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In England, Darwin observed a lot of variation in domesticated plants and animals. Farmers explained to him that, for example, some cows grew big and strong and produced a lot of milk. Others were smaller and produced far less milk. The farmer would only breed those cows that were larger and that produced more milk. These productive traits were then passed on to the following generations. Through selection of certain traits, breeders could produce a great amount of diversity.

Artificial selection is when humans choose which plants or animals get to reproduce based on traits we want. This is how we got different dog breeds and how farmers develop better crops. Humans have been doing this for thousands of years with plants and animals.

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Early scientists observed relationships among organisms and how they seemed to be well adapted for specific environments. Darwin built upon the work of these scientists to develop a theory for how evolution occurs. A theory is an explanation based on evidence that has been repeatedly confirmed through experimentation or observation. Today, we have a wide body of evidence that supports Darwin's theory of evolution.

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About six years after the publication of The Origin of Species, a little-known monk named Gregor Mendel published his research on genetics and the basic principles of heredity.

Mendel's work showed that traits are passed down from parents to offspring and that traits are inherited independently of one another. We now know that traits are coded for by genes and that alleles are different variations of the same gene. Variation in the alleles between individual organisms within a population is called genetic variation. Genetic variation is the basis for natural selection.

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Natural selection is the process by which organisms that are better adapted to their environment survive and reproduce more successfully than others. Think of it like nature's way of choosing which traits get passed on to the next generation.

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Charles Darwin was a British scientist who developed the theory of evolution through natural selection. He made his amazing discoveries while traveling around the world on a ship called the HMS Beagle, where he studied different animals and plants.

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Although Darwin had no knowledge of genetics, he observed that, with human intervention, certain individuals could be selected to produce offspring with desirable traits. When selected and allowed to breed, these individuals would pass their traits onto their offspring. In order for artificial selection to occur, the trait must be heritable. Heritability is the ability of a trait to be passed down from one generation to the next.

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In 1831, the HMS Beagle set sail from England on a five-year journey to map the coast of South America and the Pacific Islands. An invitation was extended to Charles Darwin, a recent graduate from the University of Cambridge.

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Near the end of Darwin's journey, the Beagle arrived in the Galápagos Islands. At this stop, he would make some of his most widely known observations that are still studied today. Darwin noted that the species found on one island looked different from those on nearby islands and on the mainland. He was struck by the variation of traits among similar species. Some traits seemed well suited to the animals’ environments and diets. For example, Galápagos tortoises (Geochelone elephantopus) had variations in their traits that seemed to match their environment.

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Among all of Darwin's observations, the most well known are those of the Galápagos finches. These small birds, sometimes known as Darwin's finches, are closely related but have significant differences. These observations led Darwin to infer that species must somehow be able to adapt to their surroundings.

An adaptation is a feature that allows an organism to survive and reproduce in its environment. It was this analysis that eventually helped shape Darwin's theory about how organisms change over time.

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One famous example of natural selection is the peppered moth. During the Industrial Revolution, dark-colored moths survived better than light-colored ones because they could hide better on pollution-darkened trees. Because more with dark wings survived, they passed on that gene, making the dark wings a common trait.

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Several years before Darwin landed in the Galápagos, the Beagle anchored near Bahia Blanca in Argentina. While there, hunters brought back an armadillo. This was Darwin's introduction to this strange, armored animal. While on a fossil-hunting trip in the area, he found fossils of huge animals, including Glyptodon, a giant armadillo. The fact that these fossils looked like the living species suggested that modern animals might have some relationship to fossil forms. These fossils suggested that in order for such changes to occur, Earth must be much older than previously thought.

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Genetic variation refers to the differences in DNA among individuals. This variation is essential for natural selection because it provides the raw material for evolution. Without genetic variation, all members of a species would be identical, and evolution would not be possible.

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It was the work of Thomas Malthus that inspired many of Darwin's ideas about modification by natural selection. In his work, Malthus pointed out the potential of human populations to grow exponentially if there was a constant birth rate and ideal conditions. Such conditions would include unlimited resources and an absence of predators or disease. However, populations do not grow in an unchecked way. As Malthus pointed out, human populations are limited by many factors, such as disease, war, and limited resources.

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Building on Malthus's ideas that there were limits to human population growth, Darwin reasoned that a similar struggle for resources took place in nature. The challenge is for each individual to be better at obtaining available resources, such as food, water, and shelter.

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Adaptation is a process that allows organisms to become better suited to their environment. Through adaptations, species develop traits that increase their chances of surviving and reproducing in specific environments.

A well-studied example of natural selection in jaguars is shown in the illustrations below. About 11,000 years ago, many species faced extinction. Large cats, including jaguars, faced a shortage of food due to the changing climate of that time. Fewer mammals were available to eat, so the jaguars had to eat other animals, such as reptiles. The jaguar population showed variations of jaw and tooth size that became important for survival.

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As an environment changes, different traits will become beneficial. Ecologists Peter and Rosemary Grant observed an example of natural selection acting on existing traits within a population of medium ground finches on one of the Galápagos Islands. A drought in 1977 reduced the number of the small, soft seeds that the finches preferred. However, there were still plenty of large, tough-shelled seeds.

The two graphs shown here represent the number of birds with each size of beak. In 1976, a total of 751 birds were measured. The distribution of beak size is shown in the histogram on the left. After the drought, the Grants again measured the beak sizes of the 1978 survivors. There were 90 birds measured to construct the histogram on the right. The Grants noticed that the distribution of beak sizes changed after the drought affected the types of available food in the environment.

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Over time, some organisms can develop resistance to antibiotics and pesticides through natural selection. When these chemicals are used, they kill most of the susceptible organisms, but those with resistance-surviving traits survive. These resistant organisms then reproduce, passing on the resistance genes to the next generation. As a result, the population becomes more resistant over time.

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*Not all traits are adaptations

*Natural selection does not produce individuals who are perfectly suited to their environment

*Organisms have combinations of traits that result from complex sets of tradeoffs

*Another reason natural selection does not produce ideal traits is that it acts only on traits that already exist. Genetic variation within a population is what allows for the environment to "select" for certain traits. New alleles are not made by natural selection—they occur by genetic mutations.

*some mutations lead to traits that might be advantageous to certain individuals. A mutation could change an organism's DNA in a way that leads to the production of a new type of protein. If this results in a trait that increases an organism's fitness, this trait would be selected for. Therefore, new traits can occur, but they are not created through natural selection.

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Microevolution refers to the small-scale changes that occur within a species over time. These changes are usually a result of processes like mutation, selection, gene flow, and genetic drift. Microevolution can lead to slight changes in traits, but the organisms remain the same species.

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Macroevolution involves large-scale changes that occur over long periods of time, often leading to the emergence of new species. This type of evolution can be observed in the fossil record, where species entirely disappear and new ones emerge that are significantly different from their ancestors.