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Evolutionary Fitness

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Evolutionary Fitness

In evolutionary biology, “fitness” refers to the ability to survive and reproduce. We'll see that it’s not always about being the fastest or strongest. We will discuss evolutionary fitness: its definition, its components, its relationship with environmental factors, and its role in evolutionary biology. We will also look at how it is measured by going over an example.

What is evolutionary fitness?

What is the definition of evolutionary fitness in biology?

Simply put, evolutionary fitness is the ability of an organism to survive and reproduce. It is measured by reproductive success–meaning, how well a genotype or phenotype is passed on to the next generation compared to other genotypes and phenotypes.

Genotype: refers to the genetic material that produces the phenotype.

Phenotype: the observable traits of an organism.

What are the components of evolutionary fitness?

Fitness encompasses both survival and reproduction, with an emphasis on reproduction.

Survival

For an organism to be able to reproduce, it has to survive long enough to reach reproductive age. Survival is a component of evolutionary fitness because if an organism is unable to survive, it will not be able to pass on its genotype or phenotype to succeeding generations. This means that traits that enable an organism to survive can increase evolutionary fitness.

For example, fishes produce thousands of offspring, but only a few survive. Parents invest little effort in caring for each individual. Offspring that are born with a better ability to escape predators, as well as find food and shelter have higher chances of surviving long enough to reach reproductive age. Therefore traits like coloration that help fishes hide from predators can increase fitness. The Carolina Madtom (Fig. 1) is a species of fish that uses coloration to blend in with its surroundings to hide from predators.

Figure 1. The Carolina Madtom is a small fish that blends with its surroundings to hide from predators. It also uses this adaptation to conceal its nest when breeding. Source: U.S. Fish and Wildlife Service Southeast Region, Public domain, via Wikimedia Commons.

Living longer also means that an organism has more chances to reproduce. For example, female pronghorn antelopes mate only when they are in “heat” (estrus phase of their seasonal cycle). Pronghorn antelopes that have better eyesight and endurance can outrun their predators and outlive other individuals. Living longer means that they can reproduce in multiple mating seasons.

Reproduction

Reproductive success does not only depend on an organism’s ability to survive but also its ability to attract mates and produce offspring. Reproduction is a component of evolutionary fitness because genotypes or phenotypes are passed on through reproduction. This means that traits that enable an organism to attract mates and produce offspring can increase evolutionary fitness.

A classic example is the peacock (Fig. 2). Notice how it has a large and colorful tail? The more extravagant its tail, the more mates it can attract and the more offspring it can produce. While having a more impressive tail does not increase its chance of survival, it increases its chance of reproduction. This means that sporting a larger and more colorful tail can increase fitness.

Figure 2. Peacocks use their large and colorful tails to attract mates. Source: kathypdx, CC BY-SA 4.0 , via Wikimedia Commons

What is the role of fitness in evolutionary genetics?

Genotypes that increase fitness tend to become more common in the population. This process is called natural selection. Natural selection is a process where individuals with traits that help them survive in their environment can reproduce more because of those traits.

Over time, the genetic makeup of the entire population changes, a process known as evolution. Evolution is a gradual and cumulative change in heritable traits of a population of organisms. This change takes place over the course of at least several generations.

What factors affect evolutionary fitness?

The selection of traits (meaning, which traits give an organism higher fitness and therefore are passed on at a higher frequency) is also affected by the present environment. The interaction of an organism with biotic (living) and abiotic (non-living) factors can affect its evolutionary fitness by increasing or decreasing the occurrence of a trait of a population of organisms at a given time.

Let’s say a habitat is polluted with a type of poison that can kill most marine life. While in the past, it may not have been a trait that affected their survival, tolerance for this poison during this period can increase fitness.

Additionally, a trait can have both positive and negative effects on fitness, depending on how it affects survival and/or reproduction.

For example, a peacock with a more impressive tail might attract more mates, but it might also catch the attention of more predators. On the other hand, a peacock with a less impressive tail but with stronger spurs (Fig. 3) on the back of its legs can attract fewer mates but outlive other peacocks. The peacock's spurs may not increase its chances of attracting mates, but it can increase its chances of survival, thereby increasing evolutionary fitness.

That the male peacock's tail is detrimental to its survival but is selected due to female preference is an example of sexual selection, a mode of natural selection in which mate preference influences the heritable traits of a population.

Figure 3. A male Palawan Peacock Pheasant with multiple spurs. Source: Dante Alighieri, CC BY-SA 3.0 , via Wikimedia Commons

Whether a trait increases or decreases fitness can depend on other factors in the present environment. How aggressive are their predators? How many other individuals are they competing with for a potential mate? How accessible are their food sources? How resilient are they to drought or diseases? This is why a genotype can increase fitness in one environment at a given time, but decrease fitness in another.

How is evolutionary fitness measured?

Evolutionary fitness is measured by reproductive success. It is usually expressed as absolute fitness or relative fitness.

Absolute fitness

Absolute fitness is measured based on the number of offspring produced by a genotype that would survive natural selection. It is usually denoted with (W). It can be calculated using:

When (W) > 1, this means that the genotype X is increasing over time;

When (W) = 1, this means that the genotype X remains stable over time;

When (W) < 1, this means that the genotype X is decreasing over time.

Relative fitness

Relative fitness is measured based on the proportion of the contribution of a genotype to the next generation’s gene pool compared to the contribution of other genotypes. It is denoted by (w). It can be calculated using:

The relative fitness (w) of genotype X can be interpreted as how fit it is compared to the fittest genotype.

Example of how evolutionary fitness is calculated

Let's say a population consists of individuals with genotypes A, B, and C, as presented in the table below:

No. of individuals before Selection
No. of individuals after Selection
Genotype A
100
120
Genotype B
100
60
Genotype C
100
100

Let’s calculate the absolute fitness of each genotype:

Let's calculate the absolute fitness of genotype A:

Hence, the absolute fitness of genotype A is 1.2

This means that genotype A produced an average of 1.2 offspring that survived natural selection.

Let's calculate the absolute fitness of genotype B:

Hence, the absolute fitness of genotype B is 0.6. This means that genotype B produced an average of 0.6 offspring that survived natural selection.

Let's calculate the absolute fitness of genotype C:

Hence, the absolute fitness of genotype C is 1. This means that genotype C can produce an average of 1 offspring that can survive natural selection.

The absolute fitness values of genotypes A, B, and C tell us that genotype A is increasing over time, genotype B is decreasing over time, while genotype C remains stable over time.

Now, let’s calculate the relative fitness of each genotype:

First, we need to identify the absolute fitness of the most fit genotype.

In our example, genotype A with absolute fitness of 1.2 is the fittest. It will be the standard which the other genotypes will be compared against.

Now let's calculate the relative fitness of genotype A:

Now let's calculate the relative fitness of genotype B:

Hence, genotype B is 50% as fit as genotype A.

Now let's calculate the relative fitness of genotype C:

Hence, genotype C is 83% as fit as genotype A.

Evolutionary Fitness - Key takeaways

  • Evolutionary fitness is the ability of organisms with a specific genotype to reproduce and pass on their genes to the next generation compared to those with other genotypes.
  • The main components of fitness are survival and reproduction. For an organism to be able to reproduce, it has to survive long enough to reach reproductive age.
  • Fitness can be measured as absolute fitness or relative fitness.
  • Absolute fitness is measured based on the number of offspring produced by a genotype that would survive natural selection.
  • Relative fitness is measured based on the proportion of the contribution of a genotype to the next generation’s gene pool compared to the contribution of other genotypes.

Frequently Asked Questions about Evolutionary Fitness

Evolutionary fitness measures reproductive success, or how well a genotype or phenotype is passed on to the next generation compared to other genotypes and phenotypes.

Evolutionary fitness is measured by reproductive success. It is usually expressed as absolute fitness or relative fitness. Absolute fitness is measured based on the number of offspring produced by a genotype that would survive natural selection. Relative fitness is measured based on the proportion of the contribution of a genotype to the next generation’s gene pool compared to the contribution of other genotypes.

A trait can increase evolutionary fitness if it increases the chances of survival and/or reproduction.

Coloration and other traits that help organisms live longer increase evolutionary fitness. For example, fishes produce thousands of offspring, but only a few survive. Offspring that are born with a better ability to escape predators, as well as find food and shelter have higher chances of surviving long enough to reach reproductive age. Therefore traits like coloration that help fishes hide from predators can increase fitness.

The interaction of an orrganism with biotic and abiotic factors can affect its evolutionary fitness by increasing or decreasing the occurrence of a trait of a population of organisms at a given time.

Final Evolutionary Fitness Quiz

Question

What is evolutionary fitness?

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Evolutionary fitness is the ability of organisms with a specific genotype to reproduce and pass on their genes to the next generation compared to those with other genotypes.

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How is evolutionary fitness measured?

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Evolutionary fitness is measured by reproductive success.

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What increases evolutionary fitness?

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A genotype can increase or decrease fitness, depending on its impact on survival and reproduction.

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What are the two main components of evolutionary fitness?

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Survival

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Why is survival important to reproductive success?

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For an organism to be able to reproduce, it has to survive long enough to reach reproductive age. Living longer also means that an organism has more chance to reproduce.

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What is the role of fitness in natural selection and evolution?

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Genotypes that increase fitness tend to become more common in the population in the process of natural selection. Over time, the genetic makeup of the population changes, a process known as evolution. 

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How is absolute fitness measured?

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Absolute fitness is measured based on the number of offspring produced by a genotype that would survive natural selection.  

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How is relative fitness measured?

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Relative fitness is the proportion of the contribution of a genotype to the next generation’s gene pool compared to the contribution of other genotypes.  

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What does an absolute fitness of greater than 1 imply?

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The genotype is increasing over time.

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What does an absolute fitness of less than 1 imply?

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The genotype is decreasing over time.

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What does an absolute fitness of 1 imply?

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The genotype is stable over time.

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If darker-colored peppered moths survive and leave more offspring compared to their lighter-colored counterparts, which genotype increase fitness?

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In the example, dark coloration increases the peppered moth's fitness.

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Does a genotype always have the same effect on fitness?

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No, a genotype may increase fitness in one environment but decrease fitness in another.

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What is an ecosystem?

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An ecosystem is composed of all living organisms and their interaction with the physical environment.

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What are biotic factors in an ecosystem?

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Biotic factors are living things in an ecosystem.

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What are abiotic factors in an ecosystem?

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Abiotic factors are non-living things in an ecosystem.

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What are biotic factors that produce their own food called?

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Autotrophs

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What are biotic factors that consume other organisms called?

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Heterotrophs

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What are biotic factors that feed on dead or decaying organisms?

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Detritivores

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Is temperature a biotic or an abiotic factor?

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Abiotic factor

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Are predators a biotic or an abiotic factor?

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Biotic factor

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Is the pH level of soil a biotic or an abiotic factor?

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Abiotic factor

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Coral and microscopic algae depend on each other for survival. When water temperatures get too high, the microscopic algae leave the coral tissue and the coral slowly dies. In this example, what abiotic factor affected the survival of corals?

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Water temperature

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What is the difference between biotic and abiotic factors?

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Biotic factors are living organisms while abiotic factors are non-living environmental conditions within an ecosystem.

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How are biotic and abiotic factors related?

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Biotic and abiotic factors are both selection pressures that are found within an ecosystem.

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How do biotic and abiotic factors interact?

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The interaction between biotic and abiotic factors determines the characteristics of an ecosystem. The survival and reproduction of living organisms (biotic factors) depend on abiotic factors such as temperature, wind, and water.

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Organisms benefit from each other in what type of interaction between biotic factors?

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Mutualism

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Organisms consume other organisms in what type of interaction between biotic factors?

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Predation

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What is "phenotypic variation"?

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Phenotypic variations are differences among organisms in terms of observable traits (phenotypes).

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What factors cause phenotypic variations?

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Phenotypic variations are caused by genotypic variations and environmental conditions.

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Why is phenotypic variation necessary for natural selection?

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Phenotypic variations lead to different survival and reproduction rates among organisms. Natural selection--where individuals with traits that are more adapted to the environment have more chances of survival and reproduction--can only take place when these differences are present.

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What is a phenotype?

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The phenotype of an organism refers to its observable traits.

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What is a genotype?

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A genotype is the combination of alleles (gene variants) that an organism has. The genotype contributes to the phenotype of an organism.

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When two alleles of a gene are identical, this is referred to as:

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Homozygous

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When two alleles of a gene are different, this is referred to as:

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Heterozygous

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What is mutation?

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Mutation is a change in the sequence of genes in DNA. It is the ultimate source of new alleles.

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What is gene flow?

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Gene flow is the movement of genes from one population to another. This can occur when organisms migrate and reproduce with a different population or when pollen or seeds are dispersed to a population that is geographically separated.

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How does sexual reproduction promote genotypic variation?

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Sexual reproduction promotes genotypic variation by creating new combinations of genes.

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How does the environment contribute to phenotypic variation?

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Environmental conditions such as climate, availability of food, and interactions with other organisms can influence the development of inherited traits, thus contributing to phenotypic variation.

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What are discrete variations?

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Discrete variations are traits with qualitative differences. These are distinct and separate categories with nothing ‘in between’.  

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What are continuous variations?

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Continuous variations are traits expressed as quantitative differences with a wide range of values.  

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What are polygenic traits?

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Polygenic traits are traits that are determined by more than one gene.  

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What is the difference between a dominant and a recessive allele?

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A dominant allele affects the phenotype, while the recessive allele does not.

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