Natural selection (which will discuss more in Marooned in the Galapagos) is the process in which organisms with certain characteristics are eliminated from the population are eliminated while others become more common. Natural selection acts as a filter.

Many organisms, especially those in biodiverse ecosystems like tropical rain forests (lots of competition and grazing/predation), may increase their chances of survival by making chemicals that are toxic or distasteful to grazers/predators. Environmental conditions and genetic differences within and between species may influence the types and amounts of chemicals made.

  Since evolutionary success depends on reproduction rather than mere survival, the biochemical "expense" of making toxic chemicals could conceivably be maladaptive under certain cirsumstances.

  The chemicals produced are often minor modifications (initially accidental mutations subject to natural selection) of nontoxic chemicals present in many organisms.