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Gene Pool Conservation GENE POOL CONSERVATION Con~ewationis charged with thepresetyation of a dynamic, ever. changfngsystem of vanafron contained in each species. Hampton L. Carson Modern eenetics (the stu~dv of hereditv and variation) has reached into every corner oi biology, dras~i&ll~changing old-fa\l:ioned concepts. One set of new discoveries has to do with the existence of generic variation from individual to individual in natural ~~lpulationr.The natural pwu!ation of the human species displays suchvariabilliy, and it is not hard foithe casual observer to recognize that both genes and environment are involved. Some domestic animals (for example, dogs and cattle, Canis familiari.7, Bas taurus) also show easilv observed individual neneticvariation. This is an old storv: inf&mation frjm the n& &eneri&eClend.:bur perception and indicates (hat nearly every normal, sexually reproducin~species is re lete with genetic vartntion. much of it st the molecular levcl. Alrhourli [idden from easv ohber- vntion. permanetit molecular variation from individh to individual is io all- pewasive throughout nature as to constit~tean nlmoht un:\'ersal law of life. In the last Century, wild animals and plants were frequent2 viewe? as if the individual members of a species were basically identtcal. ach mdividual was expected to conform to some kind of type; any deviation from the type was considered somehow unusual or accidental. The idea of twes is reallv a human concoction used to simpl~fythe way we deal with natuii. Fxcept pkr- haps as a device for cataloging material in museums, we mutt ab:indon the conccpt of the type, since gcneticq has shown it to he bad biology. We now know that not only is each human individual unique both in genetics and environment, but so is each dog or elephant or oak tree. It is imoortant to recoeniz; that the conservationist is not chareed with the prese&ation of types. Indeed, what is out there in nature is a dynamic, ever-cliangin~sptem of variation contained in each species. It is this flexible vGiaiiin --svstem that lies at the heart of each of the oreanisms that wc study or va!uc ss something to be preserved. As contempo;ary biologist\, ue arc view~ngthe systcm at only one moment in the progre.;? of geolopcal time. The co6servaiionist must iraoole with the difficultidea of ihe non- permanent nmre of horh the sp'kciei and the ecosystem in which it exists. Although some species and ecosystems are more stable than others, ewlu- tiona~~chan~ewlth time is alaw of life. Gkneticiariation arises through continual, irreparable, mutational changes in the D?A carried in the cells. Most of these mutations have on1 a smal effect indtv~duallv:the eenetic uniaueness of individual.; is eenerarc (I"bv genetic recombination: ihis 7s the naturally occurring ~crambling~rocess 118 tI1;tt the genes undergo each time the sex cells, egg and sperm, arc produced and then combined to form a new individual. Genetic recomhinat~onis a nearly universal accompaniment of sexual reproduction. which is the renro- ~ --~~~~~ ductive mode of almosi all of the forms of lifk that we- ----,mav un.ch.. ---. to-- mn&G-- .- -.. - . Some species, particularly among plants, have developed a capacity for clonal reproduction, in which a little piece of the mother plant becomes seoarated. as, for exam le, in a propagated cutting., The pie& can be induced io grow' into a new p7 ant that is truly genetically den tical (except for new mutations) to the plant from which it was taken. A single mother plant can be used to make thousands of such cuttings. When planted out in afield. the r~wltino - . - - .- - - .-.. plants may ]oak like a population, hut thi colleclion does not have the varr ab~lityof a population; ~tis re:dlv only one 1n(l1\,1dua1.Cloning. procedures are wholly impractical as a means for conserving gene nook: th& can merely perpeiuate one or a few combinations of genes; mo&fiedon& slightly by the accumulation of non-recombining new mutations. THE NATURAL POPULATION AND ITS POOL OF GENES The discoven, of extensive genetic variation in natural ~ooulations, , ( ~ ~~-~~.led the.... early pop~~lationgenelici.;tr to &fer to the gcrictic m:lterial carrled by the individuals of the population a< a 'pool irf genes. Ihic term is used to sug- zest the fact that a t~euo~ulation-has a so? of collective hereditam endcw- - , ~ ~~ men1 that hclongs to thi bieeding group. In a sense, each time reproduction occurs, the individuals pool their genes in the mathematical sense. It is from this pool that the next gcncrarim II~intli\.iduals derives its genetic endowment. Ai long as the population of a zpecies remains large, natural selection will tend to preserve gcnctic variahilit) :n the gene pool. POLYMORPHISM UNDER NATURAL SELECTION Contram to earlier views. variabilitv in the eene 0001 does not consist of just :I small ;lumber of variabie genes of major gffect'scattered within a mostly 1111iil)rm.or homozygous, hackground. In contrast, the heterozygous state is the rule almost throughout the DNA. Most of the effects of these genes in hnth natural and artificial populations, furthermore, are individually small. Thic does not mean that they lack importance, however. Early in this century it was ertahlishcd that most of the adantivelv imnortant characters of hoth plant, and animals (called the quantitbtivefharierers) are polygenically c~~ntrolled,mean~ng that many aenes of individually small effect contr~bute to each im~nrtantAaracter. 'Thus. there is an additive as well as an inter- active som'poncr~t,and each character is affected bv many genes. This impor- tant f:wt ha\ heen somewhat concealed by the preoccupation of the human mcdlc;tl genetic~stwith genes of large effect. These,are mostly serious acciJcnr;d mutations, amountin to defects in funct~onor structure; almost a11 are pathological in nature. -8, ey are not the raw matcnal that natural selection used to huild the human genotype. 'lhc importance ofthe small mutation was the original discoverv of the experimentd animal and plant breeders, who demonstrated the effects of artificial (man-induced) selection through their studies of the inheritance of quantitative characters. Natural selection is not different from artif~cialselection in itc basic attrihutes. Gene Pool Conservnliott \Carson 120 Accordingly, the hreeding process in both natural and art~iicialpopulartons tends to Dreserve complexgcne comhinations that serve the needs of the nr~anism(orthe ~raciicalbreeder) best in the heterozveous state. At repmducth, the exist in^ gene comhinations aiereshuffled. and novel combinanons. perhaps only slightly difierent from thcir progenitors, are eenerated. These genetic refinements are continuallv being tested in natural -populat:ons. Con&rsely, recombinations prod~.cingblocks-oi gene. in the hnmozvgous state tend to have a lower fitness. so that population su~val comes to depend more and more on individuals that are comolex multieenic heterozygotks. The result is what is called balanced genetic ~lymorphhn, a cond;tion [hat ?,sentiall) assures the mainten:lnce of eutcnsiw genetic variabilitv. At the same time. however. the svstem exacts a ~rice.sincethe individuds in the arc quite uneq;al in their cap~citvio produce progeny. The price consists of the necessary production oi a number of gene combinations that are relativeh inferior rer~roducer\.Scw laree or conwic- uous mutational changcs are in most case\'deleterious and are>limmate* by selection; or (as in the case of industrial melanism in moths, for emmple) they are retained in the presence of many other gencs ofsmall effedt that have hcen selected, so that they modifv and refine the biological properties of the main biological charaster (in this cue, protective n~imicrv).There is no such thing as "5ne gene, one adaptation." The non-geneticist tends to interpret the above emphasis on hetero- zygosity as meaning that ,qn~and an heterozygosi must be preserved at any cost in order to kee apopulation genetically hezhy. However, the main- tenance of specific "% alanced heterozygosity' which serves the needs of the individuals in the populations is the crltical point. ROTIXENECKS OF I'OPULA'I'ION SIZE AS GENETIC THREATS TO SURVIVAL When a natural population is large, balanced polymorphism is main- tained by natural selection, and therc is usually mple opportunity for the fine-tunine nf the eenetic bxis of both adaotation to the environment and rcProductLe effic'kq. A, was ment,oned above, the raw marcrial. that are incorporated are mostly genes of verv small individual effect. When the size of a natural ~ooulationb~comesreduced. however. the eenetic variabilitv on which the fldxibi~it~of adaptive properti& depends mayYhcpartly loht, oi the organization of it mav he dkturhcd due to chance loses. - Reduction in ~ouulationsize constitutes a threat to the ~ooulationand this is manifested in ;ever31 wap. In clas3ical population &neiics, emphasis was placed on loss of geneticvariability due to the fact that breeding from only a mall nurnher of individuals introduces a sampling error of reproduc- tion, such that some cne5 in low frequency in the parental population may hc h b.v chance from tie descendant population. Such a loss is termed random drift and ha.. often been referred to as the Sewall Wrieht" effect after its discoverer..~~.~~ ~~. Drift is surely an important source of variability loss, hut ,uch hottle- neeking, or unrepresentative constriction, of a population also has a disturb- ing effect on the genic equilibrium that underlies adaptive characters, cdus- ing genetic balance to undergo an equilibrium shift. The smallness of the, population provides less room for thc acc~immodationof the proce\s of trial Gene Pool Coflservation\Camon 121 recombination. The system of adaptation is blind to what may be needed in the environment, so that when the population's ability to generate lots of variants is impaired, this also adversely affects adaptive response.
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