Geographic Variation

by

Bryan Epperson

• LAST REVIEWED: 13 January 2014
• LAST MODIFIED: 13 January 2014
• DOI: 10.1093/obo/9780199941728-0034

Introduction

Evolutionary principles of geographic variation began to emerge early as the field of evolutionary biology developed within the biological sciences. Geographic variation of species, sometimes featuring distinct subspecies and often having characteristic traits, is determined by geographic variables such as climate or ecological niche as well as population genetic factors. Geographic patterns of genetic variation are generally determined by population sizes (through genetic drift); patterns, distances and rates of migration or dispersal and gene flow; and often natural selection, which is especially prominent where different geographic areas favor different phenotypes, presumably the result of past adaptations to local biotic and abiotic environmental factors. Such processes are complex by their very nature, being spatial-temporal and often stochastic (e.g., accounting for genetic drift) as well as involving other parameters such as the various forms of differential fitness and migration/dispersal rates that can themselves vary over space and time. While much can be, and has been, learned about geographic variation from studies that may be considered largely “descriptive” in terms of spatial or geographic population genetics, this article focuses on the emergent concepts and population genetic principles as they have appeared in the wake of advances in understanding geographic variation that have accrued over the past 150 years. The focus of this article is on development of principles that: (1) better detail or explain phenomena (and sometimes even following rather directly from novel mathematical approaches), or (2) show how previously uncharacterized processes can diagnostically affect geographic variation (e.g., spatial genetic structure), or (3) do as in (2) but in a new way, which may lead to new contrasts and testable hypotheses. It is difficult simply to understand the basic roles of genetic drift and migration. Appropriate models may have substantial, stochastic, and deterministic components that can interact in a wide range of ways. A recurring theme in the subject of geographic variation is the appropriate degree of biological and historical realism in models and data analyses.

The Birth of Evolutionary Biology

Importantly, the stage as it was set for Darwin in the middle of the 19th century included a strong naturalist trend in Great Britain, evident, for example, in Paley 1803, the emergence of Malthusian ideas of intraspecific competition, a much improved fossil record, and an accepted biological classification system by Carolus Linnaeus that included recognition of taxa below the species level. In addition, and in particular, the survey expeditions undertaken by Darwin (Darwin 1839) encouraged Darwin’s theoretical examination of geographic variation. Together with Darwin 1896 on domesticated animals and plants, and using comparisons with the fossil record, the author’s observations of geographic variation were used as evidence for natural selection and led to the view that geographic races can potentially evolve into closely related but separate species. Thus, for example, geographic variation is a prominent component of Darwin 1872. Domesticated varieties and geographic races provided evidence of past selection and existing geographic variation was also sometimes considered evidence of incipient allopatric speciation. Subspecies were modifiable, and, in some instances, geographic space substituted for changes over evolutionary time-scales. In Darwin 1839, the author recorded the geographical distribution of species and subspecies and, in many of his works, he expressed the hope of finding the center of creation of species.

• Darwin, C. 1839. The voyage of the Beagle. London: Smith, Elder.

  Material here has been published variably as Darwin’s journal and remarks, but the title refers to the second expedition of the HMS Beagle in 1831.

• Darwin, C. 1872. The origin of species. 6th ed. London: John Murray.

  The classic volume cited for Darwin’s theory of natural selection, Darwin published a large number of papers and volumes that have been combined in various ways and under various titles, including On the Origin of Species by Means of Natural Selection: Or the Preservation of Favoured Races in the Struggle for Life.

• Darwin, C. 1896. The variation in animals and plants under domestication. Vol. 1. New York: D. Appleton.

  DOI: 10.1017/CBO9780511709500

  Much of the material here was published earlier in various forms. This volume is fairly accessible and includes a vast array of experimental and breeding observations, some of which could still be useful for researchers working on conspecifics today.

• Paley, W. 1803. Natural theology, or, evidences of the existence and attributes of the deity, collected from the appearances of nature. London: R. Faulder.

  Reissued by Cambridge University Press in 2009; an example illustrating a link to some prevailing religious thought in 19th-century Great Britain.