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 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?
The components of evolutionary fitness encompass 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 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.
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. 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.
What is the role of fitness in evolutionary genetics?
Fitness plays a crucial role 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 the 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 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.
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 in biology?
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:
Absolute fitness of genotype (W) = The number of individuals after selection / the number of individuals before selection
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:
Relative fitness of genotype (w) = absolute fitness of genotype / absolute fitness of most fit genotype
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 try calculating the absolute fitness of each genotype.
The absolute fitness of genotype A can be calculated as follows:
- 120 individuals with genotype A after selection / 100 individuals with genotype A before selection
- 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.
The absolute fitness of genotype B can be calculated as follows:
- 60 individuals with genotype B after selection / 100 individuals with genotype B before selection
- 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.
The absolute fitness of genotype C can be calculated as follows:
- 100 individuals with genotype B after selection / 100 individuals with genotype B before selection.
- 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 try calculating 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:
- absolute fitness of genotype A / absolute fitness of genotype A
- relative fitness of genotype A = 1.2 / 1.2
- relative fitness of genotype A = 1
Now let's calculate the relative fitness of genotype B:
- absolute fitness of genotype B / absolute fitness of most fit genotype A
- relative fitness of genotype B = 0.6 / 1.2
- relative fitness of genotype B = 0.5 or 50%
- Hence, genotype B is 50% as fit as genotype A.
Now let's calculate the relative fitness of genotype C:
- absolute fitness of genotype C / absolute fitness of most fit genotype A
- relative fitness of genotype C = 1 / 1.2
- relative fitness of genotype C = 0.83 or 83%.
- 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.
References
- Figure 1: Carolina Madtom (https://commons.wikimedia.org/wiki/File:Carolina_Madtom_hiding_in_the_wild.jpg) by U.S. Fish and Wildlife Service Southeast Region, Public Domain.
- Figure 2: Peacock (https://commons.wikimedia.org/wiki/File:Peacock_-_Sapphire_Blue.jpg) by kathypdx, licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en).
Explanations 
Exams 
Magazine 
