Tuesday, February 19, 2013

Human Effects on Evolution: Focus on Fitness

Word Count: 699

The observation of evolution has different theories that explain possible operating mechanisms. While many of these theories are heavily supported, they more likely work in conjunction instead of being solely and independently correct. One of the main theories, natural selection, was popularized by Darwin's publication of "On the Origin of Species."

Natural selection is based on four basic principles, which can be paraphrased as follows:
1. The individuals of a population differ from one another in physical traits.
2. These variations can be passed on from generation to generation.
3. These individuals will compete for resources, but specific physical traits better suit an organism to surviving in the specific environment. The organisms who are more likely to survive are more likely to reproduce.
4. Therefore, survival and reproduction are not random. The individuals who are better adapted to their environment are more likely to survive and reproduce.

These can be further simplified to the following:
1. Variation among individuals of a population
2. Variations are inheritable
3. Fitness and adaptation lead to reproduction
4. Natural selection

Darwin himself recognized that variation under domestication was different than variation in the wild. Humans add another variable to evolution by acting as selectors. Fitness is no longer a concrete term because under human care, there is not a true struggle for limited resources. Therefore, genetic drift has become the main mechanism of evolution, right?

Wrong. Humans still decide which organisms to breed. They decide which ones reproduce and form the next generation. As a result, deleterious alleles may and often increase in frequency, considerably compromising the species. In their paper "Captive breeding and the genetic fitness of natural populations," Lynch and O'Hely state that these deleterious alleles can increase in frequency until fixation, resulting in extinction of the populations. Indeed, the risk runs high. If an endangered tiger is pampered in an artificial habitat for its entire life, it will forget how to live in the wild. If its offspring are raised in the same condition, there will be no learned skills passed on from parent to offspring that will be practical in the wild. Eventually, when the population is returned to nature, they will not know how to live in this natural environment.

From a genetic standpoint, suppose that a mutation that would be normally deleterious in nature (such as bones that are too brittle for pouncing on prey) is obtained. Because conservation efforts will seek to preserve all organisms, this allele may propagate through future generations and cripple the population when returned to nature. This, combined with the lack of natural experience and instinct, can result in extinction. The organisms simply are not fit and well-adapted to natural habitats.

Another example is that of the modern chicken. The majority of chickens are bred to grow to large sizes and lay many eggs. This provides the most food for humans. However, many news sources have reported that these chickens are too fat to even walk! The same situation plagues our traditional turkeys, which are simply not traditional anymore. These animals would not survive in the wild.

What then, is the impact of humans on evolution? We have skewed the process, altering fitness to fit our personal preferences. Ultimately, fitness is now a vague term with little meaning when we consider the portion of our own species that is obese and prone to premature death. However, if the human race should go extinct, biodiversity may disappear even more from the face of the earth before ecosystems restore themselves over extended periods of time. The profound effect that humans have had on nature is not ignorable. We need to consider our artificial habitats and breeding habits before we become malevolent "deities" to the creatures helpless to our influence. As insignificant as we may seem in the grand scheme of things, we have managed to veer a natural process off-track.

Does evolution run its course the same way it always has? Should we change our current breeding processes? What can we do to prevent future disaster? Are our present decisions moral, or are they immoral despite having good intentions? Consider that human-accelerated climate change is also affecting the "fitness" of natural organisms and ecosystems.


Darwin, Charles Robert. The Origin of Species. Vol. XI. The Harvard Classics. New York: P.F. Collier & Son, 1909–14; Bartleby.com, 2001.www.bartleby.com/11/. 

Lynch, Michael and O'Hely, Martin (2001) "Captive breeding and the genetic fitness of natural populations." Conservation Genetics. Kluwer Academic Publishers, the Netherlands.

Reuters. "Battery Farm Chickens Too Fat, Too Tired to Walk: Study." ABC News. ABC News, 7 Feb. 2008. Web. 16 Feb. 2013. <http://www.abc.net.au/news/2008-02-07/battery-farm-chickens-too-fat-too-tired-to-walk/1035272>

By Anthony Nguyen


  1. This is a very interesting point regarding the deleterious alleles being not selected out. However, in cases like the Florida mountain lion, the deleterious effects that surfaced because of the small population were in large part selected out in a short period of time after a small new population was introduced. If the zoos, given that the animals remembered how to survive in the wild, were to be released, why couldn't the same thing happen to them as well?

    Kelsey Wooddell

  2. The whole controversy of "playing God" in our own environment. If human didn't influence our surroundings, evolution would have occurred at a normal pace, probably less extinctions. But there are still human endeavors to protect certain species and we have a whole field of conservation biology. Perhaps we just need to balance out the harmful effects that we cause in the environment and beneficial effects.

    SoongJin Ahn

  3. Your comment about humans as selectors prompted me to think about selection among humans ourselves. I recently heard about a service that allows people to accessibly perform genetic testing on themselves (https://www.23andme.com/). This can help people discover if they are at risk for certain cancers, but it may also lead to selective mating. Do you think widespread genetic testing will be a thing of the future and what implications would it have on human evolution in the long run? There have been claims that symmetrical features are linked with increased physical fitness, which is why they're attractive to members of the opposite sex. Choosing partners with perceived qualities of optimal fitness is selection, but is using genetic mapping taking it one step too far?