Now that a basic understanding of how the scientific process works, let’s look at how the Theory of Evolution (yes, it deserves that capital T) works. Because I’d like to be somewhat comprehensive, I’m going to start small and work my way up. Because I consider a lot of this to be common knowledge, and because Wikipedia does a good enough job of explaining the things linked, I’ll probably mostly cite Wikipedia here.
Also, for an excellent lecture series by the renowned evolutionary biologist Richard Dawkins, see here (Or here for a playlist including them if youtube has pulled them or you have trouble getting to youtube). He explains things far better and more eloquently than I ever could without many more years of studying.
Evolution
For evolution to occur, several distinct things must be true.
- There must be traits that are Heritable
- There must be a mechanism for Heritability
- It must be possible for Mutations to occur (Heritability is not 100% perfect all the time).
With these simple premises, the basics of how evolution works can be explained in a short paper. Do note that I’m going to focus on getting a broad overview of how evolution works rather than every individual detail. Things will be left out in order to get the point across in a timely fashion. For example, I’m not going to cover social or sexual selection because most people either 1) Don’t know about them, or 2) are not talking about social and sexual selective pressures when they bring up evolution in a typical conversation.
Heritability
Before digging deeper into the small details of evolution, a case for heritability must be made. To say that a something is heritable is to say that the offspring resemble the parents to some degree.
The fact that offspring inherit traits from their parents was first rigorously tested by Gregor Mendel in 1822[1]. Gregor did experiments with peas and demonstrated that there is a correlation between what traits the offspring of crossbreeding certain plants have.
Picture some large family you know. Think about the natural hair colors of the children and the parents. Unless one of them is adopted, born with another parent, or has a mutation changing their hair color (it can happen), they will generally be similar. For example, in my family, the predominant colors are brown, dark brown, or blond.
This single observation forms the basis heritability. You can look anywhere to find this. Tabby cats bred with other tabby cats will have tabby kittens. Brown cows bred with other brown cows have other brown cows. The offspring are similar to their parents.
Taking a closer look at large populations of animals or people, you’ll notice that some traits tend to be more common than others. With some families, you might notice that the children have brown eyes despite one parent being brown eyed while the other is green. These traits that are more common are called Dominant Traits, and tend to “override” traits that are not (aka: Recessive).
This simple observation gives rise to a simple probabilistic method of examining heritability known as a Punnett square. While not a perfect example of how heritability works, it lets you make probabilistic predictions on what traits the children will have based on what the parents have.
At this point, it would be natural to wonder if you could test this somehow. For example, could you breed animals for a particular trait, such as coat coloration or size? The answer is yes. It wouldn’t be hard to do, although it may take longer than your lifetime to see the results depending on the animal. Just make sure the animals with the traits you like breed together while discouraging the others from breeding.
To use dogs as an example, repeat this for hundreds of generations selecting for small size and long body, and you might get a dog similar to a chihuahua. Go the other direction and select for large size, and you might get something similar to a great dane.
While this sort of thought experiment is easy to do – and in some cases, physical experimentation may be easy – it still requires some way for all this to work. If offspring resemble their parents, then how is that information about the parent transferred to the offspring? Without a mechanism through which to transfer these traits, there is no way to truly experiment and determine what causes these traits. Thankfully, a mechanism was discovered in 1869 – within a decade of Darwin publishing “On the Origin of Species”.
DNA and Cells
It is a well known scientific fact that Deoxyribonucleic Acid (DNA) exists[2] which affords us with many applications. To name just four from memory, there are
DNA is a protein found in all living organisms, the smallest of which are single-celled organisms and viruses (although there’s a great divide on whether or not viruses are alive). Cells use DNA as a “blueprint” to build the proteins required for the cell to function. The DNA is responsible for what shape the cell will be, how the cell interacts with the world around it… in short, everything that makes that cell. As the cell lives, it slowly increases in size until it becomes difficult for the cell to sustain itself. At this point, the cell usually divides into 2 different cells.
As part of this split[3], the cell makes a copy of it’s genetic material and then and gives half to one of the new cells, half to the other. To make this copy, the DNA is opened up and each half of the DNA is copied one at a time. Most of the time, the copy happens without issue and each cell gets an exact copy of the DNA.
Errors when copying the genetic material, known as mutations[4], are generally benign and cause no problems or bestow no benefit. Occasionally, however, mutations can cause changes in how the new cell looks or functions. This change in look or function could be positive or negative, an error in copying DNA the benefits a cell is still an error in copying. If the mutation causes the cell to function less efficiently, then it will die out over time. If the error confers an advantage to the cell, then it will grow better or find/consume more resources. Because it does a better job than it’s peers, it will be able to divide more often and will eventually take over it’s little niche. This is often called “Survival of the Fittest”, although a more appropriate term is “Reproduction of the Fittest”, as we’ll see later.
Cellular Origins
Evolution can adequately explain how life slowly changes from one form to the next when provided with a single cell containing heritable genetic code. Variations in DNA, the genetic code of the cell can cause it to be more or less efficient at what it does. As the cells who happened to get the not-useful or outright fatal mutations die out, the cells with useful mutations dominate. As the cells become more specialized in a particular environment, they become more and more different from their predecessors. This causes different types of cells to emerge.
As these specialized cells move into new environments, the process repeats again and again. This causes more variation in the cells and further changes to live in novel environments. When a cell has become so different from it’s progenitor, or from other cells around it, that it can no longer be considered the same type of cell, it is considered to be a new species.
Multi-Cellular Life
If mutations can explain how single celled organisms change their behavior and adapt to new environments, then how did multi-cellular life like humans arise? As you might expect, mutations in the genetic code offers an explanation as well.
Mutations can also affect how cells act towards other cells. If a cell has a mutation that changes it’s behavior with other cells to more cooperative behavior rather than competitive, cells could begin to group together. Now that a group is forming, mutations that help the group survive are also help the individual cell survive. As such mutations occur, the group becomes stronger as a whole. At some point, the cells end up as a single cooperative unit that is able to support itself.
Implications of a Shared Origin
Because an Evolutionist view of how life started requires just a single cell, there are several things that are cell specific we should see when we observe other life on our planet.
- That life should be composed of cells
- Those cells should contain DNA
- Those cells should contain similar (not 100% the same) structure
- The DNA in cells from different species should contain some similarities
The first three points are readily observable by anyone. Just take a mouth swap of yourself and a few other animals and look at the cells under a microscope. The fourth one, while not so easily observed, has also been shown by scientists who found more than a 40% DNA similarity between humans and bananas. These similarities are some observational evidence that indicate a shared origin.
In addition to evidence provided by DNA, there are other similarities we should see on a macro level when we observe the diverse life forms around us. If we all truly share a common origin, we should be able to find at minimum the following things (I’m sure there are more)
- Find relations between life and categorize it
- Determine when species branched off from each other
- Find similarities in anatomy between species such as humans and whales
- Find remains of long-dead animals that show relations between species
As the links may indicate to you, all those and more have been done.
Evolution on a Macro Level
Traits
The method for passing on traits to children are bundles of DNA known as Chromosomes that contain Genes (specific sequences of DNA). If the child has the same gene as their parent, they will have the same traits as the parent. There are many traits that can be passed on. Humans, for example, could pass on their eye-color trait. If both parents have the same colored eyes, then it’s highly likely that their child will have the same colored eyes as well.
The fact that genes are DNA sequences means the same issues affecting cell division apply here. The genes may not be copied correctly, and when offspring are made, the offspring has a gene that is slightly different. Again, like in cells, this may be beneficial, harmful, or cause no change at all.
Much like the cells they are made of, the animals most suited to their environment live healthier lives and reproduce more. This causes the traits of those best suited to their environment to flourish throughout the population and those traits that are harmful to decline. An example of this is lactose (in)tolerance[5]. In regions where dairy products are heavily consumed, larger portions of the population retain the lactase enzyme through to adulthood.
Larger changes, such as changes to the species’ form or coloration, come about the same way. If a slight change in form or color helps the species pass on more of it’s genes, then those genes will slowly pass on through the population until most or all of that species has the trait.
The most important thing to remember here is that trait’s will slowly move through the population so long as it allows the organism to reproduce more often. This is often incorrectly called “survival of the fittest”. While survival is certainly necessary in order to pass on genes, having an adaption that allows an organism to live a year longer, get more food, and reproduce one or two times more than the others will spread the gene slowly through the population.
Conclusion
While this overview may not be a complete explanation of how evolution works, I hope it helps in leveling the playing field when it comes to talking about evolution. Most religious folks that I talk to (primarily Christians where I live) don’t know the basics of how evolution works. They discount the theory entirely merely because their religious authorities tell them that it’s wrong. Because there are some arguments that show up time and time again, I’ll be covering ten of those next.