“Evolution is just a theory, there’s no proof!” argued my friend Jason, “No one has seen a new species emerging from another species”. I felt sad and angry at his naivety, as he was wrong on two different levels. First of all, the definition of the phrase ‘scientific theory’ states “A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment”. Therefore it is meaningless to state that something is ‘just a theory’. Secondly, he is mistaken in his assertion that there is no proof for evolution. The natural world is peppered with evidence for evolution – linked lineages, fossil records, molecular evidence, and direct observation of speciation! Yes, we have observed a new species evolving from an original species!!
Science had been constrained from viewing evolution in action, due to the large number of generations that it takes for a species change. It roughly takes around 100,000 generations for speciation to occur. Given that humans create a new generation every 20 years or so, it would take 20 years X 100,000 = 2 million years for human evolution, which is coincidently the time it took our ape ancestors to evolve into Homo sapiens.
However, bacteria reproduce fast, spawning out new generations every half hour. A rough calculation, 30 minutes X 100,000 = 5.7 years for new speciation. Therefore, they are the perfect playground for viewing evolution in action. Countless species of new bacteria have been observed in the lab and in nature that evolved from older species, often with much faster timescales than my rough calculation above. The most well-known are the anti-biotic resistant strains of pathogens, which evolved from more benign strains after widespread use of anti-biotics.
A few examples. In hospital environments, MRSA (Methicillin-resistant Staphylococcus Aureus) naturally evolved from Staphylococcus Aureus by gene transfer and natural selection. In 2008, Zachary et all of the National Academy of Sciences, published results of observing E. coli evolving radically, by adapting themselves to metabolise citrates. This is a profound change, as E. coli by definition are a class of organisms that can’t metabolise citrates! In 2016, biologists at the University of California, San Diego, and at Michigan State University observed another speciation event in their lab when a lambda phage virus that was cultured in hosts, evolved into two new species and the original species went extinct!
In larger organisms, obviously we have to wait longer to observe formation of new species. In the early 1900s, three species of wildflowers called salsify were introduced to the United States from Europe. By 1950s, botanists observed that these had evolved into two additional entirely new species of flowers, which were perfectly capable of reproducing with their own kind but not with any of the original three species – the classic definition of a new species. The maggotfly which is a native of the Native America solely fed on hawthorns. With the introduction of apple trees by settlers in the 1800s, the fly forked a new species, one that could feed on apples alone. The new species was substantially different from the original, in several other aspects as well – wing beat frequency, maturing time, and lifespan. And now for the most interesting of all – the case of the London Underground Mosquito. Discovered in the London Underground, it is thought to have evolved from overground mosquitos, but adapted to the warmer, darker conditions of the underground.
In still larger organisms, although we have not directly observed full speciation yet, we can clearly see micro evolution at work – gradual changes in traits. The moth Biston betularia was mainly light coloured prior to the industrial revolution, with just 2% of the population being dark coloured. After the industrial revolution, pollution darkened many light coloured surfaces, making it difficult for the light coloured moths to hide from predators. The number of dark coloured moths increased from 2% to 95% – a massive indication of natural selection. In 1971, ten Italian wall lizards were introduced to the island of Pod Mrčaru. A few decades later, the lizard population had evolved from an insectivore diet to a vegetarian diet. This change drove major structural changes – larger head, more bite strength, and new valves in intestines for digesting vegetation – all major evolutionary changes. In the isle of Dapne of the Galapagos, the medium ground finch had been established for several centuries, and it had large beaks suited for cracking large nuts. In 1982, when large finches were brought to the island, they prevented the medium Finches from eating large nuts. In a couple of decades, the medium Finches evolved smaller beaks to crack smaller nuts, which the large Finches did not want!
Apart from all the above directly observed evidence for evolution, there exists a plethora of indirect evidence. The huge amount of fossil record clearly shows one species evolving into another one, or splitting into multiple species. Most fossil records show a gradual progression from one species to another. Many living organisms have similar physiological features that indicates a common ancestry. Also, genetic analysis of currently living species unambiguously shows that all living beings are related, and can be traced backward to common ancestors. Against this backdrop of overwhelming evidence, there is absolutely no doubt of the veracity of the “Theory of Evolution”, as much as the Theory of Gravitation, Pythagoras Theorem, or the Spherical Earth Theory.