Evolution is one of the cornerstones of the life sciences. It has been demonstrated countless times in different ways, and it is supported by multiple items of evidence.

Still, some people take issue with evolution for all sorts of non-scientific reasons and present faulty arguments against it. Advocates of the pseudo-scientific “Intelligent Design” theory go as far as claiming that features in forms of life on earth were designed by an intelligent being, and were created as we find them today.

Unfortunately, these arguments appear in public discourse and sometimes in school curricula, especially in the U.S., where they are sometimes shown as explanations equally valid to those derived from scientific reasoning.

Although “Davidson Online” features several pieces explaining evolution, we have yet to tackle the arguments that challenge it. Here I will try to sort things out by presenting the main claims against evolution and then scientifically debunking them. As you will see, most of the arguments derive from basic misunderstandings of evolutionary theory or other scientific principles.

“Evolution is just a theory”
This argument stems from a misunderstanding of scientific terms. According to the scientific method, a “theory” is an explanation grounded in a large amount of evidence. Among scientists, evolution is considered a very solid, well-trusted idea, and one of the most fundamental laws of nature. In fact, evolution is a theory just like thermodynamics and gravity. They are all falsifiable, but so far no credible evidence has been offered to disprove any of them, including evolution.
Many people confuse the word “theory” with “hypothesis”(an educated guess), which still needs to be tested, experimentally or otherwise.

“Evolution explains the origin of life”
Evolution does not explain the origin of life, but how it developed after it appeared on earth. In fact, the scientific definition of evolution is “a change in the allele distribution in a gene pool”, where “allele distribution” is how often a trait appears and “gene pool” refers to the collective genetic material of a reproducing population of a life form. In other words, to have evolution there needs to be a pre-existing gene pool, meaning life already must exist. Some theories attempt to explain the origin of life, notably including the abiogenesis theory. Evolution is not concerned with this question.
 

“If people descended from apes, why are there still apes?”
In each generation, each species must face natural selection. If a species is adapted to its surroundings, meaning it can survive and produce another generation, it will remain over time. If it can’t do these things, for any reason, then it becomes extinct.

One species might evolve from another, which is called an ancestral species, but the ancestral species might still be adapted enough to its surroundings to keep existing and surviving. Moreover, the newer species might go extinct while the ancestral one might survive.

All in all, what determines the extinction of a species is natural selection, not whether new descendent species have emerged as “branches” of the ancestral population.
 

“Evolution happens only at the micro level, not the macro level”
Many evolution deniers tend to separate evolution into two levels. The “micro level” refers to changes in single traits by point mutations, such as in the evolution of antibiotic resistance in bacteria, whereas the “macro level” refers to the appearance of new organs, physiological systems and species, such as the development of the eye.
Although I have looked for a precise definition of each purported “subtype” of evolution, and for criteria to tell them apart, I could not find any.

There’s a good reason for that: The definition of “macro-evolution” changes as new evidence comes to light. Once, the “flagship” of evidence against macroevolution was the eye, but after a study was published showing how an eye could develop in a small number of steps, where each step confers an evolutionary advantage, different evidence had to be sought. Nowadays, I see that macroevolution discussions refer to “irreducible complexity”. Examples include genes encoding proteins that are claimed to have complex yet essential functions, which they could not have evolved from an ancestral gene. Once, the bacterial flagella were presented as evidence for irreducible complexity, until that was debunked as well. Nowadays, hemoglobin serves as the poster child for this claim.

My answer to this claim is simple: separating micro-evolution from macro-evolution is completely misguided. Just as the force of gravity acting on two sand grains in a vacuum is the same force acting on two galaxies in space (“micro-gravity” and “macro-gravity”, respectively), it would be wrong to arbitrarily distinguish small changes in living species from larger changes that accumulate over time.

“Evolution is wrong because there are so many missing links between species”
According to evolutionary theory, all species on earth have a common ancestor. Similar pairs of species diverged from their last common ancestor fairly recently, whereas other, more different pairs of species diverged further back in history. Some transitional populations of species exhibit traits common to both an ancestral group and its derived descendant group. Much of the information we have about transitional species comes from the fossil record. Unfortunately, fossils are hard to come by because fossilization is a very rare event that requires many different conditions in a particular order, over time. Naturally, these conditions don’t occur everywhere at all times, so only few creatures have left fossils for us to discover. Most likely, many species that existed on earth left no fossil record, but that doesn’t mean they didn’t exist.

Still, the fossil record we do have contains many transitional fossils, all of which support evolution.
 

“Evolution contradicts the second law of thermodynamics”
Of all the arguments I have seen so far, this must be the most ridiculous. The second law of thermodynamics says that for an irreversible process in an isolated system, the thermodynamic state variable known as entropy (a term associated with disorder) is always increasing. Some people, including some scientists, claim that since evolution creates order, it contradicts the second law of thermodynamics.

As the first step to challenge this, we should look at whether evolution creates order. It doesn’t. But this argument is moot for other, more important reasons.

Firstly, the second law of thermodynamics only refers to total entropy. In some local areas, however, entropy can be lowered and order can be increased, at the expense of an entropy increase in the surroundings, over time.

Secondly, the second law of thermodynamics only refers to isolated systems, which receive no energy or matter from outside, and earth is not such a system. It takes up large amounts of energy from the sun over billions of years. If we sum the earth and the sun together for the entropy calculations, we’ll find that total entropy does increase, although locally, some “islands” of order are maintained.

“Life is so complex, it must have been designed”
Here we find a logical fallacy. Why does something too complex for us to understand or explain require an intelligent being to design it? The fact we cannot explain something does not imply that someone else can.

Moreover, many design failures in living organisms are tell-tale signs for how life evolved gradually, without any preconceived plan. The vagus nerve serves as a good example for this. In mammals, it leaves the brain, extends to the heart and goes back up to the larynx. In giraffes, the anatomical constraints are so extreme, the vagus nerve must extend up to almost five meters until is reaches the larynx.

In other branches of the tree of life, such as in fish, the vagus nerve is much shorter, because the brain, the heart and the gill arches are all next to each other. But as these structures evolved over time, and as the neck formed and extended over time, the vagus nerve got “stuck” on the wrong side of the heart and had to do a long detour to enervate the larynx. Any “intelligent” design would have planned a much shorter route.

“How do organisms pre-emptively evolve traits they will only need in the future?”
Quite simply, they don’t. Populations of organisms just adapt to specific habitats all the time. Each generation, traits are put to the test of natural selection. If a trait confers a fitness advantage, it is more likely to be found in the next generation. If it confers a disadvantage, the organism is less likely to survive and reproduce, and the trait is less likely to spread throughout the population. Small changes in traits add up to larger changes. Environments change and organisms’ traits closely follow those changes, step by step, and continuously over time, never pre-emptively.
 

“So how are complex systems formed?”
A series of small changes can add up to a big change. This requires each change to confer a relative advantage compared with the previous state. When these changes add up, we can observe complex systems evolving in living organisms.

For example, let’s have a look at the eye, which everyone can agree is quite a complex system. Being able to see gives organisms a huge advantage over their blind counterparts. In 1994, a study by Nilsson and Pelger showed that an eye could develop step by step from a tissue of light-sensitive cells, where each step confers a relative fitness advantage. What’s more, they also estimated that the eye could have evolved over the course of 364,000 years, which, in evolutionary terms, is a brief moment. So, all in all, even complex systems like eyes can evolve gradually.

“How do the ‘more advanced’ life forms sexually reproduce, as they are genetically different from the rest of the population?”
Small genetic changes don’t necessarily create a reproductive barrier. Just as a Pinscher dog can mate with a Rottweiler, despite their physical differences, so an organism with a variant trait can usually reproduce with other members of the same population. If the trait precludes reproduction, it won’t be found in the next generation, and before long, it will disappear from the gene pool.

“So where do new species come from?”
A biological species is a group of organisms that can reproduce with one another. As soon as organisms from one group cannot reproduce with organisms from another group, they are defined as two separate populations of two separate species.

Let’s say we have a population of gazelles. An earthquake creates a rift valley separating the population into two groups that cannot meet each other anymore. As time goes by, each group adapts to the changes in its environment. Over time, so many genetic differences have accumulated in each of the groups that a gazelle from one group cannot reproduce with a gazelle from the other group anymore. Once this reproductive barrier is in place, the two groups will evolve separately, and are considered two separate species with a common ancestor, preceding the earthquake.
 

There are many more claims out there concerning evolution and I invite you to write them in our comments section. I will try to answer each point, and the most interesting arguments will be added to the article.

 

Erez S. Garty, Ph.D.
Editor in Chief, Davidson Online
Davidson Institute of Science Education
Weizmann Institute of Science


Article translated from Hebrew by Aviv J. Sharon, M.Sc. student at the Weizmann Institute of Science.


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