3.7: Fitness

All throughout Unit 3 we've learned about the cellular processes that keep organisms alive at the molecule level. Topic 3.7 discusses how variation in those structures helps organisms survive in varying environments.

Vocab List

• Biological fitness
  • Evolution
  • Natural selection
• Hemoglobin f
• Hemoglobin a
• Chlorophyll a
• Chlorophyll b

Written Explanation

Biological fitness is the ability of an organism to survive, reproduce, and pass on its genes to the next generation. This becomes easier with greater variation in the types and numbers of each molecule. With more types of a certain molecule, there is a higher chance that the organism will be able to adapt to a different environment and the molecule/cell will still be able to perform its necessary functions. The organism, which has survived the environmental change by having a trait which made it more adaptable to the new environment is now able to pass on its genetic material. Over time, the overall population will begin to exhibit a greater amount of that molecule. This is known as the Theory of Evolution through natural selection. (We will cover evolution more in Unit 7.)

Some examples of molecular variation and fitness include variety in hemoglobin in humans, chlorophyll in plants, and phospholipids in cell membranes:

Hemoglobin is the oxygen binding molecule in blood. When a fetus is in the womb, it produces Hemoglobin F (F for fetal), however, once the child is born they produce Hemoglobin A (A for adult). This change in gene expression is caused by a change in the environment. Hemoglobin F is much better at binding to oxygen, as any fetus is going to be in a low oxygen environment (the womb). Meanwhile, Hemoglobin A doesn't “stick” as well to oxygen, as it is much easier to become saturated with oxygen outside of the womb, and Hemoglobin A also makes it easier for the oxygen to enter the body's tissue. Hence, the transition between the type of hemoglobin molecules allows for the person to better adapt to the environments at different stages of their lives.

Furthermore, different chlorophyll molecules are able to capture different wavelengths of light better. Having more kinds of chlorophyll allows the plant to continue photosynthesizing and capturing energy from light in different seasons and times of day. For instance, chlorophyll a is better at absorbing violet and red wavelengths, while chlorophyll b is better at absorbing blue and orange wavelengths. Paired together they become more efficient at harvesting wavelengths across the entire light spectrum. In the chart below, you can see how the only visible light wavelengths that aren't absorbed are greenish, which is why most plants look green.

Finally, having multiple variations of phospholipids in the membrane of a cell, which all have different fluidity at different temperatures, allows survival in different environments. Cell membranes need some amount of fluidity to not fall apart and to maintain selective permeability. With the right proportions of phospholipids, cells and organisms are each adapted to their unique environment.