Biological Fitness

Perhaps you have heard the phrase “survival of the fittest'', which is commonly attributed to Charles Darwin, but was actually coined by a sociologist from the UK named Herbert Spencer in 1864 in reference to Darwin’s ideas. Fitness is something that we often refer to in biology, but have you ever wondered what it actually means? Is fitness always dictated by the same factors? What factors determine an individual’s fitness?

In the following, we will discuss biological fitness - what it means, why it's important, and what factors are involved.

The Definition of Fitness in Biology

In biology, fitness refers to an individual organism’s ability to successfully reproduce and submit their genes to the next generation of its species. In its most basic form, the more an organism can successfully reproduce in its lifetime, the higher its level of fitness. Specifically, this refers to the successful transmission of beneficial genes to subsequent generations, as opposed to those genes that are not transmitted. Of course, there are many other factors that can influence this fitness, most significantly overpopulation, where successful reproduction no longer results in increased fitness, but this is not common in the natural world. Sometimes, biological fitness is called Darwinian fitness.

What is the Highest Level of Biological Fitness?

The organism that can produce the highest number of offspring that survive to adulthood (breeding age) is considered to have the highest level of biological fitness. That is because these organisms are successfully passing their genes (genotypes and the phenotypes that they produce) into the next generation, while those with lesser fitness are passing their genes on at a lesser rate (or, in extreme cases, not at all).

Components of Fitness in Biology

Biological fitness can be measured in two different ways- absolute and relative.

Absolute fitness

Absolute fitness is determined by the total amount of genes or offspring (genotypes or phenotypes) submitted to the next generation within an organism’s lifespan. In order to determine the absolute fitness, we must multiply the number of successful offspring with a specific phenotype (or genotype) produced with the percent chance of surviving to adulthood.

Relative fitness

Relative fitness is concerned with determining the relative fitness rate against the maximum fitness rate. To determine relative fitness, one genotype or phenotype's fitness is compared to the more fit genotype or phenotype. The fitter genotype or phenotype is always 1 and the resulting fitness level (designated as W) will be between 1 and 0.

An Example of Fitness in Biology

Let's look at an example of absolute and relative fitness. Let's say saltwater crocodiles (Crocodylus porosus) can be either standard coloration (which may vary between light green and yellow or dark grey, depending on habitat preferences) or leucistic (reduced or lacking pigmentation, resulting in a whiteish coloration). For the sake of this article, lets say that these two phenotypes are determined by two alleles: (CC and Cc) = standard coloration, while (cc) = leucistic.

Crocodiles with the standard coloration have a 10% chance of survival to adulthood and reproduction results in an average of 50 hatchlings. Leucistic crocodiles, on the other hand, have a 1% chance of surviving to adulthood and have an average of 40 hatchlings. How do we determine absolute and relative fitness for each of these phenotypes? How do we determine which phenotype has the higher fitness level?

Determining Absolute Fitness

To determine the absolute fitness of each phenotype, we must multiply the average number of offspring of that specific phenotype produced with the chance of survival to adulthood. For this example:

Standard coloration: an average of 50 hatchlings produced x 10% survival rate

• 50x0.10 = 5 individuals

Leucistic: an average of 40 hatchlings produced x 1% survival rate

• 40x0.01= 0.4 individuals

Determining Relative Fitness

Determining the relative fitness is straightforward. The fitness (W) of the more fit phenotype is always designated as 1, by dividing the individuals produced (5/5= 1). This would be the relative fitness of the standard coloration, designated as WCC,Cc.

To determine the relative fitness of the leucistic individuals (Wcc), we simply need to divide the number of leucistic offspring (0.4) by the number of standard offspring (5), which results in 0.08. Thus...

• WCC,Cc= 5/5= 1

• Wcc= 0.4/5= 0.08

It is important to note that this is a simplified scenario and in reality things are much more complex. In fact, the overall survival rate for hatchling saltwater crocodiles in the wild is estimated to be around only 1%! This is primarily due to the high level of predation that hatchlings experience. Essentially, saltwater crocodiles begin at the bottom of the food chain and, if they survive to adulthood, end up at the top. Leucistic individuals are much easier for predators to spot, so their chance of survival would be significantly lower than 1%, but they are still occasionally encountered, as can be seen in Figure 1.

Figure 1: Leucistic crocodiles have a much lower chance of survival (lower fitness) than other individuals, likely due to increased chance of predation as hatchlings. This leucistic saltwater crocodile is present along the Adelaide River in Australia's Northern Territory. Source: Brandon Sideleau, own work

Advantages of Having a Higher Level of Biological Fitness

It should go without saying that having a higher level of biological fitness is extremely advantageous in the natural world. A higher fitness level means a better chance of survival and the passing on of genes to the next generation. In reality, determining fitness is never as simple as the examples we have discussed in this article, as there are numerous different factors influencing whether or not a genotype or phenotype is passed onto subsequent generations.

Biological Fitness and Natural Selection

To put it simply, natural selection determines an organism's level of biological fitness, since an organism's fitness is determined by how well it responds to the selective pressures of natural selection. As stated above, these selective pressures vary depending on the environment, which means that specific genotypes and their associated phenotypes may have different fitness levels depending on which environment they are found in. Therefore, natural selection determines which genes are passed onto subsequent generations.