There must very few people who do not know who Charles Robert Darwin is! He is one of the famous Scientists the world has ever known. But what exactly did Charles Darwin do that made him this famous?

Anyone can answer that and the answer will be “Evolution”, he is known among everyone for discovering the phenomenon of the process of Evolution. Charles Darwin was born on 12 February 1809 at The Mount, Shrewsbury, Shropshire, United Kingdom.

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During his trip to the Galapagos Island he saw a vast array of diversities in the different islands of Galapagos. He observed that various species in those islands looked similar to each other but had different other physical characteristics which led him to believe that they might have descended from a common ancestor but with modification which gave rise to his famous understanding and explanation of evolution “The descent with modification”. The descent with modification explains how an organism gets modified both physically and biologically or a long period of time, in which the genetic  variation plays a very important role.

It is an observable fact that an offspring slightly varies from its parents, not even twins are 100% similar there’s always a difference between the offspring of the same parents. That results due to the genetic variation and over hundreds of Generation the variation turns out to be so large that a differently phylogeny is formed. We’ve known and seen for sure, how descent with modification works in Nature, and how it can relate to the early form, the most common example being the study of fossils which reveals the link between various different organisms. Even molecular studies suggest the common link between those animals. Not only that comparative Ontogeny shows that the embryo of species which seem not related at all to the naked eyes seem to have a definite similar origin.

But what makes these changes? And what kind of changes stay and what kinds are lots??

The variation that results in offspring’s are mostly due to the Genetic variation. The most important factor that causes the modification is Genes, the genetic variation are results of mutations. The mutations can result in positive way as well negative. Those mutation which results in a way which favors the animals survival are taken as positive mutation and those mutation which hamper the survival chances of a species are negative mutation. The favorability of the mutation depends up on the environment and other factors for example lets imagine that there a lizard which feeds up on small insects, let’s say beetles. The beetles are green in color, by some mutation the new generation has few brown beetles, then as the beetles feed up on the green leaves, the brown beetles become susceptible to danger while the green ones are camouflaged against the green color of the leaves, hence increasing their chances of reproduction, thereby surviving and reproducing. Darwin termed those animals fit and said that only those species which are fit, and strong reproduce and those who aren’t don’t.  By not reproducing the non-fit animals do not get a chance to pass down their faulty genes to another generation, faulty in a sense that they won’t survive because they are not fit for that particular environment. Let’s assume the same scenario but in case of ground beetles and the beetles which live in dry leaves, dried brown leaves provide perfect camouflage for those brown beetles and in this case the green once turns out to be faulty. Through the process of Selection bestowed upon  by the nature, depending upon the  environment the animals are selected naturally to fit that environment. Those who fit live on and pass down their genes and those who don’t die.

Those animals which are prey species and can’t find a proper camouflage technique are highly susceptible for predation. During the days of Darwin, the Theory of Evolution by Natural Selection was discarded. But later on it was realized that Darwin was right. In the time of Industrial Revolution, due to the excessive smoke the lichens growing on the trees were killed and the darkened bark was exposed.  The pale (white) moths that were living in the bark which were perfectly camouflaged before became easy prey for the birds against the dark bark of the trees. But few of those mutated rare black moths who were susceptible to predation now had a better chance of survival, and they started to Reproduce rapidly. Over a short period of time the bark of the trees were filled with black moths, because the white moths were easily seen against the dark bark and easily feed upon by the birds, they became unfit for that environment and their survival was affected, but the dark once were fit and they reproduced and their population bloomed. The black species rose to almost 95 % of population at that time in England. This is a classic example of Industrial Melanism, in normal sense the effect in coloration of Species which reside near the Industrial areas. By the end of the century as the air got cleaner and cleaner and the lichens started to grow back normally on the trees, the barks started to get lighter and lighter and the white moths started to replace the black moth, since the white moths were “Fit” because they could come up with a better camouflage than the black ones  which were now sitting upon the white background and easily seen by the birds. At present as the air is clear the white moths dominate the population in England. This phenomenon is the best example Natural Selection in work.

Photo : Peppered Moths
Photo : Peppered Moths

This was realized later, but Darwin had seen this phenomenon in action some hundred years before that, he realized that in the Galapagos Island. The Galapagos island comprises of some 18 Islands, where Darwin saw different varieties of birds, finches to be exact, which looked similar but varied in shape of the beak. It is believed that the birds colonized the small islands from mainland South America and later over the period of time diverged into various species. Due to the distance between the different islands the different birds couldn’t interbreed and hence due to geographical isolation resulting in the distinctness of the species in separate islands. The different islands had different types of habitat and food sources, the birds hence got adapted to that habitat and developed a mechanism to use the resources of those islands. All the different species of birds had different types of beak suited for that specific habitat, long beaks for those birds which feed on insects from the bark, big strong beak for those who eats seeds and nuts which will easily break the nuts, sharp and large for those which feed upon the cactus as shown in the figure.

Adaptive Radiation in Finches
Adaptive Radiation in Finches

Not only birds but Darwin saw different species of Tortoises which had a common ancestor but were modified over a period of time as a result of variation due to geographical isolation. There are 11 different types of Tortoises in the Galapagos, which have different morphology suited to the particular habitat. Those tortoises which live in the island with large tall grasses have extremely long neck which helps them to feed properly, a tortoise with a small neck would probably die in that habitat because it would not be able to feed it would not be “fit” hence not be able to reproduce and die off, resulting in extinction of that species, that particular species would be week in that scenario. But the tortoise with the long neck fit perfectly and survives and passes down its good genes, and the faulty genes die off. In this way Natural selection is a process in which a heritable trait either becomes common or less common depending upon the habitat of a particular species, it’s a process where random evolutionary changes are favored or discarded by the natural forces, which aids in the evolution of a Species.

Long Neck Tortoise
Long Neck Tortoise

Evolution is gradual process and the mechanism behind Evolution are, Natural Selection, Genetic Drift, Mutation, but overall they are all connected. Genetic drift is acted upon by Natural selection, the result of the drift are monitored under selection and same case with mutation. Those with positive mutation are selected and live on those with negative are left behind.

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