Genes and Their Genes and Their Evolution: Population Evolution: Population

GeneticsGeneticsChapter 4Chapter 4

Population A population is a group of individuals of the

same species who share a geographic area and usually mate within the group

The total genetic variation of that population is the gene pool

The number of times different genes or alleles occur is the frequency

And evolution is change in allele frequency over generations

Species We only look at reproductive populations of

organisms. This is important because to be called a species, organisms must be able to mate and have fertile offspring Are these different species?

Lion and tiger

Horse and donkey

Dog and wolf

Species When reproductive isolation occurs, this

means that two populations are kept from mating

If enough time passes, these two groups will become two different species This is allopatric speciation

Example: two groups of beetles get separated by a river. Over time, enough differences arise that they become different species and would not be able to mate again

Population Genetics This is the study of changes in genetic material

More specifically, the change in allele frequency allele= different versions of genes

Frequency= how often they occur

Microevolution: small-scale; happens in a short period of time

Macroevolution: large-scale; occurs over many generations; speciation

Different Views of Evolution

Darwin thought evolution was small changes accumulating over long periods of time This is phyletic gradualism

Gould and Eldredge said evolution could have long periods of no or minor change, interrupted by sudden change, such as speciation or extinction This is punctuated equilibrium

The Two Views

More Types of Evolution

We know evolution shows common ancestry

When two related species share phenotypic traits because of common ancestry, this is parallel evolution All primates have eyes close together and 5 fingers

When distantly related species develop similar adaptations to similar environments, this is convergent evolution Crocodyles and cats have tails because they walk

on 4 legs

Chickens, bees, and bats all have wings to fly

Parallel Evolution

Convergent Evolution

Population Genetics We focus on the idea of change over time,

especially in the frequency of alleles

Example: we are looking at a trait, which we will call R The two alleles are R and r (dominant and

recessive)

Generation 1 has 50% R and 50% r

When we come back and look at Generation 2, the frequency has changed to 40% R and 60% r

This shows evolution

Population Genetics We need evidence that evolution is occurring,

and we do this by looking at the frequencies of alleles in populations

If they do not change, there is no evolution

If they do change, there is evolution

Intro to Hardy-Weinberg

If the frequency never changed, the population would be in equilibrium

So, there is an equation to test for equilibrium

If the numbers don’t change = equilibrium = no evolution

If they do change = no equilibrium = evolution

Hardy-Weinberg Godfrey Hardy and Wilhelm Weinberg developed

a way to test for equilibrium in allele frequency

In order for equilibrium to exist, you must have:

No mutations

No natural selection

Completely random mating

An infinitely large population

Each organism having the exact same number of offspring

Would this ever occur in nature?

Hardy-Weinberg Don’t let the equation intimidate you!

Focus on what each part stands for and follow the steps

There will only be a couple of questions of this on the midterm

Hardy-Weinberg Here is the equation:

p2 + 2pq + q2 = 1.00 (100% of population)

p2 = all individuals who are homozygous dominant

q2 = all individuals who are homozygous recessive

2pq = all individuals who are heterozygous

Also important: p + q = 1.00

p = the dominant allele

q = the recessive allele

Example A population of snails has a trait for either

spotted or striped shells. Striped shells are dominant. Out of 100 snails, 16 have spotted shells.

Find the frequencies of SS, Ss, and ss

The next year we return and find that 25 out of 100 have spotted shells. Why?

Hardy-Weinberg Please use the document titled “HW

Explanation” on BlackBoard to see details and how to solve a problem

Sources of Evolution So, evolution is change over time, but how do

these changes arise?

4 sources: Mutation

Natural Selection

Gene Flow

Genetic Drift

Mutation Small errors in DNA, especially during

replication

Most go unnoticed. Some can be harmful and some beneficial

They occur at random

They are the only source of new genetic variation in a population

Mutations Point mutation: a single base is changed

ATCGGTC ATCGGTA

Frameshift mutation: caused by a deletion or insertion of genetic information; causes codons to be read incorrectly

Parts of chromosomes can be mutated, or entire chromosomes can be mutated

Kleinfelter’s Syndrom: males have extra sex chromosome: XXY

Trisomy 21: person has extra 21st chromosome. AKA Down Syndrome

Mutation Example A point mutation changes the hemoglobin blood cell in

some people

This causes sickle-cell anemia. Their blood cells are deformed and cannot carry enough oxygen

It is often fatal

Why do so many sub-Saharan Africans have this mutation?

Because it protects against malaria, the #1 killer in Africa

If a person is heterozygous for the sickle-cell trait, he/she will have enough normal blood to carry oxygen and enough affected blood to kill malaria parasites that enter body

Watch: http://www.youtube.com/watch?v=1fN7rOwDyMQ

Malaria and Sickle-Cell

The malaria/sickle-cell relationship is a balanced polymorphism

The heterozygous trait balances one negative trait with another, giving the person a better chance of surviving

This is selected for, and is also an example of natural selection

Pp. 317-320

Natural Selection Survival of those best adapted to current

environmental pressures

Based on the idea of fitness: number of offspring produced in a lifetime

Has nothing to do with strength, speed, or intelligence

Is just how good are you at surviving and making babies

Natural Selection Read the box on p. 104

Who has better fitness?

Male, Harvard degree, $500,000 salary, no kids Male, high school drop

out, unemployed, 7 kids

Types of Natural Selection

Directional: selection shifts in one direction Example: large

beak sizes in finches when droughts leave only hard food to eat

Types of Natural Selection

Stabilizing: selection favors the average and is against the extremes Example: birth

weight. Babies in the normal range survive more than premature ones or obese ones

Types of Natural Selection

Disruptive: selection favors the extremes, and against the average. Leads to speciation Example: beetles

are reproductively isolated until 2 new species are created.

Example of Natural Selection

In Great Britain, most moths were light colored to blend in to the environment

With the Industrial Revolution in the 1800s, the smog and soot produced changed the environment to be darker

Did the light moths have the advantage still?

No, they were eaten and darker moths survived. This changed allele frequencies

Practical Example #2 Practical Example #2 (from lecture 2)(from lecture 2)

Can you see viruses or bacteria?Can you see viruses or bacteria?

They are alive and, like everything else, they They are alive and, like everything else, they evolveevolve

Can evolve in a matter of hoursCan evolve in a matter of hours

This is why not taking antibiotics/medication This is why not taking antibiotics/medication correctly leads to drug resistancecorrectly leads to drug resistance

Yellow = weak virusesPurple = medium virusesRed = strong viruses

When people do not take medication correctly

Yellow = weak virusesPurple = medium virusesRed = strong viruses

When people do not take medication correctly

Genetic Drift This is an over-representation or an under-

representation of traits because of a small sample size

Example: In a class of 25 people, I find that 20 have Type B blood, 3 have Type O and 2 have Type AB

Does this accurately reflect the frequency of blood types in the entire human population?

What about Type A?

Genetic Drift This shows why large populations are

healthier…there is more variation

Endogamous groups only breed within their population

Exogamous groups breed with members outside their population

Which is better for variation and health?

2 Types of Genetic Drift

1. Founder Effect: a small group breaks off from the original population and forms its own group

Will that small group accurately reflect all the variation of the original population?

Huntington’s Chorea and Tay Sachs: genetic defects that are exaggerated due to founder effect and genetic drift

2. Population Bottleneck: when a population is reduced drastically, there is not enough variation to keep it going

Can cause extinction

This is what happens to endangered species

Genetic Drift: Founder Effect

A small group of original population creates new population

Some traits will be over-represented

Some traits will be lost

Genetic Drift: Bottleneck

Severe reduction in population

Loss of variation

Gene Flow Movement of genes and mixture of them

through breeding

Not only migration: have to mate as well, in order to add variation So it is migration and nonrandom mating

If there is no gene flow between 2 populations, they could evolve into 2 different species

With global travel and more open-mindedness in cultural ideals, our human gene pool has had a large increase in gene flow and variation

Gene Flow Variation is the key to success!

Why is inbreeding so bad?

It limits variation in the gene pool and can increase harmful mutations

Discussion Read pp.113-115, including box on p. 114

Why is biodiversity important?

Why should we worry about our actions as humans?

Why should we participate in conservation?

Review Questions What is the difference between

microevolution and macroevolution?

How does the Hardy-Weinberg equation show that evolution occurs?

What are the sources of evolution? What are some examples?