What is evolution?
Evolution, as defined by Zimmer and Emlen 2012, is “the change in proportions of heritable properties of groups of organisms over the course of generations.”
This is a fairly technical definition, as most biological definitions are. First of all, the phrase “change in proportions of heritable properties” essentially means that there has been a change in the amount of a certain trait in a population (I’ll get to explaining how this change happens in a bit). The key word in that phrase, however, is “heritable.” For evolution to happen, the trait in question needs to be able to be passed down from one generation to the other, otherwise the specific trait cannot spread to the next generation and evolution would never occur. The second part of the phrase, “groups of organisms over the course of generations,” means that evolution happens in a population, not an individual, and it happens over at least one generation. So, if we put this all together, we get that evolution is the change in the amount of a given trait from a parent generation to an offspring generation.
What causes this change?
This is what we refer to as the “Four Forces of Evolution.”
(1) Mutation: mutation is solely responsible for generating all of the diversity we have on this planet. A mutation is simply an alteration in the DNA when it replicates. For example, in a germ cell (that is, a sperm or an egg cell), when the DNA from the parent replicates, sometimes the enzymes and proteins that are replicating the DNA make mistakes. These mistakes can be small, such as switching out one nucleotide (defined in the previous post on GMO’s) for another, also known as a point mutation, or they can be very big, such as reversing an entire segment of DNA. However, even the smallest mutations can cause huge changes in the offspring. So, the DNA has been mutated by a mistake in the replication and that mutated DNA is then passed onto the offspring. Therefore, the offspring has a different trait than the parent. This alone can be considered evolution! There has been a change in the amount of a certain trait (a new trait has been introduced to a population) from the parent generation to the offspring generation. However, mutation ONLY creates diversity; mutation cannot spread this new trait or make it more favorable. Mutation just introduces new genetics to a population. The other three Forces of Evolution are responsible for causing this trait to spread, generating more of a difference in the amount of the trait, and therefore generating more evolution!
(2) Gene Flow: Gene flow simply refers to new genes being introduced into a population because of immigration or genes leaving a population due to emigration. If new individuals of the same species, but from a different population and therefore with different genetics, enters a population and begins breeding with that population, it could cause a change in the amount of certain traits, especially an increase in the traits that that individual brought.
(3) Genetic Drift: This is arguably the most complicated to understand. Genetic drift simply refers to random variation. For example, if a few males have a certain trait but, for some reason, none of them mate and pass on their genes, then that trait will be significantly less in the next generation.
(4) (Natural) Selection: This is the most commonly known when discussing evolution, but is actually only one of the four Forces of Evolution. Gene Flow, Genetic Drift , and (Natural) Selection all work to spread certain traits in a population or reduce the amount of a certain trait in a population. (Natural) selection refers to when some individuals reproduce more than others because of their traits. For example, if there is a population of shellfish with two different colored shells, one that stands out against the sand and one that blends into the sand, the individuals with a shell that stands out are more likely to get eaten. Therefore, they don’t get to pass on their genes for the shell that stands out to the next generation, and therefore in the offspring generation the amount of shells that stand out are much less. This is a change in the amount of a trait across generations, and therefore it is evolution. I have been putting the word “natural” in parenthesis when talking about selection because biologists today tend to just use the phrase selection, as the “natural” does not add much anymore.
Additionally, I would like to emphasize that evolution is NOT directed and has no conscious thoughts. Evolution does not decide to keep one trait or another, the trait is kept in a population due to either gene flow, genetic drift, or selection.
Now, hopefully, you have a better understanding of the definition of evolution and why a trait MUST be heritable to evolve (because it has to be passed down to another generation) and why evolution cannot occur in an individual (because any changes in an individual will not be passed down to another generation).
Arguments Against Evolution and Misconceptions About Evolution
(1) Evolution has never been observed: This is not true; evolutionary biologists have, in fact, traced differences between generations of certain populations of organisms. In fact, an experiment performed by Buri in 1956 demonstrated that evolution does occur (he studied evolution by genetic drift). He set up 100 vials with flies that are heterozygous (this means that they have two different types of the gene for eye color: they have one for red eyes and one for white eyes; this manifests itself as orange eyes for these flies). Buri let them mate and reproduce, every generation randomly selecting 8 males and 8 females to reproduce. At the end of the experiment, 25 vials had flies with all red eyes, 25 vials had flies with all white eyes, and 50 vials had different proportions of flies with red, white, and orange. This in itself is evolution in progress.
(2) Evolution is only a theory: The idea of a scientific theory is very, very different from the colloquial use of the word theory. In every other context, theory means something that has only been thought about, but not put into practice. In science, a theory is an idea that has been rigorously tested by experimentation and shown to be true, but not proved (because nothing in science can ever be proved, not because science is not a good way to discover knowledge, but just because all scientists are naturally skeptical). To put it in perspective, gravity is also a theory. As is the idea of germs.
(3) How did life begin if all life evolved from other life? To answer this, I’d like to direct you to the first picture I posted in the “Rationale” post. Number 7: “Thou shall not argue that because of our ignorance, claim must be true or false (Ad ignorantum).” Just because scientists do not have an answer for everything, including the ever evasive question of how life began, does not mean that everything about science is false or that everything about that particular idea is false. It just means science is a continuous process.
(4) There are gaps in the fossil record: This is true, and there will always be gaps in the fossil record because we will never get fossils of every single type of creature. However, that does not mean that the fossil record is incomprehensible. We have enough data on primitive organisms, as well as observations of primitive, living organisms (also known as living fossils) to determine extreme similarities between organisms from millions of years ago and today. We can now also use techniques such as molecular and genetic data.
(5) The Second Law of Thermodynamics says evolution is false: The 2nd Law of Thermodynamics is very often misinterpreted. It says that the entropy of an isolated system is constant or increasing. Entropy is a measure of disorder and heat, and so many people take that to mean that if disorder is always increasing, then how can something more ordered come from nothing? Well, the 2nd Law of Thermodynamics discusses an isolated system, which means no energy or matter is flowing in and out of the system. This is definitely not true for Earth and for life; energy and matter are constantly flowing in and out of Earth from space and flowing in and out of habitats.
(6) There are no transitional fossils: This means that there are no fossils found in the midst of evolution. First, from what we now know about evolution, we can tell that since an individual does not evolve, we will not find a fossil that is right in the middle of becoming one organism from another. However, if this argument is trying to say that we haven’t found an organism that links two different species together (an organism in the middle of a lineage), that’s just not true. We have found fossils of species that we can identify having certain traits of more primitive organisms, but lacking traits of more modern organisms. We have also found fossils of species that have similar traits to organisms we see today.
The most important thing to keep in mind when it comes to evolution is Number 7 in the Commandments of Logic: just because we don’t know something yet does not mean that the entire theory is false; it just means that scientists are still discovering things.