“At last gleams of light have come, & I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) immutable.” — Charles Darwin in a letter to Joseph Hooker, January 11, 1844
An Introduction: What is the Theory of Evolution by Natural Selection? (The Evidence for Evolution #2)
[Part 1 of The Evidence for Evolution series, “An Intro to Natural Selection,” can be found here. Also, the reader should realize that this is an introduction, and that entire books (very long books, I might add!) have been written on the theory of evolution by natural selection, and that this is not a book! Therefore, do not expect this to be like a book!]
[Note: I’m having formatting issues with WordPress. None of the words and titles I’ve italicized are in fact italicized. I can’t seem to fix the problem. I apologize, but for the moment, it will have to do. It takes away nothing from the intent of this piece, though.]
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.” — Why Evolution is True, p. 13
There are four general points* in Coyne’s summary: (1) evolution, (2) gradualism, (3) speciation and common descent, and (4) natural selection. These are the four major cogs* of the beautiful machinery that is the theory of evolution by natural selection, so let’s look at all of them a bit closer:
Evolution is simple 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” refers to an individual’s or a species’ descendants, i.e., its offsprings’s, offsprings’s, offsprings’s, etc., all the way down.
“Modification” refers to changes (mutations) in an individual’s genetic code. Mutations can affect an individual’s ability to adapt, its “fitness” as it’s called, i.e., mutations affect an individual’s ability to survive and reproduce. They can also affect physical appearance, behavior, etc., hence the stunning diversity and complexity of the 8.7 million or so species currently residing on Earth.
Genetic mutations can be helpful, harmful, or neutral (having no effect). Helpful mutations effectively increase an individual’s fitness and/or desirable traits, so it follows that harmful mutations effectively decrease an individual’s fitness, whilst neutral mutations have no effect either way. (This is key to understanding natural selection.)
A human, e.g., will experience around thirty new genetic mutations in a lifetime, and most of these mutations will have little to no effect. However, a human will in fact experience vastly more mutations, as nearly all mutations are essentially errors in DNA replication during cell division, and errors are common! Once you consider that six feet of DNA molecule is packed into every cell’s nucleus,** and all six feet of DNA molecule has to be replicated each time a cell divides, you start to get the picture. Do not fret, however! 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!
Now that we’ve defined (the definition?) 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.”
Gradualism simply means that evolution occurs slowly, continuously, and imperceptibly. Most importantly, this means that evolution does not take giant leaps “forward,” or in any direction at all for that matter. Here’s why:
The DNA of any specific progenitor contains the genetic “plans” (code) for developing offspring which are very similar but not exact replicas of itself. (You can say the offspring are “modified descendants.”) This is why humans produce human offspring whom are very similar to their parents but not exactly the same, and chimpanzees produce chimpanzee offspring whom are very similar to their parents but not exactly the same. For a human to produce a chimpanzee, or for a chimpanzee to produce a human… Well, that’s utterly absurd, as human DNA is specifically coded for the production and function of a human, just as chimpanzee DNA is specifically coded for the production and function of a chimpanzee.
So, even if an individual chimpanzee’s genetic code could be somehow “radically mutated” (I can’t think of a better term…) to the extent that it could potentially match the genetic “plans” required for the production and function of an anatomically modern human, the probability of that ever happening is far less than the number of hydrogen atoms in the universe to 1. There’s a greater probability of my being fired out of the chair I’m currently sitting on and landing on Mars in the next five seconds than there is for a chimpanzee giving birth to a human.
5… 4… 3… 2… 1…
Well, I’m still here, and a chimpanzee still hasn’t given birth to a human yet, so I think we’re safe…
One more important point, however: if, and I stress if again, if a chimpanzee’s genetic code were somehow able to be “radically mutated” to the point where it could potentially match a human genetic code, there’s just as good of a chance for it to become “anything” else. Yes, humans share about 98.5% of their genetic code with chimpanzees. However, mutations are entirely random meaning that something which could mutate 1.5% of a chimpanzee’s genetic code to exactly match a human genetic code could also mutate it 1.1% or 0.4% or 1.49% or 1.499% or 1.4999% or 1.49999%, or 1.499999%, etc., ad infinitum, and in a virtually infinite number of directions. This is known as an infinite series in calculus. And, since there is only a finite number of specific genetic codes which could produce a human (we are all 0.1% different after all!), to quote Douglas Adams, “any finite number divided by infinity is as close to nothing as makes no odds…”
[Note: I only expounded on this point regarding chimpanzees birthing humans because of the irrational claims made by creationists in this regard. In what they consider a rhetorical question, they say, “Have you ever seen a chimpanzee give birth to a human? I haven’t either, so evolution is obviously all lies.” Ironically enough, if a chimpanzee did give birth to a human (see: above paragraph), it would actually disprove the theory of evolution! To summarize the summary, what they believe to be the one of the only pieces of evidence which could prove human evolution true, would in reality refute the entire theory of evolution by natural selection. Got to love irrationality, eh?]
Speciation can be described as the splitting of a single evolutionary lineage into two or more reproductively isolated lineages. “Reproductive isolation” is any barrier, be it geographical, genetic, behavioral, etc., which prevents (or makes it extremely difficult for) two populations capable of (or potentially capable of) interbreeding from producing offspring or from producing fertile offspring. Reproductive isolation is the key to understanding speciation, as it prevents two populations from “sharing genes” and thereby creating hybrids, leaving the two populations genetically distinct. A hypothetical demonstration:
Imagine a population of foxes doing whatever it is you imagine a population of foxes doing. Now, imagine a river begins to form in the middle of this population of foxes, and this river becomes wider and wider over time. (Also, our hypothetical foxes cannot swim! No cheating!) Since the river has split the single population of foxes into two, and because the foxes are unable to cross the river, the foxes are said to be “reproductively isolated” from one another, i.e., the two populations can potentially interbreed, but a barrier is preventing them from doing so. As time goes on, descent with modification (evolution) continues to occur, and as a result, each population of foxes builds up its own “set” of modified, and therefore distinct, genetic codes.
After a long period of time being reproductively isolated, the two populations of foxes may become so different genetically that, even if they were reunited, they may be unable to interbreed. They may also become physically and behaviorally different than one another as well, which may also prevent them from interbreeding. E.g., many animal species have complex yet specific mating rituals. If these rituals are not performed, the probability of mating is exceedingly low. Or, in the example of our hypothetical foxes, after so many thousands or millions of years of being apart, it’s highly probable that their specific mating rituals have also evolved. They are said to be “reproductively isolated,” then, and they can*** be considered different species as according to Ernst Mayr, a species is defined as “a group of actually or potentially interbreeding natural populations reproductively isolated from other such populations.” (Species Concepts and Definitions)
To sum up, then, speciation is a process by which one species becomes two or more reproductively isolated species. I would have like to have spent more time on this, but this is an introduction, and I will devote an entire piece to speciation in the future.
I need to make a mention of something, however, and that something is “common descent” or “common ancestry.” Coyne felt common ancestry was important enough to consider it as an entirely separate component from speciation in regard to of the theory of evolution by natural selection, but he also mentioned that speciation and common ancestry are “two sides of the same coin.” I agree, which is why I didn’t separate the two, but I also understand why you would.
So, two or more species are said to have common descent if they have descended down the same ancestral line at some point in the past, i.e., if a species has a common ancestor with another species, they are said to have common descent. If you had a video containing all of evolutionary history till now, you could play it in reverse, and instead of one species becoming two or more species (speciation), you would see two or more species becoming one species again (common descent). If you played this video in reverse all the way back to the beginning, you would find that every species on Earth has evolved from a single common ancestral species.
Natural Selection: If you recall, I already wrote an introduction to natural selection a few days ago. (You can find a link at the bottom of the page.) I will include in this post, however, Dr. Coyne’s definition of natural selection from Why Evolution is True again, and my example from the previous piece. I do suggest reading it in its entirety as the theory of evolution is nothing without natural selection.
“The idea of natural selection is not hard to grasp. If individuals within a species differ genetically from one another, and some of those differences affect an individual’s ability to survive and reproduce in its environment, then in the next generation the “good” genes that lead to higher survival and reproduction will have relatively more copies than the “not so good” genes. Over time, the population will gradually become more and more suited to its environment as helpful mutations arise and spread through the population while deleterious ones are weeded out. Ultimately, this process produces organisms that are well adapted to their habitats and way of life.” (Why Evolution is True*, p.11)
Picture in your mind a stick insect. (If you’re having trouble doing so, click here, then come right back). Now, imagine that this stick insect carries a gene mutation which makes it somewhat more “stick like” than another similar stick insect, i.e., your imagined stick insect is better camouflaged than another stick insect. It follows, then, that this stick insect which is somewhat better camouflaged, that is, better adapted to its environment, will have a better chance to survive and reproduce than the other stick insect as it’s less likely to be seen and thereby eaten by a predator such as a bird. When this stick insect reproduces, it’s likely to pass on this “good” mutated gene to its offspring. These offspring will then be better camouflaged (adapted) and therefore more likely to survive and reproduce than the other stick insects which don’t carry this gene. They’ll produce more offspring, who will produce more offspring, and so on and so forth. Over a long period of time, the stick insects with the original gene mutation will come to outnumber those without it due to environmental pressures, namely predation. These stick insects are then said to have been “naturally selected” as they are more adapted to their environment (they are “fitter”) than their predecessors making them more likely to survive and reproduce. So, essentially, the phrase should be “survival of the ‘fitter’,” not “survival of the fittest.”
Thanks for reading, and I sincerely hope you’ve learned something! Please comment and share if you did learn something, so that someone else may learn something, too! I’m all about spreading knowledge to as wide an audience, and I can’t do it without you, the readers! If you take anything away from this at all, it should be that species do evolve, and that natural selection is the mechanism by which they do so! Remember,
“It is not the strongest of the species that survives, nor the most intelligent, but the one most responsive to change.” ― Charles Darwin
*Coyne uses six components in his book, but I like to make things as “simple as possible, but not simpler!” Seriously, however, Coyne separates common descent and speciation into two distinct points, and he also includes a point entitled, “nonselective mechanisms of change.” I didn’t not include the latter is this is meant as a general overview, but the reader should be aware that natural selection is not the only mechanism by which evolution occurs. This will be discussed in later pieces.
**I did the calculations. 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!
***I say they “can” be considered different species, as what actually constitutes a species is still up for interpretation when two populations are very similar.
Why Evolution is True, Jerry A. Coyne
The Tangled Bank, Carl Zimmer
Species Concepts and Definitions, Ernst Mayr
Why Darwin Matters, Michael Shermer
The Blind Watchmaker, Richard Dawkins
Link to the Intro to Natural Selection: https://nonrandomevolution.wordpress.com/2013/04/24/the-evidence-for-evolution-intro-to-natural-selection/