Biological evolution is the process of living organisms changing from time to time through the process of natural selection. You may ask, “What is natural selection?” Well,before defining that, let us focus on the timeline of evolutionary thought.Even though many theories on the presence of evolution in living organisms have been well present throughout scientific history, ranging from the ideas of Aristotle to Nasir al-Dinal-Tusi to Lamarck and so on, it was Charles Darwin who explained how the evolutionary process took place.To make you understand the concept of natural selection through which the legendary Darwin explained the mechanism behind evolution, let me give you an example of a peppered moth.
Organisms that are well-suited for a specific environment adapt to it in a better way than other species or members of the same species and outlive them in evolutionary timeline.
Majority of the species of peppered moths (insects) are white in color, and hence when these moths rest on tree trunks that are covered with lichens(organisms arising from a symbiotic relationship between algae and fungi), they are very well camouﬂaged because the presence of lichens lightens the color of tree trunks and hence matches with the whitish color of the moths resting on them. This helped them hide from the predators, and in this example, helped them proliferate their population. On the other hand,genetic mutations in white moths resulted in some of them being born with blackish wings; and hence their melanic (black) forms could not survive because they were easily visible to predators. But here is the mind-boggling twist: after the industrial revolution, many of the tree trunks in towns close to the sources of air pollution darkened/blackened and hence the moths dark in color (due to mutational changes) could now hide from their predators but the non-melanic (white)moths could not. This is what natural selection is. Organisms that are well-suited for a specific environment adapt to it in a better way than other species or members of the same species and outlive them in evolutionary timeline.
There is an equally great example of natural selection which will also introduce us to Jean-Baptiste Lamarck who presented the ‘theory of acquired characteristics’ before Charles Darwin’s theory of natural selection. The theory says that an organism which may go through morphological or physiological changes in its lifetime will pass on the acquired traits to its oﬀspring. I should mention that this theory was disproved later by Charles Darwin and turned out to be wrong. To explain this, let me give you the famous ‘giraﬀe’ example.
Lamarck explained the immense length of a giraﬀe’s neck by focusing on their need for survival. He said that throughout their lifetime, short-necked giraﬀes which were present at first tried to stretch their necks in order to reach the tall branches of the trees to obtain food and throughout a long period of time, their neck evolved and evolved to reach the stage best adapted for a typical giraﬀe to gain its nutrition. This was disproved very intelligently later.
Through his theory of natural selection, Darwin explained this by saying that at first, there were some genetic variations (now known as mutations)that resulted in the long structure of a giraﬀe’s neck. In areas where there were tall trees, the long necked giraﬀes survived well because they could easily obtain their food from the higher trees, but the short-necked giraﬀes could not survive well. This resulted in the dominance of the long necked giraﬀes and in the extinction of the short-necked giraﬀes. This is exactly how natural selection works in order to select the best of the best that adapt to the environment in the most efficient, proper and reactive way possible to survive.
Another great example is that which came into being when the construction of London Underground (public transport system) took place. It came into attention when people, during the war, took shelter underground for safety, but the only reason behind why this place would catch the attention of scientists was diﬀerent altogether. Culex, a rather abundant species of mosquitoes, was reported by workers underground and, of course, scientific inquiry followed that lead to diﬀerentiating between the Culex mosquitoes living on land and the ones living underground in the transit system. The amazing twist that was discovered here was that the land-dwellers only bit birds, whereas the underground mosquitoes bit humans (as it was a pathway for humans only). Not only this, but the Culex mosquitoes which dwelled on land hibernated in winter whereas the underground mosquitoes could not hibernate mainly because there are no definite or proper seasons underground. Since the mosquitoes were trapped underground with a partition in between and could not interact with the surface members of their genus, land-dwelling Culex had great reproductive advantage but the ‘London Underground mosquito’ did not. This led to what is called ‘divergent evolution’ which requires isolation of the members of a species geographically. This means that it was time for a subspecies to form after the whole process (which was rather very quick in evolutionary terms) from one single species of Culex mosquitoes because of adapting to dissimilar environments and the diﬀerence in mating mechanism which resulted in unalike oﬀspring from the now-two species. This is a process known as ‘speciation’ in evolutionary terms. Although that is not the term that has been used for the evolution of underground Culex mosquitoes very often because according to some scientists, it lacks specificity and has explanations other than speciation or quick evolution; yet according to a large majority, it represents solid points for the theory.
This is exactly how natural selection works in order to select the best of the best that adapt to the environment in the most efficient, proper and reactive way possible to survive.
EVIDENCE FOR EVOLUTION
We, along with every other organism, have come into existence through a process of universal common descent, a process in which every organism is related to one another, and has descended from a single common ancestor. A question must be asked: what is the evidence behind this? What is the evidence behind every organism being related up to LUCA (Last Universal Common Ancestor), and mainly, what is the evidence behind evolution?
Perhaps the greatest aid to the evidence of evolution has been the fossil record of numerous living organisms throughout time. One of the most definite sights of fossils are seen in fossil layers which are made up of sediments of diﬀerent substances such as rock, deceased organisms, numerous inorganic compounds, and so on. Throughout time, sediments are deposited, layer by layer, on top of each other. The deepest layer of sedimentary rock contains the oldest fossils, whereas the layers at the surface contain the youngest fossils. Even Charles Darwin was astonished at how the remains of complex organisms were found in the recently formed layers whereas the fossils kept getting simpler in the set of old layers. This pointed towards the fact that animals have gradually changed throughout time, eventually becoming highly complex. One example can be given of the fossil samples of a once-living organism which displayed the intermediate evolution between early birds and dinosaurs. This organism is known as ‘Archaeopteryx'(oldest known bird). Its fossils date back to a time period of 150 million years ago, also known as the ‘Late Jurassic’ period. Such interminable evident samples have justified the law of evolutionary change as it has occurred.
Between organisms of diﬀerent species, one can see some remarkable resemblance that displays similarity between the basic structures of those living organisms. The diﬀerent functions of these similar organs in diﬀerent living bodies diﬀerentiate in between them, and hence these organs are called to be homologous. Examples can be given of the upper limb in human beings, ﬂipper of a whale, wing of a bat, etc. The upper limb helps human beings in grasping objects and coordination, the ﬂipper of a whale helps it to swim and the wing of a bat helps it in ﬂight. We can see the similarity between the structure of the bones and their number too but the only diﬀerence that we see is the difference of function.
It signifies the presence of common ancestry in between them
This perfect example of homology tells us how such organisms share a common ancestor in evolutionary timeline, and have successfully adapted to dissimilar environments through natural selection. The presence of analogous structures is quite remarkable too, in fact. The example of a bird and a dragonﬂy can be given to describe analogy between organisms. Here, both the bird and the dragonﬂy use their wings for ﬂying, which means that the function of the wings is absolutely similar but where the bird possesses bones in the wing and feathers on it, the dragonﬂy lacks them. This indicates that there is no recent or close common ancestry in between the species of dragonﬂies and that of Aves (birds). Hence, they have evolved through separate pathways.
Upon observation of embryos (early fetuses) related to diﬀerent species, even a common person can see how the shared ancestry shows its beauty in the beginning of the growth of such embryos. When the zygote of a human, fish, monkey, bird, or even a tortoise enters its embryonic stage, one can see the basic and fundamental similarity between their early forms. Let us take the example of gill slits in the embryo of mammals. Almost all of the mammals possess them during their embryonic stage, but lose them later as they grow. What does this denote? It signifies the presence of common ancestry in between them- in this case, fish-ancestry. Another example can be given of how young chicks have early teeth at first but they lose them as they grow which points towards their reptilian ancestry.
This has been the greatest technique, to date, that has helped geologists in determining the age of fossils to determining the age of the Earth, meteorites and whatnot. Fossils are usually present in the layers of sedimentary rocks, and these rocks contain radioactive elements which keep emitting radiation in order to change into stable elements because radioactive elements themselves are unstable. Such radioactive isotopes emit radiation at a constant rate of what is known as their ‘half-life’, which is diﬀerent for every other element. Half-life is basically the period in which an unstable element, such as uranium, emits radiation until the present value of its radioactive quantity reaches half of its original value. Since such elements are found not just within sedimentary rocks, but in the fossils of once-living organisms as isotopes too, radiometric dating can be used to date these samples in order to determine the age of present fossils. This proved Darwin’s notion that evolution has occurred over an immensely long period of time and has also proved how. Throughout time, simpler organisms evolved into complex organisms with intermediates in between them.
This relates to those organs which used to be functioning and were of immense use in the ancestor of a specific organism, but ceased to be of any use in the present possessor. From humans having the appendix, the coccyx (tailbone), ear muscles, etc. to kiwis and ostriches having ﬂightless wings of no use whatsoever, to whales and snakes having pelvic bones even though none of them have hind limbs- there are numerous examples of vestigial organs. Let us focus on ‘coccyx’ as an example. Just at the end of your vertebral column is the utterly useless tailbone that you possess. In many other mammals, external tails arise from exactly this location, but not in most of the primates like us, or chimpanzees, orangutans, apes and so on. This indicates that our mammalian ancestors could use this organ immensely well. Throughout evolution we evolved and developed a reduced tail (which is now an internal tailbone) when a new branch in the evolutionary tree occurred.
Ever since the Chimpanzee Genome Project took place, geneticists have now known for a fact that chimpanzees and Homo sapiens (human beings) share almost 99% of their genetic code of life which is called deoxyribonucleic acid (DNA). Most of the primates (including us) and even bonobos share an almost identical array of genetic material. Processes like DNA sequencing have helped us see how closely related living organisms are to each other and how beautifully theys hare a common ancestor that branches upwards to the tree of life, reaching a unifying point where the last universal common ancestor, or LUCA,existed.
This proved Darwin’s notion that evolution has occurred over an immensely long period of time and has also proved how.
Evolution has been discussed, argued and debated over countless times. Perhaps, no other theory might have faced such extreme criticism from unreasonable people and such immense appreciation from the scientific community. Nevertheless, it has been decades since evolution has proved to be a concrete fact, acting as the base of modern biology. Just like our bodies have evolved to diﬀerent environments through natural selection, may our minds evolve to retain the intellectual and reasonable capabilities it has previously been endowed with.