Evolution is easily defined by three words: descent with modification.

All species on Earth are descended from shared common ancestors that lived in the past and have been modified by natural selection and other processes to best survive in the habitats in which they live.

Scientists who study evolution seek to explain two fundamental observations of nature. First, they seek to explain the “goodness of fit” of organisms to the environments in which they live. A less fancy way of saying this is that scientists want to explain why organisms seem to be very good at doing what they do. Consider, for example, a squirrel. Its chisel-like teeth easily open the shells of hard acorns, its tail provides great balance as it runs along telephone wires, and its brownish-gray fur allows it ideal camouflage against tree trunks. Early naturalists attributed such goodness of fit to Divine design. In fact, they carefully explored nature in order to discover evidence of God’s Creation. It is from this school of study (known as Natural Theology) that Darwin’s idea of natural selection as an explanation for “goodness of fit”–better known today as adaptation–was first developed.

Second, scientists who study evolution seek to explain another fundamental observation of the living world: the great number of species, or biodiversity. In short, why are there so many different kinds of plants, animals, and other lifeforms?

Evolution as Fact and Theory

In the United States, the public considers evolution to be a controversial idea. According to polling data presented by the Pew Research Center in 2016, 62% of adults in the U.S. believe that humans evolved over time, but only 33% believe that this evolution was due only to natural processes. Roughly 34% of U.S. adults believe that humans were created in their present form by a supernatural being. In contrast with these figures, 98% of scientists accept that humans evolved over time. Even so, opponents of the idea of evolution have been effective at convincing the public that scientists disagree about evolution. Roughly half of polled individuals believe that scientists consider evolution to be true, while the other half believe that there is widespread disagreement in the scientific community about whether or not evolution explains the goodness of fit of organisms, as well as their biodiversity.

The evidence for evolution (see below) is overwhelming. While determination of whether evolution happens (or not) was once a fertile area of study, scientists have considered it an answered question for over 150 years. It is a scientific fact that evolution has occurred and it has been the central paradigm of biology for many years, so much so that in 1973, the famous geneticist Theodosius Dobzhansky titled an essay, “Nothing in Biology Makes Sense Except in the Light of Evolution.”

While the scientific community accepts that evolution has happened, research into how it works remains an incredibly active area of research. Indeed, entire departments at research universities are devoted to the study of evolution, hundreds of new scientific research reports on the mechanisms are published every year in peer-reviewed scholarly journals, and evolutionary biologists attend conferences devoted to the study of evolution every year. Indeed, because of recent technological advances–especially in terms of understanding the genetics of living organisms–there has never been a more exciting time for investigating how evolution operates, including across different organisms, environments, and timescales.

Thus, as argued by the great paleobiologist Stephen Jay Gould, it is important to recognize that evolution is both a fact and a theory. It is a fact that evolution has happened, but investigation into how it operates (theory) remains an active area of scientific research.