Evolution and Natural Selection
In biology, the theory of evolution doesn’t exactly tell us how life began on Earth, but tells us how life, once it came into existence, diversified into the many incredible forms we see now and in the fossil record. It also helps us make sense of the way in which modern creatures continue to adapt and change today. In biology, evolution can be defined as: ‘Any change in the heritable traits (those are physical traits like fur colour in mice, spots on the wings of butterflies, or instinctive behaviours like the way in which dogs greet their friends with a sniff) within a population across generations’.
This definition can be a bit confusing, so let’s see how it works. All organisms, from single celled amoebas to flowers to dolphins, are capable of reproduction. We have children, we make copies of ourselves. We do this by duplicating DNA and passing that DNA onto future generations. DNA is a chain-like chemical stored inside each one of your cells which tells them how to grow and function. Your DNA contains coded information on how to build you.
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Your DNA is different to that of say, a daffodil, which is why you look and act differently from one, and is also slightly different to that of any other fellow human beings, which is why no person acts or looks exactly the same as one another. Single celled amoebas and other simple creatures reproduce by copying their DNA inside their guts, moving both copies to either side of their body, splitting in two right down the middle, and then growing back to full size. If all goes well, the two new amoebas will be exact copies of each other. But in nature things aren’t always perfect.
When DNA is being copied, errors can occur which modify the DNA code. This is what we call a DNA mutation. These mutations, which happen completely on accident, and randomly to any part of a DNA strand, can produce variation in the body shape and function of the creature of inherits the modified DNA. In this case, our new friend might have an arm that stretches extra-long. If he survives long enough to grow and reproduce, that extra stretchy arm, which is now coded in his DNA, will be passed on to his children. Evolution: any change in the heritable traits within a population across generations, has officially occurred.
Reproduction for dolphins and badgers and people is a little more complicated. We have to find ourselves a partner. When two badgers get together and fall in love, a sperm cell from the father which contains a copy of half of his DNA, only half, combines with the egg cell of the mother, which contains half a copy of her DNA. The result is a brand new cell with all the information to divide and grow up into a brand new badger. The new child matures to be similar to our parents, but also unique, because she developed some traits from her mother’s DNA, and some from her father.
Her new combination of traits can be passed on to her children, and again. Evolution: any change in the heritable traits within a population across generations has officially occurred. Besides the recombination of her parents’ traits, she might also have developed some completely new traits of her own due to DNA mutations. For example, extra hairy ears for example. If she survives long enough to have kids, her DNA will combine with the DNA of her partner, and she’ll pass on her extra hairy ears to at least some of her children. Again, evolution has officially occurred.
So there you have it, evolution is really pretty simple. Scientists and normal folks everywhere, witness evolution happening all the time. Small changes like the ones we have seen here can add up over generations to create dramatic changes. If you were to go back in time just a few thousand years, you’d find that all dogs for example, originally evolved form an ancestral group of grey wolves. The evolution of those wolves, from generation to generation was guided by humans. People were selecting wolves with the traits they like, letting them breed, and then only keeping the puppies with the most desirable traits.
As time went on, different breeders preferred dogs with different features. Some selected from large size, some for small, some for brains, others for brawn. Today, wolves have branched out into hundreds of breeds, very few of which look and behave much like their ancestors. A massive amount of observable evidence from many different fields such as genetics, chemistry, palaeontology and mathematics overwhelmingly suggest just like all dogs share a common ancestor, all living things; me, you, puffer fish, banana trees; if you go back far enough, also share a common ancestor.
We are literally related. We don’t know what the life form was, or exactly how it came to be, but the simple process of reproduction within variation over billions of years looks to be responsible for all the diversity of life we see today. Now, you might be saying ‘wait a minute’. Isn’t evolution random? To do something functional like turn a wolf into an adorable mini poodle, random evolution had to be carefully guided by an intelligent dog breeder. Researchers claim that all mammals evolved from an ancient shrew-like creature.
But the difference between a shrew and an elephant is far greater than that of a wolf and a poodle. Who guided that process? Who was the breeder? In the mid eighteen hundreds, two men, Charles Darwin and Alfred Russell Wallace independently discovered that a breeder is not necessarily needed. There is another force capable of guiding random evolution to produce order and complex function. They called it natural selection. What is Natural Selection? Natural Selection is one of several key concepts contained within the theory of evolution.
To understand what exactly natural selection is, and why it’s so important, let’s first take a close look at two other evolutionary concept: Descent with Modification, and the overarching idea of Common Descent. Descent with modification is the observable fact that when parents have children, those children often look and behave slightly different than their parents, and slightly different than each other. They descend from their parents with modifications. The differences found in offspring are partially due to random genetic mutations. Common Descent is the idea that all living things on Earth are related.
They descended from a common ancestor. Through the gradual process of descent with modification over many, many generations, a single original species is thought to have given rise to all the life we see today. The common descent of all life on earth is not a directly observable fact. We have no way in going back in time to watch it. Instead, common descent is a conclusion based on a massive collection of facts. Facts found independently in the areas of fossils, genetics, comparative anatomy, mathematics, biochemistry and species distribution.
Because the evidence for common descent is so overwhelming, the concept has been around since ancient times. In the past however, it was rejected by many philosophers and scientists for one main reason: you cannot get order and complexity from random chaos alone. The bodies and behaviours of living things are extremely complex and orderly. Descent with modification simply produces random variation. All through history, no one could explain how complex life arose from simple life through random variation, until Charles Darwin discovered natural selection. Charles Darwin, who lived from 1909-1882, was a naturalist.
At the start of his career, he travelled the world on a ship collecting and documenting plants and animals. During his travels, Darwin became very interested in the idea of common descent. He noticed that islands contain species of plants and animals unique to those islands, they can’t be found anyplace else on Earth, but they often look and behave surprisingly similar to creatures found on nearby continents. Tortoises on the Galapagos Islands can be distinguished to those of Africa, meanwhile with the exception of size; they’re almost identical to a species found nearby in South America.
Darwin believed these similarities could be best explained through Common Descent. Long ago, a tortoise from the mainland may have drifted to the islands, possibly on a raft of storm debris, and once arriving, lay her eggs. Random changes caused by descent with modification over thousands of years eventually transformed the island creatures and the mainland creatures so much, that they could no longer be considered the same species. This idea made good sense to Darwin, except for one thing: the island creatures he found weren’t just randomly different; they were specially adapted for island life.
The Galapagos is a collection of eighteen main islands, many of which are home to tortoises. The larger islands have lots of grass and vegetation. Tortoises there grow extra heavy, and have dome-like shells. Some islands have very little grass, forcing the tortoises to feed on cactus plants. The best cactus pads grow on the tops of these plants. Fortunately, tortoises on these islands are equipped with expanded front legs and saddle-like shells, allowing them to stretch their necks extra-long to reach their food.
It’s almost as if these island creatures had been perfectly sculpted to survive within their unique environments. How did this sculpting take place? Random Descent with modification alone could never do such a thing. Darwin drew upon his knowledge of selective breeding to answer this question. For thousands of years, farmers had been taking wild plants and animals, and through the process of selective breeding have sculpted them into new domestic forms much better suited for human use and consumption. The process is slow but simple.
If a single plant produces a hundred seeds, most will grow to be nearly identical to the parent plant. A few however, will be slightly different. Some variations will be undesirable; smaller size, bitter taste, and vulnerability to disease and so on; other variations will be highly valued. Thicker, sweeter leaves, for example. If a farmer only allows the best plants to reproduce and create seeds for the next crop, small positive changes will add up over multiple generations, eventually producing a dramatically superior vegetable.
You might be surprised to hear that broccoli, cabbage, cauliflower, kale and Brussels sprouts are all just different breeds of a single type of weed commonly found along the shores of the English Channel. The evolution of this original plant into all the varieties we see today was carefully guided by different farmers around the world who simply selected for different traits. It’s important to note that the farmer doesn’t actually create anything. Random Descent with Modification creates new traits.
The farmer simply chooses which of these new creations are allowed to reproduce, and which are not. Darwin proposed that nature itself is also capable of selection. It may not have an intelligent brain like a farmer, but nature is an extremely dangerous place in which to live. There are germs which can kill you, animals that can eat you. You could die of heat exhaustion; you could die of exposure to the cold. When parents produce a variety of offspring, nature, simply by being difficult to survive in, decides which traits live on for reproduction, and which do not.
Over multiple generations, creatures become more and fit for survival and reproduction within their specific environments. Darwin called this process Natural Selection. Since Darwin first put forth his idea in the mid-1800s, natural selection has been studied and witnessed numerous times in nature and in the science lab. What started out as a mere idea is now officially an observable fact. Darwin’s discovery has greatly expanded our understanding of the natural world, it’s lead to countless new breakthroughs, and finally allowed scientists to seriously consider the idea of Common Descent.