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Issues with Trying to Measure Evolutionary “Fitness”

Frog with eggs: Photo 110649652 © Kevin Patrick |

[Originally published as The Fitness Illusion]

Evolution depends on the concept of fitness as part of its model. Organisms that survive are characterized as “more fit” and evolutionists regularly use phrases like “natural selection fits animals for their environments.” I’ve even used them myself. But we don’t often ask ourselves the question, What is fitness?

Scientists seem to assume everyone means the same thing when they talk about fitness when, in reality, they apply it quite differently. Today we will peel back the curtain a bit on how evolutionists misuse fitness in an attempt to win the origins argument.

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Most of us have heard the phrase “survival of the fittest.” But what does that mean?

To the Darwinist, teaching a class, it means differential reproductive success. In other words, in evolution classes, students are taught that fitness is measured by looking at the number of surviving offspring.

The problem here is threefold.

  1. Evolutionists are highly inconsistent with their application of this definition.
  2. Measuring fitness in this way is, in a sense, the ultimate rescuing device.
  3. Such fitness can only be measured at the organismal level, not the genetic level, even though evolutionists almost always appeal to genetic fitness.

Once you start paying attention, you will find that evolutionary concepts are not noted for their ideological or logical consistency. I often hear evolutionists refer to mutations as “beneficial” even though most of the mutations they mention have nothing to do with reproduction and therefore cannot increase fitness one iota. For example, the mutation that causes tetrachromatic vision has no reproductive advantage to the organism. Yet I’ve seen it classed as a fitness-increasing beneficial mutation.

One of the best examples of this kind of confusion about fitness is the way evolutionists look at the sickle-cell anemia mutation.

Sickle cell anemia is a disease caused by a mutation that controls the shape of red blood cells. When the mutation is present, the blood cells curve into the shape of a sickle. This significantly reduces the amount of oxygen the blood can carry. People who inherit this mutation from both parents can be badly debilitated by the disease, or even die of it. People who inherit it from only one parent are carriers but the effects from the misshapen cells are limited or non-existent.

In some countries, particularly West Africa, malaria is a great threat. People with sickle cell anemia are much less likely to get the malaria parasite, and carriers’ symptoms are generally much less than those that do not carry the trait. Thus evolutionists will sometimes argue that the sickle cell trait increases the fitness of those who carry it.

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Notice what has been done here: the definition of fitness has been changed.

Instead of involving reproductive success, now the definition is purely survival related. Evolutionists do this regularly when beneficial mutations are in question. They claim that any situational benefit to organisms is a beneficial mutation, therefore increasing fitness, when what their theory requires is an increase in reproductive fitness, which most supposed beneficial mutations are not.

Further, even if mutations were to increase reproductive success, this measurement of fitness simply punts the issue down the road. The fitness of an organism can never be measured in an ultimate sense.

Sure the number of offspring it has can sometimes be measured, but as fitness is defined, this is not enough. To determine how fit an organism was, you would need to determine the sum of all of its descendants for all time. After all, an organism may have thousands of offspring, but if all its offspring are sterile, then how reproductively fit was it? The fitness measurement of the next generation is a poor stand-in for such a far-reaching requirement.

Another problem with the appeal to fitness is that it is so often applied at the genetic level. But the level of fitness is impossibly complex to quantify because of the multitude of genes that make up an organism. Very rarely does one gene control the overall reproductive success of an organism. Reproductive success happens at the organismal level, not the genetic level. Yet what is most often examined is the fitness effect of a gene.

Fitness is an elusive idea that cannot be tied down when applied in the real world. If fitness does not exist or is inappropriately defined, perhaps we also need to re-examine our definitions of natural selection and beneficial mutations since these are so closely tied together.

Regardless, the evolutionary story is just that; a story, one best consigned to the waste basket.

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Written by Emory Moynagh

I graduated from Pensacola Christian College with a B.S. in Biology, then worked as a high school science teacher for two years before transitioning into a quality assurance role. I now do science and apologetics research. My personal interests in apologetics stem back to high school when I was introduced to the teachings of Ken Ham, ICR, CMI and others. This created a passion in me for Creation Science, the Bible, and all things science related. You can find my friends and me at In His

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