Biology  5/e   Raven/Johnson
	Student   Online Learning Center

Major features of evolution: The earth originated about 4.5 billion years ago, and life arose on it about 3.5 billion years ago. Starting from these simple bacterial beginnings, the diversity of life that exists on earth today has gradually evolved. For example, multicellular organisms evolved at least 630 million years ago, and terrestrial vertebrates evolved about 360 million years ago. The evolution of organisms has been influenced greatly by geological events such as continental drift and by drastic climate changes such as glaciation.

Communities: The populations that occur together in a given place at a given time make up a community. These populations interact and influence each other in various ways. Interspecific competition, predator-prey interactions, symbiotic relationships, and coevolution are all examples of how populations within a community may interact. All these interactions help define the community, the ecosystem to which it belongs, and even the biome.

Symbiosis: Symbiosis means living together. Two types of organisms have a symbiotic relationship if they live together consistently. The three major types of symbiotic relationships are mutualism, where both organisms benefit, commensalism, where one organism benefits and the other is not affected, and parasitism, where one organism benefits and the other is harmed. Symbiosis often leads to unique coevolutionary adaptations and helps give different communities and ecosystems their distinct characters. Different examples of symbiotic relationships can be found in all biomes.

Biogeochemical cycles: Biogeochemical cycles describe the movement of substances through ecosystems. Chemicals cycle from nonliving reservoirs such as the atmosphere, oceans, and soil, to living organisms and back to reservoirs. Water, carbon, nitrogen, and phosphorus are examples of chemicals that are vital to organisms and cycle through ecosystems. Biogeochemical cycles help define the character of an ecosystem and influence the distribution of organisms within different ecosystems.

Copyright ©1998 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.