Population Genetics

Population Genetics and Patterns of Evolution

Are these organisms the same species? In the same column = same species (they look different based on the season in which they were born). Each column is a different species from the others (names are given at top). Can we use how organisms look to determine if they are the same species? More info: http://www.nature.com/nrg/journal/v3/n6/box/nrg818_BX3.html 4 4

Are these organisms the same species? All different species Note on Horse and donkey – they are able to interbreed, but the mule offspring is not fertile. Note on dog and wolf – they can breed, but do not do so in nature, so they are not the same species 5 5

Species a group of similar organisms that are capable of producing fertile offspring. May add “under natural conditions” to the end. 6 6

Population A population is a localized group of a species in a defined area.

Biodiversity the sum total of the genetically based variety of all organisms in the biosphere

Inheritable traits coded for in DNA Genes and Variation What are genes? Inheritable traits coded for in DNA What are different forms of a gene called? alleles

Genes and Variation Although each organism has only 2 alleles for each gene, more than two alleles may exist in a population. There exists variation within a population for many of these alleles. Eye color pics 10 10

The gene pool consists of all the alleles for each gene present in a population.

We can figure out what the frequency of a particular allele is by calculating the number of times that allele appears in that population compared to others in the entire gene pool.

Relative frequency of an allele in a population is expressed in a percentage or a decimal (95% = 0.95)

What is the frequency of the black allele? 20 out of 50 0.4 What is the frequency of the brown allele? 30 out of 50 0.6

In this sample population, is the most common allele the dominant one? The most common allele does not have to be dominant!!

When a change in the relative frequency of an allele occurs in a population, “change over time” has occurred, and this is evolution on a small scale.

Polydactyly Consider alleles for in the polydactyly gene pool, the allele coding for extra digits, the polydactyly allele (P), is only 1% of the population, the frequency is 0.01. The allele for 5 fingers and toes (p) is 99% of the population, or a frequency of 0.99. If over time, extra fingers was an advantage, and natural selection selected FOR individuals with extra digits, a shift in that allele frequency might happen, and evolution on a small scale would have occurred!

Sources of Genetic Variation Mutations- a change in the DNA sequence makes a new form of gene (and new proteins). 2. Gene shuffling- because of independent assortment of chromosomes and crossing over during gamete formation. (No change in a frequency)

Selection on a Single-Gene trait A single-gene trait with two alleles will show two phenotypes (if it is not codominant or incomplete dominance). A change in frequency is easy to see in a population. Example: See Pg. 397

Genetic Equilibrium Allele frequencies in a population don’t change from generation to generation (constant) Gene frequencies will not change as long as certain factors (called Hardy-Weinberg Principles) are met.

Hardy-Weinberg Principles: No movement in or out of population Large population size No mutation Random Mating No selection (natural or artificial)

Do you think that these principles are met for most populations? No, almost never true. However, it does give us a way to take a “snapshot” of the allele frequencies at any point and compare to later to determine if “microevolution” has occurred. 22

Hardy-Weinberg Formulas: p is the frequency of the dominant allele q is the frequency of the recessive allele p + q = 1 p2 + 2pq + q2 = 1 Case study from old notes? Homozygous recessive Homozygous dominant Heterozygous 23 23

In these pigs, the allele for pink coat is dominant and the allele for black coat is recessive. Example: What is p? What is q?

Steps to this example to go through with students: 1. Calculate q2 Count the individuals that are homozygous recessive in the illustration above. Calculate the percent of the total population they represent. This is q2. 2. Find q. Take the square root of q2 to obtain q, the frequency of the recessive allele. 3. Find p. The sum of the frequencies of both alleles = 100%, p + q = l. You know q, so what is p, the frequency of the dominant allele? 4. Find 2pq. The frequency of the heterozygotes is represented by 2pq. This gives you the percent of the population that is heterozygous for white coat: Determine the percent of the pig population that is heterozygous for pink coat. 25 25

Hardy-Weinberg Formulas: You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: The frequency of the “aa” genotype: _____ 36% (As stated in the question) Case study from old notes? 26 26

Hardy-Weinberg Formulas: You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: The frequency of the “a” allele: _____ 60% (We know “aa” (or p2) is .36, then just P = .6 or 60%!) Case study from old notes? 27 27

Hardy-Weinberg Formulas: You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: 3. The frequency of the “A” allele: ____ 40 % (P + Q = 1 so .6 + x = 1) Case study from old notes? 28 28

Hardy-Weinberg Formulas: You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following: 4. The frequency of the alleles “AA” and “aa”: 16% and 48% (AA = p2 and Aa = 2pq) Case study from old notes? 29 29

Genetic Drift Genetic drift is the change in a population’s allele frequencies due to chance. There are 2 situations in which a population is shrunk and genetic drift can take place: Case study from old notes? 30 30

The Bottleneck Effect Disasters such as earthquakes, floods, droughts, and fires can greatly reduce the size of a population. Those that survive may not be representative of the original gene pool. This can greatly reduce genetic variability. Case study from old notes? 31 31

The Founder Effect Takes place when a few individuals from a larger population colonize an isolated habitat. There is very little genetic variety in the gene pool because not all genes from the original population are represented. Case study from old notes? 32 32

Read Bottleneck & Founder Effect Articles

Selection on a Polygenic Trait Polygenic trait - controlled by more than one gene. Examples: human height, weight, beak size If you were to graph the frequencies of the phenotypes, you would get a bell shaped curve. (Label your axes)

Number vs. Running speed of Rabbits Possible questions: Based just on this information, which rabbits would you predict have the greatest biological fitness? Which rabbits would probably live longer and have the most offspring? If we graphed running speed several generations later, what differences would we see? 36

Directional Selection Individuals at one end of the curve are advantaged (Higher biological fitness) Individuals at the other end are disadvantaged (Lower biological fitness) Over time the population will shift in its phenotypes to one direction. Example: Food becomes scarce and one type of beak is most efficient

Directional Selection On your graph, draw the line that shows the change!

Number of spiders vs. body size Increasing body size Explain: The species of bird on the left only eats small spiders. The species of bird on the right only eats very large spiders. Possible questions: Based just on this information, which spiders would you predict have the greatest biological fitness? Which spiders would probably live longer and have the most offspring? Which would have less offspring? If we graphed body size of spiders several generations later, what differences would we see? 39

Stabilizing selection Individuals in the middle of the curve are more advantaged than individuals at the ends. This causes the frequency of the mid-phenotypes to increase, and the ends to decrease Example: Birth weight in humans

Draw the lines!

Disruptive Selection Individuals at the ends of the curve are more advantaged than the individuals at the middle of the curve. Less common. A single curve will appear to split in two. Example: Larger and smaller seeds become more common

Selection on a Polygenic Trait Disruptive Selection- occurs when individuals at the ends of the curve are more advantaged, or have a higher fitness, than the individuals at the middle of the curve. This is less common. A single curve will appear to split in two. Example: Larger and smaller seeds become more common

Selection on a Polygenic Trait Draw the line!

Summary: types of selection on polygenic traits

Speciation What causes new species to arise?

Natural selection acts upon a population as a whole. Reproductive isolation must occur to separate the population into distinct populations for natural selection to act on them separately. The way this occur is called an isolating mechanism. The population must be separate and no longer be able to produce fertile offspring, or become reproductively isolated, in order to become officially a different species. This is speciation.

Types of Isolation Behavioral Isolation- two populations of one species are capable of mating, but they do not because of differences in mating behavior. If they do not mate, they are not interbreeding! Also, dog and wolf example from earlier Ex. The western meadowlark (left) and eastern meadowlark (right) have overlapping ranges. They do not interbreed because they have different mating songs. 48

Types of Isolation: Geographic Isolation - two populations of the same species are separated by some geologic or geographic feature and are prevented from mating.

Types of Isolation Temporal Isolation- two populations do not “mate” at the same time of year, time of day, etc.

How did speciation occur in the Galapagos? Answers to sequencing activity on next page 51

How did speciation occur in the Galapagos?

What type of beak would each bird have? Notice the beaks’ structure fits their functions

Patterns of Evolution

Extinction 99% of all species that have ever lived are now extinct. The dodo bird has been extinct for several hundred years after humans introduced predators to their habitat 99% of all species that have ever lived are now extinct. In the struggle for existence, species compete for resources - some lose, and die. Sudden changes in the environment or natural disasters can cause mass extinctions. A mass extinction allows for a new radiation of species to fill all the empty niches.

Adaptive Radiation several vastly different species arise from a single species to fill available niches.

Convergent Evolution unrelated organisms come to resemble each other because of similar environmental pressures Structures with the same functions, but not on related animals, are called analogous structures. Shark – fish; Penguin – bird, Dolphin - mammal 57

Coevolution Two species evolve along with each other based on a close relationship with each other. Plants and their pollinators, parasites with their hosts, etc.

Punctuated Equilibrium Long periods of time with stable species broken with rapid period of change.

Differentiate between the gradual change model and the Punctuated Equilibrium model using this “fossil record” as an example. 60

Complexity of the Cell Molecules needed in metabolic processes Energy conversions in organisms Evidence of the formation of simple organic molecules Development of complex molecules into DNA Cells for self-replicating