gene pool conservation
TRANSCRIPT
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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
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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
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-
. - ..
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