Evolution is blind

Note: the notion that evolution is blind has nothing to do with Darwin's eyesight. I just thought this was an amusing image.

The notion that evolution is blind has nothing to do with Darwin’s eyesight. I just thought this was an amusing image.

One of the central tenets of evolutionary biology is the concept that evolution is blind. In other words, it has not foresight or goal. This principle is extremely important for understanding how evolution works, but it’s a concept that is often misunderstood, even among people who accept evolution. Further, this lack of understanding often leads to a number of erroneous creationist arguments such as the claim that whales defy evolutionary theory. Similarly, the popular irreducible complexity argument is easily defeated once we understand that evolution is blind.

Note: in this post, I am going to use “evolution” to refer specifically to evolution by natural selection. Remember that evolution is simply a change in allele frequencies over time, and natural selection is one among several mechanisms that causes allele frequencies to change (others include things like gene flow and genetic drift). The concept of blind evolution is, however, most germane to evolution by natural selection; therefore, that is what I will be describing in this post.

Before I can explain what is meant by, “evolution is blind,” I have to give a brief primer on how evolution by natural selection works. Natural selection requires two things: heritable variation for a trait and selection for that trait. Both of these are nearly always met in real populations. In other words, in any population there is variation for a trait (e.g., not all individuals are the same height) and that variation is nearly always heritable (e.g., tall individuals tend to produce tall offspring). Further, that variation affects fitness (e.g., tall individuals may be able to get more food, which gives them more energy, which lets them produce more offspring), so certain traits get “selected” by virtue of the fact that the individuals with those traits produce more offspring than individuals without those traits. As a result, the genes for the beneficial trait will be more common in the next generation. Thus, the population will evolve because its allele frequencies will change.

There are several important clarifications to be made here. First, everyone accepts natural selection. It’s a simple mathematical certainty (even young earth creationist organizations accept it, they just place arbitrary and logically invalid limits on it). Second, evolution only acts on populations not individuals. Individuals simply cannot evolve. Third, in evolutionary terms, “fitness” refers to the number of genes that you get into the next generation, not physical strength. Generally speaking, being physically fit does give you a higher evolutionary fitness, but not always. Thus, the phrase, “survival of the fittest” is something of a misnomer. Survival is important only in that it gives you more time to produce offspring, and there are plenty of short lived species that have a high evolutionary fitness. For example, think about species like many octopuses where the females die after laying their first and only clutch of eggs. They have a lower survivorship, but a high fitness (fun fact: “octopi” = several within a species, “octopuses” = several species). Thus, natural selection only acts on traits that affect your reproductive potential. These may be traits that directly influence your survivorship, such as antipredatory behaviors, but they can also be traits like foraging ability (more food = more energy = more offspring) and the ability to attract a mate (i.e., sexual selection).

It may seem like I have digressed from my thesis, but this is all important groundwork for understanding blind evolution. They key here is that evolution has no foresight or direction. In other words, it has no goal or endpoint in mind. In each generation, it simply adapts populations to their current environment, but if that environment changes, then an adaptation that has been useful for thousands of generations can suddenly be detrimental. Let’s say, for example, that we have a group of birds who eat very small seeds that are held in little folds of the plants. Thus, they need fairly small, skinny beaks to get to the seeds. Therefore, in each generation, the birds with the beaks that are most well suited to reaching into the folds get the most food and produce the most offspring. So for many generations, evolution has been shaping the birds beaks to fit in these folds. However, one year there is a massive drought, and all of the plants that the birds usually get seeds from die, but another species with large thick seeds survives. Now, the small, petite beaks that have been so useful are suddenly detrimental, and large thick beaks are useful. This means that the birds who previously would have had a very high fitness are now going to have a very low fitness, and the thick billed birds that would previously have had a low fitness are suddenly going to have a high fitness. This is what we mean by, “evolution is blind.” It cannot anticipate the future needs of an organism. All it does is adapt a population to its present environment.

Hopefully, at this point, the problem with creationists’ whale argument is clear. For those who aren’t familiar with this argument, creationists often claim that whales are a problem for evolution because evolutionary history tells us that all land organisms evolved from a marine ancestor, but whales would have had to evolve from a terrestrial ancestor. Thus, creationists claim that whales had to evolve “backwards” or “de-evolve” because they went back to water. Similar arguments are made about species like flightless birds.

The problem with these arguments is simply that they ignore this concept of evolution being blind. There is no “forwards” or “backwards.” At one point in time, for a certain population of marine organisms, it was beneficial to be able to come out on land. Therefore traits that allowed individuals to come out on land were selected for. Then, millions of years later, for a certain population of land-dwelling mammals, it was beneficial to be able to go into the water. Therefore, nature selected the traits that allowed individuals to enter the water. This is in no way shape or form a problem for evolution because in both cases, populations were evolving to match their current environment.

It’s also worth noting that there is really no such thing as being “more evolved” because evolution has no direction. Chimps are not, for example, more evolved than a single celled bacteria living in a hot spring. Chimps are certainly more complex, and they certainly have accumulated more genetic changes, but they are not “more evolved” because that suggests that evolution is directional. Both chimps and bacteria are well adapted to their current environments, and that is all that evolution does: it adapts populations to their present environments. Think about it this way, a chimp would die in the hot springs where many bacteria thrive, and the specialized bacteria would die in the chimp’s rainforest. They are both highly adapted for their environments, but neither one is more evolved than the other. To put this another way, you can say that chimps are more evolved for a life in the forest, but you cannot make a broad comparison between them and bacteria because it is equally fair to say that certain bacteria are more evolved for a life in the hot springs.

It’s also important to realize that because evolution is blind, we don’t expect it to produce perfect organisms. Rather, we expect organisms to be a hodgepodge of former traits. In other words, we expect them to have a large number of evolutionary leftovers. We call these leftovers vestigial traits. I plan on devoting an entire post to them in the future because they provide extremely strong evidence for evolution, but to describe them briefly, these are traits that have no function or a very limited function in the current organism, but they would have been fully functional in that organism’s ancestors. Blind cave fish are the classic example of vestigial structures. These are fish that have eyes, but the eyes are no longer functional and often have a layer of skin growing over them because the fish spends its whole life inside a pitch black cave. So, at one point in time there was a population of fish living outside of a cave that had functional eyes. Then, for one reason or another, the fish ended up inside of the cave where the eyes were no longer useful. Thus, nature stopped selecting for vision, and the eyes slowly accumulated mutations to the point that they are now useless. Animals are full of examples of these structures. For example, baleen whales and certain snakes retain non-functional pelvic bones. Humans also have many vestigials. Our tail bones, goose bumps, wisdom teeth, and multiple other features are all vestiges of our evolutionary history. They are evolutionary leftovers that were beneficial in previous environments and situations but are no longer beneficial today.

Finally, I want to briefly discredit irreducible complexity. I addressed this topic in detail here, but to put it in its simplest terms, irreducible complexity states that certain biological systems are highly complex to the point that removing any one part prevents the system from function. For example, irreducible complexity claims that the bacterial flagellum couldn’t have evolved because it requires 42 proteins to function (for most species), and if any one protein is removed, it no longer functions as a flagellum. Thus, the argument is that it couldn’t have evolved because no one protein would be useful unless all of the other proteins were already in place. The problem is that this argument sets up the flagellum as some ultimate endpoint that evolution is working towards, but as you now know, that’s not the way that evolution works. Each protein doesn’t need to function as a flagellum, it just has to function. In other words, if the protein does anything useful, it will be selected for. You see, the function of a trait can change in response to new environments or as a result of new mutations. In fact, we know that all of the proteins in flagella are used for other things in the cell, and often several of them work together to perform a function. It is not hard to imagine a series of mutations that brings these functions together. In fact, we have a hypothetical pathway that would allow a flagellum to evolve step by step with each step being useful. Only the final step functions as a flagellum, but that doesn’t matter because each step still functions, and that is all that evolution needs because it is a completely blind process that acts without any forethought or anticipation of future needs.

Evolution by natural selection simply adapts populations to their current environments. It cannot anticipate future environments or needs. As a result, a trait may be selected for in one generation, and selected against in a later generation after the environment changes. Therefore, it is incorrect to describe evolution as having a “direction” because it is simply responding to the current conditions.

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