<P> Interactions between organisms can produce both conflict and cooperation . When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations . Here, the evolution of one species causes adaptations in a second species . These changes in the second species then, in turn, cause new adaptations in the first species . This cycle of selection and response is called coevolution . An example is the production of tetrodotoxin in the rough - skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake . In this predator - prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake . </P> <P> Not all co-evolved interactions between species involve conflict . Many cases of mutually beneficial interactions have evolved . For instance, an extreme cooperation exists between plants and the mycorrhizal fungi that grow on their roots and aid the plant in absorbing nutrients from the soil . This is a reciprocal relationship as the plants provide the fungi with sugars from photosynthesis . Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending signals that suppress the plant immune system . </P> <P> Coalitions between organisms of the same species have also evolved . An extreme case is the eusociality found in social insects, such as bees, termites and ants, where sterile insects feed and guard the small number of organisms in a colony that are able to reproduce . On an even smaller scale, the somatic cells that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's germ cells to produce offspring . Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die . If cells ignore these signals and multiply inappropriately, their uncontrolled growth causes cancer . </P> <P> Such cooperation within species may have evolved through the process of kin selection, which is where one organism acts to help raise a relative's offspring . This activity is selected for because if the helping individual contains alleles which promote the helping activity, it is likely that its kin will also contain these alleles and thus those alleles will be passed on . Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms . </P>

The first and most fundamental factor for evolution is