Lecture XIV: Microevolution; genetic changes at the population level

Question 1

Review: what is a population?

Answer 1

A group of individuals of the same species living in the same area.
These individuals must endure the same environmental pressures as well as compete for resources and mates.

Question 2

Evolution of populations:

Answer 2

Natural selection acts on individuals, but individuals do not evolve during their lifetime.
Evolution occurs by the differential reproductive success of individuals. Beneficial heritable traits will help an individual to produce more offspring.
Offspring in the next generation will have a higher rate of those beneficial heritable traits.
Because mating and producing offspring is critical for evolution to occur:

A population is the smallest unit of evolution

Question 3

Remember that Natural selection can only happen if:

Answer 3

1. Members within a population have variations in heritable (genetic) characteristics.
2. There are environmental pressures. Species produce more offspring than their environment can support. This leads to competition.
3. Lots of time (at least one generation).

Question 4

Gene Pool?

Answer 4

A population’s genetic makeup.

Quantifies how many of each type of allele for a given gene can be found within the population.

Question 5

Quantifying microevolution:

Answer 5

Geneticists look at:

1. Frequency of alleles for a gene
   Is one allele becoming more prevalent over generations?
2. Frequency of genotypes in the population over generations

Allele variations that are advantageous for reproductive success and/or survival will increase in frequency over time.

Question 6

Calculating genotype and Allele frequencies

Answer 6

Genotype Frequency is the proportion of the population with a particular genotype.
= # indv. with a particular genotype/ total # indv. in the population

Allele Frequency is the proportion of a specific allele in the population.
*Remember to account for diploidy. Diploid individuals have 2 alleles for every gene.

= # of copies of a specific allele for a gene in the population / Total # of alleles for a gene in the population

Question 7

So how can we tell whether a population is evolving by looking at allele and genotype frequencies?

Answer 7

Compare allele/genotype frequencies between generations.
or
Compare allele/genotype frequencies of a population with theoretical values for when no evolution is occurring (determined by a mathematical formula)

Question 8

Hardy-Weinberg Principle

Answer 8

Based on a theoretical equation that calculates what the genetic make up of a population would be if it were not evolving.

We can determine whether a population is evolving by comparing a population’s true genotypic and allelic frequencies with those predicted by the Hardy-Weinberg equation.

If they are the same, the population is not evolving for that particular gene.

TL;DR: Frequencies of alleles and genotypes in a population will remain constant from one generation to the next as long as the Mendelian laws of segregation and recombination of alleles are at work.

Question 9

Hardy-Weinberg Equation

Answer 9

Assumptions: 
1. Diploid Organisms
2. Genetic Equilibrium: 	No gene flow (immigration / emigration)
			No mutations
			Random mating within the population
			No Natural Selection
			Very large population size

A population that complies with all of the assumptions is not evolving and is said to be in: Hardy-Weinberg Equilibrium

Question 10

What is the Hardy-Weinberd Equation:

Answer 10

Allele Frequencies = p + q = 1
Genotype frequencies = p^2 + 2pq + q^2 = 1

where:
p = frequency of the dominant allele

q = frequency of the recessive allele

 p^2  = frequency of homozygous dominant genotype

2pq = frequency of the heterozygous genotype

q^2 = frequency of the homozygous recessive genotype

Question 11

What causes Microevolution

Answer 11

Minor Influences:

1. Mutations
   Rarely produce a variation that is advantageous in a short period of time.
2. Non Random Mating
   Sexual selection can alter genotype frequencies, but rarely affects allele frequencies in the population significantly between generations.

Major Influences:

                    3. Natural Selection 
		4. Genetic Drift
		5. Gene Flow

Question 12

1. Mutations:

Answer 12

Mutations can change allele frequencies within a population, but they generally do not play a significant role from one generation to the next.

Question 13

2. Non-Random Mating

Answer 13

Random mating rarely occurs in nature. Mates are often chosen based on certain characteristics, physical or behavioral. This is called sexual selection and is responsible for some rather bizarre traits! Sexual selection can affect the frequency of genotypes in a population at a broader scale.
Sexual Selection: When certain heritable phenotypic characteristics enable an individual to better acquire mates. These traits get passed on and get exaggerated sometimes.

Question 14

3. Natural Selection

Answer 14

Natural selection results in alleles being passed to the next generation in different proportions from the current generation if one allele variation proves to be advantageous for survival and reproduction over another.
Natural Selection is the only non-random major contributor to microevolution.
non random? : Natural Selection changes allele frequencies from one generation to the next in a non-random way.

Question 15

natural selection affects the population in 3 ways:

Answer 15

1. Directional Selection: One extreme variant in phenotype is favored.
2. Disruptive Selection: Both extreme variants are favored over intermediates
3. Stabilizing Selection: Intermediate variant is favored

Question 16

4. Genetic drift

Answer 16

Random fluctuations in allele frequencies from one generation to the next. Fluctuations are more substantial the smaller the population size is. Can be a major contributor to microevolution, but not directional, changes are random!

Question 17

Special Case of Genetic drift: The Founder Effect

Answer 17

When a few individuals become isolated from a population and start their own population.

Their gene pool differs from the original population because there are so few of them. Some alleles will be over represented and others will be under represented.

Question 18

Special Case of Genetic drift: Bottleneck effect

Answer 18

When a population’s numbers are drastically reduced due to a disaster
E.g. Flood or fire or mankind! Who survived is random. Alleles may be over represented or underrepresented in the survivors.

Question 19

5. geneflow

Answer 19

Transfer of alleles into or out of a population due to immigration or emigration of fertile individuals or their gametes.