Topic 10 Flashcards
(23 cards)
1
Q
the classical hypothesis
A
- Predicted that there would be little genetic variation within and among populations (little heterozygosity)
2
Q
the balance hypothesis
A
- Predicted that Balancing selection (any form of selection that acts to maintain variation) would be predominant, and heterozygosity would be high
3
Q
the selectionists
A
- Argued that selection is responsible for maintaining high levels of genetic variation and is largely responsible for the patterns observed within and among populations, as well as between species.
4
Q
the neutralists
A
- Argued that most of the variation observed in populations is neutral variation and does not affect fitness.
- Neutralists argued that most of the genetic variability in nature is a result of the interactions between neutral mutations and genetic drift.
5
Q
the neutral theory today
A
- The neutral theory provides a starting point, which is used to test alternate hypotheses (mainly concerning selection) to explain the pattern and magnitude of genetic variation observed within and between populations, as well as within and between species
6
Q
substitution
A
- The replacement of one amino acid, or nucleotide for another in the entire population (or species), which leads to fixed differences between populations or species
7
Q
polymorphism
A
- The segregation of two or more variants (alleles) of a gene or protein in a population or species
8
Q
synonomous vs nonsynonomous
A
- In the case of DNA, synonymous (or silent) nucleotide replacements do not change the amino acid sequence, while nonsynonymous differences result in an amino acid change
9
Q
what does the neutral theory state
A
- Most (not all) DNA and amino acid substitutions between species or populations are the result of drift randomly fixing neutral mutations, as opposed to selective fixation.
- Most of the polymorphisms within species and groups is the result of the interplay between drift and neutral mutations
10
Q
what the neutral theory does not state
A
- The neutral theory does not state:
1. That selection has been unimportant
2. That adaptive evolution does not occur
3. That all mutations are neutral
11
Q
mutation and substitution: neutrality
A
- The number of mutations per generation is given by 2Neu (The mutation rate per locus (u) multiplied by the number of copies of the gene in the population).
- The probability that a new neutral mutation will be ultimately fixed by drift is 1/2N, or more precisely 1/2Ne. The average time to fixation for a neutral variant is 4Ne generations
12
Q
mutation and substitution: selection
A
- With Selection: e.g., Adaptive Evolution
Probability of Fixation of an advantageous allele ≈ 2s
Rate of Fixation (Substitution) = 2Neu(2s)
13
Q
neutrality vs selection
A
- Is the fixation rate k = u, or is the fixation rate = 4Neus ?The Neutralists Claim: k = uThe Selectionists Claim: k = 4Neus
14
Q
early evidence for neutrality: molecular clock and alpha globin
A
- Each point on the graph represents a comparison between two taxa. The number of Amino Acid substitutions between the two species is given on the y-axis, and the time since they diverged (based on fossil evidence) on the x-axis. The slope gives the substitution rate.
- This suggests a constant rate of amino acid substitution over millions of years or a molecular clock
15
Q
would a molecular clock be expected under selection
A
no
16
Q
molecular clocks
A
- The Molecular Clock is one of the most important predictions of the Neutral Theory.
- The Molecular Clock Hypothesis: A given gene or protein evolves at a constant rate
17
Q
further tests of the molecular clock
A
Dispersion Tests and the Poisson Clock
- The neutral theory predicts that the accumulation of substitutions should be Poisson distributed with the mean substitution rate equal to the variance
- When additional data became available, dispersion tests indicated that the variance was greater than the mean, and the clock was over dispersed – suggesting the influence of selection
18
Q
Can Variation in the Molecular Clock Be Explained without Invoking Selection?
A
- The Generation Time Hypothesis
- The Metabolic Rate Hypothesis
19
Q
the generation time hypothesis
A
- Since fixation rates are based on generations, real time variation in the molecular clock could be accounted for by differences in generation time.
- Wen Hsiung Li was one of the first to demonstrate the existence of local molecular clocks using DNA sequence data among organisms with similar generation times (murine rodents)
20
Q
the metabolic rate hypothesis
A
- Martin and Palumbi first proposed the metabolic rate hypothesis in 1993. It suggests that organisms with higher metabolic rates will have higher substitution rates as a result of the mutagenic effects of oxygen free radicals that are produced during aerobic respiration
- Problem: metabolic rates are correlated with generation time and it is difficult to separate the effects of each
21
Q
where are molecular clocks today
A
- Most people (not me) believe they usually exist at local taxonomic levels. The question is: how far can one go beyond the species level before the molecular clock seriously breaks down?
22
Q
the neutral theory and heterozygosity
A
- The neutral theory predicts that heterozygosity should increase as a function of population size.
- Why: As 4Neu becomes larger, H^ approaches 1
- The relationship between heterozygosity and population size is not the one predicted by the neutral theory.
23
Q
environmental clines at certain loci
A
- The neutral theory predicts that there should be no environmental correlations among loci.
- Drosophila melanogaster exhibits an environmental latitudinal cline in the ADH locus (two alleles). Many other allele frequency clines have been detected among natural populations
