false? Most traits are controlled by a single gene 16. Label the two graphs to show which one represents a single gene trait and which one represents a polygenetic trait Single gene trait Polygenetic Trait Frequency of Phenotype Frequency of Phenotype (%) Phenotype (height) Widow’s peak No widow’s peak Phenotype

16-2 Evolution as Genetic Change How do we view an individual’s fitness? How do we view an evolutionary adaptation? By an organisms success in passing genes on to the next generation As any genetically controlled physiological, anatomical, or behavioral trait that increase an individual’s ability to pass on its genes

Evolution as Genetic Change Natural Selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color may be more visible to predators and therefore less likely to survive and reproduce than organisms of another color.

Natural selection acts on phenotypes, not genotypes. Example: in a forest covered in brown leaves, dirt and rocks which mouse will survive better brown or white? Brown, more hidden.

? If brown is dominant can the a predator tell the difference between: Mouse with highest fitness will have the most alleles passed on to the next generation. White mouse will have low fitness ? BB Bb

? Which mouse will have the lowest fitness? White, bb (recessive) Will the fitness of BB and Bb differ? Why? No, Both BB and Bb have the same fitness advantage of being brown ? BB Bb

Will the fitness of BB and Bb differ? Why? No, Both BB and Bb have the same fitness advantage of being brown

16-2 Evolution as Genetic Change A. Natural Selection on Single-Gene Traits can lead to changes in allele frequencies and evolution How does the color affect the fitness of the lizard? What do you predict the lizard population will look like by the 50th generation? Explain.

Pg 397. With your desk mate; answer: How does the color affect the fitness of the lizard? What do you predict the lizard population will look like by the 50th generation? Explain.

Natural Selection of Polygenic Traits The effects of natural selection are more complex when traits are controlled by more than one gene.

Three ways in which natural selection affects polygenic traits.

Individuals with highest fitness: those at one end of the curve a. Directional Selection: individuals at one end of the curve have higher fitness so evolution causes increase in individuals with that trait Individuals with highest fitness: those at one end of the curve Example: Galapagos finches – beak size Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness

Directional Selection (page 398) Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness Directional Selection

Example: human birth weight b. Stabilizing Selection: individuals at the center of the curve have highest fitness; evolution keeps center in the same position but narrows the curve 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 Individuals with highest fitness: near the center of the curve (average phenotype) Example: human birth weight

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

Individuals with highest fitness: both ends of curve c. Disruptive Selection: individuals at both ends of the curve survive better than the middle of the curve. Individuals with highest fitness: both ends of curve Example: birds where seeds are either large or small Disruptive Selection 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

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

Which type is it?

16-2 Evolution as Genetic Change Genetic Drift – Random change in allele frequency Founder effect – when allele frequencies change because of migration

Genetic Drift In small populations, individuals that carry a particular allele may leave more descendants than other individuals BY CHANCE. Over time, these alleles become more common

Video

16-2 Evolution as Genetic Change Evolution vs. Genetic Equilibrium Hardy-Weinberg principle – allele frequencies will stay constant unless some factor changes the frequency Genetic Equilibrium – when allele frequencies remain constant

16-2 Evolution as Genetic Change Five Conditions to Maintain Equilibrium Random Mating Large Population No Movement into or out of the population No Mutations No Natural Selection

16-3 The Process of Speciation Recall, a species is a group of organisms that breed with one another and produce fertile offspring

Isolating Mechanisms If the gene pools of two populations are isolated; they can not breed This can lead to the formation of new species. The populations were REPRODUCTIVELY ISOLATED

Reproductive isolation At this point, the populations have separate gene pools

16-3 The Process of Speciation Behavioral Isolation Two populations will not breed because of differences in courtship

16-3 The Process of Speciation Geographical Isolation Rivers, mountains, or bodies of water separate two populations

16-3 The Process of Speciation Temporal Isolation Different species mate at different times Times of day Times of year

16-3 The Process of Speciation Testing Natural Selection in Nature Variation Differences in population Natural Selection Which beak is best in a drought? Rapid Evolution Natural selection takes place frequently and sometimes rapidly

16-3 The Process of Speciation Speciation in Darwin’s Finches Founders Arrive How did the finches get to the Galapagos Islands? Separation of Populations How did they become separated?

16-3 The Process of Speciation Changes in the Gene Pool How did they become individual species?

16-3 The Process of Speciation Reproductive Isolation If a small beak bird moves back to the first island will it mate with a big beaked bird?

16-3 The Process of Speciation Ecological Competition What happens to the birds that are different that don’t have to compete for food? Continued Evolution How did we get all of the 13 species?

16-3 The Process of Speciation Studying Evolution Since Darwin Limitations of Research Unanswered Questions

Review The Galapagos finches are an excellent example of A) speciation B)genetic equilibrium C) stabilizing selection D) selection on single-gene traits

Review The genetic equilibrium of a population can be disturbed by all of the following EXCEPT A) Nonrandom mating B) movement into and out of the population C) a large population size D) mutations

Review Natural selection acts directly on A) alleles B) genes C) phenotypes D) mutations