What are vestigial traits and how do they provide strong evidence for evolution? Evolution has created an amazing array of living things, but it can also create flaws. Natural selection modifies species in response to their environment, but species also inherit the traits of their ancestors. And those ancestors may evolve their traits in a very different environment.
This sets up a situation in which there may be a conflict. Species may inherit traits that were useful in the previous environment, but are no longer useful in the present. Since structures are constrained by their history, the evolutionary process should therefore lead to poor designs for at least some traits. An evolutionary history may lead to poorly designed or leftover traits.
For example, the positions and nature of the keys on keyboards are due to historical processes. The positions and keys were chosen a long time ago under different conditions. The positions of the letters on manual typewriters were actually placed to reduce typing speed. Later, when keyboards became the interface for computers, Some extra keys for computer programmers were added, but the locations of the letters were not changed.
This historical process has led to the poor design of keyboards. The QWERTY arrangement of letters is less efficient than many other arrangements, in terms of how long it takes to type and the speed and accuracy that can be gained. Many keyboards also have vestigial traits, left over from those early days of computer programming. If you've ever looked at your keyboard and wondered why there is a pause, break, or scroll lock button, Those were for programmers dealing with lines of code scrolling on their computer screens. Those features of keyboards are leftover vestigial traits because of the history of keyboards.
Evolution likewise describes things changing over time, in this case living things descending from ancestors. The process is similar, however, so we expect to see examples of poor design and leftover or vestigial traits. In fact, this is a strong prediction of the theory of evolution that is distinct from a perfect design or creationist hypothesis.
All of these ideas predict fairly well-designed traits in living things, but evolution also predicts poorly designed traits. The failure to find at least some poorly designed or vestigial traits would actually be strong evidence against evolution. So what do we see when we look for poor design and vestigial traits?
It turns out there are tons of examples, and we'll look at a few in each of these three broad categories. Poor design or vestigial traits in structural traits, developmental traits, and genetic traits. An interesting example of a poorly designed trait is the coccyx that humans have at the base of their spine. This is a small set of bones that are for tails that humans no longer have, but our ancestors did have tails. The presence of these bones isn't trivial.
If you look at where it is located, the coccyx makes childbirth more difficult. In fact, One way you can tell a male skeleton from a female skeleton is that the coccyx is straighter in females to provide more room in the birth canal for babies. If you were to design a human from scratch, there's no reason to put that extra little set of bones in there. And speaking of childbirth, if you look at the skeleton of a human and you visualize where the uterus in a female would be, where should the baby emerge?
Does it make sense to design the birth canal so that it squeezes through that constrained pelvis? This delivery is a traumatic event. If you look at the skeletons of women who have had babies, you can see the damage to the pubic synthesis caused by natural childbirth. A better option exists. When we take matters into our own hands and perform an artificial delivery, we use the nice large space underneath the ribs as the exit for the baby.
The biological design of the birth canal makes no sense, but it makes more sense in an organism that walks on all fours so that gap under the ribs is much smaller. And it makes perfect sense in a fish, where the sequence of ribs runs all the way down to where the equivalent of the pelvis is. The evolutionary lineage leading to humans modified the locations of the ribs and opened up an area perfect for childbirth, but the constraints of our ancestry retain the birth canal squeezing through the pelvis.
Humans also have small muscles in our skin that can make the hair in our skin raise and cause goosebumps. This feature is fairly useful for organisms that have thick fur like the dog pictured, where raising the fur makes it appear larger and more intimidating. What makes almost no sense is goosebumps for an organism that has almost no hair.
Retaining these muscles, because we have inherited them from ancestors who did have fur, does make sense, however. You've probably heard that the human appendix is a vestigial or useless trait. Appendixes are fairly useless throughout all mammals. We're not the only ones with this problematic organ. Most mammals don't have an appendix, however.
and the ones that do tend to be plant eaters that use it to help digest cellulose. There is some debate about how functional the human appendix is, with some people saying it may have a minor role in the immune system. But it clearly isn't particularly important, because people can have their appendix removed and suffer no visible detriment. What the appendix does do is rupture and cause appendicitis. and about 5% of the population.
With modern medical care, this is no big deal, but without medical care, this would be a major source of mortality. This is a terrible design. An organ that has minimal benefit at best, but kills 1 in 20 people, makes no sense. However, possession of this organ does make sense if the ancestor of all mammals had an appendix, and we happen to be one of the species that has not lost it completely. Let's look at another famous example, the recurrent laryngeal nerve.
This nerve connects the brain and the larynx, but it does so by way of the thorax, where it loops around the ductus arteriosus, which is connected to the heart, before traveling back to the larynx. If you look at the anatomy of the fish, as shown to the left, this is actually a fairly direct route. But it's not such a direct route in mammals as shown on the right. What seems to have happened is that the gill arches in fish evolved to move down the body, thereby pulling the heart away from the larynx.
The path taken by the recurrent laryngeal nerve is stupid in humans, but gets totally ridiculous in animals with long necks like this giraffe. Again, if you were designing humans or giraffes from scratch, the recurrent laryngeal nerve would be much shorter and just go from the brain to the larynx. The current arrangement makes no sense from a good design point of view. But since humans and giraffes evolved from ancient ancestors in which the nerve was looped around the heart, the current anatomy makes sense.
Not all examples of poor design or vestigial traits are present in the adult anatomy. Sometimes we can see them during development. During development, some organisms develop traits that their ancestors have, but the adults do not. Pictured here are images from the developing wings and feet of chickens.
Chicken wings and feet have only three or four digits, but during development we can see evidence of five digits starting, while development only finishes three of them. It makes no sense for the embryo to start developing digits it's never going to use. But it does make sense if the early stages of development are following the path used by an ancestor that did have five digits.
Another example comes from early development in humans. One of these images is a human embryo at five weeks, and the other is a mouse embryo at 11 days. I won't reveal which is which right now, but you can read the description below for which is which. Notice how both of these embryos appear to have a tail.
The mouse will obviously develop that embryonic tail into a real tail. while the human will end up not developing the tail at all. Both have tails.
One keeps the tail, the other loses it, and it is not immediately obvious from this picture which is which. Why does the human embryo begin to develop a tail? It's a leftover trait from an ancestor that did develop a tail. In addition to the visible structures of our anatomy, when we look through the genome, we can find evidence of vestigial genes called pseudogenes. These are sequences that resemble functional genes, but are not expressed or have mutations that render them non-functional.
Often these pseudogenes are in sequence along the same chromosome with other functional genes. These extra, almost genes are in the genome because of a history of gene duplications. One continued to function while the other required a mutation rendering it non-functional. For example, if we look at the various globin genes, we can see a variety of functional genes and leftover non-functional pseudogenes.
These different globins are used in different tissues and during different developmental stages, which makes sense, but what doesn't make sense is for humans to have a bunch of extra genes that they no longer use. Inheriting them from ancestors that did use them under different conditions, or acquired them by random mutations, does make sense. An interesting example of a pseudogene is the gene that codes for galanalactone oxidase, which produces an enzyme that helps produce vitamin C.
The gene for this protein is functional in most mammals, but not in guinea pigs and most primates. Their genomes do contain a pseudogene, however. Why would the genomes of these species contain genetic sequence that is almost, but not quite the same as, the functional gene in their relatives?
It doesn't make any sense from a pure design point of view to have an extra, almost functional gene. On the other hand, it does make sense in a scenario in which their ancestor lost the functional gene due to a mutation, and the modern species inherited this remnant. from an ancestor.
This example is also interesting because it makes humans and guinea pigs susceptible to scurvy. Because they can't make their own vitamin C, they must consume it. For example, in fruits like limes. This is a problem for humans in some environments today, but it was unlikely to have been a problem for our primate ancestors that lived in jungles with lots of fruit available. Having a superfluous gene that allowed the production of vitamin C would not have made sense for ancestors that could consume tons of it, so natural selection did not preserve it in our ancestors, and we have inherited that loss.
In addition to pseudogenes, the genomes of many taxa are full of additional genetic elements such as lines, signs, aloe elements, and LTRs. The vast majority of these genetic elements don't appear to have any functional role, but they show similarities in related species. This is extra genetic material that you wouldn't put into a well-designed genome, but they're widespread because they are inherited from ancestors that acquired them. These were just a few of the examples of poorly designed and vestigial traits present in living things. There are plenty more that I didn't have a chance to talk about.
It's almost counterintuitive that flaws in the design of living things provide such strong evidence for evolution, but evolution is an imperfect process full of randomness, changing environments, and unpredictability. The flaws we see are some of the best markers of our biological history. Click here and there to do the YouTube things, and feel free to share this with anyone who you think might be interested.