Natural selection is a critical mechanism by which organisms evolve, alongside mutation, migration, and genetic drift.
So what is natural selection? You may be familiar with the phrase 'survival of the fittest', which has been used time and time again, sometimes even by nefarious agents falsely seeking to justify their bigotry and racism with genuine scientific concepts.
Natural selection refers to the process by which organisms that are the most adapted to an environment are more likely to survive and reproduce, while those that are less adapted tend to die out. Thus, the fittest survive to pass their genes and over the course of generations, the adapted offspring can replace entire populations and, given enough time, even form new species.
Without natural selection, it is virtually impossible to understand or explain how living things have come to exhibit their astonishing diversity and complexity.
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How does natural selection work?
You'll hear survival and reproduction mentioned frequently when discussing natural selection. That's because these factors reflect whether or not an organism or an entire population is adapted to its environment.
By looking at the rates of reproduction and mortality, one can infer how a population will change over time. The mechanism can be broken down into five basic steps: variation, inheritance, selection, time, and adaptation, abbreviated as VISTA.
Look around. You'll see people come in all shapes and sizes. Some are taller while others are short, some are Black others are White, and so on. Although we're all part of the same species, there's a great deal of variation from individual to individual. In some species, such as dogs, the variation can be even greater -- imagine the difference between a chihuahua and a Great Dane, although they're still the same species. The variations are due to mutations, which are permanent changes in the nucleotide sequence of DNA following "copying errors" when cells divide.
The traits that determine the variance in a population are encoded in the DNA and inherited from parents. When organisms reproduce, they pass on their DNA to offspring, which is why tall parents tend to have tall children.
Not everyone survives or reproduces for a reason. No environment can support unlimited populations. So, past a threshold, individuals within a population compete for food and mates, all while avoiding predators. Due to fortunate mutations, some variation allows certain individuals to be better at procuring food, securing a mate, or avoiding predators than others.
For instance, a mutation may cause an individual's skin to be a different color such that it now blends with the environment, making it easier to escape predators. This individual would be selected and eventually have better odds of passing on their genes to the next generation.
Advantageous traits take time to become prevalent across a population, requiring multiple generations before the genes of the selected individuals dominate the gene pool. These inherited traits are not random, but rather tend to be those that are slightly better suited to a particular environment. After thousands of generations, all individuals may share certain traits that help individuals survive and reproduce. Think of the zebra, for instance, where all individuals have the same black and white striped coat.
The result is a population that is better suited or adapted, to certain aspects of the environment. But since the environment is ever-changing, so are populations. Under the constant grind of natural selection, some populations go extinct, while others change and diverge eventually branching into new species.
Natural selection is so powerful that scientists believe that virtually all extinct or extant species can trace their heritage to a single, original organism that made its appearance roughly four billion years ago -- the first lifeforms on Earth.
The boldest theory in science
Although natural selection is quite straightforward to grasp compared to some other theories in science, the way it was explained by Charles Darwin is the stuff of genius.
The paradigm shift that the introduction of natural selection caused is perhaps only equaled by Nicolaus Copernicus' heliocentric model, which proposed that the Sun is the center of the solar system and the planets orbit it rather than Earth.
Similar to Copernicus who showed Earth isn't the center of the universe, Darwin showed that humans aren't above nature, but rather sit firmly within the organismic order of nature -- neither above nor below.
When Darwin's seminal book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life came out, it was a huge blow to humans' egos.
But we mustn't make the mistake of thinking that Darwin's theory simply came to him in one day in a flash. It took him decades of careful observations and studies of the natural world until his ideas were fully baked.
Pre-evolutionary ideas: close but no cigar
Darwin was the right man at the right time. Like most landmark moments in science, natural selection came about as a result of the groundwork laid by others in the past.
Earlier, paleontologists had provided compelling evidence that life had been on Earth for a long time, that it changed over time, and that many species had become extinct.
Early evolutionists like Jean Baptiste Lamarck (1744-1829) believed that life developed in a continuous upward direction, starting from dead matter, then from simple single-celled forms to complex ones over time until it reached human "perfection". Species didn't go extinct, Lamarck argued, but rather transformed into new species.
We now know that many of Lamarck's ideas were wrong, but the French biologist's theory had some concepts that were valid. According to Lamarck, organisms altered their behaviors and bodies in response to environmental changes. By changing their behavior, organisms also modified their organs, and their offspring would inherit these altered structures. For instance, Lamarck thought that giraffes developed their iconic long necks and legs due to generations of browsing on high tree leaves.
In Lamarck's view, a body structure or organ would shrink or disappear if it was used less or not all. Driven by heritable modifications, all organisms would adapt to their environments as these changed.
Lamarck was wrong in his view that evolution is a process that strives toward greater complexity and perfection. He was also mistaken in thinking that heritable traits were driven by changes in behavior, which we now know today are owed to genes and their mutations.
However, the notion that organisms inherit the traits acquired during their parents' lifetime makes sense to this day and Lamarck's contributions help put the idea of evolution on the map, rather than that of intelligent design shared by the theologists of the time.
Interestingly, Darwin and his contemporaries found inspiration for natural selection in economics. In 1798, an English clergyman named Thomas Malthus published An Essay on the Principle of Population as it affects the Future Improvement of Society, in which he argued that human society has upper limits and cannot grow without being checked by nature. This is the origin of the Malthusian argument, which posits that overpopulation is unsustainable and will be the end of us all.
Malthus wrote that the growth of a population cannot outrun its ability to feed itself. By his calculations, if every couple raised four children, the population would double in twenty-five years, and from there on it would keep doubling. In such a scenario, population growth doesn't follow a linear progression but rather a geometric one, rising by a factor of four, eight, sixteen, and so on with each iteration.
Clearing new land for farming and improving crop yield with advanced technology may raise the food supply, but it would only increase arithmetically, not geometrically. Inevitably, if left unchecked, population growth brings famine and misery.
Malthus argues that humanity has always been controlled by plagues, infanticide, or simply putting off marriage until coming of age. The Englishman then pointed out that the same forces for fertility and starvation apply to animals and plants. If there was unlimited growth, then the world would soon be knee-deep in flies and maggots. But that doesn't happen because they cannot reproduce to their full potential. Like in most other species, most flies die without having the chance to reproduce and are vulnerable to droughts, cold winters, predators, and a myriad of other assaults by the environment.
Rather than treating humans as individuals, Malthus zoomed out and looked at humans as a group, in essence being one of the first ecologists in history.
When Darwin read Malthus's ideas, it became clear that humans must evolve like any other animal in nature. The enormity of the principle of exponential growth is best evidenced by a single Escherichia coli bacterium. Assuming that cell division occurs every 30 minutes, it would take less than a week for the descendants of this one cell to exceed the mass of the Earth.
What's true for a bacterium is true for an elephant. Indeed, Darwin himself used elephants as a hypothetical example, showing that the number of descendants of a single pair would swell to more than 19,000,000 in only 750 years. But clearly, the world is not overrun by either elephants or bacteria.
The reason is simple: most offspring do not survive to produce offspring of their own. The greater a species' proclivity for massive overproduction, also known as 'superfecundity', the larger the share of offspring that fail to reproduce.
Speaking of human society, clues pertaining to natural selection abounded in artificial selection or selective breeding -- evolution directed by the human hand. The history of selective breeding can be traced to about 10,000 years ago when hunter-gatherers began to keep flocks and herds and grow crops.
Individual animals that are more social, less aggressive, and easily manipulated were favored by humans and allowed to mate, paving the way to domestication. Today's sheep, cows, dogs, and house cats are the result of thousands of years of selective breeding.
Although there are important differences between natural selection and selective breeding, chief among them being that selective breeding favors alleles (forms of a gene) that do not contribute favorably to survival in the wild, there is great wisdom to be learned from farmers and livestock breeders.
Darwin spent quite a lot of time with pigeon breeders. He learned that pigeon breeders select individual birds to reproduce in order to produce a neck ruffle. The British naturalist reasoned that nature must unconsciously select individuals better suited to survival in their local environment. Given enough time, natural selection could augment a species, leading to new body parts from wings to eyes.
Two giants of science: Darwin vs Wallace
Many associate Charles Darwin with the introduction of the concept of evolution, but he wasn't the only scientist on the job. His British colleague Alfred Russel Wallace independently proposed the theory of evolution by natural selection. The two had a great relationship of mutual respect and even published scientific papers together.
Wallace traveled across the world studying nature to gather evidence for his evolutionary theories. He is best known for his studies of warning colorations in animals, particularly in the golden birdwing butterfly (Ornithoptera croesus). Another major contribution of Wallace is the theory of speciation, which says that when two populations of one species become increasingly different in phenotype that they can no longer interbreed to form fertile offspring, this can lead to the formation of two distinct species.
Wallace's proposed theory of evolution matched the unpublished ideas Darwin had been working on in secret for nearly 20 years. The two corresponded, traded ideas, and ultimately published their scientific ideas jointly in 1858, a year before Darwin published On the Origin of Species.
Darwin and the Beagle
The ideas presented in On the Origin of Species were largely based on Darwin's direct observations while traveling around the globe between 1831 and 1836. During this time, the British naturalist was part of a survey expedition carried out by the crew of the ship HMS Beagle, which made stops in South America, Australia, and the Southern tip of Africa.
At each stop, Darwin sampled and cataloged local plants and animals, which he contrasted and compared. This led him to uncover hidden patterns in the distribution and features of organisms, particularly during his observations of the Galápagos Islands off the coast of Ecuador.
The nearby islands of Galápagos had similar but nonidentical species of finches on them. Although they differed ever so slightly, each type of finch was well-suited to their particular environments.
Some finches ate large seeds so they tended to have large, tough beaks, while others ate insects and had thin, sharp beaks. Moreover, finches on the Galápagos were similar to those living on the nearby mainland of Ecuador but differed greatly from those found elsewhere in the world.
Gradually, Darwin understood the pattern of the related but different finches. He asserted that the Galápagos Islands must have been populated by bids from the neighboring mainland. Once there, on each island, the finches gradually adapted to the local conditions. Over the course of many generations, natural selection led to speciation.
Strikingly, when scientists revisited Darwin's finches they not only confirmed his observation, they witnessed evolution happening before their very own eyes. In 1972, biologists went to Daphne Major, one of the volcanic islands in the Galápagos archipelago, and found natural selection drove changes in the beak shape and size of two species, the medium ground finch (Geospiza fortis) and common cactus finch (Geospiza scandens).
For 30 years, scientists have been assessing cactus finches and in that time the size of their beaks has fluctuated, eventually decreasing in size over a period of 15 years. This is one of many direct pieces of evidence of natural selection that scientists now have at their disposal. Darwin thought that natural selection was only visible over an exceedingly long period of time, but we now know that it can be seen in motion within a lifetime.
The most common misconceptions about natural selection
'So you mean to say that humans descend from monkeys? Preposterous'. This is the kind of misconstrued criticism that Darwin faced in his lifetime and which some people hold to this day.
Humans did not evolve from chimpanzees or any of the other great apes that live today. Instead, we share a common ancestor with chimps that lived roughly 10 million years ago.
But this is just one of many misconceptions surrounding natural selection or evolution at large, for that matter. For instance, many people misinterpret the phrase "survival of the fittest" to mean "only the strong survive". The phrase was coined by British economist Herber Spencer and Darwin included it in the fifth edition of On the Origin.
By "survival of the fittest", both Spencer and Darwin referred to organisms that are "best suited to a particular environment", rather than the "most physically fit". The simplest way to view fitness from an evolutionary standpoint is to think of any factor that enhances reproductive success here and now. That's because what may be advantageous now may be disadvantageous a number of generations later as the environment changes.
Another common misconception is the idea that natural selection is random. What's random are mutations, however if these mutations increase fitness, they will be selected by natural selection and passed down to the next generation. In time, the proportion of the variation introduced by these mutations will increase in the population. Thus, natural selection doesn't happen 'by chance' because you can predict fairly confidently which traits will be selected.
That's not to say that there are guarantees either. Natural selection is a probabilistic process by which some traits make it more likely that organisms possessing them will successfully reproduce. Even a small difference in reproductive success, such as 1%, is enough to produce a gradual increase in the frequency of a trait over many generations.
There is no example of fossils that disagree with the principles of natural selection. Quite on the contrary, there are quite a few examples of transitional fossils (an extinct species that exhibits traits common to both an ancestral group and its derived descendant group), such as Archaeopteryx (between reptiles and birds), Ambulocetus (terrestrial mammals and cetaceans), or Tiktaalik (fish and tetrapods).
Natural selection is a crucial component of the modern theory of evolution and has withstood the test of time for more than 150 years. It is our single, most important theory that explains how and why life is so marvelously diverse.