Natural selection illustrates how organisms that survive pass along the traits that aid in their survival. As population pressure makes survival more difficult, organisms learn to take advantage of others using the same traits that they use. Some organisms that entered into a facultative, or take-it-or-leave-it, relationship evolved over time and were unable to survive on their own without the food or other benefit they gained from another organism -- or symbiosis.
Symbiosis also may be a natural tool in the form of an evolutionary bypass system. Why should a tree evolve a deeper root system over thousands of years when it can work out a deal with fungi living nearby? The fungi already pull nutrients from deep in the ground, so the tree adapts to incorporate what the fungi already do.
How are different alleles created?
Answered by Science Channel
What are vestigial and atavistic traits?
Answered by Discovery Channel
What is symbiogenesis?
Answered by Planet Green