The Theory of Evolution by natural selection was first formulated in Charles Darwin's book "On the Origin of Species (opens in new tab)" published in 1859. In his book, Darwin describes how organisms evolve over generations through the inheritance of physical or behavioral traits, as National Geographic (opens in new tab) explains. The theory starts with the premise that within a population, there is variation in traits, such as beak shape in one of the Galapagos finches Darwin studied.
According to the theory, individuals with traits that enable them to adapt to their environments will help them survive and have more offspring, which will inherit those traits. Individuals with less adaptive traits will less frequently survive to pass them on. Over time, the traits that enable species to survive and reproduce will become more frequent in the population and the population will change, or evolve, according to BioMed Central (opens in new tab). Through natural selection, Darwin suggested, genetically diverse species could arise from a common ancestor.
Darwin did not know the mechanism by which traits were passed on, according to National Geographic. He did not know about genetics, the mechanism by which genes encode for certain traits and those traits are passed from one generation to the next. He also did not know about genetic mutation, which is the source of natural variation. But future research by geneticists provided the mechanism and additional evidence for evolution by natural selection
What is natural selection?
Darwin chose the term "natural selection" to be in contrast with "artificial selection," in which animal breeders select for particular traits that they deem desirable. In natural selection, it's the natural environment, rather than a human being, that does the selecting.
Put simply, the theory of evolution by means of natural selection can be described as "descent with modification," said Briana Pobiner (opens in new tab), an anthropologist and educator at the Smithsonian National Museum of Natural History in Washington, D.C., who specializes in the study of human origins. The theory is sometimes described as "survival of the fittest," but that characterization can be misleading, Pobiner said. Here, "fitness" refers not to an organism's strength or athleticism but rather its ability to survive and reproduce.
Natural selection can alter a species in small ways, causing a population to change color or size over the course of several generations, according to The Natural History Museum (opens in new tab). When this process happens over a relatively short period of time and in a species or small group of organisms, scientists call it "microevolution."
(Image credit: Leonello Calvetti/Getty Images) (opens in new tab)
But when given enough time and accumulated changes, natural selection can create entirely new species, a process known as "macroevolution," according to Derek Turner and Joyce C. havstad in "The Philosophy of Macroevolution (opens in new tab)." This long-term process is what turned dinosaurs into birds, amphibious mammals (such as an animal called Indohyus) into whales and a common ancestor of apes and humans into the people, chimps and gorillas we know today.
Darwin also described a form of natural selection that depends on an organism's success at attracting a mate — a process known as sexual selection, according to Nature Education (opens in new tab). The colorful plumage of peacocks and the antlers of male deer are both examples of traits that evolved under this type of selection.
How did whales evolve?
One of the best examples scientists have of natural selection, is the evolution of whales. By using Darwin's theory as a guide, and understanding how natural selection works, biologists determined that the transition of early whales from land to water occurred in a series of predictable steps.
The evolution of the blowhole, for example, might have started with random genetic changes that resulted in at least one whale having its nostrils farther back on its head, according to Phys.org (opens in new tab).
The whales with this adaptation would have been better suited to a marine lifestyle, since they would not have had to completely surface to breathe. Such individuals were more successful and had more offspring. In later generations, more genetic changes occurred, moving the nose farther back on the head.
Other body parts of early whales also changed. Front legs became flippers. Back legs disappeared. Their bodies became more streamlined, and they developed tail flukes to better propel themselves through water, according to the Natural History Museum (opens in new tab).
Even though scientists could predict what early whales should look like, for a long time they lacked the fossil evidence to back up their claim. Creationists viewed this absence, not just with regard to whale evolution but more generally, as proof that evolution didn't occur, as pointed out in a Scientific American article (opens in new tab).
(Image credit: Illustration by Robert W. Boessenecker)
However, since the early 1990s, scientists have found evidence from paleontology, developmental biology and genetics to support the idea that whales evolved from land mammals. These same lines of evidence support the theory of evolution as a whole.
In the first edition of "On the Origin of Species," Darwin speculated about how natural selection could cause a land mammal to turn into a whale. As a hypothetical example, Darwin used North American black bears (Ursus americanus), which were known to catch insects by swimming in the water with their mouths open, according to the Darwin Correspondence Project (opens in new tab).
"I can see no difficulty in a race of bears being rendered, by natural selection, more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale," he speculated.
The idea didn't go over very well with the public or with other scientists. Darwin was so embarrassed by the ridicule he received (opens in new tab) that the swimming-bear passage was removed from later editions of the book. Scientists now know that Darwin had the right idea but the wrong animal. Instead of looking at bears, he should have been looking at cows and hippopotamuses.
Other theories of evolution
Darwin wasn't the first or only scientist to develop a theory of evolution. Around the same time as Darwin, British biologist Alfred Russel Wallace independently came up with the theory of evolution by natural selection, according to the Natural History Museum (opens in new tab). However this had little impact.
"The concept of evolution as a historical event was a hot topic among biologists and geologists prior to Darwin’s book because there was so much evidence accumulating, but I suspect biological evolution hadn’t really impinged on people outside of the academic bunker," Dr. P John D. Lambshead, a retired science research leader in marine biodiversity, ecology, and evolution at The Natural History Museum, London, told All About History Magazine (opens in new tab). "As long as science knew of no mechanism to explain how evolution happened it could be safely dismissed as a crank idea."
Meanwhile, French biologist Jean-Baptiste Lamarck proposed that an organism could pass on traits to its offspring, though he was wrong about some of the details, according to the University of California’s Museum of Paleontology (opens in new tab).
Like Darwin, Lamarck believed that organisms adapted to their environments and passed on those adaptations. He thought organisms did this by changing their behavior and, therefore, their bodies — like an athlete working out and getting buff — and that those changes were passed on to offspring.
(Image credit: Anup Shah/Getty Images) (opens in new tab)
For example, Lamarck thought that giraffes originally had shorter necks but that, as trees around them grew taller, they stretched their necks to reach the tasty leaves and their offspring gradually evolved longer and longer necks. Lamarck also believed that life was somehow driven to evolve through the generations from simple to more complex forms, according to Understanding Evolution (opens in new tab), an educational resource from the University of California Museum of Paleontology (opens in new tab).
Though Darwin wasn't sure of the mechanism by which traits were passed on, he did not believe that evolution necessarily moved toward greater complexity, according to Understanding Evolution — rather, he believed that complexity arose through natural selection.
A Darwinian view of giraffe evolution, according to Quanta Magazine (opens in new tab), would be that giraffes had natural variation in their neck lengths, and that those with longer necks were better able to survive and reproduce in environments full of tall trees, so that subsequent generations had more and more long-necked giraffes.
The main difference between the Lamarckian and Darwinian ideas of giraffe evolution is that there's nothing in the Darwinian explanation about giraffes stretching their necks and passing on an acquired characteristic.
What is modern evolutionary synthesis?
According to Pobiner, Darwin did not know anything about genetics. "He observed the pattern of evolution, but he didn't really know about the mechanism," she said. That came later, with the discovery of how genes encode different biological or behavioral traits, and how genes are passed down from parents to offspring. The incorporation of genetics into Darwin's theory is known as "modern evolutionary synthesis."
The physical and behavioral changes that make natural selection possible happen at the level of DNA and genes within the gametes, the sperm or egg cells through which parents pass on genetic material to their offspring. Such changes are called mutations. "Mutations are basically the raw material on which evolution acts," Pobiner said.
Mutations can be caused by random errors in DNA replication or repair, or by chemical or radiation damage, according to Nature Education (opens in new tab). Usually, mutations are either harmful or neutral, but in rare instances, a mutation might prove beneficial to the organism. If so, it will become more prevalent in the next generation and spread throughout the population.
In this way, natural selection guides the evolutionary process, preserving and adding up the beneficial mutations and rejecting the bad ones. "Mutations are random, but selection for them is not random," Pobiner said.
(Image credit: Shutterstock)
But natural selection isn't the only mechanism by which organisms evolve, she said. For example, genes can be transferred from one population to another when organisms migrate or immigrate — a process known as gene flow. And the frequency of certain genes can also change at random, which is called genetic drift.
The reason Lamarck's theory of evolution is generally wrong is that acquired characteristics don't affect the DNA of sperm and eggs. A giraffe's gametes, for example, aren't affected by whether it stretches its neck; they simply reflect the genes the giraffe inherited from its parents. But as Quanta reported (opens in new tab), some aspects of evolution are Lamarckian.
For example, a Swedish study published in 2002 in the European Journal of Human Genetics (opens in new tab) found that the grandchildren of men who starved as children during a famine passed on better cardiovascular health to their grandchildren. Researchers hypothesize that although experiences such as food deprivation don't change the DNA sequences in the gametes, they may result in external modifications to DNA that turn genes "on" or "off."
Such changes, called epigenetic changes, do not modify the actual DNA sequence itself. For instance, a chemical modification called methylation can affect which genes are turned on or off. Such epigenetic changes can be passed down to offspring. In this way, a person's experiences could affect the DNA he or she passes down, analogous to the way Lamarck thought a giraffe craning its neck would affect the neck length of its offspring.
What is the evidence for evolution?
The Theory of Evolution is one of the best-substantiated theories in the history of science. It is supported by evidence from a wide variety of scientific disciplines, including genetics, which shows that different species have similarities in their DNA.
There is also evidence supporting the Theory of Evolution in paleontology and geology. This is through the fossil record, which shows how that species that existed in the past are different from those present today, according to Bruce S. Lieberman and Roger L. Kaesler in "Prehistoric Life: Evolution and the Fossil Record (opens in new tab)" (Wiley, 2010).
There is also evidence for Darwin's theory found in developmental biology. It has been discovered that species that seem very different as adults pass through similar stages of embryological development, suggesting a shared evolutionary past, according to the open-access textbook "Concepts of Biology (opens in new tab)."
Evidence for whale evolution from paleontology
(Image credit: Nobumichi Tamura/Stocktrek Images via Getty Images) (opens in new tab)
The critical piece of evidence was discovered in 1994, when paleontologists found the fossilized remains of Ambulocetus natans, which means "swimming-walking whale," according to a 2009 review published in the journal Evolution: Education and Outreach (opens in new tab). Its forelimbs had fingers and small hooves, but its hind feet were enormous relative to its size. The animal was clearly adapted for swimming, but it was also capable of moving clumsily on land, much like a seal.
When it swam, the ancient creature moved like an otter, pushing back with its hind feet and undulating its spine and tail.
Modern whales propel themselves through the water with powerful beats of their horizontal tail flukes, but A. natans still had a whip-like tail and had to use its legs to provide most of the propulsive force needed to move through water.
In recent years, more and more of these transitional species, or "missing links," have been discovered, lending further support to Darwin's theory. For example, in 2007, a geologist discovered the fossil of an extinct aquatic mammal, called Indohyus (opens in new tab), that was about the size of a cat and had hooves and a long tail.
Scientists think the animal belonged to a group related to cetaceans such as Ambulocetus natans. This creature is considered a "missing link" between artiodactyls — a group of hoofed mammals (even-toed ungulates) that includes hippos, pigs, and cows — and whales, according to the National Science Foundation (opens in new tab).
Researchers knew that whales were related to artiodactyls, but until the discovery of this fossil, there were no known artiodactyls that shared physical characteristics with whales. After all, hippos, thought to be cetaceans' closest living relatives, are very different from whales. Indohyus, on the other hand, was an artiodactyl, indicated by the structure of its hooves and ankles, and it also had some similarities to whales, in the structure of its ears, for example.
(Image credit: NOAA)
Genetic evidence also supports the idea that whales evolved from land mammals and provides information about the exact branching of the evolutionary tree. For instance, in 1999, researchers reported in the journal Proceedings of the National Academy of Sciences (opens in new tab) that according to genetic analysis of "jumping gene" sequences, which copy and paste themselves into genomes, hippos were whales' closest living relatives. Before 1985, researchers thought pigs were more closely related to whales, but this 1999 study overturned that idea, as the Associated Press (opens in new tab) reported.
In 2019, researchers reported in the journal Science Advances (opens in new tab) about which genes within the whale genome were inactivated during the process of the creature's evolution from land mammals, as Science Friday (opens in new tab) reported. The researchers could tell that certain genes, including one involved in making saliva, had been inactivated because there are remnants of them, which the researchers call genomic fossils, in whale genomes. This indicates that whales evolved from a salivating creature.
There's also evidence of cetacean evolution from developmental biology. Developmental biology illustrates the fact that animals that are very different as adults share similarities as embryos because they are evolutionarily related. For example, as embryos, cetaceans started to develop hind limbs, which disappear later in development, while the forelimbs remain and develop into flippers, according to the journal Evolution: Education and Outreach (opens in new tab). This suggests that cetaceans evolved from a four-legged ancestor.
Is the theory of evolution controversial?
Despite the wealth of evidence from the fossil record, genetics and other fields of science, some people still question the theory of evolution's validity. Some politicians and religious leaders denounce the theory, invoking a higher being as a designer to explain the complex world of living things, especially humans.
School boards debate whether the theory of evolution should be taught alongside other ideas, such as intelligent design or creationism.
Mainstream scientists see no controversy. "A lot of people have deep religious beliefs and also accept evolution," Pobiner said, adding, "there can be real reconciliation."
Evolution is well supported by many examples of changes in various species leading to the diversity of life seen today. "Natural selection, or to put it another way — variation, heredity, and differential fitness — is the core theory of modern biology," John Lambshead explains. "It is to biology what, say quantum mechanics and special relativity are to physics or the atomic model is to chemistry."
Additional reporting by contributors Alina Bradford, Ashley P. Taylor and Callum McKelvie