species as historical individuals

8/14/2019 Species as Historical Individuals

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Sp ec i e s a s i s tor i ca l Ind iv idua l sA R N O L D G . K L U G EM useum of Zoology an d Depar tm ent o f Bio logyUniversity o f MichiganA nn Arbor Michigan 48109U.S.A.

ABST RAC T: The species category is defined as the smallest historical individual withinwhich there is a p arental pattern of anc estry and descen t. The use of historical individual nthis definition is consistent with the prevailing notion that species per se are not involved inprocesses -- they are effects, not effectors. Rep roductive isolation distinguishes biparentalhistorical species from their parts, and it provides a basis for understanding the natu re ofthe evidence use d to discove r historical individuals,KEY W OR DS: Phylogenetic ystematics, e productive isolation,species.

S p e c i e s is o n e o f t h e m o s t i m p o r t a n t c o n c e p t s i n o r g a n i s m i c b io l o g y .R e f e r e n c e t o t h e s p e c ie s c a t e g o r y o r s o m e b i n o m i n a l t a x o n, s u c h a s H o m os a p i e n s is c o m m o n p l a c e , a n d s o m e e v e n c o n s i d e r s p e c i es t o b e t h e u n it o ff u n d a m e n t a l e v o l u t i o n a r y s ig n i fi c a nc e E l d r e d g e 1 9 8 5 , p. 1 59 ; H o w a r d1 9 8 8 ; P a t t e r s o n 1 9 8 8 , p . 6 6 ) . A s p e c i e s c o n c e p t c a n b e t r a c e d a t l e a s t t oA r i s t o t le D e w e y 1 9 1 0 ), a n d i t h a s b e e n d e b a t e d v i g o r o u s l y f r o m s e e m -i n g ly a ll p e r s p e c t iv e s : p h i l o s o p h i c a l , o p e r a t i o n a l , t y p o l o g i c a l , p o p u l a t i o n a la n d h i s t o r i c a l . T h e m o s t r e c e n t d i s c o u r s e c o n c e r n s t h e c l a s s - i n d i v i d u a ld i c h o t o m y G h i s e l i n 1 9 7 4 ; H u l l 1 9 7 8 ) . I t h a s b e e n e s p e c i a l ly h e l p f u lb e c a u s e t h e s p e c i e s c a t e g o r y i s d i s t i n g u i s h e d a s a c l a s s f r o m p a r t i c u l a rs p e c i e s w h i c h a r e i n s ta n c e s o f t h a t k i n d s e e , h o w e v e r , R i e p p e l 1 9 8 6 ;C r a c r a f t 1 9 8 7 , p. 3 4 3 ; M i s h le r a n d B r a n d o n 1 9 8 9) .

S t il l, t h e s p ec i e s p ro b l e m p e r s i s ts , an d i t is co m p l i ca t ed b y : 1 ) co n -f u s i n g t h e o n t o l o g i c a l s t a t u s o f s p e c i e s w i t h h o w t o i n d i v i d u a t e s p e c i e so p e r a t i o n a l l y ; 2 ) d i s a g r e e i n g o v e r t h e r o l e s p e c ie s p l a y i n e v o l u t i o n a r y -e c o l o g i c a l t h e o r y ; 3 ) a p p l y i n g d i f f e r e n t s p e c i e s c o n c e p t s t o b i p a r e n t a l a n du n i p a r e n t a l o r g a n i s m s ; 4 ) a n d i n s i st in g o n a p p l y i n g t h e ru l e o f m o n o p h y l yt o a ll sp e c i es . T h e f i rs t th r e e t o p i c s a r e r e v i e w e d i n th e b o d y o f th e t e x t t of o l lo w ; t h e f o u r t h c o m p l i c a t i o n i s c o n s i d e r e d o n l y in N o t e s 1 , 3 , 5 a n d 8s ee a l s o K l u g e 1 9 8 9 a ) .

I. BIOLOGICAL AND PHYLOG ENETIC SPECIES CONCEPTST h e p r e v a i l i n g o p i n i o n i s t h a t g e n e a l o g i c a l t a x o n o m i e s m u s t b e r e s t r i c t e d


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4 1 8 A R N O L D G . K L U G Eto ind iv id u a l s b ecau se o n ly th ey h av e h i s to ry (Crac r a f t 1 9 8 7 , p . 3 5 1 ; F ro s ta n d W r i g h t 1 9 8 8 , p . 2 0 1 ) . F o r e x a m p l e , M i s h l e r a n d D o n o g h u e ( 1 9 8 2 ) ,C r a c r a f t ( 1 9 8 3 , 1 9 8 7 ) , D o n o g h u e ( 1 9 8 5 ) , M i s h l e r a n d B r a n d o n ( 1 9 8 7 ) ,an d McKi t r i ck an d Z in k (1 9 8 8 ) r ev iewed th e p o p u la r b io lo g ica l sp ec ie sco nce p t (e .g ,, M ayr 1942 , p. 120 ; spec ie s a re g rou ps o f ac tua l ly o rp o ten t i a l ly in t e rb r eed in g n a tu r a l p o p u la t io n s , wh ich a r e r ep ro d u c t iv e lyi so lat ed f ro m o th e r su ch g ro u p s ) , an d th e p r in c ip a l p ro b lem th eyid en t i f i ed wi th th a t co n cep t i s t h a t r ep ro d u c t iv e co m m u n i t i e s d o n o tn ecessa r i ly co r r e sp o n d to we l l co r ro b o ra t ed c l ad es . Su ch in co n s i s t en cyseem s to b e esp ec ia l ly t ru e in p l an ts (Ch r i s Hu m p h r ie s , p e t s . co m m . ).

Al l t h e a fo r em en t io n ed r ev iewer s a rg u ed fo r an a l t e rn a t iv e p h y lo g en e t i c( ev o lu t io n a ry ) sp ec ie s co n cep t , wh ich h as th r ee th in g s in co m m o n : (1 )em p i r i ca l ly in v es t ig a t in g sp ec ie s w i th a c l ad i s t i c an a ly s i s o f sy n ap o m o r -p h ie s sco red am o n g in d iv id u a l o rg an i sm s (wh e re id ea l ly o rg an i sm m ean sen t i re l i f e -cy c le ) o r l o ca l p o p u la t io n s o f i n t e rb r eed in g o rg an i sm s (d em es)a s t e rm in a l t ax a ( e . g . , K lu g e 1 9 6 9 , 1 9 8 8 ; d e Qu e i ro z an d Do n o g h u e1 9 8 8 ) ; ( 2 ) em p lo y in g th e ru l e o f m o n o p h y ly to ach iev e a co n s i s t en t ,g en ea lo g ica l t ax o n o m y ; ~ (3 ) a t t r ib u t in g th e c a t eg o ry sp ec ie s to o n e o f th eleas t inc lus ive c lade s wi th in which there i s a paren ta l pa t te rn o f anc es t ryan d d escen t (Crac r a f t 1 9 8 3 , p . 1 7 0 ).

A c c o r d i n g t o M i s h l e r a n d D o n o g h u e ( 1 9 8 2 ; s e e a l s o M i s h l e r a n dB ran d o n 1 9 8 7 , p . 4 0 6 ; 1 9 8 9 ) , t h e sp ec ie s ca t eg o ry is t h e sm a l l e s t' im p o r t an t ' l in eag e d ee m ed w o r th y o f f o rm a l r eco g n i t io n , wh e re ' im p o r -t an t ' re f e r s to th e ac t io n o f th o se p ro ces se s th a t a r e d o m in an t in p ro d u c in gan d m a in ta in in g l in eag es in a p a r t i cu la r ca se . D o n o g h u e (1 9 8 5 , p. 1 7 9 )w o u ld s im p ly g iv e sp ec ie s r an k to th e sm a l l e s t r eco g n izab le m o n o p h y le t i co r u n re so lv ed u n i t s ( see a l so R o se n 1 9 7 8 , 1 9 7 9 ; N e l so n an d P la tn i ck1 9 8 1 ; Crac r a f t 1 9 8 3 , 1 9 8 7 ; McKi t r i ck an d Z in k 1 9 8 8 ) . Mish le r an dD o n o g h u e ' s ( 1 9 8 2 ; s e e a l so M i s h le r a n d B r a n d o n 1 9 8 7 ; d e Q u e i r o z a n dDo n o g h u e 1 9 8 8 ) t r ea tm en t i s u n sa t i s f ac to ry b ecau se th e p lu r a l i s t i c so lu -t io n th ey p ro p o se fo r t h e ' sp ec ie s p ro b lem ' is t h e 'n u ll h y p o th e s i s ' t h a t wesh o u ld a t t em p t to r e fu te (So b e r 1 9 8 4 a , p. 3 4 1 ; see a l so Gh i se l in 1 9 8 7 ,1 9 8 9 ; F ro s t an d W r ig h t 1 9 8 8 ) . I b e l i ev e th e r e a r e two r ea so n s wh yDo n o g h u e ' s ( 1 9 8 5 ) p o s i t io n wi l l n o t b e accep ted wid e ly . As I w i l l d i scu ssin d e ta i l b e lo w, i t d o es n o t t ak e in to acco u n t th e f ac t t h a t sp ec ie s a r eg en e ra l ly co n s id e r ed to b e p ro d u c t s o f ev o lu t io n ( l i n eag es ) , b u t n o t t h eun i ts par t ic ipa t ing in p rocesses (ne i ther the rep l ica to rs o r in te rac to rs ; Hul l1 9 8 0 ) . Seco n d ly , a s th e sm a l l e s t c l ad e , t h e sp ec ie s ca t eg o ry i s l i k e ly to b et r iv ia l ized by the la rge numbers o f taxa tha t wi l l be recognized , and suchtax a a r e l i k e ly to b e u n s t ab le n o m en c la to r i a l ly . Fu r th e r , E re sh e f sk y (1 9 8 9 )h as c r it i c ized a ll th e cu r r en t f o rm u la t io n s o f th e p h y lo g en e t i c sp ec ie sco n cep t o n th e g ro u n d s th a t t h ey t each u s l i t t l e ab o u t th e n a tu r e o fspec ies .


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S P E C I E S A S H I S T O R I C A L I N D I V I D U A L S 4 1 9II. SPEC IES AS H ISTOR ICAL INDIVIDUALSI t i s c lear tha t spec ies a re no t c lasses (e .g . , Ghise l in 1974; Hul l 1978 ,1 9 8 0 ; s e e , h o w e v e r , N e l s o n 1 9 8 9 a ) . M o r e o v e r , a s n o t e d a b o v e , t h esp ec ie s - a s - in d iv id u a l s p e r sp ec t iv e seem s to b e ca r ry in g th e d ay ( see a l soSo b e r 1 9 8 4 a ; Gh i se l in 1 9 8 7 , 1 9 8 8 ; Hu l l 1 9 8 7 ; Rid ley , 1 9 8 9 ) . Ho wev e r ,a re spec ies typ ica l ind iv idua ls?

M i s h le r a n d B r a n d o n ( 1 9 8 7 ; s e e a l so d e Q u e i r o z a n d D o n o g h u e 1 9 8 8 )p ro v id ed a r ecen t r ev iew o f th e c l a ss -in d iv id u a l d i ch o to m y in wh ich th eyd is t ingu ished fou r con di t ion s o f ind iv idua l ity : (1 ) spa t ia l boun dar ie s , (2 )t em p o ra l b o u n d a r i e s , ( 3 ) co h es io n , an d (4 ) in t eg ra t io n . Th ey p o in ted o u t(p. 399 ) tha t the fo rm er [1 - -2 ] re fe r to 'pa t te rns , ' i . e ., e f fec ts o f b io log ica lp ro cess e s , an d th e l a t te r [ 3 - -4 ] r e f e r d i r ec t ly to th e ac t io n o f p ro cesse s( see Crac r a f t 1 9 8 9 a , f o r a l en g th y d i scu ss io n o f co h es io n a n d in teg ra tio n ).A l l o f t h e m o re in c lu s iv e c l ad es , su ch a s h ig h e r m o n o p h y le t i c t ax a, wh ichmight b e g iven the ca tego r ica l rank of genus , fami ly , e tc ., a re spa t io -t em p o ra l ly r e s t r i c ted ; h o w ev e r , p ro cesse s o f co h e s io n an d in t eg ra t io n h av en o t b een a sso c ia t ed co n v in c in g ly wi th th o se g ro u p s . Acco rd in g to Crac r a f t(1 9 8 9 a , p . 4 2 ) , t h e o n ly ev id en ce fo r su g g es tin g th a t ' co h es iv e p ro cesse s 'm ig h t b e o p e rab le is th e t em p o ra l an d sp a ti a l co n t in u i ty o f a p a r t i cu la rp a tt e~n o f ch a ra c te r v a r i a t io n . Th e r ea l it y o f o n e o r m o re ' co h es iv ep ro ce sse s ' is an o th e r q u es t io n a lto g e th e r . To b e su r e , t h e co n g ru en tsy n ap o m o rp h ie s ex h ib i t ed b y h ig h e r t ax a m ig h t b e in t e rp r e t ed a s c i r cu m -s tant ia l ev id en ce o f co h es io n , a s t ick y p ro c ess (E ld r ed g e 1 9 8 5 ) , b u tc o m m o n a n c e s t r y ex p la i ns t h e p r e s e n c e o f s h a r e d a p o m o r p h i e s e q u a l lywe l l ( d e Qu e i ro z an d Do n o g h u e 1 9 8 8 ) . Th u s , a s Gh i se l in (1 9 8 7 , p . 1 4 1 )su cc in c t ly su m m a r ized o u r cu r r en t u n d e r s t an d in g , g en e ra an d h ig h e r t ax an ev e r d o an y th in g .In co n t r a s t t o h ig h e r t ax a , a l l f o u r co n d i t io n s o f i n d iv id u a l i ty ap p ly tod em es . Wh i l e th e u n i ty o f d em es i s o b v io u s ly a f u n c t io n o f th e ac t io n o fco h es iv e an d in t eg ra t iv e p ro cesse s , su ch a s m a t in g an d r ep ro d u c t io n ,d em es a r e a l so sp a t io t em p o ra l ly r e s t r ic t ed , i f o n ly fo r re l a tiv e ly sh o r tp e r io d s o f tim e (F ig u re lb , c ). Pe rh ap s o u r b e s t ex am p les o f i n d iv id u a l sa r e o rg an i sm s (H u l l 1 9 7 8 ) .

W i l e y ( 1 9 8 1 b , p p . 7 4 - - 7 6 ) r e f e r r e d t o h i g h e r m o n o p h y l e t i c t a x a a sh i s to r ica l g ro u p s b ec au se o n ly th e two co n d i t io n s o f p a t t e rn ap p ly . I

b e l i ev e an eq u a l ly s t r o n g ca se can b e m ad e fo r co n s id e r in g sp ec ie s t ax a tob e h i s to r ica l g ro u p s . I n th e r em a in d e r o f th i s p ap e r , I r e f e r t o an ym o n o p h y le t i c t ax o n th a t i s n o t i n v o lv ed in p ro cesse s a s a h i s to r i ca lin d iv id u a l ; t h e t e rm co n tem p o ra ry in d iv id u a l i s r e se rv ed fo r th o se en t i t i e sth a t a r e sp a t io t em p o ra l ly r e s t r i c t ed an d a r e d i r ec t ly in v o lv ed in co h es iv ean d in t eg ra t iv e p ro cesse s (o th e r t e rm s h av e b een u sed b y Hu l l 1 9 7 8 ; an dW i ley 1 9 8 0 , 1 9 8 1 b ) . As a r ul e , a cco rd in g to Crac r a f t ( 1 9 8 9 a ) , i f t h e


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4 2 0 A R N O L D G . K L U G Ea c t i o n s o f a n i n d iv i d u a l c a n b e a s c r i b e d t o t h e a d d i t i v e a c t i o n s o f i n c l u d e d ,s m a l l e r , u n i ts o f d i v e r s i t y t h e n t h a t i n d i v i d u a l i s t e r m e d h i s t o r ic a l .

T h e c a t e g o r y s p e c i e s , a s a c l a s s c o n c e p t , i s d e f i n e d . M o r e o v e r , a sG h i s e l i n ( 1 9 8 8 , p p . 4 6 8 - - 4 6 9 ; s e e a l s o C r a c r a f t 1 9 8 9 a , p . 4 4 ) s t a t e d ,

w h a t i n d iv i d u a ls p o p u l a t e t h e w o r l d a n d w h a t r o l e s th e y p l a y in e v o l u t i o na n d o t h e r p r o c e s s e s a r e e m p i r i c a l q u e s t i o n s t o b e a n s w e r e d b y r e f e r e n c et o t h e d a t a o f e x p e r i e n c e . T h e r e f o r e , it is a p p r o p r i a t e t o s u m m a r i z ec o m m o n k n o w l e d g e p e r t a i n i n g t o s p e c ie s ( a m o r e d e t a i l e d r e v i e w w illb e p r e s e n t e d i n K l u g e , m s . ) b e f o r e d e f i n i n g t h e s p e c i e s c a t e g o r y . F o re x a m p l e , b i p a r e n t a l s p e c i e s a r e v i e w e d g e n e r a l l y a s f o r m i n g l i n e a g e s(F ig u re 1 ), an d I t h in k i t i s f a i r t o s ay th a t b ip a ren t a l s p ec i e s a re a l s ou n d e r s t o o d t o b e p o l y m o r p h i c ( F i g u r e 2 ; H e n n i g 1 9 6 6 , F i g u r e 6 ) , a n do f t e n p o l y t y p i c w h e n g e o g r a p h i c a l l y w i d e s p r e a d ( H u l l 1 9 8 0 , p . 3 2 4 ).R a r e l y i s a s i n g l e , u n i f o r m d e m e r e f e r r e d t o a s a s p e c i e s . A l s o , o n l y f i v ec l as s es o f p r o c e s s e s s e e m t o h a v e b e e n a t t r i b u t e d f r e q u e n t l y t o s p e c ie s :e v o l u t i o n ( a n a g e n e s i s ) , s p e c i a t i o n ( c l a d o g e n e s i s ) , e x t i n c t i o n , d i s p e r s a l , a n de c o l o g i c a l i n t e r a c t i o n s , s u c h a s c o m p e t i t i o n ( G h i s e l i n 1 9 8 7 ; C r a c r a f t1 9 8 9 a ) .

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b cF i g 1 The relationship between ana genesis and cladogenesis in biparental organisms. (a)Tw o novelties (x and y) originate independently, and spread am ong the pa rts of a dem e. Ahorizontal line denotes mating; a ver tical lin e symbolizes an organism's ontogeny fromconception to dea th. (b) The novelties spread am on g demes. The coalescing lines depictthe temp orary na tur e of d em es, due to dispersal of organisms or the union of entirereproductive comm unities. (c) The n ovelties cannot spread beyond the bou nds of repro-ductively isolated lineages. M odified after Sim pson (1955, Figure 4 8).


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SPECIES AS HISTORICAL INDIVIDUALS 421S p e c i e s D i f f e r e n c e s

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Fig 2 The total structure of hologenetic relationships and the differences in formassociatcd with its individual parts (Hennig 1966, p. 31). Modificd aftcr Hennig (1966,Figurc 6).Lewontin (1970) outlined the few conditions necessary and sufficient

for evolution by natural selection, and he recognized species as one of theunits in the traditional organizational hierarchy that could possibly beinvolved in that process. However, Hull (1980; see also Damuth 1985)concluded that species is an unlikely candidate because the necessaryconditions of replication and interaction do not seem to apply to thatunit} Various group selection arguments and examples (e.g., Stanley 1975;Jablonski 1987) have been examined, and none appears to be convincing(Levinton 1988; Russell and Lindberg 1988). The same conclusionpertains to species adaptations; while possible, they don't appear to apply(Williams 1966; Sober 1984b). Vrba's (1980) effect hypothesis, whichdoes not assume adaptation (Schoch 1986), seems to have fared no betterin explaining interspecific macroevolutionary patterns (Damuth andHeisler 1988).The familiar notion o f new species originating from ancestral species byanagenesis alone, the non-lineage concept of speciation held commonly bypaleontologists (see, however, Gauthier e t a l 1988), might also suggesta process in which species p e r s e are participants. However, upon closeinspection all the examples of common ancestral species seem to be


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4 2 2 A R N O L D G . K L U G Eam b ig u o u s (e .g ., t h e s t r a to p h en e t i c c o m m o n a n ces to r s o f G in g e r i ch 1 9 7 9 ) ,o r t h e c o m m o n a n c e s t o r s c a n b e d e s c r i b e d m o r e s i m p l y a s p ar t s o f s p e c i e s(i.e ., i n d iv id u a l o r g an i sm s o r d em es ; Mish le r an d B r an d o n 1 9 8 7 , p . 4 0 9 ) .

Wh a t t h en i s sp ec i a t io n ? Acco r d in g to Hen n ig ( 1 9 6 6 , F ig u r e 6 ; see a l soFu tu y m a 1 9 8 7 , p . 4 6 7 ) , i n b ip a r en ta l o r g an i sm s i t i s t h e p r o d u c t io n o finheren t ly h ie ra rch ica l l ineages . Fur ther , the ances t ry o f such h is to r ica lin d iv id u a l s is t o b e f o u n d am o n g th e p a r t s o f sp ec i e s , n o t am o n g sp ec ie sper se an d th e p a t t e r n s o f p a r en t - o f f sp r in g r e l a t io n sh ip ( to k o g en e t i c )a t tha t leve l a re inhe ren t ly non-h ierarch ica l (C racraf t 198 9a , p . 49) . 3C r a c r a f t ( 1 9 8 3 , p p . 1 7 4 - - 1 8 4 , 1 9 8 9 b ; s e e a l so W i le y 1 9 8 1 b ) d i s c u s s edn o v e l way s to b eg in s tu d y in g sp ec ia t io n ( sen su Hen n ig ' s m o d e l ) . Th eyin v o lv e a s se ss in g a r ea s o f en d em ism an d th e i r co n g r u en ce , w i th th eo b jec t iv e o f d i s ti n g u ish in g b e tw een d i sp e r sa l an d v i ca r i an ce h y p o th es es( Klu g e 1 9 8 9 b ) . I n co n t r a s t , t h e p o p u la t io n a l v i ew o f sp ec i a t io n , t h a t wh ichacco m p an ie s t h e b io lo g ica l sp ec i e s co n ce p t , i n v o lv es h y p o th e ses o f f u tu r eev o lu t io n - - t h e ac tu a l it y o r p o ten t i a l it y o f i n t e r b r eed in g ( Dar r e l F r o s t ,p e r s . co m m . ) , wh ich m ay ex p la in wh y g en e r a l i za t io n s ab o u t p o p u la t io n a ispec ia t ion have been so d i f f icu l t to d iscover (e .g . , Bar igozz i 1982; Vrba1 9 8 5 ) . A l th o u g h o u r cu r r en t u n d e r s t an d in g m ay b e p r e l im in a r y an dimper fec t , spec ia t ion i s su f f ic ien t to exp la in the o r ig in o f the d ivers i tys u b s u m e d b y all h ig h e r m o n o p h y l e t i c t ax a ( N e l s o n 1 9 8 9 a ) . A s H e n n i g( 1 9 8 1 , p . 4 ) s t a t ed Al l r ecen t sp ec i e s a r e th e p h y s ica l d e sc en d an t s o fan c ien t sp ec i e s , an d so a s we g o b ack f u r th e r i n to th e p as t we a r e b o u n dto en co u n te r sp ec i e s t h a t h av e g iv en r i se t o m o r e an d m o r e ex ten s iv eg r o u p s o f r ecen t sp ec i e s .

Usu a l ly , co n tem p o r a r y in d iv id u a l s a r e ac tu a l ly i n v o lv ed wh en sp ec ie sex t in c t io n , d i sp e r sa l an d eco lo g ica l i n t e r ac t io n s a r e co n s id e r ed . I n d eed , i ti s r a re tha t a l l par t s o f a spec ies go ex t inc t a t once ; the causes o f ex t inc t ionten d to b e lo ca l , n o t g lo b a l , p h en o m en a . I n d iv id u a l o r g an i sm s m ay b eex p ec ted to d i sp e r se , b u t i t i s r a r e t h a t a wh o le sp ec i e s m o v es in a s in g u la rm an n e r ( K lu g e 1 9 8 9 b ) . To say th a t sp ec i e s co m p e te ( p a r a s i t i z e , p r ed a te ,e t c . ) s e e m s t o b e e v e n m o r e o f a n o v e r s t a t e m e n t . F o r e x a m p l e , M a r g a l e f( 1 9 6 8 ) a r g u ed co n v in c in g ly th a t sp ec i e s d o n o t h av e an y th in g th a t co u ldb e co n s id e r ed a u n i t a r y eco lo g y , an d th e co m p e t in g en t i t i e s a r e l i k e ly tob e s e m a p h o r o n t s ( H e n n i g 1 9 6 6 ) . E v e n t h e p r o c e s s o f c o m p e t i n g r e p r o -d u c t iv e ly , wh ich Gh i se l in ( 1 9 7 4 ) a t t r i b u ted to sp ec i e s , s eem s to b e m o r ea c c u r a te l y u n d e r s t o o d a s a n e p i p h e n o m e n o n o n i n d i vi d u al e f f ec t s ( R u s e1 9 8 7 ) . I n su m m ar y , ex cep t f o r g eo g r ap h ica l ly r e s t r i c t ed m o n o ty p ic t ax a ,sp ec i e s d o n o t ap p ea r t o p a r t i c ip a t e d i r ec t ly i n ev o lu t io n a r y an d eco lo g ica lp r o cesse s , o n ly th e i r p a r t s d o . As su ch , sp ec i e s u su a l ly d o n o t ac t a sco h es iv e wh o le s ( see a l so Cr ac r a f t 1 9 8 9 a ; E r e sch e f sk y , m s .).

N o w , t h e q u e s t io n m a y b e r a i s e d a s t o w h y t h e s p e c i e s c a t e g o r y s h o u l dn o t b e d e f in ed a s t h e sm a l l e s t c l ad e . I n t h e p r eced in g d i scu ss io n I d i s t i n -g u i sh ed b e tw een h is to r ica l an d co n te m p o r a r y in d iv id u a l s , an d co n c lu d e d


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S P E C I ES A S H I S T O R I C A L I N D I V I D U A L S 4 2 3t h a t s p e c i e s a r e e x a m p l e s o f t h e f o r m e r . N o n e t h e l e s s , c o n t e m p o r a r yi n d i v i d u a l s , s u c h a s d e m e s , h a v e b e e n d i a g n o s e d o n o c c a s i o n , a n d s u c hd e l i m i t a t i o n s m a y b e e x p e c t e d t o i n c r e a s e w i t h t h e d i s c o v e r y o f n e wm o l e c u l a r t e c h n i q u e s . I n t h o s e i n s t a n c e s , t h e s m a l l e s t c l a d e w o u l d b e t o os ma l l f o r s p ec i e s , a s h i s t o r i ca l i n d i v i d u a l s (Fu t u y ma 1 9 8 7 , p . 4 6 7 ) .F u r t h e r , g r e a t e r t a x o n o m i c i n st a b il it y is e x p e c t e d w h e n c o n t e m p o r a r yi n d i v i d u a l s , s u c h a s d e m e s , a r e e q u a t e d w i t h s p e c i e s t a x a t h a n w h e n t h ec h o s e n e n t it ie s a r e h i s t o ri c a l in d i v i d u a ls . T h e r e a p p e a r t o b e t w o r e a s o n sf o r t h e i n s ta b i li ty : ( 1 ) e v i d e n c e o f h i s t o r y m a y b e d i f fi c u lt t o d i s c o v e rb e c a u s e o f t h e m i c r o e v o l u t i o n a r y n a t u r e o f n o v e lt ie s ( c o n t i n u o u s a n dp o l y m o r p h i c v a r i a ti o n s ; A r n o l d 1 9 8 1 , p p . 1 6 - - 1 9 ) ; ( 2) i n c o m p l e t e l yi s o l a t e d l i n e a g e s t e n d t o g e n e r a t e h o m o p l a s y ( e . g . , i n c o n g r u e n c e d u e t os e g r e ga t io n a n d r e c o m b i n a t i o n ) a n d u n r e s o l v e d h y p o t h e s e s o f t a x o n o m i cr e l a t io n s h i p s r e s u lt b e c a u s e o f c o n f l ic t i n g c h a r a c t e r e v i d e n c e . S p e c i e s a st h e s m a l l e s t c l a d e w i ll g r e a t l y in c r e a s e t h e n u m b e r o f s p e c ie s t a x a , b u t t h a td o e s n o t n e c e s s a r i l y m e a n t h e r e w i l l b e a n i m p r o v e d u n d e r s t a n d i n g o fp a t t e r n a n d p r o c e s s contra C r a c r a f t 1 9 8 9 b) . T h e q u a li ty a n d t h e a m o u n to f th e e v i d e n c e o n w h i c h o u r p h y l o g e n e t ic h y p o t h e s e s a r e b a s e d m u s t a l sob e t a k e n i n t o c o n s i d e r a t i o n .

T h u s , I d e f i n e t h e s p e c i e s c a t e g o r y a s t h e c l a s s w h o s e m e m b e r s a r e t h es ma l l e s t h i s t o r i ca l i n d i v i d u a l s w i t h i n w h i ch t h e re i s a p a ren t a l p a t t e rn o fa n c e s t r y a n d d e s c e n t ? O p e r a t i o n a l l y , p a r t i c u l a r s p e c i e s , l i k e o t h e r h i s -t o r i c a l g r o u p s , a r e d e l i m i t e d w i t h s y n a p o m o r p h i e s ; r e c o g n i t i o n o f s p e c i e si s n o t a m a t t e r o f h a v i n g a d e f i n i n g p r o p e r t y o r s e t o f p r o p e r t i e s contraN e l s o n a n d P l a t n i c k 1 9 8 1 ) . M y d e f in i t i o n o f t h e s p e c ie s c a t e g o r y e m p h a -s i ze s H en n i g ' s (1 9 6 6 ) v i ew t h a t t h e re i s a b r eak a t t h e s p ec i e s l ev e l , w i t hr e t ic u l a t in g g e n e a l o g i c a l r e l a t i o n s h i p s p r e d o m i n a t i n g a m o n g t h e p a r t s o f as p e c i e s , b u t n o t a m o n g s p e c i e s ( F i g u r e 2 ) . U n d e r m y d e f i n i t i o n , s p e c i e sa r e r e a l , a t l e a s t a s m u c h a s a n y m o n o p h y l e t i c h i g h e r t a x o n o m i c h y p o t h e s i sis re a l, b u t s p e c ie s a r e p r o b a b l y n o t t h e f u n d a m e n t a l u n i t o f e v o l u t i o n - -n e i t h e r r e p l i c a t o r o r i n t e r a c t o r . A c c o r d i n g t o C r a c r a f t ( 1 9 8 9 a , p . 5 0 ) ,

S p e c ie s a r e m u c h l ik e d e v e l o p m e n t a l p a t h w a y s w i t hi n th e o n t o g e n y o fi n d i v i d u a l o r g a n i s m s : b o t h a r e e f f e c t s r a t h e r t h a n e f f e c t o r s , T h e y a r ee p i p h e n o m e n a , d e v e l o p e d o r e v o l v e d f r o m l o w e r - le v e l p r o c es s e s. B o t h a r et h u s h i s t o r i ca l en t i t i e s o r ' b y -p ro d u c t s ' .D o e s t h e s p e c i e s c a t e g o r y b e c o m e l e s s i m p o r t a n t w h e n d e f i n e d a s ah i s to r i c a l in d i v i d u a l ? I n o n e s e n s e i t d o e s , b u t i n a n o t h e r it d o e s n o t . W i t hf e w e x c e p t i o n s , s p e c i e s c a n b e n o l o n g e r v i e w e d a s t h e p l a y e r s i n t h ee v o l u t i o n a r y a n d e c o l o g ic a l t h e a t r e ( K a w a t a 1 9 8 7 , p . 4 1 9 ) , a n d c o n -s e q u e n t l y w e l o s e t h e o p p o r t u n i t y t o f o r m u l a t e n a t u r a l l a w s a b o u t t h ec l a s s s p ec i e s i n t h o s e co n t ex t s . N o n e t h e l e s s , s p ec i e s a r e t h e l i n eag es w h i chc h a n g e i n d e f i n it e l y t h r o u g h t i m e a s t h e r e s u l t o f t h o s e p r o c e s s e s ( H u l l1 9 8 0 , p . 3 2 7 ) , a n d l a w s r e m a i n t o b e d i s c o v e r e d a t l o w e r , p o p u l a t i o n -o rg an i s m, l ev e l s (G h i s e l i n 1 9 8 8 ) . A t l e a s t co n cep t u a l l y , s p ec i e s , a s t h e


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4 2 4 A R N O L D G . K L U G Esmal les t h is to r ica l ind iv idua ls , wi l l be the same among c lades , and such ac o n c e p t u a l i z a t i o n p r o v i d e s a c o m p a r a b l e u n i t w i t h w h i c h t a x o n o m i cd iv e r s i ty m ig h t b e s tu d ied q u an t i t a t iv e ly . Mo reo v e r , m y d e f in i t i o n r eq u i r e sth a t p a t t e rn b e d i s tin g u i sh ed f ro m p ro cess , an d in th a t r eg a rd I b e l i ev e o u ru n d e r s t an d in g o f th e n a tu r e o f d iv e r s i ty will im p ro v e . A n y d e f in i t i o n th a tco n fo u n d s th e tw o will su r e ly l ead to m isu n d e r s t an d in g .

IlL REPRODUCTIVEISOLATIONI ag ree th a t r ep ro d u c t iv e i so l a t io n sh o u ld n o t b e a p a r t o f a sp ec ie sd e f in i t i o n (McKi t r i ck an d Z in k 1 9 8 8 , p p . 5 , 8 ; see a l so Mish le r an dD o n o g h u e 1 9 8 2 , p . 4 9 8 ) b e c a u s e r e p r o d u c t i v e i s o l a t i o n a n d t h e c l a d i s t i ch ie r a r ch y m ay n o t b e p e r f ec t ly co r r e l a t ed (F ig u re 3 ) . Ho wev e r , I a rg u eth a t r ep ro d u c t iv e i so l a t io n i s an im p o r t an t r e l a t io n (n o t i n t e rb r eed in g ) ind i s t in g u i sh in g h i s to r i ca l f r o m co n tem p o ra ry in d iv id u a l s ( see a l so McKi t r i ckan d Z in k 1 9 8 8 ; Rid ley 1 9 8 9 ) , 5 an d i t p ro v id e s a b as i s f o r u n d e r s t an d in gt h e n a t u r e o f t h e e v i d e n c e u s e d t o d i s c o v e r h i st o ri c al g r o u p s . M a y r ( 1 9 6 3 ;see a l so Crac r a f t 1 9 8 3 , p . 1 6 3 ; E ld r ed g e 1 9 8 5 ) a l so r eco g n ized ex p l i c i t l ym a n y y e a r s a g o t h e g e n e r a l i m p o r t a n c e o f r e p r o d u c t i v e i s o l a t i o n . M a y rs t a t ed (p. 3 1 ) th at t h e p r im ary c r i t e r io n o f sp ec ie s r an k o f a n a tu r a lp o p u la t io n i s r ep ro d u c t iv e i so l a t io n . Th e d eg ree o f m o rp h o lo g ica l d i f f e r -e n c e d i s p l a y e d b y a n a t u r a l p o p u l a t i o n i s a b y - p r o d u c t o f t h e g e n e t i cd i sco n t in u i ty r e su lt in g f ro m r ep ro d u c t iv e i so la t io n . ( I a s su m e M ay r ' s

d e g r e e o f m o r p h o l o g i c a l d if f e re n c e c a n b e e q u a t e d t o n u m b e r o f

B C B C

F i g 3 (a) The true phylogeny. (lo) The pattern of relationships ba se d on interbreeding(taxa A and C interbreed, but neither do es with B). Mod ified after M cKitrick and Zink(1988 , Figure 1).


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S P E C I E S S H I S T O R I C L I N D I V I D U L S 425c ong r ue n t s yna pom or ph i e s . ) S t i l l , t h e pa r t s o f a s pe c i e s m a y no t ha vea ny t h i ng i n c om m on bu t r e p r odu c t i ve is o la ti on .A dm i t t e d l y , d i ve r ge nc e a nd r e p r oduc t i ve i s o l a t i on a r e no t pe r f e c t l yc oup l e d be c a u s e t he f o r m e r c a n o c c u r i n t he a bs e n c e o f the l a t te r , i.e ., int he c on t e x t o f ge og r a ph i c i so l a ti on a l one . A s C r a c r a f t ( 1989a , p . 53 )po in t ed o u t , a ll r epro du c t ive i so la t i on mani fe s t s d i f fe ren t i a t ion , bu t .no ta ll d i f fe ren t i a ti on is mani fe s t ed a s repro du c t ive i so la t ion . Al so , a s no t edabov e , t he l ea s t i nc lus ive c l ade can be repro duc t ive ly i so l a ted , bu t t ha tdoe s no t m e a n m or e p l e s i om or ph i c s i s te r ta xa a r e r e p r odu c t i ve l y i s o la t edf rom each o the r (F igure 3 ; McKi t r i ck and Z ink 1988 , p . 8 ) . S t i l l , r epro-duc t ive i so l a t i on i s t he on ly process t ha t guarantees f o r t h e i n d e t e r m i n a t efu ture , t he i nd iv idua l i t y o f l i neages o f o rgan i sms ; geographic i so l a t i on byi ts e lf doe s no t ( F u t uym a 1987 , p . 467 ) . A s M i s h le r a nd D on og hu e ( 1982 ,p . 495 ) s t a te d , t he i m po r t a nc e o f r e p r oduc t i ve ba r r ie r s is t ha t g r oups t ha ta r e r e p r oduc t i ve l y i s o l a t e d f o r l ong pe r i ods o f t i m e a r e a t l e a s t e vo l u -t i ona r i l y i nde pe nde n t ( w he t he r o r no t t he y d i ve r ge m or pho l og i c a l l y ) ,m aking the m e f fec t i ve ly sepa ra t e en t i ti e s . I t is i n th i s wa y tha t r ep rod uc -t ive iso l a t ion and h i s to r ica l ind iv idua l i ty a re conn ec t ed . A m or e l imi t edspec i e s concep t t ha t de f ines spec i e s on ly a s t he sma l l e s t c l ade (e .g . ,C r a c r a f t 1983 ; D on og hu e 19 85 ) doe s no t d i st ingu i sh r e p r odu c t i ve l y a ndgeog raphica l l y i so l a ted i nd iv idua l s .T h e s ha r e d t ra i ts t ha t p r ov i de s y s te m a t i s ts w i th e v i de nc e o f s is te r g r oupr e l a t i ons h i p s a r e a s s um e d t o o r i g i na t e by m u t a t i on a nd / o r r e c om bi na t i on(F igure l a ). Th e t axo no m ic gene ra l i t y o f t ra i t s is a f fec t ed pro x im a l ly byi n t e r b r e e d i ng a n d / o r r e p r od uc t i on , a nd u l ti m a t e l y by r e p r odu c t i ve i so l a-t i on . T he f i r s t t w o p r oc e s s e s a c c oun t f o r t he i nc r e a s e d a ppe a r a nc e o f anove l ty , hor i zon ta l l y and ve r t i ca l l y (F igure l a ) , whi l e t he l a s t s t ops i t ss p r e a d . I n b i pa r e n t a l o r ga n i s m s , i n t e r de m i c ge og r a ph i c i s o l a t i on ( F i gu r el b ) t e nds t o r e t a r d t he d i f f u s i on p r oc e s s o f t r a i t s , bu t on l y r e p r oduc t i vei so l a t i on ac tua l l y conf ines i t (F igure l c ) . I t i s i n t h i s way tha t d ive rgence ,l eve ls o f c h a r a c t e r ge ne r a li ty , a nd r e p r o duc t i ve is o l at ion a r e c on ne c t e d .

T he r e s e e m t o be t h r e e pos s i b l e e xp l a na t i ons f o r t he p r e s e nc e o fs yna pom or ph i e s a t t he s a m e l e ve l o f t a xonom i c ge ne r a l i t y : l a c k o f c ha r -a c t e r i nde pe nde nc e , s a m p l i ng a r t i f a c t , a nd r e p r oduc t i ve i s o l a t i on . P r o -c e s s e s s uc h a s ge ne t i c l i nka ge c a n p r oduc e s i m i l a r pa t t e r n s o f c ha r a c t e rc ova r i a ti on ; how e ve r , s y s te m a t i st s do no t know i ng ly u s e s uc h c h a r a c t e r s ina s s e s s i ng c l a de s be c a us e t he y a r e no t i nde pe nde n t e v i de nc e o f h i s t o r y(Kluge 1989c) . Likewise , fa i lure to sample a l l l ineages , extant or ext inc t ,c a n be v i e w e d a s a m e t hodo l og i c a l p r ob l e m ove r w h i c h t he i nve s t i ga t o rha s s o m e c on t r o l . T h e e f f e c t o f r e p r od uc t i ve i so l a ti on is a no t he r m a t t e r - -t he l eng th of t ime a l i neage of b ipa ren t a l o rgan i sms i s i so l a t ed and thee f fec t iveness o f t he i so l a t i on be ing t he c r i t ic a l pa ram e te rs . Th a t is , two o rm or e nov e l ti e s t ha t a ppe a r i nd e pe nde n t l y i n d i ff e r e n t pa r t s o f a r e p r o duc -t ive c om m un i t y ( F i gu r e l a ) w ill in time pr oc e e d t o t he s a m e l i m i t s e t by


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4 2 6 A R N O L D G . K L U G Er ep r o d u c t iv e i so l a t io n ( F ig u r e l c ) J ' Su ch ex ac t ch a r ac t e r co n g r u en ce ( co n -f i rmat ion) migh t be usefu l in opera t iona l iz ing my spec ies-as-h is to r ica l -in d iv id u a l d e f in i ti o n b eca u se i t im p l i e s t h e p r e se n ce o f e f f ec t iv e iso l a t io nan d h i s to r i ca l i n d iv id u a l i t y . A l so , em p h as i z in g co r r o b o r a t ed t ax a en su r e ss o m e m e a s u r e o f t a x o n o m i c s t a b i l i t y , s e n s u t h e sp a t io t em p o r a l l im i t s o fthe hypothes ized h is to r ica l en t i t ies .M i s h l e r a n d B r a n d o n ' s ( 1 9 8 7 , p . 3 9 7 ; s e e a l s o d e Q u e i r o z a n dDo n o g h u e 1 9 8 8 ) r an k in g c r i t e r io n o f sp ec i e s i s p lu r a l i s t i c , i n t h e sen set ha t w h a t e v e r c a u s al p r o c e s s e s a r e d e e m e d m o s t i m p o r t a n t in p r o d u c i n gan d m a in ta inin g l i n eag es in a p a r t i cu la r ca se a r e to b e em p lo y e d . M yd ef in i t i o n o f t h e sp ec i e s ca t eg o r y i s m o n i s t i c ; h o wev e r , t h e r e i s m o r e th ano n e way i t c an b e o p e r a t io n a l i zed . Fo r ex am p le , ap p l i ca t io n o f Ne l so n ' s( 1 9 8 3 ) c l ad i s t i c m e th o d o lo g y f o r d i sco v e r in g h y b r id s m ig h t su g g es t t h ep r e sen c e o f in e f f ec tiv e iso l a tio n , an d th e ca t eg o r i ca l r an k o f sp ec i e s wo u ldth en b e a t t r i b u ted to a m o r e in c lu s iv e c l ad e , o n e th a t d o es n o t ap p ea r t ob e in v o lv ed in h y b r id i za tio n . A b r u p t ch an g es in a h o m o p la sy p r o f i le 7( Klu g e 1 9 7 1 ) m ig h t a l so b e u sed to ad ju s t t h e r an k o f sp ec i e s t o a l a r g e rc l ad e . I t i s wo r th n o t in g th a t h y b r id i za t io n an d h o m o p la sy an a ly se sp r o v id e e s t im a te s o f co h e s io n an d in t eg r a tio n ( see Mish le r an d Br a n d o n1 9 8 7 , p p . 3 9 9 - - 4 0 0 ) . A l s o , b o t h a n a l y s e s e m p l o y t h e s a m e e v i d e n t i a lb as i s u sed to d i sco v e r t h e g r o u p s in t h e f i r s t p l ace ( sy n ap o m o r p h ie s a sp u ta t iv e h o m o lo g u es ) , an d in th a t sen se th e im p o r t an ce o f t h o se an a ly se sin ad ju s t in g th e r an k o f sp ec i e s f o l lo ws f r o m c l ad i s t i c p r in c ip l e s . Sy s t em a-t i s t s in te res ted in spec ies p e r s e m ig h t a l so sea r ch f o r ap o m o r p h ie s t h a ta r e cau sa l ly co n n ec ted to r ep r o d u c t iv e i so l a t in g m ech an i sm s ( in c lu d in gsp ec if i c m a te r eco g n i t io n c h a r ac t e r s , s e n s u P a t e r s o n , 1 9 8 5 ; s e e al s o C o y n ee t a l . 1 9 8 8 , p . 1 9 3 ) , a n d t h o s e a p o m o r p h i e s c o u l d b e u s e d t o a s s e m b l ed e m e s i n t o c l a d e s . T h u s , a d d i t i o n a l i m p o r t a n c e m a y b e a t t a c h e d t o m yp a r t i cu l a r m o n i s t i c v i ew o f t h e sp ec i e s ca t eg o r y b ecau se i t i s p a r t o f al a r g e r c o h e r e n t t h e o r y , p h y l o g e n e t i c s y s t e m a t i c s ( M i s h l e r a n d D o n o g h u e1 9 8 2 , p . 4 9 5 ) . Of co u r se , t h e s im p le s t o p e r a t io n a l co n tex t i s t o o b se r v ep a r t s o f s i s t e r l i n eag es in sy m p a t r y , wh ich wo u ld b e co n s id e r ed p r i m af a c i e ev id en c e f o r r ep r o d u c t iv e c lo su r e an d h i s to r i ca l i n d iv id u a li t y .

IV. UNIPARENTALSPECIEST h e p r e c e d i n g d i s c u s s i o n s o f t h e p h y l o g e n e t ic s p e c i e s c o n c e p t , s p e c i e s -a s -h i s to r i ca l - in d iv id u a l d e f in i t i o n , an d r ep r o d u c t iv e i so l a t io n a r e co u ch ed int e r m s o f b ip a r en ta l o r g an i sm s , an d g r ea t e r g en e r a l i t y wo u ld o b ta in i f m yideas a l so app l ied to un iparen ta ls . To beg in wi th , there i s no reason tob e l i ev e th a t u n ip a r en ta l s can ' t b e an a ly zed c l adi s ti ca lly , ev en th o u g h th eyd o n ' t f o r m l in eag es s e n s u Hu l l 1 9 8 0 ) , an d th e r e i s ev id en ce th a t u n i -paren ta l and b iparen ta l taxa exh ib i t s imi la r un i ty (Mish ler 1985) . In the


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S P E C I E S A S H I S T O R I C A L I N D I V I D U A L S 4 2 7ab se n se o f h o r i zo n ta l t r an s f e r o f n o v e lt i e s, o n e m ig h t ex p ec t t o f in dn u m ero u s l a rg e p o ly to m ies r e l a t iv e ly m o re f r eq u en t ly in u n ip a ren ta l s . S t ill ,t h e r e sh o u ld b e n o m o re d i f fi cu l ty ap p ly in g th e ru l e o f m o n o p h y ly tou n ip a ren ta l s t h an to b ip a r en ta l s b ecau se th a t co n v en t io n i s b l in d tosexual ity . Fu r ther , c lade s o f un ipa ren ta l o rgan isms are h is to r ica l ind iv id -u a ls , l ik e h ig h e r tax a , b ec au s e acco rd in g to Gh i se l in (1 9 8 7 , p . 1 4 1 ) th eyd o n ' t d o an y th in g e i th e r . Th e re fo re , a u n ip a ren ta l sp ec ie s wo u ld b e th esmal les t c lade , and as h is to r ica l ind iv idua ls , un iparen ta l spec ies a re nolo n g e r p seu d o s p ec ie s (Gh i se lin 1 9 8 7 , p . 1 3 8 ; 1 9 8 9 )Y

A C K N O W L E D G E M E N T S

T h i s p a p e r w a s p r e p a r e d f o r B r e n t M i s h l e r ' s s y m p o s i u m o n s p e c i e s , h e l da t t h e S to ck h o lm m ee t in g s o f th e Wi lli H en n ig So c ie ty , 1 9 8 8 , w h ich I wasu n ab le to a t t en d . I t h an k Do u g Beg le , Pau lo Bu ck u p , Ch r i s Hu m p h r ie s ,M i c h a e l D o n o g h u e , D o u g E e r n i s s e , M a r c E r e s h e f s k y , D a r r e l F r o s t ,Mich ae l Gh i se l in , Dav id Hi l l i s , F r ed Krau s , Mary McKi t r i ck , Bren tMish le r , Wi l l i am Mo o re , E l l io t t So b e r , P r i sc i l l a Tu ck e r , D ick Van e -W r ig h t , an d E d W i ley fo r r ead in g o n e o r m o re o f th e ea r ly d r a f ts o f t h em an t / sc r ip t . Th e i r c r i t i c i sm s h e lp ed to sh a rp en m y a rg u m en t s ; h o wev e r , Ia s su m e so le r e sp o n s ib i l i t y f o r a l l t h o se e r ro r s an d am b ig u i t i e s th a t r em a in .Th i s in v es tig a tio n was su p p o r t ed in p a r t b y N . S . F . g r an t B SR -8 8 2 2 6 5 6 .

N O T E ST h e m o n o p h y l y c o n v e n t i o n e n s u r e s t h a t ta x o n o m i e s a r e c o m p l e t e s y s t e m s o f s i st e r

g r o u p r e l a t i o n s h i p s , w h e r e e a c h s u c h s y s t e m c o n t a i n s a l l t h e d e s c e n d a n t s o f t h e m o s tr e c e n t c o m m o n a n c e s t o r o f t h e s y s te m . T h u s , m o n o p h y l y a c h i e v e s lo g i ca l ly c o n s i s t e n tp h y l o g e n e t i c h y p o t h e s e s a n d t a x o n o m i e s ( H u l l 1 9 6 4 ; W i l e y 1 9 8 1 a ) , w h i c h is v i e w e d b ys o m e a s t h e c o r n e r s t o n e o f c l a d is t ic s ( N e l s o n 1 9 8 9 a ; s e e , h o w e v e r , N o t e 3 ). L i k e N e l s o n( 1 9 8 9 b ) , I se e n o p u r p o s e i n c o n s i d e r i n g m o n o p h y l y s i s to b e a n a t u r a l p r o c e s s c o n t r a d eQ u e i r o z a n d D o n o g h u e 1 9 8 8 ; s e e a l so M i s h l e r a n d B r a n d o n 1 9 8 7 ).2 E r e s h e f s k y ( 1 9 8 8 , p . 2 2 1 ; se e , h o w e v e r , C r a c r a f t 1 9 8 9 a , p . 4 3 ) h a s a r g u e d t h a t t h ei n d i v i d u a l i t y o f a g r o u p i s n e i t h e r n e c e s s a r y n o r s u f f i c i e n t f o r i t s b e i n g a u n i t o f s e le c t i o n ,a n d t h a t t h e in d i v i d u a l i t y o f a s p e c i e s is n e i t h e r n e c e s s a r y n o r s u f f ic i e n t f o r t h e t r u t h o ft h e T h e o r y o f P u n c t u a t e d E q u i l i b r iu m .3 I t is w i d e l y h e l d t h a t s p e c i e s e v o l v e f r o m o t h e r s p e c i e s , a n d t h i s m e a n s t h a t s p e c i e s t a x ac a n ' t al l b e r e q u i r e d t o b e m o n o p h y l e t i c . T h u s , i n th e p h y l o g e n e t i c s y s te m a l l m o n o p h y l e t i ct a x a a r e h y p o t h e s i z e d i n d i v id u a l s , b u t n o t a l l in d i v i d u a l s c a n b e m o n o p h y l e t i c c o n t r aR o w e 1 9 8 7 , p . 2 1 0 ) . H e n n i g u n d e r s t o o d t h i s a s y m m e t r y p e r f e c t ly ( 1 9 8 1 , p . 2 9 ). T h a ta n c e s t r a l s p e ci e s c a n ' t b e m o n o p h y l e t i c s h o u l d p r e s e n t f e w i f a n y p r a c t i c a l p r o b l e m s i nt a x o n o m y b e c a u s e a n c e s t r a l s p e c i e s a r e d i f f i c u l t t o i n d i v i d u a t e ( D o n o g h u e e t a l . 1 9 8 9 ) . I na n y c a s e , s p e c i a l t a x o n o m i c c o n v e n t i o n s a r e a v a i l a b l e t o f l a g n o n - m o n o p h y l e t i c a n c e s t r a ls p e c i e s t a x a if t h e y w e r e t o b e d i s c o v e r e d ( W i l e y 1 9 8 1 b ; K l u g e 1 9 8 9 a ) .4 M y d e f i n i t i o n o f s p e c i e s is s i m i l a r t o S i m p s o n ' s ( 1 9 6 1 ; s e e a ls o W i l e y 1 9 7 8 ) - - a n


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4 2 8 A R N O L D G . K L U G Ee v o l u t i o n a r y s p e c ie s i s a l i n ea g e ( a n a n c e s t r a l - d e s c e n d a n t s e q u e n c e o f p o p u l a t io n s ) e v o l v -ing separa t e ly f rom o the r s and wi th i t s own un i t a ry evo lu t ionary ro l e and t endenc ies .H o w e v e r , t h e c o n c e p t u a l c o n t e x t s i n w h i c h t h e d e f i n it i o n s a re d e v e l o p e d a r e q u i t e d i f fe r e n t(Frost and Hi l l i s 1989) .

D o n o g h u e ' s ( 1 9 8 5 ) m e t a s p e c i e s a n d W i l e y 's (1 9 8 l b ) sedis mutabilis c o n v e n t i o n s c a n b eused to d i s t i ngu i sh non-monophy le t i c spec ies t axa when the t e rmina l t axa , i nd iv idua lo r g a n i s m s o r d e m e s , f o r m u n r e s o l v e d p l e s i o m o r p h i c s i s t e r a s s e m b l a g e s ( K l u g e 1 9 8 9 b ) .T h i s o f c o u r s e a s s u m e s t h a t g e o g r a p h i c i s o l a t i o n w i t h i n a r e p r o d u c t i v e c o m m u n i t y i s

o n l y t e m p o r a r y .7 H o m o p l a s y i s a s s u m e d t o b e g r e a t e r a m o n g t h e o r g a n i s m s a n d d e m e s w i t h i n a r e p r o -duc t ive ly i so l a t ed l i neage than i t i s among r ep roduc t ive ly i so l a t ed l i neages because r ep ro -d u c t i v e c l o s u r e n e c e s s a r i l y l i m i t s t h e a m o u n t o f h o m o p l a s y t h a t c a n r e s u l t f r o m i n d e p e n -d e n t s e g r e g a t i o n a n d r e c o m b i n a t i o n .s S p e c i a l p r o b l e m s i n t h e t a x o n o m y o f u n i p a r e n t a l s a r e r e v i e w e d b y F r o s t a n d W r i g h t(1988 ; see a l so Eche l l e 1990) .

R E F E R E N C E SA r n o l d , E . N . : 1 9 8 1 , ' E s t i m a t i n g P h y l o g e n i e s a t L o w T a x o n o m i c L e v e l s ' , Zeitschrift fiirzoologische Systematik un d E volutionsforschung 1 9 , 1 - - 3 5 .Ba r igozzi , C. (ed. ): 1982, Mechanism s o fSpec ia tion L i s s, N e w Y o r k .C o y n e , J . A . , H . A . O r r , a n d D . J . F u t u y m a : 1 9 8 8 , ' D o W e N e e d a N e w S p e c i e s C o n c e p t ? 'Systematic Zoology 3 7 , 1 9 0 - - 2 0 0 .Crac ra f t , J . : 1983 , 'Spec ies Concep t s and Spec ia t ion Ana lys i s ' , i n R . F . Johns ton , CurrentOrnithology 1 , P l e n u m P r e s s, N e w Y o r k , p p . 1 5 9 - - 1 8 7 .C r a c r a f t , J . : 1 9 8 7 , ' S p e c i e s C o n c e p t s a n d t h e O n t o l o g y o f E v o l u t i o n ' , Biology and Phi-losophy 2 , 3 2 9 - - 3 4 6 .Crac ra f t , J .: 1989a , 'Spec ies a s En t i t i e s o f Bio log ica l Th eory ' , in M. Ruse , What thePhilosophy o f Biology Is -- Essays fo r Da vid Hull K l u w e r , N e t h e r l a n d s , p p . 3 3 - - 5 4 .C r a c r a f t , J .: 1 9 8 9 b , S p e c i at io n a n d I ts O n t o l o g y : T h e E m p i r i c a l C o n s e q u e n c e s o f A l t e r n a -

t i v e S p e c i e s C o n c e p t s f o r U n d e r s t a n d i n g P a t t e r n s a n d P r o c e s s e s o f D i f f e r e n t ia t i o n ' , inD . O t t e a n d J . A . E n d l e r , Speciation and Its Consequences S i n a u e r , S u n d e r l a n d ,M a s s a c h u s e t ts , p p . 2 8 - - 5 9 .

D a m u t h , J .: 1 9 8 5 , ' S e l e c t i o n a m o n g ' S p e c i e s ': A F o r m u l a t i o n i n T e r m s o f N a t u r a l F u n c -t iona l Un i t ' , Evolut ion 3 9 , 1 1 3 2 - - 1 1 4 6 .D a m u t h , J . a n d I . L . H e i s l e r: 1 9 8 8 , ' A l t e r n a t i v e F o r m u l a t i o n s o f M u l t il e v e l S e l e c t i o n ',Biology and Philosophy 3 , 4 0 7 - - 4 3 0 .d e Q u e i r o z , K . a n d M . J . D o n o g h u e : 1 9 8 8 , ' P h y l o g e n e t i c S y s t e m a t i c s a n d t h e S p e c i e sP r o b l e m ' , Cladistics 4 , 3 1 7 - - 3 3 8 .Dewey , J . : 1910 , The Inf luence o f Darwin on Ph ilosophy and O ther Essays in Contem -porary Thought H o l t , N e w Y o r k .D o n o g h u e , M . J .: 1 9 8 5 , ' A C r i t i q u e o f t h e B i o lo g i c a l S p e c i e s C o n c e p t a n d R e c o m m e n d a -

t ions fo r a Phy logene t i c Al t e rn a t ive ' , Bryologist 8 8 , 1 7 2 - - 1 8 1 .D o n o g h u e , M . J ., J . A . D o y l e , J . G a u t h i e r , A . G . K l u g e, a n d T . R o w e : 1 9 8 9 , ' T h eI m p o r t a n c e o f F o s s i l s i n P h y l o g e n y R e c o n s t r u c t i o n ' , Ann ua l Rev i ew o f Ecology andSystematics 2 0 , 4 3 1 - - 4 6 0 .

E c h e l l e , A . A . : 1 9 9 0 , ' N o m e n c l a t u r e a n d N o n - M e n d e l i a n ( C l o n a l ) V e r t e b r a t e s ' , Sys tem-atic Zoolog y 3 9 , 7 0 - - 7 8 .Eld redge , N . : 1985 , Unfinished Synthesis. Biological Hierarchies and Modern EvolutionaryThought O x f o r d U n i v e r s i t y P r e s s, N e w Y o r k .E r e s c h e f s k y , M .: 1 9 8 8 , ' I n d i v i d u a l it y a n d M a c r o e v o l u t i o n a r y T h e o r y ' , Phi losophy o fScience Association I , 2 1 6 - - 2 2 2 .


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S P E C I E S A S H I S T O R I C A L I N D I V I D U A L S 4 2 9E r e s c h e fs k y , M . : 1 9 8 9 , ' W h e r e ' s t h e S p e c i e s ? Co m m e n t s o n t h e P h y lo g e n e t ic S p ec i es

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1 , 3 1 1 - - 3 3 2 .Hul l , D. L .: 198 7 , 'Gen ea log ica l A c to r s in Eco log ica l Ro les ' , Biology and Philosophy 21 6 8 - - 1 8 4 .Jab lonsk i , D. : 1987 , 'H er i tab i l i ty a t the Spec ies Leve l : An a lys is o f Geog raph ic Ran ges o fCr e t a c e o u s M o l lu s k s ', Science 2 3 8 , 3 6 0 - - 3 6 3 .Kaw ata , M. : 1987 , 'Un i t s and Passages: A V iew fo r Evo lu t iona ry Bio logy and Eco logy ,"Biology and P hilosophy 2 , 4 1 5 - - 4 3 4 .K lu g e , A . G . : 1 9 6 9 , ' T h e E v o lu t i o n a n d G e o g r a p h i c a l O r ig in o f t h e N e w W o r ldHemidactylus mabouia-brookii Co m p le x ( G e k k o n id a e , S a u r i a ) ' , Miscellaneous Publica-t ions M useum of Zoology University of Michigan N o . 1 3 8 , 1 - 7 8 .K lu g e , A . G . : 1 9 7 1 , ' A Q u a n t i t a t i v e A p p r o a c h t o t h e B io lo g i ca l S p e c ie s Co n c e p t ' ,A m erican Phi losophical Society Year Bo ok 19 7 1 32 9 -- 333.K lu g e , A . G . : 1 9 8 8 , ' t h e Ch a r a c t e r i z a t i o n o f O n to g e n y ' , i n C . J. H u m p h r i e s , Ontogeny andSystematics C o l u m b i a U n i v e r i s ty P r es s, N e w Y o r k , p p . 5 7 - - 8 1 .Kluge , A. G . : 1989a , 'M etac lad is t ic s ' , Cladistics 5 , 2 9 1 - - 2 9 4 .K lu g e , A . G . : 1 9 8 9 b , ' P a r s im o n y i n V ic a r i a n c e B io g e o g r a p hy : A Q u a n t i t a t i v e M e th o d a n da G r e a t e r A n t i l l e a n E x a m p l e ' , Systematic Zoology 3 7 , 3 1 5 - - 3 2 8 .K lu g e , A . G . : 1 9 8 9 c , ' A Co n c e r n f o r E v id e n c e a n d a P h y lo g e n e t ic H y p o th e s i s o f Re l a t i o n -s h ip s a m o n g Epicrates (Bo idae , Se rpen tes ) ' , Systematic Zoology 3 8 , 7 - - 2 5 .Kluge , A. G. : ms . 'Spec ies The or ies f rom Ar is to t le to C lad is t ic s ' , in p repara t ion ,Lev in ton , J. : 1988 , Ge netic s Paleontology a nd Macroevolution C a m b r i d g e U n i v e r s i t yPress , New Y ork .L e w to n in , R . C .: 1 9 7 0 , ' T h e U n i t s o f S e l e c ti o n ' , An nua l Rev iew o f Ecology and Systematics1 , 1 - - 1 8 .


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species as historical individuals

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