The theory of evolution is a scientific theory that essentially states that species change over time. There are many different ways species change, but most of them can be described by the idea of natural selection. The theory of evolution through natural selection was the first scientific theory that put together evidence of change through time as well as a mechanism for how it happens.

The idea that traits are passed down from parents to offspring has been around since the ancient Greek philosophers' time. In the middle 1700s, Carolus Linnaeus came up with his taxonomic naming system, which grouped like species together and implied there was an evolutionary connection between species within the same group.

The late 1700s saw the first theories that species changed over time. Scientists like the Comte de Buffon and Charles Darwin's grandfather, Erasmus Darwin, both proposed that species changed over time, but neither man could explain how or why they changed. They also kept their ideas under wraps due to how controversial the thoughts were compared to accepted religious views at the time.

John Baptiste Lamarck, a student of the Comte de Buffon, was the first to publicly state species changed over time. However, part of his theory was incorrect. Lamarck proposed that acquired traits were passed down to offspring. Georges Cuvier was able to prove that part of the theory incorrect, but he also had evidence that there were once living species that had evolved and gone extinct.

Cuvier believed in catastrophism, meaning these changes and extinctions in nature happened suddenly and violently. James Hutton and Charles Lyell countered Cuvier's argument with the idea of uniformitarianism. This theory said changes happen slowly and accumulate over time.

Sometimes called "survival of the fittest," natural selection was most famously explained by Charles Darwin in his book On the Origin of Species. In the book, Darwin proposed that individuals with traits most suitable to their environments lived long enough to reproduce and passed down those desirable traits to their offspring. If an individual had less than favorable traits, they would die and not pass on those traits. Over time, only the "fittest" traits of the species survived. Eventually, after enough time passed, these small adaptations would add up to create new species. These changes are precisely what makes us human.

Darwin was not the only person to come up with this idea at that time. Alfred Russel Wallace also had evidence and came to the same conclusions as Darwin around the same time. They collaborated for a short time and jointly presented their findings. Armed with evidence from all over the world due to their various travels, Darwin and Wallace received favorable responses in the scientific community about their ideas. The partnership ended when Darwin published his book.

One very important part of the theory of evolution through natural selection is the understanding that individuals cannot evolve; they can only adapt to their environments. Those adaptations add up over time and, eventually, the entire species has evolved from what it was like earlier. This can lead to new species forming and sometimes extinction of older species.

There are many pieces of evidence that support the theory of evolution. Darwin relied on the similar anatomies of species to link them. He also had some fossil evidence that showed slight changes in the body structure of the species over time, often leading to vestigial structures. Of course, the fossil record is incomplete and has "missing links." With today's technology, there are many other types of evidence for evolution. This includes similarities in the embryos of different species, the same DNA sequences found across all species, and an understanding of how DNA mutationswork in microevolution. More fossil evidence has also been found since Darwin's time, although there are still many gaps in the fossil record.

Today, the theory of evolution is often portrayed in the media as a controversial subject. Primate evolution and the idea that humans evolved from monkeys has been a major point of friction between scientific and religious communities. Politicians and court decisions have debated whether or not schools should teach evolution or if they should also teach alternate points of view like intelligent design or creationism.

The State of Tennessee v. Scopes, or the Scopes "Monkey" Trial, was a famous court battle over teaching evolution in the classroom. In 1925, a substitute teacher named John Scopes was arrested for illegally teaching evolution in a Tennessee science class. This was the first major court battle over evolution, and it brought attention to a formerly taboo subject.

The theory of evolution is often seen as the main overarching theme that ties all topics of biology together. It includes genetics, population biology, anatomy and physiology, and embryology, among others. While the theory has itself evolved and expanded over time, the principles laid out by Darwin in the 1800s still hold true today.

Excerpt from:

A Brief Explanation of Evolution - ThoughtCo