Creation, Flood and Biogeography: Part 1

Introduction: What is Biogeography?

Biogeography is the study of the distributions of living organisms. The major goal of biogeography is to explain why different regions of the earth are inhabited by different types of organisms. Biogeographers seek to discover what historical and ecological factors explain why a species lives in one particular area but not in another area.

Biogeography is largely a historical science, rather than an experimental science. Distributions can be observed, but the initial conditions that led to those distributions are unknown. Biogeographers seek to infer initial conditions of distribution by studying the principles of ecology, the fossil record, the present and past relationships of the continents, and the processes of speciation and adaptation. Because of its historical nature, much of the data is unavailable, and considerable speculation may be involved in reconstructions of the past.

The study of biogeography can be conducted at various scales, depending on the purpose of the study. This necessitates that we identify the taxonomic unit being studied and the geographic units comprising its distribution. If a part of the group or a part of its distribution is ignored, we may easily arrive at incorrect conclusions. Alternatively, combining two natural groups into one artificial group is likely to lead to incorrect interpretations. In this blog, we will be concerned with large-scale patterns, focusing on how groups of species are distributed over the surface of the earth. For our purposes we will mostly use the taxonomic family as the taxonomic unit representing a lineage and the continents as the geographic units.

Biogeographical Patterns and Processes

Most distributional patterns fit into one of three categories: widespread, narrowly endemic, or disjunct. Widespread species are those found on several continents. The common egret (Ardea alba) is a widespread species, while the cat family (Felidae) is a widespread family. Narrow endemics are restricted to a single area, often small in extent. The Torrey pine (Pinus torreyana) is endemic to a small area of southwestern California and the nearby island of Santa Rosa. The kangaroo family (Macropodidae) is endemic to Australia. Disjunct distributions are those with a major gap in the distribution, especially at the continental or sub-continental scale. Creosote bushes (genus Larrea) have a disjunct distribution, being found in the deserts of North America and in South America, with a large gap in between. The family of “clawed” frogs (Pipidae) is disjunctly distributed in South America and Africa. Biogeography attempts to explain the origins of these various kinds of distributional patterns.

Three biogeographical processes are thought to be responsible for the observed patterns of species distributions: dispersal, extinction, and speciation. Dispersal is the movement of populations from one region to another. This may be accomplished by a gradual expansion of range, such as the opossum in North America, or by an unusual circumstance, such as when the cattle egret crossed the Atlantic Ocean from Africa to South America in the late 1930s. Dispersal normally occurs continuously until blocked by some condition acting as a dispersal barrier. Excellent dispersal potential may produce a widespread distributional pattern. Failure to disperse may result in a narrowly endemic distribution. One or more accidental dispersals may produce a disjunct distribution.

Extinction and speciation may alter distributional patterns. Extinction in part of a species’ range (extirpation) may leave a single relictual population that is narrowly endemic, or a small number of disjunct populations. Speciation may result in morphological divergence of two isolated populations, producing a new species[1] with a narrowly restricted distribution. Extensive speciation and extinction may make it difficult to interpret the biogeographical history of a group.

Ecological factors are also important in explaining present distributions, especially those of subcontinental scale. For example, parrots are largely restricted to tropical areas because they depend on a year-around supply of fruit, which is typically not available in more temperate areas. However, parrots are found in temperate New Zealand and temperate South America, and the Carolina parakeet once lived in the southeastern United States. Distributions of all living species are influenced by ecological factors.

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L. James Gibson

Geoscience Research Institute

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[1] Note: development of new species is compatible with creation theory, assuming that many original kinds were created, from which numerous species may have descended, with minor changes.

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