Interactions among organisms play key roles in determining the distribution and abundance of species. These interactions can take many forms, but we’ll cover just three broad categories that include nearly all of them.

Competition – two organisms using the same limited resource
Predation – one organism using another as a resource
Symbiosis[*] – two organisms existing in intimate association

Keep in mind that these interactions typically involve many species[#](Molles 2004). Competition certainly occurs (and is strongest) among conspecificsMembers of the same species, but they also compete with members of other species. They may use many other species as food, and must avoid being eaten by perhaps several other species as well. Some species live in intimate association with others, increasing energy intake or improving survival. All of these interactions, not just one or two, impact the evolution of species. In addition, changing the environment can influence the balance of the interaction[#](Connell 1961b).

And Nuh is the letter I use to spell Nutches
Who live in small caves, known as Nitches, for hutches
These Nutches have trouble, the biggest of which is
The fact there are many more Nutches than Nitches

Each Nutch in a Nitch knows that some other Nutch
Would like to move into his Nitch very much
So each Nutch in a Nitch has to watch that small Nitch
Or Nutches who haven’t got Nitches will snitch
                                                      - Dr. Suess, On Beyond Zebra!

This amusing rhyme from Dr. Suess is not intended to be an accurate description of an ecological concept, of course, but it does describe intraspecific competitionCompetition among members of the same species over a limited resource. It also illustrates a common misunderstanding concerning the term niche. A niche is not a physical location, but it does include the range of physical, chemical and biological conditions occupied by a species. Rather, a niche is an organism’s functional role in the biotic environment and its relations to food and enemies[#](Molles 2004)(Elton 1927)(Grinnell 1917). Members of the same species share the same niche within a community.

The niche concept is key to understanding competition and its role in evolution, because it describes the many factors necessary for survival and reproduction. When these factors are limited (scarce relative to demand; not infinite), individuals compete. This is not a formalized competition; in most cases individuals don’t even know they are competing (just as they don’t know their population is evolving). Individuals simply do what they can to survive and reproduce. The needs of some are met, the needs of others are not.

The effect that competition has on a species’ niche was described by GE Hutchinson[#](Hutchinson 1957). He made a distinction between a theoretical concept of a niche and the reality of what a species actually experiences within a community. The difference between these two things - the fundamental niche versus the realized niche - is due to competition and other biotic interactions.

Competition, then, reduces the availability of a resource to other individuals[#](Molles 2004). Competition occurs over many resources, not just food. Plants compete for space to grow, sunlight, water, nutrients in the soil. Consumers compete for food, nest sites, territory. Males compete for females and females compete for males. Because they share the same niche, individuals within a species all require the same things. This means that intraspecific competition is stronger than interspecific competitionCompetition among members of different species, and competition is weakest between species that share the fewest niche requirements. As a result, two similar species rarely occupy similar niches[#](Gause 1934). Typically competing species end up using different resources, or using different parts of a resource, a phenomenon known as resource partitioning[#](May and MacArthur 1972). And prolonged competition for limiting resources may lead to permanent changes in phenotypeThe outward expression (physical, behavioral or physiological traits) of a genotype[#](Grant 1999).

Just as physical conditions, such as long-term fluctuations in climate, are ‘tracked’ by evolution as populations of animals and plants become adapted to local conditions, long-term changes in predators will be ‘tracked’ by evolutionary changes in their prey[#](Alonzo 2002). In either case, adaptations to local conditions can affect morphology, physiology, and behavior[#](Futuyma 1998).

The difference, of course, is that climate does not evolve, but predators do. Take, for example, running speed and agility between predator and prey. If faster prey (whose offspring are likely to be fast) are more likely to survive and reproduce, a prey population will get a little faster as it evolves. But the faster members of the predator population may still be successful - and therefore likely to produce offspring - even if slower predators are not. The predator population thus gets a little faster as it evolves. The process, described as an "arms race", can continue on a timescale of hundreds of thousands of years. Yet there will be long stretches of time in which no evolutionary change at all takes place. One reason is that species may have multiple “enemies” (and simultaneous arms races), and these other enemies may be enemies of each other[#](Dawkins 1986)(Van Valen 1973).

An important point in these arms races is that there no change in the long-term success rate on either side of the arms race, even with substantial changes in some aspect(s) of the phenotype on both sides. The principle of no change in success rate, no matter how great the evolutionary change in “weapons,” is known as the “Red Queen effect.”[#](Van Valen 1973)

"Well, in our country," said Alice, still panting a little, "you'd generally get to somewhere else - if you ran very fast for a long time as we've been doing." "A slow sort of country!" said the Queen. "Now, here, you see. it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"
                                                      - Lewis Carroll, Alice's Adventures in Wonderland

How do arms races end? They may end with one side going extinct, and perhaps the other as a consequence. In other cases, economic pressures (energy restrictions) on both sides may halt an arms race. Using our “speed and agility” example, when both predator and prey reach the maximum running speed that they can “afford,” the arms race between them will come to a relatively stable end[#](Dawkins 1986)(Van Valen 1973).

A mutualism is sort of the opposite to competition - a mutually positive interaction between members of different species, an association that increases the fitness of both involved[#](Molles 2004).

Perhaps the most obvious example of a mutualistic relationship (although intermittent rather than long-term) is between flowering plants and their pollinators. Pollinators use flowers (specifically its nectar and pollen) as a food resource. As they move among flowers, pollinators unwittingly introduce pollen to the stigmaThe terminal portion of a flower's female reproductive structure (carpel), that is fitted to receive pollen of a flower, fertilizing the ovule.

Bacteria (called rhizobia) which live in nodules on the roots of legumes (plants in the pea family) are another example. These bacteria convert nitrogen (an essential and limited nutrient) from an inert form into nitrogen compounds that they - and the plant - can use, and the bacteria obtain carbohydratesA family of organic molecules consisting of carbon, hydrogen and oxygen. A source of energy and a structural component of living cells produced by the plant via photosynthesisThe process by which plants, algae and some bacteria absorb light energy and use it to synthesize carbohydrates[#](Taiz & Zeiger 2002).

Bacteria are also found within the digestive system of termites (and some other herbivores, such as ruminants). The cell walls of plants are made of a sugar called cellulose, a rich source of energy but undigestable by animals. Symbiotic protozoa in the digestive tract have a hospitable environment, and they can digest cellulose. And termites get access to otherwise-unavailable energy[#](Abe, et al. 2000).

Commensalism is similar to mutualism, except that one receives a positive benefit and the other is neither helped nor harmed. For instance, pilotfish often travel with sharks, eating small scraps of food released as the shark feeds. Pilotfish also may conserve energy by riding the wake of the shark. The shark is apparently neither helped nor hindered[#](Claro & Parenti 2001).

Parasites live in or on another organism and “steal” resources from the host, but do not (usually) kill their host. Common examples include hookworms and other intestinal parasites, and Ichneumonid wasp larvae, which are endoparasites of a wood-boring beetle larva[#](Wahl 1993). Mistletoe is one of a small number of parasitic plants[#](Kuijt 1969).

Endosymbiotic Theory
An endosymbiont is any organism that lives within the body or cells of another organism. Endosymbionts can be harmful, such as intestinal parasites, but they can also be beneficial.

Endosymbiosis has had important consequences in the evolution of new taxa, specifically the origins of mitochondriaMembrane-enclosed organelle in most eukaryotic cells; they generate adenosine triphosphate (ATP), used as a source of chemical energy. and chloroplastsA chlorophyll-containing structure found in algae and plant cells. Photosynthesis takes place here., organelles within eukaryoticAn organism whose cells have their DNA contained in a nucleus, separated from the other contents of the cell. cells[#](Margulis 1992)(Sagan 1969). According to this theory, these organelles originated as separate prokaryotic“Before the nucleus,” the term applies to single-celled organisms, such as bacteria, without a nucleus segregating the DNA from the rest of the cell organisms which were incorporated by the cell as endosymbionts.

There is much evidence in support of this theory. For example, although most of a cell's DNA is contained in the nucleus, mitochondria and plastids (such as chloroplasts) have their own independent genomeThe hereditary information of an organism encoded in the DNA. This includes the genes and the non-coding sequences of the DNA. Mitochondrial and plastid DNA shows similarity to bacterial genomes, and new mitochondria and plastids are formed only through a process similar to binary fission. Proteins made by organelles, like those of bacteria, use N-formylmethionine as the initiating amino acidAn organic molecule that contains both amine and carboxyl functional groups; Amino acids are the building blocks of proteins. Finally, much of the structure and biochemistry of plastids, notably the presence of chlorophylls, is very similar to that of cyanobacteria[#](Margulis 1992).

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Hutchinson, G. E. 1957. Concluding remarks. Cold Spring Harbor Symp. Quant. Biol. 22:415–427