As a creationists, Charles Darwin had believed that everything on Earth was the creation of a divine being. But upon his voyage to the Galapagos Islands, he made an unanticipated discovery that changed the course of science. Darwin, in his book "Origin of the Species", proposed a rather controversial idea about how the organisms on Earth came to be as they are. He suggested that, in principle, all life forms on Earth could be traced back to a common ancestor. We diverged from this common ancestor because of the slight variations that each offspring has from their parents. He proposed that no two organisms were alike, and that this slight variation contributed to the process of natural selection.
The concept of natural selection that Darwin proposed contributed to evolution is difficult to observe because it often takes years for the variation in an organism to become truly distinct. But luckily, natural selection can be observed "at a snail's pace" in the little European snail, Cepaea nemoralis.
The Cepaea nemoralis comes in three different colors, and can have up to five horizontal dark bands going across them. The pattern and color of a shell is dependent on the genetic information that the snails have, and throughout their life span these colors reamin unchanged and unaffected by any environmental factors. In the 1950s and 1960s, a large-scale breeding programme in this snail studied the genetic components that were involved in coding the characteristics of the snail's shells (Patel). Similar to Gregor Mendel's experiment, the phenotypic ratio of these snails were the same as the offsprings in Mendel's pea plants. Gene C, with three alternative alleles contributed as the determining factor for these snail's shell color and band. The phenotypic characteristics varied from one snail to the next, and distribution of the different colored snails across Europe can be studied to observe the process of natural selection occurring over the course of just a few decades.
When Darwin coined the term natural selection, he proposed that organisms that were best suited to their environment would survive to reproduce and produce more offsprings, thus passing on the distinctive trait that helped them survive. The different colored shells in the European snails have different thermal properties. The darker shells can absorb solar radiation more efficiently than yellow shells (Patel). This distinctive characteristics makes the darker snails more suited to a cooler climate, and unsuited to warmer conditions. The yellow shelled snails on the other hand can withstand sunshine without much trouble. Biologists have studied these snails and a significant correlation exists between the color of the their shells and their distribution pattern across Europe. The population of snails with darker shells are concentrated around areas around northern Europe, because they do better in cooler climate. And the population of snails with yellow shells were found to be most populated in areas around southern Europe, where it's hot and sunny. The maladaptive shell colors are selected against. The darker shelled snails do not survive as well in hot climate, and thus do not reproduce as much as the light colored snails do. Without offsprings they cannot pass on their alleles, and their traits are not passed down to the next generation. The selective process that these snails go through demonstrates to us the process of evolution in a nutshell. The distinctive adaptive skills that these snails have, over time have caused evolution to occur. The better adapted individuals survive and pass on their traits to the next generations.
Natural selection sometimes involves many mutations leading to significant changes. In New South Wales, Australia, the yellow bellied three toed skink is able to use reproductive methods that allows it to give live birth to their young or lay eggs. Scientists have discovered that skinks that live near coastal areas, where it's warm, tend to lay eggs as a reproductive method. But move higher up into the colder mountains, the skinks that live there are almost all giving birth to live young. Since the skinks can either lay egg or give live birth it gives scientists a chance to study the conditions necessary for live birth. The deciding factor depends on how the young gets their nourishment before birth. In egg laying species the embryo gets its nutrients from the yolk. In mammalian species, a highly specialized placenta connects the fetus to its mother, allowing the baby to take up oxygen and nutrients from the mother's blood and pass back waste. In reptiles, eggs are formed, and as development of the embryo progresses the shell begins to thin out, until the babies are born covered with only a thin layer of membrane. Both these birthing styles come with an evolutionary trade off, eggs are more vulnerable to the predators and harsh weather, but babies are more physically demanding for the mother. For the skinks, mothers in warmer climate tend to lay eggs onto the ground before the final week of development. Moms that live on the mountains feel that it is necessary to protect their young by keeping them longer inside their bodies. The variation in reproductive style, allows each skink to evolve to use the method which is best suits them. This increases the chances that their young will go on to reproduce. Scientists have predicted that these two populations will at some point, separate into two different species, as each population becomes fixed with its reproductive strategy.
To survive, a species must find a way to adapt to its physical surroundings. An organism that is able to find the best way to adjust to its environment will be the one that is likely to reproduce and pass on the characteristic traits that they have to their offsprings. Genetic variation from more organism to the next, allows for this to occur. Over time, as the trait becomes more distinct, speciation may occur and a species will diverge into new, and more fit organisms.
"8 Examples of Evolution in Action." Listverse. N.p., n.d. Web. 07 Oct. 2012. <http://listverse.com/2011/11/19/8-examples-of-evolution-in-action/>
Handwerk, Brian. "Evolution in Action: Lizard Moving From Eggs to Live Birth."National Geographic. National Geographic News, 1 Sept. 2010. Web. 7 Oct. 2012. <http://news.nationalgeographic.com/news/2010/09/100901>.
Patel, Simit. "Evolution at a Snail's Pace." Biological Sciences (2010): 26-29. Biological Sciences Review. Web. 04 Oct. 2012. <http://www.rougemontschool.co.uk/.../938-evolution-at-a-snails-pace>.
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