What Is Evolutionary Fitness? (Example, Problem & Explain)

Evolutionary Fitness

Evolutionary fitness theory explains how modern organisms evolved from their ancestors over long periods. Then this is one of those big things. Evolution believes that the earth has been around for significantly long periods. So organisms change over time, and the ultimate source of all change is mutations.

Mutations are going to be variations that exist in populations. Also, mutations will be changes in the DNA sequence. Some mutations ultimately become more prevalent. So more abundant is another bird for that if they improve the fitness of an organism. Evolutionary biologists work closely with many similar disciplines, and people might often call themselves paleontologists or evolutionary ecologists.

They usually study how a single trait like a brain or a nervous system can change between organisms or change over time. They also study the genetic basis for different characteristics how do other species vary genetically or genomically. For example, it’s often quoted that around 15 or 20 different genes are involved in human skin color. Each of them has different alleles, and the combination of all of those alleles results in a wide variety of skin colors.

What is evolutionary fitness?

What was Darwin working with when he was building his case? When he was in South America, he found the giant sloth fossils, and he was used to seeing sloths as really tiny creatures. But, when he saw the structure, it looked like a sloth except much bigger. So fossils were a vast finding for him. DNA is molecular evidence of evolution.

If you compare DNA from a chimp in a human, you’ll find it there roughly 98% similar. It’s an observation that’s proven true that organisms change over time through mutation through adaptation to their environment. Evolution is natural selection, which means selecting an organism based on its fitness when a mutation helps an adaptation organism.

Darwin discussed a lot about evolutionary fitness or survival of the fittest. Fitness is not the fastest or lifts the most weight. An organism can merely reproduce and create viable offspring. For example, Suppose organism “A” ends up finding a mate and making babies an organism “B” does not. Then, A has more fitness biologically than organism “B.”

Suppose organism “A” creates awesome babies and survive to make lots of their babies. Organism “B” babies don’t do that so much again organism “A” is more fit. Also, the traits allow them to become fitter and have those viable offspring that make them fit. Viable offspring are offspring that live to maturity that can reproduce themselves.

So Fitness is how well you make children and how your children make more children. Some mutations become prevalent in the “A” population because they improve an organism. Fitness is then called adaptations.

There are a couple of types of adaptations:

  • Physical adaptations like color, eyesight, defense mechanisms, etc.
  • Behavioral adaptations like hunting behavior that allows catching more prey. Grouping of organisms in hurt or pack versus being solitary is group behaviors altruism. When you think of altruism, you should be thinking of being selfless doing something nice for others. That is a behavioral adaptation.

Formula: Fitness equals the multiplication of survival percentage with the average number of offspring.

Fitness (W) = Percentage of survival × Average number of offspring

Evolutionary fitness has to do with the ability to survive, and the idea is that going to have adaptations or survive. Darwinian Fitness’s statement has to do the best set of adaptations. In other words, how much at any given individual contributes to the gene pool.

Gene flow brings in new genes or takes away genes for the population. It has to do with how much pressure the environment pulls on those genes. Then there’s also the concept of relative fitness. It compares the fitness of different members of the same species and tries to understand how much each is likely to contribute to the next generation.

Fitness has to do with how much contributes to the current gene pool. Then, fitness has to do with how much it will contribute to the next generation of the gene pool. Also, it has to do not with having the best set of adaptations and increasing the chances of having a longer lifespan reproducing faster or more. They also succeed in the offspring; otherwise, the offspring lives to do precisely the same.

Evolutionary fitness is all about the fittest, who tend to survive through natural selection. Natural selection has something to do with artificial selection. It’s like the same thing an official selection is that humans do. Populations tend to grow exponentially, but the thing is that environments have limited resources.

So not all population members get to live, and they will struggle to survive. The more differences among them will make the difference, and those with the best adaptations will live longer. They have more children than survive to the same, and therefore they will contribute more to the next generation and become more common in the gene pool. Genetic drift causes changes in the population.

Selection works overtime, leading to species differentiation across populations. Selective pressure animals compete for a lot of things. They compete for their niche or their roles in the environment.

Therefore there will be fighting, and the strongest will occupy that niche. So that’s the idea of the actual niche versus the realized niche. There’s also sexual pressure or reproduction pressure to have a better chance of carrying the offspring. All of these things will lead to sexual selection.

Animals have tolerance levels and can’t expand beyond a specific niche. So they won’t be able to live if the environment changes too fast. They compete for mates, shelter, and all kinds of different things, and that’s true even in humans.

Example of evolutionary fitness

Consider many frogs sitting on logs in their habitat. Let’s assume these are the same frog species to breed with each other and pass down their DNA to their offspring. Naturally, there is variety in these frogs. Some of these frogs are darker green, maybe almost brown. Some of them are lighter green.

There is a variety of traits even in the same species. So back to the log. Do you know what else is in this habitat? Predators! The predators find that the lighter frogs are much easier to see in this habitat than the darker frogs. So, in this particular environment, the darker frogs have an easier time surviving and potentially more fitness if they breed.

Evolutionary fitness

In the biological sense, Fitness is determined not by how long they live but by how many offspring they have. These darker frogs pass down their DNA to their offspring so that the new baby frogs will have DNA from their parents. The lighter frogs are selected since they are easier to see in this habitat. Over an extended period, you could expect to see a higher frequency of darker frogs. It could even result in all frogs in this area being darker if it continues for a very long time.

Evolution changes over time and could take place because natural selection has occurred. Natural selection is a mechanism of evolution. It doesn’t necessarily mean that the allele for lighter color is gone completely.

The allele could be recessive and carried within the population. Because of processes like crossing over and mutations, there are opportunities for variety. But if this habitat and predators do not change, darker frogs will continue more fitness. Variation or mutations are not things that a frog can “will” itself to have.

Mutations and variations are random. It’s possible they might not affect an organism’s fitness. So, in that case, the genes are being passed on if that organism happens to reproduce. Or variations and mutations could be harmful.

If they are negative and negatively affect the organism’s fitness, that trait will not be passed down. But if they positively affect fitness, then that frog may have more babies than average because that trait helps them survive and reproduce. More babies will receive the passed those genes. Over time, that trait that is an advantage will be more frequent in the population.

Problem & Solution

Problem: Assume 50% of dark moths survive, and each produces an average of 10 offspring, while light moths have 80% survival and produce an average of 0.9. Solve for absolute fitness of both phenotypes.

Solution: Multiply those numbers! So the fitness of the dark moths would be,

W (Dark) = 0.5 × 10 = 5
W (Light) = 0.8 × 9 = 7.2

Light color moths have a higher fitness rate. Its survival rate is higher and has
roughly the same reproductive rate.

Relative fitness is the reproductive success of an organism compared to the fittest.

For light moths, the relative fitness is 1.

Relative fitness of light moths = 7.2/7.2 = 1
Relative fitness of dark moths = 5/7.2 = 0.69

The dark moths are 69% as fit as the standard light moths.

Unfitness of the dark moths = 1- 0.69 = 0.31, So 31% organisms are not surviving.
Unfitness of the light moths = 1 – 1 = 0, No unfitness of light moths.

This is also called the selection coefficient, which is denoted by S.

Slection coefficint, S = 1 – W

Darwin coined the phrase descent with modification.

Every species, living or dead, must have descended by reproduction from pre-existing species, and that species must change over time. – Darwin

Alfred Wallace came to a similar conclusion at the same time as Darwin. Darwin delineated the necessary parts of natural selection, which drove evolution. There are four main parts. First overproduction, more offspring are produced than can survive. This creates some competition. Then there must be genetic variation individuals have different traits. It is important for natural selection because traits must be selected from the existing variety.

There is a struggle to survive. Animals with adaptations may have better survival than those who do not. Then differential reproduction animals with the best adaptations may survive and reproduce in natural selection. Fitness is ultimately measured by whether the genetic material is passed on to offspring or not. If an individual has lots of viable offspring, they’re fit. If they have no offspring, they’re not fit because their genes are gone from the population.

Read More:


Wassersug, J. D., and R. J. Wassersug. Fitness fallacies. Natural History 3:34–37.
Maynard-Smith, J. Evolutionary Genetics ISBN 978-0-19-854215-5
Hartl, D. L. (1981) A Primer of Population Genetics
Kimura, James F. Crow, Motoo. An introduction to population genetics theory.

Julia Rose

My name is Julia Rose. I'm a registered clinical therapist, researcher, and coach. I'm the author of this blog. There are also two authors: Dr. Monica Ciagne, a registered psychologist and motivational coach, and Douglas Jones, a university lecturer & science researcher. I would love to hear your opinion, question, suggestions, please let me know. We will try to help you.

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