Biological evolution is the central organizing principle of modern biology.

The study of biological evolution has transformed our understanding of life on this planet. Evolution provides a scientific explanation for why there are so many different kinds of organisms on Earth and how all organisms on this planet are part of an evolutionary lineage. It demonstrates why some organisms that look quite different are in fact related, while other organisms that may look similar are only distantly related. It accounts for the appearance of humans on Earth and reveals our species’ biological connections with other living things. It details how different groups of humans are related to each other and how we acquired many of our traits. It enables the development of effective new ways to protect ourselves against constantly evolving bacteria and viruses.

[Trait: A physical or behavioral characteristic of an organism.]

Biological evolution refers to changes in the traits of organisms over multiple generations. Until the development of the science of genetics at the beginning of the 20th century, biologists did not understand the mechanisms responsible for the inheritance of traits from parents to offspring. The study of genetics showed that heritable traits originate from the DNA that is passed from one generation to the next. DNA contains segments called genes that direct the production of proteins required for the growth and function of cells. Genes also orchestrate the development of a single-celled egg into a multicellular organism. DNA is therefore responsible for the continuity of biological form and function across generations.

[DNA: Deoxyribonucleic acid. A biological molecule composed of subunits known as nucleotides strung together in long chains. The sequences of these nucleotides contain the information that cells need in order to grow, to divide into daughter cells, and to manufacture new proteins.]

[Protein: A large molecule consisting of a chain of smaller molecules called amino acids. The sequence of amino acids and the molecule’s three-dimensional structure determine a protein’s specific function in cells or organisms.]

However, offspring are not always exactly like their parents. Most organisms in any species, including humans, are genetically variable to some extent. In sexually reproducing species, where each parent contributes only one-half of its genetic information to its offspring (the offspring receives the full amount of genetic information when a sperm cell and an egg cell fuse), the DNA of the two parents is combined in new ways in the offspring. In addition, DNA can undergo changes known as mutations from one generation to the next, both in sexually reproducing and asexually reproducing organisms (such as bacteria).

[Mutation: A change in the sequence of nucleotides in DNA. Such changes can alter the structure of proteins or the regulation of protein production.]

When a mutation occurs in the DNA of an organism, several things can happen. The mutation may result in an altered trait that harms the organism, making it less likely to survive or produce offspring than other organisms in the population to which it belongs. Another possibility is that the mutation makes no difference to the well-being or reproductive success of an organism. Or the new mutation may result in a trait that enables an organism to take better advantage of the resources in its environment, thereby enhancing its ability to survive and produce offspring. For example, a fish might appear with a small modification to its fins that enables it to move more easily through shallow water (as occurred in the lineage leading to Tiktaalik); an insect might

[Population: A group of organisms of the same species that are in close enough proximity to allow them to interbreed.]

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