1. Stream A and Stream B are located on two isolated islands with similar characteristics. How do these two stream beds differ?

2. Suppose a fish that varies in color from a light shade to darker shade is introduced from Stream A into Stream B. How might the color of the fish population in Stream B change over time?

FORM GROUPS OF FOUR

• Genes determine most of an individual’s features, such as tooth shape or flower color.

• If an organism has a feature that is poorly adapted to its environment, the organism may be unable to survive and reproduce.

GENETICS AND EVOLUTION

• Evolution occurs as a population’s genes and their frequencies change over time.

•This can take millions of years for a species to change

GENETICS AND EVOLUTION

• How can a population’s genes change over time?

• Picture all of the alleles of the population’s genes as being together in a large pool called a gene pool.

• The percentage of any specific allele in the gene pool is called the allelic frequency.

GENETICS AND EVOLUTION

• They refer to a population in which the frequency of alleles remains the same over generations as

being in genetic equilibrium.

GENETICS AND EVOLUTION

• A population that is in genetic equilibrium is NOT evolving.

The Hardy-Weinberg principle is like a Punnett square for populations, instead of individuals. A Punnett square can predict the probability of offspring's genotype based on parents' genotype. Likewise, the Hardy-Weinberg principle is a tool we can use to calculate the frequency

of particular alleles in a population.

A aAa

AA AaAa aa

Evolution is simply a change in frequencies of alleles in the gene pool of a population. 

• Alleles:

•Gene Pool:

• Population:

One of two or more forms of a gene that code for different versions of the same trait.

All possible genes and genetic combinations in a population.

A group of organisms from the same species and the geographic location.

B=brownb=blonde

Let us assume that there is a trait that is determined by the inheritance of a gene with two alleles--B and b. 

If the parent generation had 92% B and 8% b and their offspring collectively had 90% B and 10% b, it would be

evident that evolution had occurred between the generations. 

This definition of evolution was developed in the early 20th century by

Godfrey Hardy, an English mathematician, and Wilhelm Weinberg, a German physician. 

Hardy Weinberg

Hardy-Weinberg Equilibrium maintains five basic assumptions:

3.  mutation is not occurring

Under these conditions it is obvious that evolution would not occur. 

There are no mechanisms of evolution acting on the population, so the process cannot happen--the gene pool frequencies will remain unchanged.

5.  natural selection is not occurring

1.  the population is infinitely large, and that genetic drift is not an issue within the population.

4. all mating is totally random

2.  there is no gene flow, or migration in or out of the population

However, since it is highly unlikely that any one of these seven conditions, let alone all of them, will happen in the

real world, evolution is inevitable.Hardy and Weinberg went on to develop a simple equation that can be used to discover the probable genotype frequencies in a

population and to track their changes from one generation to another. 

This is the Hardy-Weinberg equilibrium equation.

p² + 2pq + q² = 1

p is defined as the frequency of the dominant allele

q is the frequency of the recessive allele

In other words, p equals all of the alleles in individuals who are homozygous dominant (AA) and half of the alleles in people who are heterozygous (Aa) for this trait in a population.  In mathematical terms, this is

p = AA + ½AaLikewise, q equals all of the alleles in individuals who are homozygous recessive (aa) and the other half of the alleles in people who are heterozygous (Aa).

q = aa + ½Aa

Because there are only two alleles in this case, the frequency of one plus the frequency of the

other must equal 100%, so…p + q = 1

Since p+q=1, then logically p=1-qThere were only a few short steps from this knowledge for Hardy and Weinberg before they realized that the chances of all possible combinations of alleles occurring randomly is

p² + 2pq + q² = 1

In this equation:

p² is the predicted frequency of homozygous dominant (AA) organisms in a population.

2pq is the predicted frequency of heterozygous (Aa) organisms

q² is the predicted frequency of homozygous recessive (aa) ones!

Tudaaaaaaah!

Albinism is a rare genetically inherited trait that is only expressed in the phenotype of homozygous recessive individuals (aa). 

The most characteristic symptom is a marked deficiency in the skin and hair pigment melanin. 

                                       

                                            

This condition can occur among any human group as well as

among other animal species. 

The average human frequency of albinism in North America is only

about 1 in 20,000.

The Hardy-Weinberg equation (p² + 2pq + q² = 1), and the frequency of homozygous recessive individuals (aa) in a population is q².  Therefore, in North America the following must be true for albinism: q² = 1/20,000 = .00005By taking the square root of both sides of this equation, we get:   

q = .007 (rounded)     

Knowing one of the two variables (q) in the Hardy-Weinberg equation, it is easy to solve for the other (p).

p = 1 – q

p = 1 - .007

p = .993

The frequency of the dominant, normal allele (A) is, therefore, .99293 or about 99 in 100.The next step is to plug the frequencies of p and q into the Hardy-Weinberg equation:

p² + 2pq + q² = 1(.993)² + 2 (.993)(.007) + (.007)² = 1

.986 + .014 + .00005 = 1This gives us the frequencies for each of the three genotypes for

this trait in the population:

p² = AA = .986 = 98.6%

2pq = Aa = .014 = 1.4%

q² = aa = .00005 = .005%

You have sampled a population in which you know that the percentage of the homozygous

recessive genotype (aa) is 36%. Using that 36%, calculate the following:

1. The frequency of the "aa" genotype.

The frequency of the “aa” genotype is given in the problem as 36%!

2. The frequency of the "a" allele. The frequency of aa is 36%, which means that q2 =

0.36, by definition. If q2 = 0.36, then q = 0.6, again by definition.

Since q equals the frequency of the a allele, then the frequency is 60%.

3. The frequency of the "A" allele.

Since q = 0.6, and p + q = 1, then p = 0.4; the frequency of A is by definition equal to p, so

the answer is 40%.

4. The frequencies of the genotypes "AA" and "Aa."

The frequency of AA is equal to p2, and the frequency of Aa is equal to 2pq.

So, using the information above, the frequency of AA is 16%

(p2 = 0.4 x 0.4 = 0.16) and Aa is 48%

(2pq = 2 x 0.4 x 0.6 = 0.48)

5. The frequencies of the two possible phenotypes if "A" is completely dominant over "a."

Because "A" is totally dominate over "a", the dominant phenotype will show if either the homozygous "AA" or heterozygous "Aa" genotypes occur.

The recessive phenotype is controlled by the homozygous aa genotype. Therefore, the frequency of the dominant phenotype equals the sum of the frequencies of AA and Aa, and the recessive phenotype is simply the frequency of aa.

Therefore, the dominant frequency is 64% and, in the first part of this question above, you have already shown that the recessive frequency is 36%.

• Any factor that affects the genes in the gene pool can change frequencies of a certain trait.

• This disrupts a population’s genetic equilibrium, which results in the process of evolution.

GENETICS AND EVOLUTION

• One mechanism for genetic change is mutation.

• Environmental factors, such as radiation or chemicals, cause many mutations, but other mutations

occur by chance.

GENETICS AND EVOLUTION

• However, occasionally, a mutation results in a useful variation, and the new gene becomes part of the population’s gene pool by the process of natural

selection.

• Many mutations are lethal. GENETICS AND EVOLUTION

• Another mechanism that disrupts a population’s

genetic equilibrium is genetic drift—the alteration of

allelic frequencies by chance events.

GENETICS AND EVOLUTION

• Genetic drift has been observed in some small human populations that have become isolated due to reasons such as religious practices

and belief systems.

PRACTICE

HARDY-WEINBERG!