The Evolution of Populations

Populations

• It is a common misconception that evolution happens to one organism over the course of their lifetime– Remember adaptations

have to be inherited

• Natural selection overall affects the populations of organisms that are in a particular habitat

Populations

• A population is a group of individuals of the same species that live in a similar area

• A population might be the number of black bears in New Jersey

• Or it might be the species of bacteria that live under your fingernails

Populations

• Some populations are isolated from other populations– Like a population

trapped on an island

• Some populations are very fluid and have members entering and exiting constantly– Insects in a forest

Populations

• When looking at a population it is important to look at the total gene pool

• The gene pool is the total of the genes in a population at any given time

Darwin Awards• The following stories are tragic because they involve

a human removing themselves from our population’s gene pool.

• Most times they are tragic losses of life. • However other times they are good lessons in how to

think about problems before approaching them

Darwin Awards

• (1998, Buenos Aires) Did he win the argument? It happened in February 1998 in a working-class Boedo neighborhood. During a heated marital dispute, a 25-year-old man picked up his 20-year-old wife and threw her off their eighth-floor apartment balcony. To his dismay, she became tangled in the power lines below. He immediately leapt from the balcony and fell towards his wife. We can only speculate as to his reasons. Was he angrily trying to finish the job, or was he remorsefully hoping to rescue her? He did not accomplish either goal. He missed the power lines completely, and plunged to his death.

• The woman managed to swing over to a nearby balcony and was saved.

Darwin Awards

Darwin Awards

• Some men will got to extraordinary lengths to prove how macho they are. Polish farmer Krystof Azninski, who staked a strong claim to being Europe's most macho man. Azninski, 30, had been drinking with friends when it was suggested they play “macho men games". Initially they hit each other over the head with frozen turnips, but then one man upped the ante by seizing a chainsaw and cutting off the end of his foot. Not to be outdone, Azninski grabbed the saw and, shouting "Watch this then," he swung at his own head and chopped it off.

• "It's funny," said one companion, "when he was young he would put on his sister's underwear. But he died like a man."

Microevolution

• When we focus our studies of evolution to a single population we are branching into microevolution

• This is evolution on a small scale that studies the frequencies of alleles within a population

Microevolution

• The subtle differences in traits that lead to the variations in a population are all caused by different alleles

• These alleles can be attributed to our genetics

Microevolution

• However a change in alleles can be attributed to two main factors…

• 1) Mutation can change alleles by creating new alleles for the population

• This is the ultimate source of new alleles for a population

Microevolution

• 2) Sexual reproduction causes an increase in the number of different traits in a population

• The combination of different alleles will produce different traits

Hardy-Weinberg

• It is possible to show if a population is changing the alleles within that population

• Lets take an example• To the right you see a

Blue Footed Boobie

Hardy-Weinberg

• Notice their feet• The foot webbing seen

on the BFB is controlled by a single gene

• Non webbing (W) is dominant over webbing (w)

• Now we can test the frequency of the alleles

Hardy-Weinberg

• The Hardy-Weinberg principle is the equation that allows us to test alleles in a population at equilibrium

• The equation is shown to the right

• p stands for the dominant allele

• q stands for the recessive allele

Hardy-Weinberg• The Hardy-Weinberg principle is

only valid when…• No mutations must occur • No gene flow can occur (i.e. no

migration of individuals into, or out of, the population).

• Random mating must occur (i.e. individuals must pair by chance)

• The population must be large so that no genetic drift (random chance) can cause the allele frequencies to change.

• No selection can occur so that certain alleles are not selected for, or against.

Hardy-Weinberg (Cheat Sheet)

• These are helpful tips for Hardy-Weinberg

• p2 is equal to (WW)%• 2pq is equal to (Ww)%• q2 is equal to (ww)%

• p + q = 1

• ALWAYS write down variables given to you

• ALWAYS write down variables you are looking for

Hardy-Weinberg

• Lets go back to our BFB example

• If we knew the number of BFBs with webbed feet we could figure out the allele frequency for both the dominant and the recessive alleles

Hardy-Weinberg

• Scientists have counted that there are 500 birds, however only 20 of them have webbed feet

• Try to find the allele frequency for both p and q

Hardy-Weinberg

• Workspace…

Practice Problems

• There are 100 students in a class. Ninety-six did well in the course and four blew it totally and received a grade of F. In the highly unlikely event that these traits are genetic rather than environmental, if these traits involve dominant and recessive alleles, and if the four (4%) represent the frequency of the homozygous recessive condition, please calculate the following:

• The frequency of the recessive allele.• The frequency of the dominant allele.• The frequency of heterozygous individuals.

Hardy-Weinberg

• Workspace…

Practice Problems• Sickle-cell anemia is a genetic disease. Normal homozygous individuals

(SS) have normal blood cells that are easily infected with the malarial parasite. Thus, many of these individuals become very ill from the parasite and many die. Individuals homozygous for the sickle-cell trait (ss) have red blood cells that readily collapse when deoxygenated. Although malaria cannot grow in these red blood cells, individuals often die because of the genetic defect. However, individuals with the heterozygous condition (Ss) have some sickling of red blood cells, but generally not enough to cause mortality. In addition, malaria cannot survive well within these "partially defective" red blood cells. Thus, heterozygotes tend to survive better than either of the homozygous conditions. If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?

Hardy-Weinberg

• Workspace…

Mechanisms of Microevolution

• There were five different things needed for Hardy Weinberg to be a valid way to assess allele frequencies

• Changing any of those parameters can throw off the equilibrium out of whack and can make the equilibrium cease to be useful

Mechanisms of Microevolution

• To understand the next topic, have a partner or yourself take out a coin

• Flip that coin 10 times• What are the results?• Would 5 heads and 5 tails

be an uncommon result?• Would 7 heads and 3 tails

be an uncommon result?

Mechanisms of Microevolution

• Lets say I had you flip the coin 5000 times

• Would 2500 heads and 2500 tails be an uncommon result?

• Would 3500 heads and 1500 tails be an uncommon result

• These are the same scenarios as when I asked you to flip the coin 10 times

Genetic Drift

• A larger sample size has a more stable result than a smaller sample

• This also happens with a larger population vs a smaller population

• The larger a population, the better the chance the allele frequencies will remain stable

Genetic Drift

• Genetic drift is the idea that chance events will cause the allele frequencies to fluctuate unpredictably

• The larger the population, the smaller the overall change

• The smaller the population, the larger the overall change

Genetic Drift

• Things such as earthquakes, floods and fires are all examples of events that might cause genetic drift

• This leaves a small percentage of the original population and is likely to cause a change in the allele frequencies

Genetic Drift

• Often times the event that causes genetic drift is so large that it drastically reduces the population

• If this is the case there is an extremely low amount of genetic variation among the species

Genetic Drift

• When a population undergoes a massive loss and only a few genetic makeups remain it is called the bottleneck effect

• This is similar if you were to fill a soda bottle with multicolored marbles and try to pour it out

• Only a few colors from the original population of marbles would be able to make it out of the bottle

Genetic Drift

• Often times there does not need to be a huge event to cause a loss in genetic variation

• Another event could be when a small section of the population discovers a previously unused habitat

• When happens we call it the founders effect

Genetic Drift

• If a small section of the population accesses a new section of habitat

• Those are the sections of the population that will be seen in that location in the future

• This lower amount of genetic variation will be because the original population that was available was lower in genetic diversity

Types of Selection

• Many times natural selection will affect the makeup of a population

• It will rapidly (or slowly) change the allele frequencies, genetic variation and phenotype make up of a population

Types of Selection

• Most ranges in traits are represented by a bell curve

• A bell curve is a graph that shows that most organisms are similar in their genetic variations

• There are some organisms that are outliers in a bell curve

Types of Selection

• Stabilizing selection favors the majority of the organisms and removes the outliers

• Directional selection shifts the range of traits to one of the extremes

• Disruptive selection favors the outlying individuals and removes the bulk of the population

Sexual Selection

• Take a look at the next few slides and decide if the organisms that are listed are of the same species…

Male Peacock

Female Peacock

Male Sperm Whale

Female Sperm Whale

Female Wasp Spider

Male Wasp Spider

Sexual Selection

• Darwin proposed that males and females within a population could exhibit different traits due to the fact that they are in competition for mates

• He proposed this idea as sexual selection

Sexual Selection• Darwin stated that males

and females of the same species are looking for different things in a mate

• Most males are looking to spread their genes with little investment in the raising of the offspring (depending on the species)

• Most females are looking for a mate that has good genes because they are putting a large investment into offspring (most species)

Life Video

• Mammals – Humpback Whales

Sexual Selection

• Since males and females are looking for different things, males and females in populations will start to look different

• This is an example of selection within a population

Sexual Selection

• Sexual dimorphism is a distinction in appearance between male and female organisms

• Often times it is a direct hindrance to a male

• However if it causes females to select them over other males, they will be more fit

Video

• http://www.youtube.com/watch?v=gKybAp--n7M

• http://www.youtube.com/watch?v=A-05Z-GsGfI