A genetic record of population isolation in pocket gophers during Holocene climatic change - PubMed (original) (raw)

A genetic record of population isolation in pocket gophers during Holocene climatic change

E A Hadly et al. Proc Natl Acad Sci U S A. 1998.

Abstract

A long-standing question in Quaternary paleontology is whether climate-induced, population-level phenotypic change is a result of large-scale migration or evolution in isolation. To directly measure genetic variation through time, ancient DNA and morphologic variation was measured over 2,400 years in a Holocene sequence of pocket gophers (Thomomys talpoides) from Lamar Cave, Yellowstone National Park, Wyoming. Ancient specimens and modern samples collected near Lamar Cave share mitochondrial cytochrome b sequences that are absent from adjacent localities, suggesting that the population was isolated for the entire period. In contrast, diastemal length, a morphologic character correlated with body size and nutritional level, changed predictably in response to climatic change. Our results demonstrate that small mammal populations can experience the long-term isolation assumed by many theoretical models of microevolutionary change.

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Figures

Figure 1

Figure 1

Average depth of stratigraphic levels in Lamar Cave, number of modern (n = 7 specimens of T. t. tenellus and T. t. fuscus) and ancient specimens (n = 73) from Lamar Cave that were subjected to DNA analysis, respective age of the specimens, and summary of SSCP and sequence data. (A) The age-versus-depth curve for the deposits based on 18 calibrated radiocarbon dates with 95% confidence intervals shown (3). (B) Illustration of a lower left jaw with the measurements taken to analyze morphologic characters. (C) Changes in diastemal length (•) and toothrow length (□) for comparison with haplotypes A–C identified by SSCP analysis. We examined 88 ancient specimens spanning all 16 stratigraphic levels. In 73 cases (83%) we obtained a 164-bp PCR product of the mitochondrial cytochrome b gene (cyp b) 5′ end. Level 16 (2,400 to 3,000 years) did not yield any PCR product. The sample size column lists the number of specimens that were examined, and in parentheses, the number screened for 164 bp by using SSCP (first number), individuals directly sequenced for 63 bp (second number), and individuals cloned and sequenced for 134 bp (third number). Note: cloned individuals also have been sequenced directly with the exception of level 6. At level 7 morphologic change is most pronounced, coincident with changes in temperature and/or moisture regimes in the region.

Figure 2

Figure 2

Sampling localities and genetic relationships of the modern neighboring populations of pocket gophers to the Lamar Cave population. Shaded areas illustrate the present range of six nominal subspecies of T. talpoides (light stipple), and two subspecies of T. idahoensis (dense stipple) in and around Yellowstone National Park. Numbers above lines denote the number of observed substitutions between haplotypes connected by a minimum spanning network (6) based on 63 bp of cytochrome b sequence. An alternative link is shown in parentheses. Haplotype numbers are indicated at localities where they are found (Table 1).

Figure 3

Figure 3

SSCP typing of T. talpoides from Lamar Cave and surrounding localities. Lanes 1 and 2, level 3, haplotype A; lanes 3 and 4, level 6, haplotype A; lanes 5 and 6, level 12, haplotype A; lanes 7 and 11, recent Lamar Cave, haplotype A; lane 10, recent Lamar Cave, haplotype C; lanes 8, 9, and 12, recent from localities outside Lamar Cave, all having unique haplotypes differing by 1–8 substitutions.

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References

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