Without decomposers the plant would be filled with garbage. Include may familiar species. Plant are multicelled producers and most are photosynthetic. Animals are multicelled consumers that eat both plants and animal. Source of variation Mutations are a change in the genetic material.
Mutations can be harmful, have no effect, or have a survival advantage.
Light colored moths survive better on light colored trees. The DNA in the moth must change during some point in time. The environment must select for the change and there will be gradual change in color of the moth population with time. Natural Selection is the outcome of survival of populations with different traits. Those organism that survive were select to remain in the population and can pass on their genes to the next generation.