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<br /> <br />8 <br />7 <br />6 <br />5 <br />H4 <br /> <br />3 <br />2 <br />1 <br /> <br />SOULE/CHAPTER 9 <br />THRESHOLDS FOR SURVIVAL <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br /> <br />FIGURE 2. Relationship between <br />area and percent heterozygosity <br />(H) among island populations of <br />the lizard Lacerta melisellensis. <br />(Data from Gorman etal., 1975) <br /> <br />10 100 1000 10,000 <br />AREA (ha) <br /> <br />rate, it cannot prevent a gradual attrition of genetic variation. Sooner or <br />later, depending on ~, the population will become virtually homozygous, <br />although there is evidence for an increase in heterozygote advantage at <br />higher values of f (Frankel and Soule, in press, Chapter 3). This is why <br />the basic rule is a short-term criterion. A population held in check at <br />~ = 50 for 20 to 30 generations will have lost about one-fourth of its <br />genetic variation, and along with it, much of its capacity to adapt to <br />changing conditions. <br />The effects of a chronically small population on genetic variation <br />seem to be indicated by the "small island effect" shown in Figures 2 and <br />3. These figures suggest that decreasing island size has little effect on <br />heterozygosity until a critical area is reached-about five hectares for liz- <br />ards. Below this size the populations appear to lose heterozygosity. For <br />the species shown, a five hectare island has somewhere between 250 and <br />2000 individuals. These particular islands were formed by rising eustatic <br />sea levels between eight and ten thousand years ago. Some or all of the <br />populations might be that old; some or all might be much younger, hav- <br />ing been established by one (gravid) or more founders at a later date. In <br />any case, the founder effect alone is not a sufficient or likely explanation <br /> <br />. <br /> <br />. <br /> <br />10 <br /> <br />8 <br /> <br />H 6 <br />4 <br /> <br />2 <br /> <br /> <br />. <br />. <br /> <br />. <br /> <br />Anolis cristate/lus <br /> <br />FIGURE 3. Relationship between <br />area and percent heterozygosity <br />(H) among island populations of <br />the lizard Anolis cristatellus. <br />(Data from Gorman et al.. 1979) <br /> <br />10 100 1000 100,000 <br />AREA (ha) <br /> <br />161 <br />