Darwin's Theory Of Evolution: Definition & Evidence
The theory of evolution by natural selection, first formulated in Charles Darwin's book "On the Origin of Species" in 1859, describes how organisms evolve over generations through the inheritance of physical or behavioral traits, as National Geographic 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. Individuals with traits that allow 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 allow species to survive and reproduce will become more frequent in the population and the population will change, or evolve. Through natural selection, Darwin suggested, a diverse life-forms could arise from a common ancestor.

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, according to National Geographic. In natural selection, it's the natural environment, rather than a human being, that does the selecting.

Put simply, the theory can be described as "descent with modification," said Briana Pobiner, 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.

Darwin did not know the mechanism by which traits were passed on, according to National Geographic; that is, 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 didn’t 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 (see “Modern Understanding,” below). It is one of the best-substantiated theories in the history of science, supported by evidence from a wide variety of scientific disciplines, including not just genetics (which shows that different species have similarities in their DNA) but also paleontology and geology (through the fossil record, which shows how that species that existed in the past are different from those present today), and developmental biology (species that seem very different as adults pass through similar stages of embryological development, suggesting a shared evolutionary past). (See the open-access textbook Concepts of Biology for more information about these lines of evidence.)

How did whales evolve?
In the first edition of "On the Origin of Species," published in 1859, 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.

"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 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.

The story of the origin of whales is one of evolution's most fascinating tales and one of the best examples scientists have of natural selection.

What is natural selection?
Archaeopteryx, shown here in this illustration, is considered the first bird-like dinosaur on record, dating to about 150 million years ago during the Jurassic period. (Image credit: Leonello Calvetti/Getty Images) To understand the origin of whales, you need a basic understanding of how natural selection works. Natural selection can alter a species in small ways, causing a population to change color or size over the course of several generations. When this process happens over a relatively short period of time and in a species or small group of organisms, scientists call it "microevolution."

But when given enough time and accumulated changes, natural selection can create entirely new species, a process known as "macroevolution." 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. Take the example of whales: By using evolution as a guide and understanding how natural selection works, biologists knew 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. 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.

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. The colorful plumage of peacocks and the antlers of male deer are both examples of traits that evolved under this type of selection.

But 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, while 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.

Difference between Darwinian and Lamarckian evolution
Maasai giraffe browses on leaves of a tall tree in the Maasai Mara National Reserve, Kenya. (Image credit: Anup Shah/Getty Images) 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. 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, an educational resource from the University of California Museum of Paleontology.

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, 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 theDarwinian explanation about giraffes stretching their necks and passing on an acquired characteristic.

What is modern evolutionary synthesis?
Darwin didn't know anything about genetics, Pobiner said. "He observed the pattern of evolution, but he didn't really know about the mechanism," Pobiner 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. 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.

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, some aspects of evolution are Lamarckian. For example, a Swedish study published in 2002 in the European Journal of Human Genetics 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?
Even though scientists could predict what early whales should look like, 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. But 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.

Evidence for whale evolution from paleontology
Ambulocetus natans swimming underwater. (Image credit: Nobumichi Tamura/Stocktrek Images via Getty Images) 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. 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, 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.

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.

Evidence for whale evolution from genetics & developmental biology

The last shore-dwelling ancestor of modern whales was Sinonyx, top left, a hyena-like animal. Over 60 million years, several transitional forms evolved: from top to bottom, Indohyus, Ambulocetus, Rodhocetus, Basilosaurus, Dorudon, and finally, the modern humpback whale. (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 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 reported. In 2019, researchers reported in the journal Science Advances about which genes within the whale genome were inactivated during the process of the creature's evolution from land mammals, as Science Friday 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. 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.
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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.

Additional reporting by contributors Alina Bradford and Ashley P. Taylor.

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