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Organismal Diversity

Module by: Ian Harrison, Melina Laverty, Eleanor Sterling. E-mail the authors

Thus far we have discussed variation of the "genotype;" genotypic variation is the variation that exists between the genetic constitution of different individuals. Genes code for the outward expression of physical traits of the organism but the environment can also modify the way in which the genes are physically expressed in the organism. The physical constitution of an organism that results from its genetic constitution (genotype) and the action of the environment on the expression of the genes, is termed the phenotype.

Organismal diversity, therefore, refers to the variation of the physical traits, or phenotypic characters of the organism, such as differences in anatomical, physiological or behavioral characteristics. As noted above, organismal diversity is, in part, a product of genetic diversity. Nevertheless, the phenotypic characters of organismal diversity represent an important measure of the adaptation of the organism to its environment because it is these phenotypic characters that interact with the biotic and abiotic (i.e., living and non-living) factors of the environment. Additionally, it is the combination of phenotypic characters of different organisms that comprise diversity at higher levels, such as community and ecosystem diversity.

The extent to which genetic variation between organisms is expressed in their phenotypes can be quite variable for different characteristics. Genetic variation between some features might be expressed as very subtle differences in their phenotype; for example, populations and subspecies of the herring gull (Larus argentatus) and the lesser black-backed gull (Larus fuscus) show a range of slight differences in coloration between different organisms. Genetic variation can also be expressed more distinctly; for example, some species include unusually diminutive forms.

Distinctive anatomical, physiological, and behavioral characters are the product of complex interrelationships between the form and function of various body tissues and organs. For example, the physical properties of light limit the minimum size for vertebrate eyes, consequently the eyes are relatively large in minute vertebrates (such as species of amphibians and fishes). This, in turn, may affect the development and function of adjacent organs in the head, where there is a "competition" for head space (Hanken, 1983 [link] ; Harrison, 1996 [link]).

As noted above, local environmental conditions can alter organismal characters. The physiological (and anatomical) characteristics of the kidney in fishes, for example, can vary depending on the environment. Rainbow trout (Oncorhynchus mykiss) and flounder (Platichthys flesus) filter fluid through their kidneys at different rates depending on the salinity of the water in which the fish are immersed (see Harrison, 1996 [link] for references). In plants, leaf shape can show significant variability among individuals occupying different habitats (e.g., dry versus wet sites, or sunny versus shaded sites). Thus, any discussion of organismal diversity should account for the interrelationships between anatomical structure and function, for organisms living in different habitats.

Evidently phenotypic features represent more complex, and potentially less precise measures of diversity than genetics. However, analyses of organismal diversity can in some cases be more informative than genetic studies because they provide direct information about the relationship between the diversity of the organism and the environment.

Behavioral characteristics - which are part of organismal diversity - may also define population, community, and ecosystem diversity. The herding behavior of some mammals such as elephants, or wildebeest, helps determine the size and activity of populations. Moreover, the activity of these herds (e.g. 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 section on Biogeographic Diversity).

Glossary

Genotypic variation:
the variation that exists between the genetic constitution of different individuals
Phenotype:
the physical constitution of an organism that results from its genetic constitution (genotype) and the action of the environment on the expression of the genes
Organismal diversity:
the variation of the physical traits, or phenotypic characters of the organism, such as differences in anatomical, physiological or behavioral characteristics

References

  1. Harrison, I.J. (1996). Interface areas in small fish. In Symposia of the Zoological Society of London. (Vol. 69, pp. 15-45).

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