BOREL OPINION COLUMN: How Environment Impacts Evolution
Environment: Prime Mover in Evolution
by Helen Borel, PhD
With minor editorial adjustments, the ideas in this article are substantially preserved and fully supportable for my 21st Century readers. Surprisingly so, since it was written over three decades ago as my term paper for an Anthropology course.
Evolutionary Consequences of Environmental Pressures How environmental influences stress organisms, that ultimately alter the latter, is of paramount interest to geneticists as well as to anthropologists. For both are concerned with the adaptive changes manifested in genotypes and their ultimate expression in phenotypes.
When the pressures of environment - i.e., climate, locale, altitude, proximity to predators, and types of available food - are expressed in altered genetic material or as mutants, new characteristics become available to a population. And, after a considerable period of time elapses, such mutant genes may express themselves in new phenotypes.
This process is commonly referred to as "evolution." Consequently, it is thus evident that environment is a primary stimulus to heredity and, therefore, plays a key role in ultimate evolutionary adaptations.
Genetic Damage in Humans Due to Chemical Interference by LSD A tragic example1 of environmental interference with genetic material was pregnant (pun intended) with mutational grief and watchful concern by scientists and society at large in the 60s, when "the Flower Children" were tripping on LSD. Because its implications - that is, LSD's effect on fetal development - could be predicted to not only produce contemporaneous anomalies, but to also have far-reaching evolutionary consequences.
The case concerned the first known occurrence, at the time, of deformity in a newborn due to the ingestion of d-lysergic acid ethylamide (LSD) by the 19-year-old mother; the LSD taken four times during gestation.
Since many of the "Flower Children" who experimented with LSD were of child-bearing age, the findings in this case were especially significant for geneticists who felt compelled to warn the public against taking LSD, unless they wished to produce freaks due to genetic damage.
In the case cited above, the infant was born with a deformed lower limb which was attached at an abnormal bodily angle. This physically disabling (and ultimately emotionally and socially compromising) anomaly along with other deformities suffered by this baby, was attributed by medical scientists to chromosomal breaks resulting from the young mother's ingestion of the hallucinogen.
This was one instance where chemical interference with genetic material - specifically by LSD - resulted in a strange new phenotype.
Genetic Change in Bacteria Due to Chemical Interference by Antibiotics The way in which bacterial organisms respond to antibiotics provides the student of evolution with another good example of the primary role played by environment in genetic change.
And we are reaping the hazardous fruits of this process in the many untreatable infections and resulting deaths seen today in the so-called "superbug" epidemics afflicting hospital patients, caught with their defenses down, exemplified by the marauding spread of MRSA (methicillin-resistant Staphylococcus aureus). A bacterium once highly susceptible to methicillin. Now, powerfully mutated to resist it.
Because, at first, an antibiotic will kill off a bacterial population - saving a human life. However, the next time around, the same antibiotic may no longer subdue this, now mutated, bacterium. This is because the antibiotic environment - particularly prevalent in hospital settings where patients are liberally treated for existing infections as well as prophylactically to prevent, for example, postoperative infections - has forced mutation to occur within the bacterial population. The latter so that the bacterium will save itself from annihilation through such adaptive changes.
Although the original bacteral population, unexposed to the antibiotic, had a selective advantage, the now mutant bacterium in the antibiotic medium will selectively survive while those unable to adapt will be eliminated. This constantly evolving challenge is a headache to physicians trying to keep pace with these wily microscopic invaders.
Because such evolutionary phenomena of bacterial flora keeps doctors and drug manufacturers very busy in perpetual search for ever newer antibiotics to eliminate the new mutants which won't succumb to the older antibiotics. There isn't a clearer lesson to demonstrate how environmental stress produces hereditary change and ultimate adaptive phenotypes.
Diet as an Environmental Stimulus in an Evolutionary Trend The effects of an abrasive diet on the dentition of a Mesolithic population from the Sudan have been reported by Greene and associates.2 From their studies, they conclude, "The Mesolithic population...was subjected to rigorous selective pressures favoring large and/or morphologically complex teeth." Which they attributed to "intensive wear...caused by large amounts of grit in the diet."
They pointed out that simple but large teeth or complex teeth with added "supernumery cusps and additional ridges and wrinkles" allowed these Mesolithics to resist dental attrition. These were dentition mutations, resulting from the available food, that allowed the new phenotype to survive.
The Role of Heredity in Evolution Huxley3 claimed that the science of modern genetics has served to strengthen Darwin's central theory of natural selection. However, he comments that "[Darwin's] idea of the struggle for existence...which involved the all-or-nothing alternatives of survival or death, has been replaced by that of the differential survival of variants. New favorable combinations of small mutations will enjoy a slight advantage, so that old alleles in the hereditary constitution will gradually be replaced by new, and the average of the type will change slowly over the generations."
With advances in medical science that preserve types which were once eliminated by natural selection, humans have added another determinant to the evolutionary heritage of the future - our own role as controller of the environment.
(My final illustration of the function of environment in adaptedness also demonstrates the potential positive results achievable when humans "tamper" with environmental forces, as well as exemplifying a paradoxical adaptation.)
Genetic Alteration from Repeated, Widespread Infection - Malaria --> Sickle- Cell Anemia This final example of genetic adaptation to environmental pressure includes individuals with genes for Sickle-Cell Trait. Regarding the specific environmental pressure producing this trait, Dunn4 stated, "...in the case of sickle-cell anemia and of thalassemia, the distribution of the genes appears to be related to tangible factors of the environments of populations in which they occur. The evidence is compelling that natural selection is the paramount force determining the frequencies of these genes."
What is Sickle-Cell Anemia? Briefly, Sickle-Cell Anemia is the fatal disease manifested in the offspring of parents with the Sickle-Cell Trait. Sickle-Cell disease eliminates those who have it before child-bearing age is reached because of the profound anemia it produces. The anemia results from the severely reduced oxygen-carrying capacity of the afflicted's red blood cells which, instead of being normally round, are constricted into the shape of a sickle, curved and arc-like and oxygen deprived.
Couples who are carriers of the trait should be counselled against ever having children lest they perpetuate this tragic mutation producing more carriers and/or more tragically-doomed children.
How the Malarial Environment Caused the Sickle-Cell Adaptation The environmental role in the genetic mutation that produced the Sickle-Cell Trait gene was scientifically proven. Investigators found that presence of the mutant gene was an adaptive compromise with a Mediterranean environment particularly vulnerable to malaria.4,5
They further discovered that, when actually exposed to malaria, many individuals with a sickle gene-cell gene were unusually insulated against it. One example of how repeated infection of a population with the same pathogenic environment ultimately resulted in a mutation that expressed itself in altered blood cells.
Although this cellular alteration protects the organism from malaria, when an offspring is created by two parents carrying the trait, its dominant expression is fatal. Thus, the organism may adapt only so far to the threat of malaria without its own harsh adaptive mechanisms killing itself off.
How Humans Can Impact the Environment Positively In this particular instance of the sickling trait, humans have been able to control and probably reverse the genetic impact and consequences of lethal environmental factors by (1) preventing the birth of potentially afflicted individuals via genetic counselling and (2) eradicating malaria wherever possible; thereby eliminating malarial environmental pressure and, so, preventing its further expression in genetic sickling of red blood cells.
Summary There are numberless other examples of environmental pressures affecting evolutionary trends. These include the fact that (1) blood groups differ in their concentrations in different geographic regions; (2) skin pigmentation, dark in Africa, pale in Northern Europe - is certainly related to increased and decreased sun exposure, respectively; and (3) stereoscopic vision was environmentally determined as primates became erect and depth perception became essential to early humans.
(There is one outstanding, possibly humorous, question: How come human ears were so conveniently placed for the wearing of eyeglasses long before lenses to improve vision were ever invented or used?)
In summation, there can be little doubt of the prime role environment plays in genetic adaptation and its ultimate evolutionary consequences.
1. News Item, The New York Times, New York, (Nov. 22) 1967.
2. Greene, D.L.; Ewing, G.H., and Armelagos, G.J.: Physical Anthropology 27 :41-55 (July) 1967.
3. Huxley, J.S.: "Darwin and the Idea of Evolution," in A Book that Shook the World, Anniversary Essays on Charles Darwin's Origin of Species, University of Pittsburgh Press, 1958, pp. 1-12.
4. Dunn, L.C.: Heredity and Evolution in Human Populations, Harvard University Press, Cambridge, Mass., 1959, 157 pp.
5. Buettner-Janusch, J.: Origins of Man, Physical Anthropology, John Wiley & Sons, Inc., New York, 1966, pp. 540-55.
Background bibliography includes 7 additional sources which are available on request. (See email contact below.)
(c) copyright 2008 Dr. Helen Borel. All rights reserved.
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