Biology 152: Introduction to Biology II Prof. Rounds: Exam 1 Study Guide Weeks 1-3
Defn, the accumulation of differences among species, resulting in descendants that differ from their ancestors, where new species arise from existing ones
Evolution is random and caused by many things: mutations, selection, gene flow, etc. Anything that can change the genes of a population overtime.
Mutations are random but once developed, can be positively selected for if beneficial
The number of complex animals has grown overtime, but the number of non-complex, minimal complexity animals have also been growing
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Evolution by Natural Selection is driven by four main principles:
Variation in traits must exist Don't forget about the age old question of How can we control so many processes happening in the cytoplasm?
Differences must be heritable
- More offspring must be produced than what is needed to replace the parents
Offspring must have an advantage in fitness
Gene Flow Defn. where a new allele is introduced into a population by migrating specimens
Cline: A decrease in allelic frequency as the population increases in distance from the causative environment
E. Coli Cultures Question: A population of E. coli was grown in a limited glucose environment, where the main source of energy was citrate (a material that the E. coli could not readily use). The graph to the right shows their population over 35,000 generations in the solution. What does the change in population / optical density (OD) show about the population of E. coli?
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A. It shows how many bacteria are in the solution B. It shows how many generations of E. coli have divided C. It shows the ability of the E. coli to use citrate D. It shows whether the E. coli are myopic
Answer: C. The dramatic change in the population levels of the E. coli, signalled by the optical density of the culture, shows that at approximately 33,000 generations, the E. coli mutated and
gained the ability to use the citrate as energy. With this new energy source, their population was able to explode.
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E. Coli Cultures 2: In the culture represented on the graph above, what causes the dramatic jump in population / optical density after 33,000 generations of E. coli?
A. The cells divide more rapidly in a 24 hour period B. The cells grow in size more rapidly C. The cells are able to use citrate D. The cells no longer use glucose
Answer: A & C. Because of a random genetic mutation, the E. coli adapted the ability to use citrate as a source of energy. Because of this, they now have the resources to grow their population at a much faster rate.
Natural Selection Defn. the gradual, non-random process where heritable traits become more or less common in a species based on differential reproduction, the reproductive Don't forget about the age old question of Gene flow exchanges what?
success of members of a population best suited for the environment
Artificial Selection Defn. the usually sped-up process where chosen heritable traits become more or less common in a species based on how humans organize breeding
Adaptation Defn. a heritable, phenotypic trait that changes in a population over generations of selection, and allows those with the trait to survive in certain environments (Different from Acclimation!)
Acclimation Defn. a non-heritable, temporary physiological change to an individual over a short period of time
Fitness Defn. the ability of an individual to contribute the most to composition of the following generations and pass on their genes
Positive Selection Defn. where a trait is beneficial to a population's fitness so over time, the trait becomes more common, may eventually fixate (where it becomes the only allele for the trait present in the population)
Negative Selection Defn. where a trait is dangerous to a population's fitness and thus becomes less common over time, may lead to loss of the causative alleles
Differential Selection Defn. where positive or negative benefits skew the frequency of alleles towards one extreme
Stabilizing Selection Defn. where positive selection reinforces the mean/average allelic frequency and increases it overall
Disruptive Selection Defn. where positive selection favors the extremes of the allelic frequencies and selects against the mean frequency
Frequency of individuals
Phenotypes (fur color)
(c) Stabilizing selection
(a) Directional selection
(b) Diversitying selection COMO Educo, le publishing as an Ouma
E. Coli Cultures 4: What is true about the E. coli observed above?
A. The bacteria evolved over several generations B. This is an example of natural selection C. This is an example if artificial selection D. The bacteria did not evolve because a new species has not been created
Answer: A & C*. The bacteria evolved over time due to a random change in their genetics that allowed them to adapt to their artificial, man-made environment.
Growth of Cit- (blue triangles) and Cit+ (red diamonds)
cells in DM25 medium.
10 12 14 Time (h)
20 22 24
E. Coli Cultures 6: What is true about the culture that is able to use citrate?
A. The Cit+ bacteria have adapted to the citrate medium B. The Cit+ cells have acclimated to the medium C. All of the Above
Answer: C. The Cit+ E. coli already had the beneficial change in their genome that allowed them to use citrate. However, at the point where the population finally consumes all of the glucose, they must then acclimate to the new criteria for survival, that being the use of citrate, and change the mechanisms that they use to process energy. The change from using glucose to citrate is physiological, but the ability to use citrate in the first place is a genetic adaption.
Hardy-Weinberg Equilibrium & Allelic Frequency:
The HW Equilibrium shows at what frequency of alleles a population should be at if not evolving. Evolution cannot be occurring if there is HW Eq.
The frequency of hetero and homozygous alleles for a trait is mathematically represented by (p+q)2 = p+ 2pq+q? , where p is the frequency that alleles AA (AA = p?) show, and q is the frequency that alleles aa (aa =q2) are seen. The frequency at which Aa is seen is equal to 2pq, as it is the middle part of the expanded form.
Follow the equation below to calculate overall allelic frequency per allele:
Finding Recessive Alleles (a): aa% + 12 Aa% = overall a%
Finding Dominant Alleles (A): AA% + 12 Aa% = overall A%
Overall Allele % multiplied by number of alleles of total individuals
Each individual has 2 alleles
Use allelic frequencies previous calculated above in the Hardy-Weinberg Equilibrium Equation and compare to the actual results in the population. If the results are not the same as the actual data, then evolution is currently occurring in the population
Stable Equilibrium: where a disturbed system returns to its original equilibrium state
Neutral Equilibrium: where a disturbed system reaches equilibrium again, but at a slightly different and new state than the equilibrium state it was once at.
Ex. 300 Butterflies are examined for a monomial trait that affects striation of their wing pattern. 40% are identified as being homozygous dominant (SS), while 50% are heterozygous (Ss) and the other 10% are homozygous recessive (ss). What is the allelic frequency of the recessive allele?
Recessive Allele Frequency = aa% + Aa%
= 10% + 12 50%
= 10% + 25%
= 35% of the alleles of that gene present in the population are recessive
= 35% of the 600 alleles (300 individuals x 2 alleles each) are recessive
= 210 alleles are recessive in the population
= 390 alleles are dominant in the population
Ex. 300 Butterflies are examined for a monomial trait that affects striation of their wing pattern. 40% are identified as being homozygous dominant (SS), while 50% are heterozygous (Ss) and the other 10% are homozygous recessive (ss). We already know that 35% of the alleles are recessive and 65% are dominant. Calculate if evolution is occurring:
Use (p + q)2 = p< + 2pq+q?, where p = dominant alleles S, and q = recessive alleles
(0.65 +0.35)2 = (0.65 +0.35)2 = (0.65)2 + 2(0.65)(0.35) + (0.35)2
1 = 0.4225 +0.455 +0.1225
AA = 42.25%, Aa = 45.5%, aa = 12.25%
These numbers, though close, DO NOT match the observed genotypic frequencies and thus, evolution IS occurring, if only slowly. Just because the numbers sum up to 1 does not mean that they are in Hardy-Weinberg Equilibrium