Day 39 Population Evolution
A population is a group that includes all the members of a species living in a given area
-Remember that individuals do not evolve; Populations evolve
To understand evolution we need to study how alleles interact with each other in populations
-When talking about allele frequencies in a population we talk about the individual alleles, not the individuals
-Remember everyone has two of every allele
-Gene Pool- A gene pool is all the alleles in a given population
-A population can only reproduce and choose from the gene pool it has
-Gene Flow- Gene flow is if alleles are brought in to the population from another population
To calculate if evolution (Change of allele frequency) is taking place we can use the Hardy-Weinberg principle
If a population is not evolving then we say it is an Equilibrium population
-In order to have a non-evolving population these 5 rules must be true
-There must be no mutations
-There must be no gene flow
-The population must be very large
-All mating must be random
-There must be no natural selection (All genotypes have an equal chance of success)
Under these conditions allele frequencies in a population will remain the same
-Since there is no such thing as a population that meets all 5 of the rules for an equilibrium population, we say that an equilibrium population is a hypothetical population (that is just one we talk about but never really exists)
Hardy-Weinberg Equation
p^2 + 2pq + q^2 = 1
-The Hardy-Weinberg equation is used to test if a population is evolving or not
-A population is a group of the same species
-We can characterize a population by describing its gene pool
For example human hair texture is an incomplete dominant trait
Lets say that:
C= Curly
S= Straight
CS= Wavy
Because hair texture is an incomplete dominant trait we can tell what an individual's genotype is based on their phenotype
Lets describe the gene pool for hair texture at this high school
-Assuming there are 500 people at this school we will have 1,000 alleles for hair texture since we are all diploid
-We have 320 people with straight hair, 160 with wavy hair, and 20 with curly hair
-This gives us 800 alleles for straight hair (320 x 2= 640 + 160 from the heterozygotes = 800)
-800/1000 means that our gene pool is 80% straight hair
-The rest has to be for the curly allele so 20% for curly
If a population is in Hardy-Weinberg equilibrium then the allele frequencies will remain constant over time
-If mating is random then we can think of it as if all these alleles are in a pool and randomly thrown together
-In a random mating situation we can use the Hardy-Weinberg equation to figure out what the chances are of having each type of offspring
In our hair population we can figure out the chances of each genotype
-For the homozygous Straight hair alleles we take p x p = p^2 so that is .80 x .80 = .64
-If random mating takes place then 64% of our population should be homozygous for straight hair
-For the homozygous Curly hair we take q x q = q^2 so that is .20 x .20 = .04
-We get 4% with curly hair
-To get the percentage for heterozygous we have to remember that there are two possibilities for heterozygous, p x q and q x p. which is 2pq.
-So we take .8 x .2 = .16 and .2 x .8 = .16 we add both of these together to get 32% wavy hair
If these allele frequencies were to remain relatively the same generation after generation then the population would be in a Hardy-Weinberg Equilibrium. In that case we would say that the population is not evolving
In order for a population to be in a Hardy-Weinberg Equilibrium these 5 conditions must apply
1) No mutations
2) Random Mating
3) No natural Selection
4) Extremely Large Population Size
5) No Gene Flow
Time for a quick writing exercise!
-Write a paragraph explaining why each of these 5 conditions must be present for evolution to not occur
With our hair texture example we could tell how many heterozygous individuals there were because it shows in their phenotype. When it is unclear if an individual is heterozygous we have to guess
There is a disease called PKU (phenylketonuria) that is contracted when you are homozygous recessive for the disease.
-PKU means that you can not ingest any phenylalanine otherwise it results in mental disability.
-We know that the disease affects 1 in every 10,000 births so that would mean q^2 = .0001 so q = the square root of .0001 which is .01
-p = 1 - q which is 1 - .01 = 0.99
2pq = 2 x .99 x .01 = 0.0198 (almost 2%)
homozygous recessive are .01% of the population
Heterozygous is 2% of the population
Homozygous dominant are 98% of the population
The smallest unit of evolution is a population
-A population is a group of the same species
-The change of an allele frequency over time within a population is known as microevolution
-Genetic variation is what makes evolution possible. If there were no variation within a population one trait cannot reproduce more than another.
-Although genetic variation is what makes evolution possible it is phenotypic variation that evolution works with. If a gene is not expressed it usually has no determination on reproductive success.
All genetic variation originates with mutation. A mutation is something the organism is born with. It has to happen during the formation of the gametes during meiosis or it has to happen during fertilization. Once an organism is more than one cell a mutated cell is simply a damaged cell (like cancer)
A population can only choose from the alleles that are available in that population. If alleles are brought in from another population or emigrated to another population this is referred to as gene flow.
-Gene flow is a good thing. It allows for variation in a population giving them more opportunities to survive stress.
Natural selection is the only mechanism that consistently causes adaptive evolution
-Sexual selection is not adaptive
-Genetic drift is not adaptive
-and artificial selection is not adaptive
-Relative fitness refers to an individuals contribution to the next generation's gene pool
Natural selection can lead to three modes of selection
-Directional selection
-Disruptive selection
-Stabilizing selection
-The normal distribution has an average bell curve
-The average is in the middle with the majority of the population and to each end are the few extremes
-If the population starts selecting for one of the extremes then the population will go through directional selection
-If the population selects for the two extremes and does not favor the average then it will go through disruptive selection
-If the average is selected for than the extremes will become less sever and the population will go through stabilizing selection
-Directional selection affects the evolution of a population
-Disruptive selection could lead to speciation
-Stabilizing selection can lead to a more uniform population