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<br /> <br />SOULE/CHAPTER 9 <br />THRESHOLDS FOR SURVIVAL <br /> <br />the causal or mechanistic basis of the relationship remains obscure). <br />These studies can be broken down into two principal categories: compari- <br />sons among individuals within populations and comparisons between <br />populations. Many of the former involve age- or size-class comparisons, <br />whereas the latter depend on differences in mean heterozygosity and <br />fitness between populations of the same species. Some of the technical <br />and theoretical issues raised by these studies are discussed elsewhere <br />(Frankel and Soule, in press). <br />Table I summarizes the results of 13 studies. In 11 of these studies the <br />relationship between various measures of fitness and heterozygosity was <br />or could be tested within populations. Data in 10 out of these 11 reports <br />could be interpreted as supporting such a relationship, although the au- <br />thors of some of these studies advance alternative hypotheses. One of the <br />most complete and convincing investigations was that of Schaal and <br />Levin (1976) on the herb Liatris cylindracea. Liatris is a perennial com- <br />posite of the dry prairies. Its perennating organ is a corm which can be <br />aged in some populations by counting the rings of pigmented cells depos- <br />ited periodically. Survival is related to heterozygosity as shown in Figure <br />1. Among young individuals there is a large excess of homozygotes, proba- <br />bly because of inbreeding, but the mortality of the most homozygous <br />plants is disproportionately high. Greenhouse tests performed on seed- <br />lings by Schaal and Levin established that age to sexual maturity as well <br />as reproductive output and vegetative output were all significantly corre- <br />lated with individual heterozygosity. <br />Table I also refers to a somewhat similar study in animals, one on <br /> <br /> -2.00 <br /> -3.00 <br /> -4.00 <br />0 -5.00 <br /> -6.00 <br /> -7.00 <br /> -8.00 <br /> <br /> <br />FIGURE 1. The decrease in <br />heterozygote deficiency with age <br />in a natural population of the herb <br />Liatris cylindracea. Ordinate (D) is <br />deviation from expectation <br />assuming random mating. (Data <br />from Schaal and Levin, 1976) <br /> <br />1 3 5 <br />(1-5) 2 (11-15) 4 (21-25) 6 <br />(6-10) (16-20) (>26) <br /> <br />AGE CLASS (years) <br /> <br />153 <br />