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GENE POOL CONSERV TION

Con~ewations charged with thepresetyation of a dynam ic, ever.

changfngsystem of vanafron contained in each species.

Hampton

L.

Carson

Mo dern eenetics (the stu~ d vf hereditv and variation) has reached into

every corner oi bio logy , d r a s~ i& l l~hanging old-fa\l:ioned concepts. One set

of new discoveries has

to

d o with the existence of generic variation from

individual to individual in natural ~ ~ lp u la ti o n r .

he

natural pwu ation o f the

hu ma n species displays suchvariabilliy, and it

is

not hard fo ith e casual

observer to recognize th at both genes and environment ar e involved. Some

domes tic animals (for example, dogs and cattle,

anis

amiliari.7, Bas taurus

also show easilv observed individual nen eticvariation. This is an old storv:

inf&mation fr jm the n& &eneri&eClend.:bur perception and indicates (hat

nearly every normal, sexually re pr od uc in ~ pecies is re lete with gene tic

vartntion. much of it s t the molecu lar levcl. A lrhourli [idden fro m easv ohber-

vntion. perm anetit molecular variation from in d iv id h to individual is i o all-

pewasive throu ghou t nature as to co n st it ~ te n nlmoht un:\'ersal law of life.

In the last Century, wild animals and plants we re freq uen t2 viewe? as if

the individual mem bers of a species were basically identtcal. ach mdividual

was expected to co nform to som e kind of type; any deviation from th e type

was considered somehow unusual or accidental. The idea of tw es is reallv a

human concoction used to simpl~fyhe way we deal with natuii. Fxcept pkr-

haps as a device for cataloging material in museums, we mutt ab:indon the

con ccpt 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

environm ent, but so is each dog or elephant or oak tree.

t

is imo ortan t to recoeniz; tha t the conservationist is no t chareed with

the prese&ation of types. Ind eed , what is out there in nature is a dynamic,

eve r-cli ang in~ pt em of variation co ntained in each species. It is this

flexible vGiai iin svstem that lies at the h eart of each o f the oreanisms that

wc study or va uc ss som ethin g to be preserved. As contempo;ary biologist\,

u e

arc view~nghe systcm at only one mo ment in the progre.;? of geo lop cal

time. Th e co6servaiionist must ir aoo le with the difficultid ea of ihe non-

perm anen t n m r e of horh the sp'kciei and th e ecosystem in which it exists.

Although som e species and ecosystems are more stable than oth ers, ew lu-

t i o n a ~ ~ c h a n ~ e

lth

time is al aw of life.

Gk net icia riatio n arises through continual, irreparable, mutational

changes in the D A carried in the cells. Most of

these

mu tations have on1 a

smal effect indtv~duallv: he e enetic uniauen ess of individual.; is eenerarc

(I v

genetic recombination: i h i s 7s the naturally occurring ~c ra m bl in g~ ro ce ss

8

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t I1; t t

the genes und ergo each time the sex cells,

egg

and sperm, arc produced

and then com bined to form a new individual. Genetic rec om hinat~ ons a

nearly universal accom panimen t of sexual reproduction. which is the re nro-

~ ~ ~ ~

ductive mode of almo si all of the forms of lifk tha t we mav un.ch t m n G

----

. . .

Som e species, particularly am ong plants, have developed a capacity for clo nal

reproduction, in which a little piece of the m other plant become s seoarated.

as, for exam le, in a propagated cutting., The pie can be induced i o grow '

into a new p nt that is truly genetically den tical (except for new m utations)

to the plant from which it was taken. A single mother plant ca n be used to

make thousands of such cuttings. When planted out in afield. th e r ~ w l t i n o

-

-

. - ..

plants may ]oak like a population, hut

t h i

colleclion does not have the varr

ab ~l ity f a po pulation; ~t s re:dlv only o ne 1n(l1\,1dua1. loning. procedures

ar e wholly impractical as a means for conserving gene nook: th can

merely perpeiuate o ne or a few combinations of genes; mo fied on slightly

by the accumulation of non-recombining new mutations.

THE N TUR L POPUL TION ND

ITS

POOL OF

GENES

Th e discoven, of extensive genetic variation in natural ~o ou la ti on sed the

~ ~ - ~ ~ .

...

ear ly pop~~la t ionenelici.;tr to fer to the gcrictic m:lterial carrled by the

individuals of the popu lation

a

a 'pool

i r f genes. Ihic term is used to sug-

zest th e fact that t ~ eo~ ula tion -ha s so? of collective hereditam endcw-

men1 that hclongs to t h i bieeding group. In a sense, each time reproduction

occurs, the individuals pool their genes

n

the mathematical sense. It is

from this pool that the next gcn cra rim ~ntli\.iduals derives its gene tic

endowment.

Ai

long s the population of a zpecies remains large, n atural

selection will tend to preserve gcn ctic variahilit) :n the gene pool.

POLYMORPHISM UNDER N TUR L SELECTION

Co ntra m to ea rlier views. variabilitv in th e ee ne 0001 doe s not consist of

just :I small ;lumber of variabie genes of major gffect'scattered within a mostly

1111iil)rm.r homozygous, hackground. In contrast, the heterozygous sta te

is the rule almost throughout the

DNA.

Most o f the effects of these ge nes in

hnth natural and artificial populations , furthermore, a re individually small.

Thic does not m ean that they lack importance, however. Early in this century

t was ertahlishcd that most of the adantivelv imnortant cha racter s of hoth

plant, and animals (called the qu ant itbt ive fha rier ers ) are polygenically

c ~ ~ n t r o l l e d ,e a n ~ n ghat many aenes of individually small effect co nt r~ bu te

to each im ~ n rt an t aracter . 'Thus. there isan addit ive as well s an inter-

active som'pon cr~t, nd each cha racter is affected bv many genes. This impor-

tant f:wt ha\ he en somewha t concealed by the preo ccup ation of th e hum an

mcdlc;tl genetic~stwith genes of large effect. The se,ar e mostly serious

acciJcnr;d m utations, amoun tin to defects in fu nc t~ on r structure; almost

a11 are pathological in nature. -8 ey are not the raw matcnal that natural

selection used to huild the human genotype. ' l hc importance of th e 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 differe nt from artif~ cial election in itc basic attrihutes.

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  enePool onservnliott \ arson

12

Accordingly, the hreeding process in both natural and art~ iicia l opulartons

tends to Dreserve com plexg cne com hinations that serve the needs of the

n r ~ a n i s m ( o rhe ~r ac ii ca l reeder) best in the heterozveous state.

A t r e p m d u c t h , t he

exist in

gene comhinations ai eres huf fled . and novel

combinanons. perhap s only slightly difierent from thcir progenitors, ar e

eenerated. These gen etic refinements a re continuallv being tested in natura l

opulat:ons. Con&rsely, recombinations prod~.cingblocks-oi gene.

in

the

hnmozvgous state tend to have a lower fitness. s o that

population

s u ~ v a l

comes

t o

depend more and more o n individuals that ar e comolex multieenic

heterozygotks. Th e result is what is called balanced genetic ~ l y m o r p h h n ,

a cond;tion [hat ?,sentiall) assures the mainten:lnce of eutcnsiw genetic

variabilitv. At th e same time. however. the svstem exacts a ~r ic e.s in ce he

indiv iduds in the ar c quite uneq;al in their ca p~ ci tvo produce

progeny. The price consists of the necessary production

oi

a number o f gene

combinations that a re relativeh inferior rer~roduc er\.Scw laree or conwic-

uous mutational changcs are in most case\'deleter ious and are>limmate* by

selection

or (as

i n

the case of ind ust rial melanism in moths, for em mp le)

they are retained in the p resence of many othe r gencs ofsma ll effedt that

have hcen selected, so that they modifv and refine the biological prop erties

of the main biological charaster (in this c u e , protective n~im icrv ).There is

n o such thing as 5ne gene, on e adaptation.

Th e non-geneticist tends to interpre t the above emphasis on h etero-

zygosity as meaning that qn~and n heterozygosi must b e preserved a t any

cost in order to kee apopu lation genetically he zh y . However, the main-

tenance of specific alanced heterozygosity' which serves th e needs of the

individuals in th e populations is the crltical point.

ROTIXENEC KS O F I OPULA I ION SIZ E

AS GENETIC THREATS

TO

SURVIVAL

When a natural population is large, balanced polymorphism is main-

tained by natural selection, and therc is usually m p l e opportunity for the

fine-tunine nf the eenetic bx is of both adaotation to the environment and

rcProductLe e ff ic 'kq. A was ment ,on ed above, the raw marcrial. that are

incorporated are m ostly genes of verv small individual effect. When the size

o f a n a tu ra l ~ o o u l a t i o n b ~ c o m e seduced. how ever. the eenetic variabilitv on

which the f ld xib i~i t~f adaptive properti& dep end s mayYhcpartly loht, o i he

organization of i t mav he dkturhcd d ue to chance lose s.

Re duct ion in ~o uu la t ion ize const itutes a threat to the ~ oo ul a t io n nd

this is manifested in ;ever31 w a p . In clas3ical population &n eii cs, emphasis

w s placed o n loss of geneticvariability d ue to the fact that breeding from

only a m a l l nurnher of individuals introduces

a

sampling error of reproduc-

tion, such that some cne5 in low frequency in th e parental po pulati on may hc

.v chance from ti e

descendant

population. Such a loss is termed random

drif t and ha.. often b een r eferred to as the Sewall Wri eht effect after its

discoverer.

~

~~

Drift is surely an imp ortant sou rce of variability loss, hut ,uch hottle -

neeking, or unrepresentative con striction, of a population als o has a disturb-

ing effect on the genic

equilibrium

that underlies adaptive characters , cdus-

ing genetic balance to undergo an equilibrium shift. Th e smallness of the ,

population provides less room for thc acc~imm odatio n f the proce\s of

trial

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  ene

ool

oflservation amon 2

recombination. Th e system of adaptation is blind to what may be needed in

the environme nt, so that when th e population's ability to gene rate lots of

variants is impaired, this also adversely affects adaptive respo nse. n rief,

a good adaptwe response and efficient reproduction of a population depend

on the main tenan ce of balanced genetic polymorphisms. This balance 1s up-

set in populations that have been redu ced to a small size.

Following a population bottleneck , several outcomes are possible. Th e

first, and th e one th at concerns the conservationist the most, is that th e pop-

ulation may be threatened w ith extinction. Some of the genetic details that

underlie th e de ath of a population or species are well modeled in what hap-

pens in experimental populations an d will be considered below. Basically,

there is a loss of both genetic variability and its organization. If the reduc-

tion in p hat occur

i n

larger pop~.liltions.

n

sumc css et, howe\,er, the altered pop ulation nv shou

new conspicu ou\ ch ar xt er s (i or extm ple, size, color of hmr) that reflect the

underlying gene tic shift. Indeed, s thcury of species forniat~tm as heen de-

wlu p ed a ro un d th is ~~h c n o m c n l~ nf genetic reorg3nizatinn b rec om hi ns ~i ~m .

In m n m a ty , dr as ti c r e d ~~ c t i c ~nn po pul xiu n sile nit,) be llonc d by:

I

an apparent nm;~inten:~rii~~f the .srurrr.\ qiro; 2 ) wd rlcn ~h i i tn impur-

tant chttracrer,; or

3

cwtincti~m.Extinction is ccrt:~inlvh\ idr the must

probable outcome of severe population bottlenecks.

THE GENE POOL OF

ARTIFICIAL OR CAPTIVE POPULATIONS

The r.onsc.nutioni>t s pr~niarilv oncerned w i t h popu1;ttions cxist ~n dn a

.itsre o f n:tturc, and t l i i b is the rearon for the c m ph a~ t\ laced a h w e nn

natural populations. O n the other hand, there a?e noG an increasing numb er

of instances where the sizes of the natu ral nooulations of valued suec ies have

hecome so low that speci:,l or e\trhurdin:lry measu res ;,re ireq ~e n't ly ropth .4

for saving them. I'rumincnt : m m g these is thr. rem w al of all specim ens from

nature and re.wrting. at 1cad teml1~3rily.o p r o p ~ g ~ t i o n

n

captivitv. Ex-

amples arc the ( '3 lifornia condor and the 1lhnaii; tn crow (G w ~ r n o ~ p \

T h e cxpcctcd genr.tic reil.lr< of cnp tiw breeding or [ ~ r o p a g : h n re

hrietlv outlined IieIu~.. he geneticist is i n reasonahl . good pobitiun to

make judgments o n such procedures, since there is an exiensive literature on

the eenetlcs of artificial oooulations of dome stic animals and olants. includ-

ing &ensive experimcnial '\ rork on lr lhnratnq pn ~ ~ u la ti u n ~

Illat one uf the in;tjor pruhlcins ariring

i n

m:Jl c:tpti\e populations is hreak-

du un in the mating s steni,; this >criously interfe res nit11 the reproductive

process.

n some birds

d

nsects. mide choice (eithe r fem ;~l e hoice or 1 1 1 . k

choice) depen ds on the e \i> ten ce o i a genetic;~ll v;tri:~hle ield ui indi\.idu;lls

that 3re suhject to the t)stein oiclioice. Such ii d &

of

mating clmice

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Gene

ool

onservation

arson

23

of events has been pro ~o se d s a mode wherebv new s~ ec ie s volve: however.

shifts induced in 'x~)criment;t l p ~ ~ ~ ~ u l a t i o n sre hot th i

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Gene Pool Consen~a~iort\Camon 124

increasingly severe population bottlenecks

now being

imposed by widespread

environmental destruction.

Verv few

soecies.

however. can be

successfullv

pro

agared

in

the lahoratory,

f eid

c x p e h e n t a l f ar m, o r mo.

l hei;

ecokg ical requirements are often to o spectal. Only a minority of Hawaiian

Droso>hila

soecies. for examole.

can

be br ed in thelaborator? even

for

those'that can he

so

hred, iniportant natural characteristics a ie evidently

10% Sh tu re is

I

better stockkee er than we are, and this principle would

appear

to

apply to many kinds oforganisrns. From the gm point of view,

the estahlisllment

of

manv

reht ively

small

reserv es ma serv e ar viable

reiugia

ior

lawaiian plants and

an~mals .&e size

of successful

rehgiurn

in Hawhi'i should be determined by what i t takes to provide maximal

insula-

tiun of [he ecosystem from encroachment b

aliens

and to rotect it from

oth er adjacent degrading for ces (see also d l t , his volume .

A

large number

o i

refuria of

ec~~loeicnllvound size

will

~ r o b a b l verve Hawaiian consewa-

tion needs better than