Natural selection is when organisms that are best adapted to an environment survive and reproduce more than others.
There are six different principals of evolution, natural selection itself being one.
The second principal is overproduction, it happens when each species produces more offspring that can survive.
The third principal is competition. This is when individuals compete for limited resources, in this case, food, both the rattlesnake and iguana eat insects.
Insects are limited!!!
The butterflies vary from yellow to orange to both yellow AND orange.
The fourth principal is variation. This is when each individual has a unique combination of inherited traits, color for this example.
The fifth principal is adaption. Adaption is an inherited trait that increases an organisms chance or survival. For example this polar bear has a four-inch layer of blubber. It's fat that helps keep it warm while swimming.