Evolution: Lecture 3 Flashcards
What is an allele?
An allele is an individual variant of a gene at some locus.
ex. R or r
What is a genotype?
A genotype is an individual’s combination of two alleles at a locus (for diploid species)
ex. Rr
What is a population?
A localized (typically geographically clustered) group of interbreeding and interacting species.
- They are more likely to breed with their own population rather than those from other populations.
Each species is made up of?
One or more populations that can interbreed when they meet.
Populations evolve?
Populations are what evolve over generations, experiencing changes.
What is a gene pool?
The gene pool is the total genetic variability in populations, including all alleles at all gene loci in all individuals.
What are fixed alleles?
Fixed alleles occur when the whole population is homozygous at a locus for the same allele.
What are polymorphic loci?
When there are 2 or more alleles in a population, each being present in the population at some frequency.
Where are polymorphic loci found?
The standard sexual population contains mostly polymorphic loci.
How many polymorphic loci do most populations have?
Most populations have thousands of polymorphic loci.
Where do we see variation?
We see genetic variation at polymorphic loci, which is where natural selection comes in and acts on it.
Source of Genetic Variation?
New alleles arise by mutation in existing alleles. A single mutation can result in a new allele.
- New allele creates variation
Mutations occurring in a particular environment?
Most mutations don’t meaningfully affect fitness - neutral variation in respect to natural selection (called neutral alleles)
Some reduce fitness: harmful alleles
(a.k.a. ‘deleterious’ mutations/alleles)
A very few increase fitness: beneficial alleles
Microevolution?
Microevolution is the change in the frequencies of alleles over generations
Extreme evolutionary change?
At the extreme, ‘change’ can mean the fixation of an allele, or the loss (extinction) of an allele.
Example of having 1 locus with 2 alleles?
Incomplete Dominance in a flower population.
Red: 320 RR
Pink: 160 RW
White: 20 WW
n = 500
Genotypic frequency?
Number of observed / total number
Ex. RR - 320/500 = 0.64
Allele frequencies?
Total R and r / total number times 2
Total R is number of homozygous individuals X 2 plus number of heterozygous individuals.
Allele frequencies: p and q?
p + q = 1
p is one allele frequency
q is the other allele’s frequency
p?
p = frequency of RR + frequency of RW/2
q?
q = frequency of WW + frequency of RW/2
H-W Principle uses?
Calculating genotype and phenotype frequencies and can be done by using census data of a population
What is the Hardy-Weinberg Principle?
Describes expected relationships between allele and genotype frequencies in an idealized population when there is no evolution.
When is H-W applicable?
Occurs under certain assumptions, including random mating.
Model of random mating?
We can model random mating by taking two random alleles out of the population pool and sticking them together to create a combination of alleles.
Hardy-Weinberg equation?
For two alleles, it is as follows:
p^2 + 2pq + q^2 = 1
- p^2 and q^2 are the expected frequencies of the two homozygote genotypes
- 2pq is the expected frequency of heterozygotes
What does the Hardy-Weinberg equation let us do?
Lets us estimate the proportion of a population that are carrier of a recessive genetic disorder.
Uses of the H-W principle?
We can estimate the allele and genotype frequencies. Example: finding the prevalence of carriers (heterozygotes) of recessive genetic disorders.
Cystic Fibrosis - Allele and Genotype frequencies?
Cystic fibrosis is a single-locus recessive disorder, which affects 1 in 2500 people of European descent
(i.e. frequency is 1/2500 = 0.0004)
What is the estimated frequency of
carriers (i.e. heterozygotes) in the European population? 1 in 2500 have the disorder, 0.0004.
q2 is frequency of disorder genotype = 0.0004
q (frequency of disorder allele) = √0.0004 = 0.02
p (frequency of normal allele) = 1 - q
p = 1 - 0.02 = 0.98
Frequency of heterozygote genotype = 2pq
= 2 x 0.98 x 0.02 = 0.0392
So an estimated ~4% (1 in 25) are carriers
Other uses of Hardy-Weinberg principle?
Populations with genotype frequencies that conform to the equation are said to be in Hardy-Weinberg equilibrium at that locus.
We can look at population genotype frequencies to see if they are actually in Hardy-Weinberg equilibrium.
Is this population in Hardy-Weinberg equilibrium?
1000 individuals:
400 RR
200 RW
400 WW
ACTUAL frequencies of genotypes:
{RR} = 400/1000 = 0.4
{RW} = 200/1000 = 0.2
{WW} = 400/1000 = 0.4
ACTUAL frequencies of alleles:
p = {RR} + {RW}/2 = 0.4 + 0.1 = 0.5 q = 1 - p = 0.5
Hardy-Weinberg equation: p2 + 2pq + q2 = 1
EXPECTED genotype frequencies (when p = 0.5 & q = 0.5):
{RR} = p2 = 0.52 = 0.25
{RW} = 2pq = 2 x 0.5 x 0.5 = 0.5
{WW} = q2 = 0.52 = 0.25
What are the assumptions of the Hardy-Weinberg principle?
1) No net mutations
2) Random mating
3) No natural selection
4) Very large (infinite) population size
5) No migration
What happens if there is a violation of one of these assumptions?
Violation of any of these assumptions usually signals evolutionary change.
What are the three causes of microevolution?
Natural selection
Gene flow
Genetic drift
Natural selection violates?
No natural selection
Gene flow violates?
No migration
Genetic drift violates?
Very large (infinite) population size
Natural selection is the only?
Of these causes of microevolution, only natural selection results in adaptive evolution. That is, it is the only thing that causes greater suitability for the environment.
What is gene flow?
Migration, where an individual moves from one population to another (population of the same species) or interbreeds with a member of another population.
Gene flow is also?
Dispersal of gametes (e.g. pollen) or migration
- Gene flow from population with different allele frequencies causes a change in allele frequencies, causing evolutionary change
- Gene flow can introduce new alleles to a population
Assumption: very large population?
Frequencies of alleles should not change if it is a large population.
What is Random Genetic Drift?
A fluctuation from one generation to the next in the frequency of alleles within one population.
Random Genetic Drift?
‘Sampling error’: random changes in allele frequencies over generations
- Can lead to fixation (or extinction) of alleles in the population with the absence of natural selection
Rate of drift depends on?
The rate of drift strongly depends on the size of the population. It can be ignored in very large populations but can be detected in smaller populations in just a few generations.
- The time taken to have an allele go extinct or become fixed.
Genetic drift is random:
A neutral allele at frequency 0.5 is equally likely to eventually be fixed or to go extinct, as it is equally far away from both.
[In theory, chance of eventual fixation of a neutral allele is the same as its frequency]
What is a genetic bottleneck?
Breeding population is very small for a time
– Genetic drift powerful: allele frequencies change, many alleles fixed or go extinct.
Genetic bottleneck diversity?
Lower genetic diversity overall, even if population later expands in numbers as it won’t recover well
Can rare alleles frequencies change?
Some rare alleles can increase in frequency
= high frequencies of harmful alleles possible (even fixation of slightly harmful alleles)
Founder effect in progress?
A few individuals found a new population from the ancestral population. A new population grows, and the gene pool of new population reflects the small sample of alleles present in the founders population.
What is the founder effect?
Some previously rare alleles end up being much more common in the new population
e.g. High prevalence of particular genetic diseases in isolated human populations
Greater Prairie Chickens of Illinois?
Went through a genetic bottleneck effect.
- There was a lower genetic variability than larger populations
- They had much reduced reproductive success: fitness lowered by accumulated harmful alleles.
Endangered Populations/species?
- Genetic diversity (gene pool) can be increased by adding individuals from other populations
- Captive breeding programs manage matings to preserve remaining genetic diversity
