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Because of this, we can use information from a single offspring generation to test the hypothesis that individuals in the parental generation were all equally likely to survive and to produce surviving offspring and, therefore, that the population is not evolving with respect to this locus.
If parents are all equally likely to survive and to produce surviving offspring, then the allele frequencies of the offspring generation can be used as proxy for the allele frequencies of the parental generation. They should be identical if the no evolution condition is met.
Now we can use these hypothetical parental generation allele frequencies, that are based on the assumption of no evolution, in the Hardy-Weinberg equation to calculate the genotype frequencies we would see in the offspring generation if the population were not evolving with respect to these alleles.
Finally, we can draw a conclusion about whether a population is subject to an agent of evolution by comparing the genotype frequencies we calculated using Hardy-Weinberg to those we actually observe in the offspring generation.
To test your understanding of these relationships, answer the questions below.
What would you conclude about a) the actual similarity of the parental and offspring generation allele frequencies and b) whether the population was evolving if
Now, let’s apply this logic to an actual problem.
In the mid-1990's, researchers discovered that despite repeated exposure to HIV-1, a strain of the Human immunodeficiency virus (HIV), some individuals remained uninfected (Samson et al. , 1996).
Subsequent investigation revealed the existence of an allele that confers immunity to HIV-1 infection in homozygotes. This allele, known as delta- ccr-5 or CCR5-delta-32, is a mutant version of the cell-surface receptor protein CCR-5. It inhibits HIV infection because it codes for a form of the CCR-5 receptor to which HIV-1 viruses are unable to bind and, thus, to enter white blood cells and thereby establish an infection (Samson et al. , 1996).
For reasons unrelated to its effects on susceptibility to HIV-1 infection, this allele is found most commonly in caucasian Europeans and is absent or virtually absent from African, Asian, Middle Eastern and American Indian populations (Galvani and Slatkin, 2003). Table 1 contains original data from Samson et al. (1996) documenting the genotypes of 704 caucasian Europeans. Use these data to answer the questions that follow.
| Genotype | Number of Individuals |
| CCR-5/CCR-5 | 582 |
| CCR-5/ ccr-5 | 114 |
| ccr-5 / ccr-5 | 8 |
| Total | 704 |
Is this population of Europeans evolving with respect to this allele? How do you know? To answer this question, work your way through the steps below.
1. Review the question. Using information from earlier in this module or in related modules, complete the chart below on a piece of paper.
| What do I need to know to answer this question? | How do I get this information? |
To answer this question, you need to know
To get this information, you need to
Complete these steps and move on to the following problems to confirm your answers.
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