Introduction to Hardy- Weinberg Principle How do we know the population is evolving? We can tell if the population is evolving if we measure genetic change using Hardy- Weinberg equilibrium equation.
Think back to yesterday’s bird beak lab. What beak survived better than other beaks while foraging for cat food? Did the bird population change over time?
Now, in real populations in nature, it’s not that easy to observe change. If we were to go into Sauerman Woods, and attempt to capture all the rabbits, it’s not possible. We need to capture as many rabbits as possible because we need a large sample. However, capture is difficult, so we use a Population Sampling. Population Sampling is a technique that uses part of a population to represent a whole population.
Once we have a population sample, we can evaluate whether the population is in fact, evolving. Scientists attempt to measure genetic change using the Hardy-Weinberg principle. “Under certain conditions, frequency of the dominant and recessive alleles will remain the same generation after generation.”
Common Misconceptions The dominant allele of a trait will always have the highest frequency in a population. The recessive allele of a trait will always have the lowest frequency in a population. Polydactyl is a condition of having more than 5 digits—inherited as an autosomal dominant phenotype.
Once we have the population sample to actually measure Hardy- Weinberg principle, we can use a gene pool (all of the genes of every individual in a population sample) to study the frequency with which certain alleles change over time.
Hardy-Weinberg Equation Allele frequency (gene frequency) = for example, A:80% a:20% Genotype frequency = AA Aa aa (the number of specific types of alleles / the total number of alleles in the gene pool) P = frequency of the dominant allele Q = frequency of the recessive allele P + Q = 1 P 2 + 2pq + q 2 = 1 P 2 = homozygous dominant individuals (percentage of homozygous dominant individuals) 2pq = heterozygous individuals (percentage of heterozygous individuals) Q 2 = homozygous recessive individuals (percentage of homozygous recessive individuals)
Does Hardy-Weinberg Exist in Reality? What does the Hardy-Weinberg principle say? Hardy-Weinberg says that genes and gene frequency remain stable and at equilibrium. Equilibrium means that the genotypes or phenotypes in a population remain constant over time. If this principle is in fact, true, NO EVOLUTION is occurring in the population!
Does Hardy-Weinberg Exist in Reality? In order for Hardy-Weinberg principle to be actually occurring, the following conditions must be present in order for Hardy-Weinberg equilibrium to be actually occurring: The population is large (no emmigration or immigration) Mates are chosen at random Mutation is not present Natural Selection forces are absent (no “Survival of the Fittest”
Hardy-Weinberg is a Hypothetical Situation! This is a hypothetical or imaginary situation because….. Mates are really not chosen at random. Mutations are often present in a population. Natural Selection forces are often present in a population.
Are the 4 conditions of Hardy-Weinberg principle ever met in nature? Very rarely are these conditions met in nature—so evolution is usually occurring. However, sharks and turtles come very close to meeting the 4 requirements.
Would you expect sharks and turtles to exhibit relatively constant allele frequencies? Yes! Researchers have determined that genetic information in mammals changes about 3% every 1 million years, while genetic information of sharks changes at a rate 7 to 8 times slower than other mammals. Do any populations ever fulfill all of the conditions perfectly? No. The Hardy- Weinberg principle describes an imaginary situation in which no selection forces are acting on organisms.
Each population has a life-span of 5 million years—when genes change too much, the population may die out. Scientists often attempt to prove something is not true—in order to prove something else is true! This is why Hardy- Weinberg principle is so convenient.