population genetics population genetics and patterns of evolution

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  • Slide 1
  • Slide 2
  • Population Genetics
  • Slide 3
  • Population Genetics and Patterns of Evolution
  • Slide 4
  • Are these organisms the same species?
  • Slide 5
  • Slide 6
  • Species a group of similar organisms that are capable of producing fertile offspring.
  • Slide 7
  • Population A population is a localized group of a species in a defined area.
  • Slide 8
  • Biodiversity the sum total of the genetically based variety of all organisms in the biosphere
  • Slide 9
  • Genes and Variation What are genes? Inheritable traits coded for in DNA What are different forms of a gene called? alleles
  • Slide 10
  • 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.
  • Slide 11
  • The gene pool consists of all the alleles for each gene present in a population.
  • Slide 12
  • 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.
  • Slide 13
  • Relative frequency of an allele in a population is expressed in a percentage or a decimal (95% = 0.95)
  • Slide 14
  • 1. What is the frequency of the black allele? 20 out of 50 0.4 2. What is the frequency of the brown allele? 30 out of 50 0.6
  • Slide 15
  • In this sample population, is the most common allele the dominant one? The most common allele does not have to be dominant!!
  • Slide 16
  • 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.
  • Slide 17
  • 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!
  • Slide 18
  • Sources of Genetic Variation 1. 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)
  • Slide 19
  • 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
  • Slide 20
  • Genetic Genetic Equilibrium Allele frequencies in a population dont change from generation to generation (constant) Gene frequencies will not change as long as certain factors (called Hardy-Weinberg Principles) are met.
  • Slide 21
  • Hardy-Weinberg Principles: No movement in or out of population Large population size No mutation Random Mating No selection (natural or artificial)
  • Slide 22
  • Do you think that these principles are met for most populations?
  • Slide 23
  • Hardy-Weinberg Formulas: p is the frequency of the dominant allele q is the frequency of the recessive allele p + q = 1 p 2 + 2pq + q 2 = 1 Homozygous dominantHeterozygous Homozygous recessive
  • Slide 24
  • Example: In these pigs, the allele for pink coat is dominant and the allele for black coat is recessive. What is p? What is q?
  • Slide 25
  • Determine the percent of the pig population that is heterozygous for pink coat.
  • Slide 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: 1. The frequency of the aa genotype: _____ 36% (As stated in the question)
  • Slide 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: 2. The frequency of the a allele: _____ 60% (We know aa (or p 2 ) is.36, then just P =.6 or 60%!)
  • Slide 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: 3. The frequency of the A allele: ____ 40 % (P + Q = 1 so.6 + x = 1)
  • Slide 29
  • 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 = p 2 and Aa = 2pq)
  • Slide 30
  • Genetic Drift Genetic drift is the change in a populations allele frequencies due to chance. There are 2 situations in which a population is shrunk and genetic drift can take place:
  • Slide 31
  • 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.
  • Slide 32
  • 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.
  • Slide 33
  • Read Bottleneck & Founder Effect Articles
  • Slide 34
  • 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)
  • Slide 35
  • Slide 36
  • Number vs. Running speed of Rabbits
  • Slide 37
  • 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
  • Slide 38
  • On your graph, draw the line that shows the change! Directional Selection
  • Slide 39
  • Number of spiders vs. body size Increasing body size
  • Slide 40
  • 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 Stabilizing selection
  • Slide 41
  • Draw the lines!
  • Slide 42
  • 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 Disruptive Selection
  • Slide 43
  • 3. 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
  • Slide 44
  • Draw the line!
  • Slide 45
  • Summary: types of selection on polygenic traits
  • Slide 46
  • Speciation What causes new species to arise?
  • Slide 47
  • 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.
  • Slide 48
  • Types of Isolation 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! Ex. The western meadowlark (left) and eastern meadowlark (right) have overlapping ranges. They do not interbreed because they have different mating songs.
  • Slide 49
  • Types of Isolation: 2. Geographic Isolation - two populations of the same species are separated by some geologic or geographic feature and are prevented from mating.
  • Slide 50
  • Types of Isolation 3. Temporal Isolation- two populations do not mate at the same time of year, time of day, etc.
  • Slide 51
  • How did speciation occur in the Galapagos?
  • Slide 52
  • Slide 53
  • What type of beak would each bird have? Notice the beaks structure fits their functions
  • Slide 54
  • Patterns of Evolution
  • Slide 55
  • Extinction 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. The dodo bird has been extinct for several hundred years after humans introduced predators to their habitat
  • Slide 56
  • Adaptive Radiation several vastly different species arise from a single species to fill available niches.
  • Slide 57
  • 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.
  • Slide 58
  • 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.
  • Slide 59
  • Punctuated Equilibrium Long periods of time with stable species broken with rapid period of change.
  • Slide 60
  • Slide 61
  • 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