Problem 5 - Animal Physiology

Question

At least three hemoglobin alleles in human populations alter hemoglobin structure in such a way as to impair the transport of \(\mathrm{O}_{2}\) by the blood but enhance resistance of red blood cells to parasitization by malaria parasites. Explain how such alleles exemplify pleiotropy, and discuss whether such alleles could lead to nonadaptive evolution of blood \(\mathrm{O}_{2}\) transport in certain situations.

Step-by-Step Solution

Answer

Hemoglobin alleles that alter O2 transport and provide resistance to malaria exemplify pleiotropy as they influence these two, seemingly unrelated, traits. Nonadaptive evolution could occur in certain conditions, as the increase in frequency of these alleles might not be a result of natural selection considering the overall survival and well-being of the organism (adaptive), but rather a result of chance spread considering only localized disease resistance (nonadaptive).

Step 1: Understanding Pleiotropy

Pleiotropy refers to the phenomenon where one gene influences multiple, seemingly unrelated, phenotypic traits. This exercise refers to a type of pleiotropy where alterations in hemoglobin alleles not only affect O2 transport but also increase resistance to malaria.

Step 2: Impact of Pleiotropic Hemoglobin Alleles

Because of this alteration in hemoglobin allele, there could be impairment in O2 transport by the blood. Despite this disadvantage, the alteration provides an advantage of the red blood cells being more resistant to malaria infection. This clearly shows an example of pleiotropy as two different traits are affected due to one gene alteration.

Step 3: Discussion on Nonadaptive Evolution

Nonadaptive evolution refers to changes in a population's gene pool due to chance, rather than the result of natural selection favoring survival-promoting traits. Given these hemoglobin allele alterations, it's possible for nonadaptive evolution to occur as the advantage of resistance to malaria might be promoted in regions where malaria is prevalent, despite it impairing the O2 transport. Regions free of Malaria could witness prevalence of regular O2 transport-aiding genes. This chance occurrence would depend on the random spread of the alleles and not on natural selection considering the overall well-being of the organism.