Detecting Evolution and Biological Inference Flashcards
How does Independent Assortment affect Probability
Under Independent assortment during gametogenesis (meiosis) in a diploid germ cell, each copy/allele at a locus is equally likely to be passed to a given gamete.
or
Probability of a gamete carrying a particular allele is inversely proportional to the number of alternative alleles
probability = 1/number of possibilities
How can you define evolution
Evolution can be defined as the change in the average value of genetically bases characteristics over time
or the change in allele frequency over time
What is Hardy-weinberg Equilibrium
In a diploid and sexually reproducing population with random mating, expected genotype frequency are predictable from allele frequencies:
p^2 + 2pq + q^2 = 1
When is HWE constant
- no mutation
- no gene flow
- extremely large population size
- random survival
- random mating
What is HWE used for
So HWE is the null or default expectation in the absence of evolution
Used to detect the strength of evolutionary forces
What are Hypotheses
Hypotheses are at the discretion on the scientists/student
Empirical studies are those that require the collection of data to test hypotheses
Hypotheses are only useful if there are multiple alternatives
(ie. make different testable predictions)
Why do biologists use statistical tests
in most cases, one cannot definitively prove a hypothesis is true/correct
- data is typically just a sample of possible observations, so any patterns observed also reflect the effects of chance
Statistical tests aim to assign some probability (i.e a P-value which ranges from 0.0-1.0) to describe how likely an observed pattern or result would be generated purely by chance.
A small P-value expresses how ‘unusual’ the data are;
ex a P-value of 0.001 means you could get this data by chance one in a thousand random studies/samples
What is X^2
The X^2 statistic - a test tool for detecting unusual counts
X^2 = ∑(observed - expected)^2 / expected
X = chi
Expected = expected counts (E) under a scenario of pure chance for each group
Observed = observed counts (O) from the actual study
What is HWE
Hardy-Weinberg Equilibrium (HWE):
In a diploid and sexually reproducing population with random mating
$p^2 + 2pq+q^2 = 1$
It is the benchmark expectation of allele frequency outside of evolution.
Plug in allele frequency to get genotype frequency of following generations.
How do you figure out how significant the expected vs observed numbers are
To find out how significant these numbers are we use the X^2 formula
X^2 = ∑(observed - expected)^2 /expected = 15.75
And the P value would be P = 0.0003
meaning 3 in 10,000 experiments would give this result if the results were random
What is the recipe for the chi squared test
- the test is designed for counts (not proportions, no negative numbers)
- Is only useful after you have data (counts for more then one group)
- Asks “how different is what is observed vs expected”
- Requires the scientist to decide appropriate expected counts
When does HWE apply
HWE is the null or default expectation in the absence of evolution when there is:
- no mutation
- no gene flow
- extremely large population size
- random survival
- random mating
What does Recombination do
Generates new combinations of alleles at different loci
What does genetic drift do
Adds and removes copies at random
What does natural selection do
Adds and removes copies depending on phenotypes
