Trial, Error, and Polymorphism
Tamarinda Barry Figueroa
Stories of Evolution & Visa Versa
Professor Anne Dalke
Trial, Error, and Polymorphism
The idea of evolution as a function of trial and error is fueled by the appearance of new genes in any given population. Produced solely by the occurrence of mutation, these genes and their respective alleles work to either promote the adaptation of an organism to its environment, or act as a detriment, limiting the individual’s ability to survive and reproduce. Charles Darwin compiled observations of such occurrences to formulate his theory of natural selection, summarized by Ernst Mayr and many others as “differential reproductive success” (Mayr, p. 138). Though unaware of the underlying genetic mechanisms, Darwin’s theory of evolution by trial and error has held constant, and is widely supported by studies around the globe. Even so, it is important to consider the existence of polymorphic traits such as that for Sickle Cell Anemia, as they challenge the claim that deleterious traits arise and are weeded out shortly thereafter.
Sickle Cell Anemia (SCA) appeared as the result of a point mutation on chromosome eleven of the human genome, where one nucleotide was substituted for another, thereby coding for an incorrect protein. This alteration creates a mutant form of the polypeptide beta globin, which transforms circular red blood cells into de-oxygenated, pointed sickles. Prominent among African descendants, the sickle cell gene is believed to have appeared and disappeared within the population several times before it was permanently established. Once in place, the trait became autosomal recessive, negatively affecting those who inherited two recessive alleles from their parents. Carriers, heterozygous for Sickle Cell Anemia, possessed only one recessive allele and were not prone to suffering through “anemia, joint pain, swelling of the spleen, and frequent, severe infections” like their homozygous counterparts (Lewis, p. 1). Additionally, these individuals tended to live long lives, as they were resistant to a prominent strain of malaria, responsible for the majority of deaths in Africa during the 1940s. When infected by a malaria-causing parasite (transferred from the salivary glands of an affected mosquito) red blood cells that contained some abnormal hemoglobin would sickle and flow through to the spleen. The irregular shaped cells were then eradicated from the blood stream, preventing the parasite within them from affecting their host.
As a polymorphic trait, the “superiority of the heterozygous carrier favors the retention of the rarer gene in a population” (Mayr, p. 89). That being said, we must think back on our idea of evolution with regards to trial and error and natural selection. One often discusses evolution as a function of mutation, or the accumulation of variants within a population over time. This general accumulation is often illustrated in a branched, linear, or at least outwardly expanding manner. These branches result from the existence of at least three types of mutations, including those that are beneficial, neutral, and deleterious. Deleterious mutations, such as Sickle Cell disease, are those that will be selected against and eliminated over time, as they are often lethal. It is interesting to ask, then: “How can a ‘bad’ gene….also be beneficial” to the evolutionary process (Nagal, 1)?
The passing down of the recessive allele provided a large number of individuals with a survival advantage when facing the malaria epidemic. The gene, as a result, persisted at a high frequency. Yet, every so often, two heterozygous parents would face the 25 percent chance of reproducing to form a homozygous child with SCA. That child, though seemingly resistant to malaria, would most likely suffer a different, but equally painful death. It seems that this example negates our current understanding of the evolutionary process with regards to the repetition of trial and error sequences. As a polymorphism, this trait serves both a positive and negative purpose, and is thus, difficult to classify. Heterozygous carriers in Africa had and continue to be naturally selected for; their homozygous offspring the result of unfavorable chance. Perhaps, then, both the passage of time and the diffusion of the sickle cell allele across seas has helped to both eradicate malaria in Africa while causing sickness abroad.
The advantage of the heterozygous individual over the parasite Plasmodium Falciparum is lost when that individual moves to a malaria-free region. Yet, in countries such as the United States, as many as 1 in 12 African Americans and 1 in 17 Hispanic Americans are carriers (4, p.1). This prevalence of recessive alleles can only be seen as detrimental, as it provides ample opportunity for the production of homozygous recessive (and therefore, Sickle Cell Anemia-obtaining) individuals within a country where malaria is not an issue. As time passes, however, the trait is likely to dissipate further, eventually disappearing from the sum of human genomes in America. This is interesting, as it illustrates the deleterious nature of a primarily positive adaptation. If evolution truly is a trial and error process, then the loss of the recessive sickle cell gene represents a correction unto itself. But does that necessarily identify the polymorphic trait as an error?
As malaria claimed a large majority of African populations up to and during the 1940s, the frequency of the sickle cell allele, within 35 generations, rose from 0.1 to 45 percent (Lewis, p. 1). Though reproduction resulted in a large number of SCA offspring, the carrier trait prevented and therefore decreased the deaths of hundreds of thousands of people.
Though Africa is still plagued by both this disease and the malaria-causing parasite, it is safe to claim the relative importance of the mutant strain that initiated the formation of the sickle cell allele. This gene, unlike most, works to both promote the adaptation of an organism to its environment, while simultaneously limiting another’s ability to survive and reproduce. It is in light of this that we must emphasize the importance of the population as a unit of evolution, as adaptations do not always follow the same trial and error sequence once removed from the environment from which they were derived.
1. Mayr, Ernst, What Evolution Is, New York: Basic, 2001
2. Unknown, "A Mutation Story," WGBH Educational Foundation and Clear Blue Sky Productions, Inc. 2001, pp. 1-1
3. Lewis, Ricki, "Human Genetics: Concepts and Application," Human Genetics: Concepts and Applications, Second edition, 1997 pp. 247-248 http://www.pbs.org/wgbh/evolution/educators/course/session7/explain_b_pop1
4. Unknown, "HBB: The Gene Associated with Sickle Cell Anemia," U.S. Department of Energy (DOE), Human Genome Project, 2003, pp. 1-1