Examples of Natural Selection

Natural Selection

The theory of evolution presented by Charles Darwin in his book On the Origin of Species by Means of Natural Selection (1859) is one of the most controversial and revolutionary concepts in the history of humanity. It changed how we perceived the world and the belief that God created everything around us in seven days as presented in the Bible was consequently put into question.

Darwin’s theory holds that all the living things around us are related. It states that a common ancestor in the distant past gave rise to the diverse life we see and that through time, several phenotypic modifications occurred enabling organisms to cope up with the ever changing environment.

Natural selection is a major driving force in evolution. According to Darwin, it has four components – Variation, Inheritance, Population Growth and Differential Survival and Reproduction. In order for natural selection to operate, organisms in the population must show variation in their characteristics. The differences in size, shape as well as color for example can give others an advantage over the rest, and thus those with more favorable traits survive to pass on their genes to the next generation. And as more offspring are produced, the rate of competition for resources increases considerably and struggle for existence occurs wherein the less suited are weeded out and the fit survive.

The Examples

Industrial Melanism

One of the most common examples of natural selection is the industrial melanism that occurred in England during the Industrial Revolution. This phenomenon affected over 70 species of moth and was most studied among the Biston betularia (peppered moth). Prior to 1800, the typical peppered moth prevalent in the area was the lighter-colored variety. As the industrial age progressed, pollution became an increasing concern as more soot collected and accumulated on rooftops, walls of buildings, leaves and tree trunks. As a result, the lighter-colored peppered moth appeared more visible against their darker background allowing predators to see them easily. The increased risk of predation caused a decline in their number and later on drove them away from the city. On the other hand, the condition became more favorable to the rarer dark-colored peppered moth. After several decades, it was observed that the darker variety became more prevalent in industrial areas since the darker environment caused by soot accumulation ensured their safety against predators.

Finches’ Beaks

During Darwin’s voyage in Galapagos Islands, he noticed that the beaks of finches varied from island to island. In his book (On the Origin of Species), he hypothesized that starling-like birds came to the Galapagos, evolved, and various groups resorted to different diets. He reasoned out that as birds reproduced, their food source became limited. As  a result, each species needed to compete with one another in order to survive or to change their diet and resort to other food resources. As it happened, the shape and size of their beak changed over time in response to the type of diet they are into. The warbler finch for example has a thin, sharp beak appropriate for spearing insects while the cactus finch posses a long, pointed beak useful for picking seeds out of cactus fruits. Ground finches on the other hand have a shorter, stouter and stronger beak best suited for eating seed from the ground.

A study by researchers at Harvard Medical School found out that a protein called calmodulin controls the shape of the finches’ beaks by binding and activating enzymes capable of turning certain genes on and off. This study revealed that calmodulin is associated with longer beaks since higher levels was expressed among cactus finches as compared to those of the more blunt-beaked ground finches.

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Sickle Cell Trait

This is a heterozygous condition in which the gene of a person coding for hemoglobin has a recessive allele that codes for Hemoglobin S instead of the normal Hemoglobin A. This type of hemoglobin which occurs in people with sickle cell anemia causes the red blood cells to sickle. The sickle-shaped RBC give them smaller area to bind oxygen, thereby reducing the blood’s capacity to deliver oxygen throughout the body. Although considered a disorder, the sickle cell trait became advantageous to people living in places with high incidence of malaria such as Africa. The precise mechanism of sickle cell’s resistance to malaria hasn’t been fully understood yet but several factors had been cited by researchers to explain its defensive properties against the disease.

RBC in people with sickle cell trait do not sickle considerably at normal oxygen levels. But if infected with Plasmodium palcifarum, the RBCs sickle significantly as the parasite reduces the oxygen tension during its metabolism. The deformation of these sickled cells make them abnormal and thus targeted by phagocytes found in the blood stream for destruction. In vitro experiments also found out that infected RBCs sickle more readily compared to uninfected cells. And as these abnormal cells are removed from the blood, the burden of carrying P. falcifarum is reduced thereby increasing the survival of patients against malarial infections. Further studies also show that infected sickle cells incubated at low levels of oxygen tension could lead to slower growth rate, proliferation and even death of P. falcifarum as a result of extensive vacuole formation and oxygen radical formation such as  superoxide anion and hydgrogen peroxide.

Deer Mouse’s Coat

Deer mice (Peromyscus maniculatus) got their name from their two-toned or bicolored coats that resemble that of a white-tailed deer. The dorsal coat is usually dark which varies from reddish brown to grayish to allow them to blend to their habitat and evade predatory hawks and owls. In contrast, most of the deer mice found in Sand Hills, Nebraska posses a pale coat. A recent study conducted by researchers at Harvard University and the University of California investigated the difference in coat coloration of deer mice found in Sand Hills and those that thrive in dark-soiled areas. The scientists found out that the light colored-coat is coded by a gene  called Agouti where it is expressed at a higher rate in deer mice living in Sand Hills. Furthermore, the researchers discovered that the Agouti gene appeared in the mouse’s genome just a few years after the then dark colored mice settled in the area. Because of this, the researchers deducted that the light-colored coat coding gene wasn’t in their genome  before their colonization. The trait developed from a new gene which waited for a mutation to happen and then natural selection took action and increased the newly evolved gene’s frequency in the population.[ad#afterpost]

References

• http://evolution.berkeley.edu/evolibrary/article
• http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html
• http://www.biology-online.org/2/10_natural_selection.htm
• http://curiosity.discovery.com/topic/ecology-and-evolution/10-examples-natural-selection.htm
• http://www.blackwellpublishing.com/ridley/tutorials/Natural_selection_and_variation5.asp
• http://www.news.harvard.edu/gazette/2006/08.24/31-finches.html
• http://www.env.gov.yk.ca/wildlifebiodiversity/mammals/deermouse.php
• http://www.sdnhm.org/fieldguide/mammals/pero-man.html
• http://news.bbc.co.uk/earth/hi/earth_news/newsid_8225000/8225219.stm
• http://www.news.harvard.edu/gazette/2006/08.24/31-finches.html
• http://sickle.bwh.harvard.edu/malaria_sickle.html