Name: ______Period: ______
Population Genetics
Species - a group of similar organisms that are capable of producing fertile offspring.
Example: ______
Population is a localized group of species in a defined area.
Example: ______
Biodiversity the total of the genetically based variety of all organisms on the Earth.
Genes and Variation
1. Inheritable traits are coded for within our genes.
a. Alleles – different forms of a gene
· What are some different genes within humans?
______
b. There are many different alleles for the same gene within a population.
· What are some different alleles for a particular gene within humans?
______
2. The gene pool consists of all the alleles for each gene present in a population.
3. 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.
a. The relative frequency of an allele in a population is often expressed in a percentage or a decimal (95% = 0.95).
b. Remember: the most common allele doesn’t have to be dominant!
c. 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.
Sources of Genetic Variation
1. Mutations
· How does a mutation affect the DNA sequence and protein formation?
______
2. Gene shuffling - because of independent assortment of chromosomes and crossing over during gamete formation. Note: This does not change gene frequency.
Genetic Equilibrium (Non-evolving populations) - The alleles frequencies in a population don’t change from generation to generation. (The allele frequency is constant).
Five conditions must be true for this to occur (called Hardy-Weinberg principles):
1. No movement into or out of the pop.
2. Large population size
3. No mutations
4. Random mating
5. No selection (natural or artificial)
Hardy-Weinberg formulas:
p = frequency of the dominant allele in the population
q = frequency of the recessive allele in the population
p2 = percentage of homozygous dominant individuals
q2 = percentage of homozygous recessive individuals
2pq = percentage of heterozygous individuals
p + q = 1
p2 + 2pq + q2 = 1
Hardy-Weinberg Practice Problems:
1. 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. ______
· The frequency of the "a" allele. ______
· The frequency of the "A" allele. ______
· The frequencies of the genotypes "AA" and "Aa." ______
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.
1. The Bottleneck Effect
o Disasters such as earthquakes, flood, droughts and fires can greatly reduce the size of a population. Those that survive may not be representative of the original gene pool.
o This can greatly reduce genetic variability.
2. The Founder Effect
o Takes place when a few individuals from a larger population colonize an isolated habitat.
o There is very little genetic variety in the gene pool because not all genes from the original population are represented.
Selection on Polygenic Traits: (for evolving populations)
· Graphing the frequencies of the phenotypes within a population, produces a bell shaped curve.
· Natural selection can affect the frequency of a heritable trait in a population and thus change the frequencies of the phenotypes in 3 different ways:
Type of selection / Which part of the phenotype spectrum is selected for? / Graphical Representation(you draw in the change!)
Directional Selection / Individuals at one end of the curve are advantaged, and selection against the other end occurs. Phenotype at one end of the spectrum increases. (favors those that at one time were seen as rare) /
Stabilizing selection / Individuals in the middle of the curve are more advantaged than individuals at the ends. The frequencies of the mid-phenotypes increases, and the ends to decrease. /
Disruptive Selection / Individuals at the ends of the curve are more advantaged than the individuals at the middle of the curve. This is less common. A single curve will appear to split in two. /
Examples:
1. Infants born around 3-4 kg (the average birth weight) are generally healthier and are less likely to die as infants than those with much smaller or larger birth weights.
Type of Selection? ______
2. In Cameroon, West Africa, the black-bellied finches with large beaks can crack the hard seeds and the finches with the small beaks crack the soft seeds. Those with intermediate-sized beaks can crack both seeds but do so extremely inefficiently. Those finches with the intermediate sized beak are not as successful in this environment.
Type of Selection? ______
3. Due to a drought in the Galapagos, there is no more fruit available to the various Finches on the island. Those birds with the largest beaks are the most efficient at cracking open any nuts found and thus their population thrives on the island.
Type of Selection? ______
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 occurs 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. (speciation)
Isolation that can lead to speciation:
1. 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. **Often depends on the elaborate courtship rituals of a particular species. **
2. Geographic Isolation- two populations of the same species become separated by some geologic or geographic feature and are prevented from mating.
3. Temporal Isolation- two populations do not mate at the same time of year, time of day, etc. Brainstorm with your neighbor to think of an example of temporal isolation.
Patterns of Evolution
1. Extinction
· 99% of all species that have ever lived are now extinct.
§ Why do you think that is?
______
· Mass Extinction: Sudden changes in the environment or natural disasters can cause a significant number within the population to die out.
§ A mass extinction allows for a new radiation of species to fill all the empty niches.
2. Adaptive Radiation - several vastly different species arise from a single species to fill available niches.
3. Convergent Evolution - unrelated organisms come to resemble each other because of similar environmental pressures. What kind of structures arise as a result of convergent evolution? Why? ______
4. Coevolution - two species evolve along with each other based on a close relationship with each other. Examples: Plants and their pollinators, parasites with their hosts, etc. Brainstorm with your neighbor to think about organisms that might have developed through coevolution. Record your ideas here:
______
5. Punctuated equilibrium - long periods of time with stable species broken with rapid period of change.
What is the difference between gradualism and punctuated equilibrium? Record your thoughts here:
______
Label the pictures as Gradualism or Punctuated Equilibrium
Practice Problems
Set I: Gene Frequencies
Initial Population - Circles are used to represent genes in this diagram of a population. Individuals are diploid, so two circles are used to represent an individual.
1. What is the frequency of A? ______
2. What is the frequency of a? ______
Gene frequency: Generation II - Suppose these represent the genes in the second generation of this population:
3. What is the frequency of A? ______
4. What is the frequency of a? ______
5. Has “small-scale” evolution occurred between generation I and generation II (look at the gene frequency)? ______Explain your answer.______
In order for question number 5 to be true, according to the Hardy-Weinberg principle, what 5 conditions must be met?
1. ______
2. ______
3. ______
4. ______
5. ______
Gene frequency: Generation VII - Suppose these represent the genes in the third generation of this population:
6. What is the frequency of A? ______
7. What is the frequency of a? ______
8. Has “small-scale” evolution occurred between generation II and generation VII? ______Explain your answer.
______
9. What are some factors that might have caused the change seen between Generation II and Generation VII?
______
Set II: Types of Selection
** Remember to label all diagrams**
1. The Siberian Husky is a dog bred for working in the snow. The Siberian Husky is a medium dog, males weighing 16-27kg (35-60lbs). These dogs have strong pectoral and leg muscles, allowing it to move through dense snow. If the Siberian Husky had heavier muscles, it would sink deeper into the snow and they would move slower get stuck. Yet if the Siberian Husky had lighter muscles, it would not be strong enough to pull sleds and equipment, so the dog would have little value as a working dog. What type of selection does this illustrate? Fill in the graph to show the change in the population (remember to label the X and Y axis).
______
2. In a species of African butterfly Pseudacraea eurytus, the colorations range from a reddish yellow to blue. In both cases, these extremes of color, from different ends of the spectrum, look like (mimic) other species of butterflies that are not normally the prey of other the local predator group of birds and other insects. Accordingly, selection favors the extremes in coloration within the Pseudacraea eurytus population. Those butterflies that are moderate in coloration are eaten in far greater numbers that those at the extremes of the color spectrum. As a consequence, those butterflies with extremes of coloration survive as a greater percentage of the population available to pass on those genes for coloration to the next generation. What type of selection does this illustrate? Fill in the graph to show the change in the population (remember to label the X and Y axis).
______
3. The greyhound breed of dog was originally used to hunt the fastest of game, fox and deer. Their breed dates to Egypt in 3BC. Early breeders were interested in dog with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 64km/h (40mph). What type of selection does this illustrate? Fill in the graph to show the change in the population (remember to label the X and Y axis).
______
4. ______5. ______6. ______
Example / What type of isolation is it?7. There are 2 different species of cricket that would be indistinguishable unless you heard their mating song: they are of a noticeably different pitch. Members of the separate species do not interbreed because of the different mating songs.
8. There are 4 groups of Chinook salmon who spawn at different times of the year along California's central coast: winter, spring, summer and fall. The populations who breed in one season never breed with those in another season.
9. Male fireflies of various species signal to females of their kind by blinking their lights in particular rhythms. The females respond only to signals characteristics of their own species, flashing back and attracting males.
10. The geographic ranges of the Western Spotted Skunk and the Eastern Spotted Skunk over lap but the two very similar species do not interbreed because the Western Spotted Skunk mates in late Summer and the Eastern Spotted Skunk mates in late Winter.
11.Two species of garter snakes never mate because one lives mainly in water and the other is primarily terrestrial (on land).
12. Geologic evidence indicates that most of Death Valley, CA was covered by a huge lake during the last ice age. When the ice age ended, the region became dry. Only small, spring fed ponds remained. Members of a fish species that previously formed a single population in the lake became isolated in different ponds. The environments of the isolated ponds differed enough that natural selection acted on the separate populations. Eventually the fish in the different ponds diverged so much genetically that they could no longer interbreed even if brought together.
Set III: Bottlneck or Founder Effect?
1. Cheetahs were once widespread in Africa and Asia. Their numbers fell drastically during the last ice age about 10,000 years ago. Those few that survive reproduced and made up the population that exists today.
______
2. Northern elephant seals have reduced genetic variation. This is probably due to over-hunting that reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000. Because of the over-hunting, the Northern Elephant seals have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.