Introduction to Hardy-Introduction to Hardy-Weinberg PrincipleWeinberg PrincipleHow do we know the population is How do we know the population is

evolving? evolving? We can tell if the population is evolving if We can tell if the population is evolving if we measure genetic change using Hardy-we measure genetic change using Hardy-

Weinberg equilibrium equation.Weinberg equilibrium equation.

Think back to Think back to yesterday’s bird beak yesterday’s bird beak lab. What beak lab. What beak survived better than survived better than other beaks while other beaks while foraging for cat food?foraging for cat food?

Did the bird population Did the bird population change over time?change over time?

Now, in real populations in nature, it’s not that Now, in real populations in nature, it’s not that easy to observe change.easy to observe change.

If we were to go into Sauerman Woods, and If we were to go into Sauerman Woods, and attempt to capture all the rabbits, it’s not attempt to capture all the rabbits, it’s not possible. We need to capture as many rabbits possible. We need to capture as many rabbits as possible because we need a large sample. as possible because we need a large sample. However, capture is difficult, so we use a However, capture is difficult, so we use a Population Sampling.Population Sampling.

Population Sampling is a technique that uses Population Sampling is a technique that uses part of a population to represent a whole part of a population to represent a whole population. population.

Once we have a population sample, we Once we have a population sample, we can evaluate whether the population is in can evaluate whether the population is in fact, evolving. fact, evolving.

Scientists attempt to measure genetic Scientists attempt to measure genetic change using the Hardy-Weinberg change using the Hardy-Weinberg principle. “Under certain conditions, principle. “Under certain conditions, frequency of the dominant and recessive frequency of the dominant and recessive alleles will remain the same generation alleles will remain the same generation after generation.”after generation.”

Common MisconceptionsCommon Misconceptions

The dominant allele of a The dominant allele of a trait will always have the trait will always have the highest frequency in a highest frequency in a population.population.

The recessive allele of a The recessive allele of a trait will always have the trait will always have the lowest frequency in a lowest frequency in a population.population.

Polydactyl is a condition Polydactyl is a condition of having more than 5 of having more than 5 digits—inherited as an digits—inherited as an autosomal dominant autosomal dominant phenotype.phenotype.

Once we have the Once we have the population sample to population sample to actually measure Hardy-actually measure Hardy-Weinberg principle, we Weinberg principle, we can use a gene pool (all can use a gene pool (all of the genes of every of the genes of every individual in a population individual in a population sample) to study the sample) to study the frequency with which frequency with which certain alleles change certain alleles change over time. over time.

Hardy-Weinberg EquationHardy-Weinberg Equation Allele frequency (gene frequency) = for example, A:80% a:20%Allele frequency (gene frequency) = for example, A:80% a:20% Genotype frequency = AA Aa aaGenotype frequency = AA Aa aa (the number of specific types of alleles / the total number of alleles in (the number of specific types of alleles / the total number of alleles in

the gene pool)the gene pool) P = frequency of the dominant alleleP = frequency of the dominant allele Q = frequency of the recessive alleleQ = frequency of the recessive allele P + Q = 1P + Q = 1 PP2 2 ++ 2pq + q2pq + q2 2 = 1= 1 PP2 2 = homozygous dominant individuals= homozygous dominant individuals(percentage of homozygous dominant individuals)(percentage of homozygous dominant individuals) 2pq = heterozygous individuals2pq = heterozygous individuals(percentage of heterozygous individuals)(percentage of heterozygous individuals) QQ2 2 = homozygous recessive individuals= homozygous recessive individuals(percentage of homozygous recessive individuals)(percentage of homozygous recessive individuals)

Does Hardy-Weinberg Exist in Does Hardy-Weinberg Exist in Reality?Reality?

What does the Hardy-Weinberg principle say?What does the Hardy-Weinberg principle say? Hardy-Weinberg says that genes and gene Hardy-Weinberg says that genes and gene

frequency remain stable and at equilibrium. frequency remain stable and at equilibrium. Equilibrium means that the genotypes or Equilibrium means that the genotypes or

phenotypes in a population remain constant over phenotypes in a population remain constant over time.time.

If this principle is in fact, true, If this principle is in fact, true, NO EVOLUTIONNO EVOLUTION is occurring in the population!is occurring in the population!

Does Hardy-Weinberg Exist in Does Hardy-Weinberg Exist in Reality?Reality?

In order for Hardy-Weinberg principle to be In order for Hardy-Weinberg principle to be actually occurring, the following conditions actually occurring, the following conditions must be present in order for Hardy-Weinberg must be present in order for Hardy-Weinberg equilibrium to be actually occurring:equilibrium to be actually occurring:

The population is largeThe population is large (no emmigration or (no emmigration or immigration)immigration)

Mates are chosen at randomMates are chosen at random Mutation is not presentMutation is not present Natural Selection forces are absentNatural Selection forces are absent (no (no

“Survival of the Fittest”“Survival of the Fittest”

Hardy-Weinberg is a Hypothetical Hardy-Weinberg is a Hypothetical Situation!Situation!

This is a hypothetical or imaginary situation because…..This is a hypothetical or imaginary situation because…..

Mates are really not chosen at random.Mates are really not chosen at random. Mutations are often present in a population.Mutations are often present in a population. Natural Selection forces are often present in a population.Natural Selection forces are often present in a population.

Are the 4 conditions of Hardy-Weinberg Are the 4 conditions of Hardy-Weinberg principle ever met in nature?principle ever met in nature?

Very rarely are these Very rarely are these conditions met in conditions met in nature—so evolution nature—so evolution is usually occurring.is usually occurring.

However, sharks and However, sharks and turtles come very turtles come very close to meeting the 4 close to meeting the 4 requirements.requirements.

Would you expect sharks and turtles to exhibit relatively Would you expect sharks and turtles to exhibit relatively constant allele frequencies? Yes! Researchers have constant allele frequencies? Yes! Researchers have

determined that genetic information in mammals changes determined that genetic information in mammals changes about 3% every 1 million years, while genetic information of about 3% every 1 million years, while genetic information of

sharks changes at a rate 7 to 8 times slower than other sharks changes at a rate 7 to 8 times slower than other mammals. mammals.

Do any populations ever fulfill all of the Do any populations ever fulfill all of the conditions perfectly? No. The Hardy-conditions perfectly? No. The Hardy-Weinberg principle describes an imaginary Weinberg principle describes an imaginary situation in which no selection forces are situation in which no selection forces are acting on organisms. acting on organisms.

Each population has a life-span of 5 Each population has a life-span of 5 million years—when genes change too million years—when genes change too much, the population may die out.much, the population may die out.

Scientists often attempt to prove Scientists often attempt to prove something is not true—in order to prove something is not true—in order to prove something else is true! This is why Hardy-something else is true! This is why Hardy-Weinberg principle is so convenient.Weinberg principle is so convenient.