Organismal Diversity

Organismal diversity refers to any variation in the anatomical, physiological, or behavioral characteristics of different individual organisms. These are called phenotypic characters, or physical traits. They represent the outward expression of genes and the action of the environment on the way those genes are expressed in an organism. It is this phenotypic diversity that overwhelmingly interacts with biotic and abiotic (that is, living and nonliving) factors to create higher levels of biodiversity, such as community and ecosystem diversity. The phe-notypic characters of organismal diversity, therefore, represent an important measure of the adaptation of the organism to its environment. Similar to genetic characters, variation of organismal characters can be used to measure the amount of diversity between individuals of the same population, different populations, or different species.

Variation in the genes that control certain features may be expressed as quite distinct phenotypes. For example, two organisms might be different sizes or colors as a result of genetic variation. However, that is not always the case. For some features, the phenotypic expression of genetic variation may be very subtle and difficult to detect.

Distinctive anatomical, physiological, and behavioral characters are the product of complex interrelationships between the form and function of various organs. For example, the distinctive appearance of some muscles might be closely correlated with their position, orientation, and function relative to adjacent muscles. Local environments can significantly alter organismal characters. The physiological (and anatomical) characteristics of the kidney in fishes, for example, can vary depending on the environment. Rainbow trout and flounder filter fluid through their kidneys at different rates, depending on the salinity of the water in which the fish are immersed (see Harrison [1996] for references). Therefore, interpreting the relationship between what something looks like and its underlying diversity is difficult. Phenotypic features can be less precise measures of diversity than genetics. However, analyses of organismal diversity can be more informative than genetic studies, because they provide direct information about the relationship between the diversity of the organism and the environment.

The behavior of an organism is controlled by genetic diversity. In some cases, the behavior of whole populations is closely related to the genetic diversity of the individuals in the population. For example, in some eusocial insects such as ants, which have a "queen" producing "worker" daughters, the daughters share three-quarters of their genes. Rather than produce daughters of their own, these workers can ensure that more of their genes are passed on through the population by assisting the queen in the care of new generations of their own sisters. Community or ecosystem diversity also shapes some behaviors. For example, feeding behavior is dependent on the relative availability of different types of prey.

Behavioral characteristics define population, community, and ecosystem diversity. The herding behavior of some mammals such as elephants or wildebeests helps determine the size and activity of populations. Moreover, the activity of these herds (for example, seasonal migrations) can significantly affect the overall ecology of an ecosystem.

Behavioral patterns are also associated with landscape/seascape and biogeographic diversity. For example, the long-range spawning migrations of eels are perhaps associated with the biogeographic and, hence, evolutionary history of the species (see Biogeographic Diversity). Similarly, the annual migrations of wildebeests are associated with physiographic aspects of landscape (for example, seasonal variation of climate) and biogeographic diversity.

The behavioral patterns of species are sometimes included in taxonomic and phylogenetic studies. One of the most difficult problems in applying behavioral characters to phylogenetic studies is how one establishes whether behavioral traits, shared by different species, are sim ilar because of descent from a common ancestor (that is, homology), or whether the characters originated independently in phyloge-netically unrelated taxa (that is, homoplasy) (Wenzel, 1992). McLennan (1993) mapped behavioral characters onto a phylogenetic tree for sticklebacks and showed where there was independent, convergent evolution of similar behavioral characters in unrelated species. This information is useful to behavioral ecologists because it indicates where further investigation of the characters would be informative, as well as analysis of the relationships between the organism and the environment.

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