|
A
<br />
<br />
<br />
<br />tance in adaptive evolution, other types of genetic variation also However, there is little direct evidence that heterozygosity per se
<br />should be considered, such as the recessive lethal component of increases fitness (that is, that heterozygocc advantage at single loci is
<br />inbreeding depression, selectively neutral polymorphism (that may common), beyond simply avoiding inbreeding depression and al-
<br />be adaptive in an altered environment), and single genes of large lowing adaptation m environmental change (12). For populations
<br />effect conferring resistance to specific selective agents causing sus- tracking a moving optimum phenotype in a fluctuating environ-
<br />mined high mortality (such as pesticides or diseases). For example, ment, too much genetic variation can be detrimental since, in any
<br />the maintenance of substantial heterozygosity by mutation at neutral generation, individuals deviating from the optimum phenotype are
<br />loci may require N, larger than IW individuals because single loci selected against; the optimal amount of heritable variation in a
<br />usually have low mutation rates on the order of 10-6 per gamere per quantimrive character depends on the magnitude and pattern of
<br />generation (5). The relatively high mutability of quantitative trairs fluctuations in the optimum phenotype and on the strength of
<br />implies thar, following a period of small population size during stabilizing selection within generations (27).
<br />which most genetic variability has been lost, if the population Extinction is fundamentally a demographic process, influenced by
<br />regains a large size then rypical levels of heritable variance can be genetic anenvironmen ctors. If a population becomes actin
<br />restored by mutation on a time scale of 10= or 103 generations, or demographic reasons, such as bitat destruction, the amount of
<br />which is much faster than the 1W or 106 generations for restoration genetic variation it has is irrelevant- For example, natural extinctions
<br />of heterozygosiry by mutation at neunal loci (5). Low genetic of small populations of butterflies in California appear to be
<br />polymorphism in soluble proteins reported in several species of large unrelated to their levels of protein polymorphism (28). Even for
<br />mammals (23), therefore, does not necessarily mean that the popula- closely managed populations in a controlled emironmcm where
<br />tion is devoid of heritable variance in quantitative characters, or of genetics plays a leading role, danography cannot be neglected in
<br />inbreeding depression. Substantial independence of different apes achieving a stable population size and age distribution (29). For wild
<br />of genetic variation undermines the "genetic uniformitarianism" of populations in natural or seminatural environments, demogra by is
<br />Soule (8), which supposes that levels of all kinds of generic variation fit& o more rmm ate tx -enrna in d_aer-
<br />hi
<br />arc proportional. c
<br />conning pop non ,7a ahty. Below rs an outline o p
<br />Attempts to establish the minimum size for a viable population on of ctors eruct to a persistence of small populations.
<br />genetic grounds alone are highly questionable for several reasons.
<br />The management goal of preserving maximum genetic variability .?.?
<br />? dprn??ra?4c7tr-s
<br />within populations is based on the assumption that the rate of Demography - i4ipw f d a r
<br />evolution in a changing environment is limited by the amount of
<br />genetic variation (7, 8). This assumption has been previously
<br />l
<br />i
<br />h
<br />l
<br />i
<br />=
<br />f
<br />f
<br />? Allee e ea. In many species, individuals in populations declining
<br />and reproduction
<br />erience diminished viabilit
<br />be
<br />ect
<br />on), as t
<br />e
<br />(natura
<br />se
<br />avor o
<br />ecological oppomm
<br />n
<br />rejected, in
<br />at least in morphological evolution
<br />primary rate-controlling factor y
<br />to ow num
<br />rs exp
<br />for nongenetic reasons, and there may be a threshold densirv or
<br />,
<br />(26). Recent writings on genetics and conservation also espouse the number of individuals frbm below whi epopulation cannot
<br />?
<br />` view that genetic variation is adaptive in and of itself (7, 8). he
<br />*=vcr. Known as an Allee cffi:ct, thiscan Tie caused hv
<br /> ,)rganisms physically or chemically modifi•ing their environment by
<br /> L. d ' d d marin success For
<br />1.0
<br />ot-
<br />Y 0.8
<br />r
<br />c
<br />0.6
<br />A
<br />0.4
<br />0.2
<br />=? r 0
<br />social mterarnon or y enstty- epen rnt g
<br />example, some aquatic microorganisms condition their medium by
<br />releasing substances that stimulate growth of eonspecifics. Social
<br />'animals frequently increase individual survival by group defense
<br />against predators and competitors. In very sparse populations, social
<br />interaction necessary for reproduction may be lacking, or it may be
<br />difficult to find a mace (30).
<br />Stochastic demography. Extinction of single populations is influ-
<br />enced by two kinds of random demographic factors. "Demo hic C
<br />stochasticirv" arises because, at anv time, individuals o a given age „G y
<br />or c evc oPMrntal stage have probabilities (or rates) of survival and
<br />reproduction, called vital rates. Assuming that these apply indepen-
<br />dcntly to each individual, demographic stochasticity produces sam-
<br />pling variances of the vital rates inversely proportional to pc =,: uiataon
<br />size. In contrast, "emironmental stochasticity" is represented by
<br />temporal changes in the v u rates at ect all individuals of a
<br />given age or stage similarly; the sampling variances of the vital rates
<br />arc then nearly independent of population size. For this reason, and
<br />because most populations undergo substantial fluctuations due to
<br />changes in weather and the abundances of interacting species,
<br />environmental stochasticity is generally considered to dominate
<br />` 0 0.2 0.4 0.6 0.8 1 0 demographic stochasticity in populations larger than about 100
<br />' h ?erMr,?l individuals (31, 32). This conclusion is supported by observations of
<br />r0?? , 1;;., 0 ? Gr?x ?,?,?ps?? -Of Ca?la?,.;? h6;0 It
<br />Fig. 1. The proportion of suitable habitat occupied at demographic equilib- birds on islands, which, except for very small populations (initially
<br />rium, p, for a territorial age-structured population in a patchv environment; less than 30 breeding pairs), become extinct at rates far greater than
<br />h is the proportion of a large region composed of patches of suitable habitat, predicted by demographic st:ochasticity alone (31).
<br />-i ch the size of individual territories, that arc assumed to be randomly or Simple analytical models describing the stochastic civnamics of
<br />".eniv distributed in space. The demographic potential of the population, k, density-dependent populations without age structure (31. 32), or
<br />c. the cUwGhnum occupancv in a complctcly suitable region, as derer-
<br />w.cd by the lite history and dispersal behavior. (Reproduced with permis- densirv-independent populations with age structure (33), yield
<br />l nm, yualirative insights into the importance of different patterns of
|