Evolutionary Fitness

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.

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

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

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.