Evolutionary Fitness

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.

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.

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.

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.

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.

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:

$AbsolutefitnessofgenotypeX=\frac{No.ofindividualswithgenotypeXafterselection}{No.ofindividualswithgenotypeXbeforeselection}$

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:

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.