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Natural Selection

Written by tutor Karen M.

Natural selection is one of the foundations of biology and, in its simplest terms, can be defined as a mechanism of evolution. It is a concept that was brought to light by Charles Darwin in his monumental book On the Origin of Species by Means of Natural Selection (1859). Darwin’s pivotal book was based on his notion of unity; that all organisms were related and descendants of common ancestors, and that these generations of descendants underwent changes over time as they move further from their base ancestor. This idea can best be envisioned by imagining a tree with various branches extending from the same trunk, and many smaller branches extending from one large branch, and many twigs extending from one small branch. Today, this process is what biologists commonly term “evolution”, however, Darwin originally referred to evolution as “descent with modification”. Descendants would give rise to further generations that would undergo various modifications or adaptations that would allow them to better fit into the specific ways of life that their habitats required. These populations of individual species are best suited for their local environment, their survival allows them to reproduce, and it is said that they are naturally selected for due to the benefit of their modified traits. Conversely, populations with traits that provide a disadvantage for survival and/or reproduction in an environment are said to be naturally selected against.

The alternative to natural selection is artificial selection, sometimes referred to as selective breeding. In this process, humans play the role of the environment and dictate the characteristics or traits that they would like to see in the selected organisms. Since this does not occur naturally, and is determined by human involvement, it is said to be artificial selection. Examples of this would include purposeful breeding of dogs to select for certain characteristics allowing the ability to herd sheep or cattle, pull sleds or retrieve game, or simply physical traits such as long hair or small size, or to produce a dog that doesn’t shed. Additionally, many of our foods have undergone artificial selection in order to select for accentuation of specific portions of an edible plant. Examples would include broccoli, cauliflower and kale that all share the common ancestor of wild mustard.

There are a number of key points necessary to understanding the process of natural selection. First, variations or changes occur within a population. These variations typically arise randomly through the means of mutations or recombination on the genetic level. Given that these genetic events occur by chance, the variations may have a positive or negative effect on survival, or may not result in any effect whatsoever (i.e., neutral). The fact that these changes occur on the genetic level is significant to the process of natural selection because a second key point is that the variation must be heritable. This is to say that the most advantageous of mutations in an individual serves no benefit to the overall evolving population if it cannot be passed on to offspring and further generations. While an individual may non-genetically acquire characteristics that allow the organism to adapt to its environment over its own lifetime, this is not considered to be natural selection as it is not heritable to the next generation. Central to this is the fact that an individual itself does not evolve, populations evolve over generations.

A third key point is that while the variations occur on the genetic level in a random fashion, natural selection as a process does not occur by chance. Natural selection occurs as a result of the interaction between the environment and the population. It is on this level that the first two key points described above converge. A random mutation provides an advantage to an individual within a particular environment/habitat (key point 1), this individual is better able to survive and therefore reproduce (higher reproductive success) within that environment and pass the advantageous trait onto its offspring and future generations (key point 2). Over time, the advantageous trait will accumulate within the population, and thus the population gradually changes and is considered to have evolved through the natural selection or favoring of one trait over others.

A fourth key point is that environments vary vastly. The selection that occurs within a population in one environment can be very different from what would occur within the population in a slightly different environment or even the exact same location at a different point in time. Certain variations may be advantageous to one environment, while they may confer no advantage or be disadvantageous in another. This plays a role in creating the great diversity that we see in all forms of life throughout history and the planet.

One does not have to look far to find examples of natural selection in everyday life. Antibiotic-resistance demonstrated by bacteria is a demonstration of natural selection that has provided a great advantage to the bacteria, while causing significant health concerns to humans. Random mutations occur at a basal level within the bacteria. One or more of these chance mutations confers resistance to a particular antibiotic, therefore allowing that bacterial cell to survive while the other cells are killed by the antibiotic. This bacterial cell survives and is able to reproduce, passing on the mutation conferring the antibiotic-resistance to its progeny. This carries on for generations, which for bacteria can occur rather rapidly, and eventually the original population of antibiotic-susceptible bacteria has been killed by the antibiotic while the progeny of the mutated antibiotic-resistant bacterial cell flourish. Similarly speaking, events such as these can give rise to pollution-degrading bacteria and pesticide-resistant insects.

Another popular example used to demonstrate natural selection is the English peppered moth, Biston betularia. Prior to the industrial revolution, the light-colored form of the moth was the most abundant form in England. Its speckled, light colored wings blended in very easily with the lighter colored bark found on many of the common trees of England, while it would stand out against a darker colored background. Its color served to camouflage it on the trees to hide from predation by birds. However, a new dark-colored form of moth was observed in the mid-1800s and this dark-colored moth became the most abundant form in certain environments by near the turn of the century. The dark-colored moth is extremely difficult to see against a darker background, however would stand out in a lighter area. This change in the population of the moth from light- to dark-colored was attributed to natural selection for the dark-colored variety as a result of the production of soot from the burning of coal at the start of the industrial revolution. The soot covered certain areas of England, creating very dark/black surfaces that the light-colored form of the moth could not “blend into,” and the dark-colored moths were provided an advantage for survival with their dark-colored camouflage against soot covered surfaces. The lighter-colored moths were more common in non-sooty regions, while the dark-colored forms of the moth became the majority in soot-covered areas. Air quality regulations in recent years has greatly reduced the prevalence of soot and, as one might expect, the numbers of the dark-colored form of moths have substantially decreased and may be near extinction within the next few decades (Miller, 1999).

While these key points were described individually, it is important to understand that they are all interconnected. A final key point that serves as a reemphasis of the ideas discussed above is that natural selection occurs over generations and therefore is a gradual process that, in reality, results in the slow accumulation of very small changes. It is a process that is never ending and will persist as long as there is life on Earth. Selection and re-selection for and against population variations will continue as environments and habitats undergo change, as evidenced by the English peppered moth. Temperature fluctuations, habitat changes (i.e., deforestation, water shortages, contamination), the introduction of invasive species, and different predator-prey interactions are just a few examples of the ways that environments change and force their populations to endure natural selection challenges. Diversity, and the natural selection events that help to drive it, will forever be a work in progress.

References:
Darwin, Charles. On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. London: John Murray, Printed by W. Clowes and Sons, 1859.
Miller, Ken. The Peppered Moth. (August, 1999). Retrieved January 23, 2013. www.millerandlevine.com/km/evol/Moths/moths.html

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