In “Why Evolution is True,” evolutionary biologist Jerry A. Coyne of the University of Chicago summarizes the theory of evolution by natural selection as follows:
Life on Earth evolved gradually beginning with one primitive species - perhaps one self-replicating molecule - that lived more than 3.5 billion years ago. It then branched out over time, throwing off many new and diverse species, and the mechanism for most (but not all) of evolutionary change is natural selection.
In order to understand exactly what is being said here, there are four terms that need to be understood (these are terms that everyone needs to know): (1) evolution, (2) gradualism, (3) speciation and common descent (4) natural selection. These are the four major "cogs" of the beautiful machinery that is the theory of evolution by natural selection. But let’s take a closer look at what "evolution" actually means.
Key Terms for Understanding Evolution:
"Evolution" is really rather easy to understand. It essentially means that organisms change (over time). For our uses here, though, we will define evolution in five words: descent with modification over time. Let’s break this definition down:
“Descent” (this one is pretty obvious) refers to an individual’s or a species’ descendants.
“Modification” refers to changes (mutations) in an individual’s genetic code. Mutations can impact an individual’s ability to adapt (this is generally referred to as its “fitness”). Ultimately, mutations impact an individual’s ability to survive and reproduce. Notably, genetic mutations can be helpful, harmful, or neutral (having no effect). Helpful mutations effectively increase an individual’s fitness and/or desirable traits. Conversely, harmful mutations effectively decrease an individual’s fitness, and neutral mutations have no effect either way (this is key to understanding natural selection). Mutations can also alter a plethora of things: Physical appearance, behavior, etc., These modifications created the stunning diversity and complexity of the 8.7 million (or so) species currently residing on Earth.
How rare are these mutations? Well, they probably aren't as uncommon as you might thing. A human will experience around thirty new genetic mutations in a lifetime. Most of these mutations will have little to no effect. On a day to day basis, humans probably experience a lot more mutations, but these are corrected by the body's built in protection systems. Ultimately, nearly all mutations are just errors in DNA replication during cell division, and errors are common. Once you consider that six feet of a DNA molecule is packed into every cell’s nucleus, and that all six feet of this DNA molecule has to be replicated each time a cell divides, you start to get the picture of how common mistakes are. But as was already stated, DNA has an extraordinary built-in error-correcting mechanism, which exhibits exemplary performance in its duties, catching and correcting nearly every error.
“Over time” refers to the necessarily long periods of time over which descent with modification (evolution) takes place. On Earth, evolution has been an ongoing process for over 3.5 billion years! That’s a lot of time to produce all kinds of diversity and complexity, and we need a lot of time with evolution.
Now that we’ve defined (the definition?) of evolution by natural selection (the individual parts of “descent with modification over time”), let’s define evolution one more time. Ernst Mayr’s summarization of evolution is wonderfully simple, yet wonderfully descriptive,
Evolution is change in the adaptation (fitness) and in the diversity of populations of organisms.
So there you have it. That's what evolution is. Ultimately, it is important to understand this definition, as many people get muddled up and reject the science behind evolution because of all sorts of misunderstandings. For example, many people argue that evolution can't explain how life started; however, if you take another look at the definitions given, you'll see that (in fact) evolution doesn’t explain anything about how life originated. Is this a problem with evolution? No. Because evolution isn’t trying to explain how life started. To complain that evolution doesn’t explain the origin of life is like complaining that the gas laws don’t explain gravity.
How life originated is something completely different, called abiogenesis. What evolution does try to explain (and does a good job of explaining) is how life developed and continues to develop. You see, evolution starts from the assumption that life is already there, and it has nothing to do with how it got there. Therefore it isn’t a flaw that evolution does not explain abiogenesis–that is a completely separate discussion. Ultimately, understanding the terms is the first step towards understanding the science.
Fun fact: Considering that you have six feet of DNA in each cell nucleus, and you have around 100 trillion cells in your body, that means if you were able to remove all of your DNA and lay it out from end to end, it would be over 1200 AU (Astronomical Units). The average distance from the Earth to the Sun is 1 AU, hence the term, meaning that you could wrap your DNA around the Earth and Sun 1200 times! If you took the DNA from every member of the human species and laid it out end to end, it would be over 135 million light years long!