AP Biology 3.7 - Fitness

In this section of the AP Biology curriculum, we’ll be turning our focus to evolution. Specifically, we’ll be looking at the term “fitness” and what it means to have fitness as a biological organism. We’ll start by reviewing what fitness is, and how it fits into the theory of evolution by natural selection. Since most heritable adaptations that confer fitness are related directly to the biological macromolecules in a cell, we’ll see exactly how this process works. After we’ve made the connection from DNA through proteins to actual traits, we’ll take a look at a few examples. First, we’ll see how bacterial colonies can increase their fitness by gaining antibiotic resistance. Then, we’ll see how Peter and Rosemary Grant quantified the fitness conferred by different beak sizes and shapes in Galapagos finches!

Video Tutorial

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Resources for this Standard

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Naturally occurring diversity among and between components within biological systems affects interactions with the environment.

Explain the connection between variation in the number and types of molecules within cells to the ability of the organism to survive and/or reproduce in different environments.

Variation at the molecular level provides organisms with the ability to respond to a variety of environmental stimuli.

Variation in the number and types of molecules within cells provides organisms a greater ability to survive and/or reproduce in different environments.

3.7 Fitness Overview

Fitness describes an organism’s ability to survive and reproduce in a given environment. Since every organism is just an expression of a DNA molecule in a particular environment, fitness is really a measure of how well a DNA molecule can replicate itself. Let’s consider a few examples.

These two adult giraffes feed on the same tree. Which one has a higher fitness?
These two adult giraffes feed on the same tree. Which one has a higher fitness?

Consider these two giraffes and the single tree that makes up their environment. If both of these giraffes are adults, the giraffe on the right has a greater fitness. Can you see why? The giraffe on the left is shorter, and can only reach the lowest leaves on the tree. The giraffe on the right is much taller, allowing it to reach the entire canopy! Therefore, this giraffe will be able to get more food, grow faster, and have the excess energy needed to create more offspring! But, in real life, the environment is never this simple.

Mice try to hide from the fox on a dark-colored background
Mice try to hide from the fox on a dark-colored background

Let’s consider another example: the fitness of mice being hunted by a fox. The fitness of each mouse, in this case, is directly related to the mouse’s coat color. In this environment, mice with a white coat color are easily spotted by the fox. These mice have a lower fitness because they do not blend in with their surroundings. In fact, we directly see how much lower their fitness is compared to their black counterparts by looking at what happens to the population over time. Since all of the white mice get eaten, the population has shifted to more black mice. The way populations change due to differences in fitness is known as evolution through natural selection, and we will cover that topic further in section 7.

In this section, we are looking at how fitness is impacted by not only the molecules organisms create but also by the environment they live in. So, what if these mice existed in a much lighter-colored environment? In this case, the black mice would be much easier to spot. Therefore, the black mice would be eaten more often by the fox – leading to a decrease in the proportion of black mice in the future population.

Mice try to hide from the fox on a light-colored background
Mice try to hide from the fox on a light-colored background

So, what is fitness? Fitness is an expression describing how well a DNA molecule and the molecules it creates in an organism can survive and reproduce in a given environment!

In the context of evolution, it is important that traits conferring fitness can be passed onto offspring. Otherwise, populations of organisms would never have been able to evolve. Since DNA ultimately controls the number and type of molecules in every organism, DNA directly contributes to an organism’s overall fitness. Let’s take a closer look at how this actually works.

Consider a species of pea plant that can produce two colors of flowers – purple and white. The variation in color is caused by a single gene, the “B” gene. While we use shorthand like “capital B” or “lowercase b” to denote the different alleles of a gene, what’s actually happening at the molecular level is literally controlling the number and type of molecules present in the cells of each flower.

In the case of the capital B allele, the genetic variant codes for a protein that produces purple pigment molecules. The DNA is transcribed into RNA, translated into a protein, and released into the cytoplasm. In the cytoplasm, this protein folds into a specific shape. This enzyme is responsible for the final step in the reaction that creates pigment molecules. As it catalyzes this reaction, the newly formed products reflect purple light. As this pigment molecule fills up the cytosol within each cell, this ultimately creates purple flowers.

This Punnett square shows how different alleles can create different traits
This Punnett square shows how different alleles can create different traits

However, the lowercase “b” allele actually denotes a mutation within the DNA. This mutation could be tiny, but it could be enough to cause a change in the sequence of amino acids within a protein. This change in the primary structure of the protein makes the protein effectively non-functional. Without this protein, the pigment molecules cannot be converted to their final form. Therefore, the flowers with only lowercase “b” alleles have no way to produce purple pigment and are all white!

Now, what does all this molecular genetics have to do with fitness? Well, the color of a flower almost always affects the reproductive success of a plant. Consider why plants make flowers: to attract bees of course! If the bees in an area are only attracted to purple flowers, the plant with the white flower mutation will have a lower chance of getting fertilized, and therefore a lower overall fitness. On the other hand, it’s entirely possible that bees would be more attracted to the white flower, thereby increasing the fitness of the plant with white flowers.

As this clearly demonstrates, an organism’s overall fitness is a combination of both the number and types of molecules within an organism’s cells and the environment it lives in.

For the remainder of this lesson, let’s consider a few examples of how fitness works in different organisms and populations. To begin, let’s consider a population of organisms that doctors are constantly trying to eliminate the fitness of: bacteria. Antibiotics are designed specifically to attack a specific part of a bacterial cell. For instance, the antibiotic may bind irreversibly to a bacterial cell’s ribosomes, rendering it totally incapable of expressing the DNA.

However, there are many mutations that could overcome this theoretical mutation. In fact, it’s incredibly easy to see just how common some of these mutations can be, and how a single mutation can create a whole population of organisms with a high enough fitness to survive in a new environment. Check this out!

What’s interesting to notice in this video is the speed at which these populations grow across antibiotic-laced agar. Clearly, the variation present in the original population was enough to create entire populations of antibiotic-resistant bacteria at the center of the plate.

More often than not, the traits that contribute to an organism’s fitness are controlled by many genes interacting with the environment in complex ways. No one knows this better than Peter and Rosemary Grant, two famous evolutionary biologists who have studied Darwin’s finches on the Galapagos Islands for the past several decades. In fact, the Grants saw exactly how fitness works in changing environmental conditions.

For instance, when the Grants first arrived at one of the Galapagos islands, they found an environment thick with trees, shrubs, and plentiful with insects. When they started documenting the finch species present, they found that the large majority of the finches were tree finches – finches with beaks adapted to eating the insects and fruit found on trees.

The many different types of finch that exist in the Galapagos archipelago
The many different types of finch that exist in the Galapagos archipelago

However, shortly after they started documenting the finches, the island experienced a terrible drought. All of the freshwater dried up in a few months. In the course of a year, the entire island had changed from a tropical paradise to a scorching desert. When the Grants remeasured which finches were present on the island, they found almost exclusively cactus finches. The cactus finch still had a high fitness because it could easily gain access to the juicy flesh of the cactus using its large, sharp beak.

Other finches, like the seed finches with round beaks, had a low fitness after the drought because they could not obtain enough food to grow and reproduce. While beak size is a complex trait controlled by many genes, it still impacts a finch’s overall fitness and only works in certain environments.