Natural Selection

Definition of fitness: The ability to survive and reproduce in an environment.

The traits, or adaptations, that contribute to fitness for the marine iguana: 

• Bigger males have more success when reproducing with females

• Males near the center of territories are more likely to find a mate

• A good fitness in general is good for the male iguanas, because they fight over territory by interlocking heads while wrestling

Zoomed in pics:

Adaptations:

The size of an iguana determines the chances of reproducing, the bigger they are, the more chances they have of reproduction. This is beneficial because it helps iguanas pass on their genes.

(These marine iguanas are in the center of the territory,

as you can tell by the shade that they're getting)

Marine Iguanas in the center of the territory have a higher success rate with reproduction, because it is more convenient for the females. There is more access to shade in the center of the territory as well as more places to sunbathe.

Being bigger and more fit like the iguana shown above helps the marine iguanas when wrestling, and it also attracts more female iguanas. This, in turn, allows them to pass on their genes.

Marine iguanas weren't always their own population, they split off from the iguana population on the Galapagos Islands, which actually eventually lead to them being an entirely different species. Since the marine iguanas do not represent the whole entire iguana population, this is the founder effect.

Change in genes that led to change in phenotype:

A change in the genes allowed the marine iguanas to shrink their bodies by reabsorbing their bones. This reduced their overall body length and weight, which is beneficial for them because as temperature increases, they need more and more food. However, the rise in temperature killed off all the algae (their primary food source).

Because of this, having a small body is beneficial for survival, but decreases the chance of reproduction. This can be seen as an example of directional selection, because the smaller iguanas have a higher chance of survival.

Example of genetic drift:

A random occurrence in the genes allowed some, but not all marine iguanas to change colors around mating season. However, there was no difference in the success of finding a mate with marine iguanas that could change color and iguanas that couldn't. This is an example of genetic drift.

These traits were all naturally selected for because of selective pressures like finding a mate or surviving with little food. Since only the fit iguanas were able to find mates, they were the only ones to pass on their genes to their offspring, also known as descent with modification.