Chapter 6: Evolutionary Biology

Darwin's Four Postulates of natural selection

1. Individuals in a population vary in their traits

2. Some of these differences are heritable; they are passed on to offspring

3. In each generation, many more offspring are produced than can survive

4. Individuals with certain heritable traits are more likely to survive and reproduce

Darwin's four postulates restated

1) Allelic variation exists among individuals

2) Alleles are passed down from parent to offspring by meiosis and fertilization

3) More young are born than can survive

4) Some allelic combination are more fit than others and these survive to reproduce more often

Definition of evolution

change in allele frequencies in a population over time

Microevolution

Evolution at the population level which is the level evolution acts at

Hardy-Weinberg equilibrium Principle

Allele and genotype frequencies in a population will remain constant from generation to generation when evolution is not occuring

What is a population?

A group of individuals that interbreed and their offspring

Life cycle of humans

Adults produce gametes which combine to make zygotes which grow up to become the next generation of adults

Tracking mendelian genes

Finding out if particular alleles become more or less common over time in generations of populations

Genetic drift

A change in the allele frequency of a population as a result of chance events rather than natural selection.

All possible genotype frequencies will add up to xxx

One

Allele frequencies that are the same as in the first generation and the population does not evolve are said to be in....

Hardy-Weinberg Equilibrium

What is the hardy weinberg equation?

p^2 + 2pq + q^2 = 1

Two fundamental conclusions of hardy weinburg

1) The allele frequencies in a population will not change, generation after generation

2) If the allele frequencies in a population are given by p and q, the genotype frequencies will be given by p2, 2pq, and q2

Why do we use the hardy-weinburg equilibrium principle?

It shows that evolution does not happen without evolutionary forces it is a null model to test evolution against

What are the assumptions of hardy weinburg?

1) No selection

2) No mutation

3) No migration

4) Infinitely large population size (no drift)

5) Panmixia ( mates are randomly chosen)

Why do we use HWE ( hardy weinberg equil)

Allows prediction of genotype frequencies given allele frequencies, binomial distribution after one generation of random mating, if we know the allele frequencies in generation 1, we can predict the genotype frequencies in generation 2

Cat toe example

Cats typically have 5 toes per hand by can also be polydactyl with 6-7 toes per hand, this form of polydactyly is an autosomal dominant trait caused by a variant in the Pd gene

If a population of 100 cats has 60 polydactyl and 40 normal individuals..

Then the frequencies of polydactyl and normal phenotypes are 0.60 and 0.40 respectively the dominant allele is designated as p and the recessive allele is designated q (p+q=1)

HWE allows for...

Calculation of allele frequencies with the assumption that population is in HWE

What happens if HWE assumptions are violated?

Evolution which is change in allele frequency over time

How do you test if HWE is broken?

A Chi-squared test with the formula X2= Epsilon (observed-expected)2/expected

Empirical study of Drosophila

Cavener and Clegg, two alleles for alcohol dehydrogenase locus, breakdown alcohol at different rates, maintained two populations of flies spiked with alcohol and two without alcohol, took random sample of flies to determine genotype

Conclusions of Drosophila study

Control populations appeared to be in HWE and were not evolving, populations under selection pressure ( alcohol) showed decline in Adhs allele, HW conclusion 1 did not hold in the experimental populations because they evolved to better break down alcohol

Malaria & pregnancy

Pregnant woman are more susceptible to malaria if a pregnant woman contracts the parasite it then invades the placenta and causes inflammation and usually death of the child (most common in endemic areas of africa)

How do we determine whether the difference between actual genotype frequencies and HWE expected frequencies is significant?

A chi squared test

How does natural selection affect the two conclusions of HWE?

1) Selection will cause populations to evolve if there is a trait to be selected for

2) If there is evolution from selection frequencies do not change but genotype frequencies cannot be calculated by HWE

Flour beetles

Two alleles are + and l

++ or +l are normal but ll is a recessive lethal allele

Dawson started colonies with heterozygotes allele frequency 0.5 for each

Conclusion of flour beetle study

Because II have lower fitness expect populations to evolve at lower l frequencies, measured allele frequency over 12 generations I allele dropped very low but was not eliminated

Dominance and allele frequency interaction

If recessive allele is common, evolution is rapid when it is rare evolution is very slow when it is rare recessive alleles are typically hidden from selection

Selection coefficients

w = fitness of an allele, ranges from 0-1

s = strength of selection on an allele

Positive s is selection in favor of phenotype

Negative s is selection against phenotype

Heterozygote fitness

When one allele is dominant and one is recessive heterozygous fitness is equal to that of one kind of homozygote

Intermediate heterozygote fitness

Fitness is intermediate to two homozygotes which changes the rate of evolution and eventually one allele that is more fit will be fixed and the other lost

Superior or inferior heterozygote fitness

Different evolutionary outcomes are produced depending on how the fitness related to either homozygote

Drosophila melanogaster

Single locus, Homozygotes for V allele are viable, Homozygotes for L allele are not viable, heterozygotes were used to start two populations, frequency of viable allele increased or decreased rapidly at first but the rate slowed and viable allele reached equilibrium at 0.79

Heterozygote superiority

Heterozygotes have higher fitness than homozygotes which creates overdominance that maintains genetic diversity even at lower allele frequencies

Heterozygotes may have lower fitness than...

homozygotes

Drosophila with compound chromosomes

Homologous chromosomes that have swapped entire arms, during meiosis compound chromosomes may not separate and four types of gametes can be produced

Drosophila compound chromosomes

When two compounds mate 1/4 of zygotes have correct chromosome dose and are viable the other 3/4 have an incorrect number and are not viable, If compound fly mates with normal fly no zygotes are viable

Compound chromosome study

13 mixed populations, populations with higher initial frequency rose to fixation, populations with lower frequency were lost

Heterozygote Inferiority

reduces genetic diversity within populations by pushing alleles to fixation

What is the scientific name of the fish that rips scales off of other fish for food?

Perissodus microlepis

What determines the handedness of Perissodus microlepis fish?

A single locus

Which type of Perissodus microlepis fish is dominant?

Right-handed fish

What percentage of the time are scale eaters successful in catching prey?

20%

How does the abundance of right-handed Perissodus microlepis affect prey fish?

Prey will be more vigilant from the left side

Why is it better for Perissodus microlepis fish to be the rarer type?

To catch prey unaware

What happens to the population of Perissodus microlepis fish over time?

It evolves for a higher frequency of the rarer type

What is frequency dependent selection?

The fitness of a phenotype depends on how common it is in the population, this helps to maintain genetic diversity

Mutation alone is not..

A potent evolutionary force

Model mouse population

Locus A frequency of allele A = 0.9 and frequency of a = 0.1, a is recessive loss of function mutation, Copies of A are converted to a at a high mutation rate, mutations happen in gametes in the gene pool

How do you determine what the frequency of a particular allele will be in the future based on a particular mutation rate?

Pn = P0e-mn

pn = frequency of A in generation n

p0 = frequency of A in generation 0

m = mutation rate

Mutation..

can cause evolution but it typically happens slowly, alone it cannot cause great changes in allele frequency but is still important to evolution, in combination with selection it can be a potent evolutionary force

What strain of E. coli did Lenski study?

A strain incapable of conjugation.

What was the only form of genetic variation in Lenski's E. coli study?

Mutation.

How many colonies did Lenski grow in minimal salts medium?

12 colonies.

How many cells did Lenski grow and transfer to a new medium?

Billions of cells, transferring 5 million from each.

How long did the daily transfers in Lenski's study last?

1500 days.

What happened to fitness and cell size in response to natural selection in Lenski's study?

Both increased.

What effect did mutation have on bacteria in Lenski's study?

Bacteria divided faster and increased in size.

What is the ultimate source of genetic variation according to Lenski's study?

Mutation.

Overview of mutation

The ultimate source of genetic variation, selection eliminates these mutations, mutations are created anew

Mutation selection balance

When rate of deleterious alleles being eliminated by selection equals rate of creation by mutation

When are deleterious recessive alleles at equilibrium?

q = square root (m/s)

m= mutation rate

s = selection coefficient

Selection and mutation equilibrium relationship

If selection is small and mutation is high equilibrium frequency of allele will be high

If selection is high and mutation is low, equilibrium frequency will be low

Cystic fibrosis

Recessive loss of function allele on locus on chromosome 7, Cystic fibrosis transmembrane conductance regulator (CFTR) normally destroys Pseudomonas aeruginosa, disease causes chronic lung infections and few people survive to reproductive age or are infertile

Heterozygosity Cystic fibrosis

Researchers discovered that the high frequency is maintained by heterozygote advantage, heterozygotes were partially resistant to typhoid fever infection, cystic fibrosis is maintained by mutation and over dominance