昆虫体系学6 系統推定の方法論 祖先形質状態の復元

吉澤和徳

子供 乳 嘴 毛

魚 卵 × なし なし

鳥 卵 × あり なし

カモノハシ 卵 ○ あり あり

哺乳類 仔 ○ なし あり

祖先形質 派生形質

外 群

魚

哺乳類カモノハシ鳥

魚哺乳類カモノハシ鳥

口→嘴

口→嘴 x 2

Total 4 steps

Total 5 steps

鱗→毛 x 2 餌→乳 x 2

鱗→毛 餌→乳

子供 乳 嘴 毛

魚 卵 × なし なし

鳥 卵 × あり なし

カモノハシ 卵 ○ あり あり

哺乳類 仔 ○ なし あり

祖先形質 派生形質

外 群

子供 乳 嘴 毛

魚 0 0 0 0

鳥 0 0 1 0

カモノハシ 0 1 1 1

哺乳類 1 1 0 1

カモノハシ＋哺乳類＝２ カモノハシ＋鳥＝１

A

BC

X

A

BC

X

D

Y

A

BC

X

D

Y

1

00

X

• Key words • マンハッタン距離⇔ユークリッド距離

•中央値⇔平均値

0

11

X

1

01

X

0

Y

A

B

C

X

D

Y

無根系統樹 有根系統樹

＝

A BC

X

D

Y

1

0

1

1

0

0

無根系統樹 有根系統樹

＝

1 01

1

0

00⇄1 0→1

1

00

X

0

Y

•うまく決まらない場合もある

1 100

H1

H2

•降路１：H1 •上の 0と1から 0,1

1 100

H1

H2

0,1

•降路２：H2 •上の 0とH2 の 0,1 から暫定０ • [(0,0),(0,1)]

1 100

H1

H2

0,10

•昇路１：H2 •外群の１，上の 0，H2 の 0,1 から 0,1

1 100

H1

H2

0,10,1

•昇路２：H1 • H2 の0,1，上の 0 と 1 から 0,1

1 100

H1

H2

0,10,1

•逆転（赤の場合）と平行（青の場合）が等価に最節約的（どちらも２ステップ）だから起きる

1 100

H1

H2

１→０

０→１１→０

１→０

Hennig の補助原則

•対立する証拠が無い限り，常に相同性を仮定し， 収斂や平行を仮定しては ならない

O P I N I ON

Did stick insects really regain their wings?

J . W . H . TRUEMAN , 1 B . E . PFE IL 1 , 3 * , S . A . KELCHNER1 andD . K . YEATES 2

1School of Botany and Zoology, Australian National University, Canberra, Australia,2CSIRO Entomology, Canberra, Australia and3CSIRO Plant Industry, Canberra, Australia

In a recent issue of Nature, Whiting et al. (2003) report asurprising and novel result: the re-evolution of insect wingsfrom wingless ancestors in Phasmatodea (stick insects).If true, this represents an important advance in ourunderstanding of insect and gene evolution. It overturns along-held notion that wings evolved just once in insects andhave been lost many times. The allegedly re-evolved wingshave exactly the same detailed structure as other insectwings (Whiting et al., 2003, their Fig. 1b) and if thisfeature truly has been regained in stick insects itwould demonstrate a remarkable maintenance of ‘winggenes’ despite inactivation of the developmental pathwayover long periods of evolutionary time. Whiting et al.’sconclusions have already been promulgated in the popularliterature (New Scientist, Scientific American, The New YorkTimes, Anon, 2003a, b, c) but before this extraordinaryevolutionary scenario reaches the entomology textbooks are-examination is in order.Two lines of evidence are essential to Whiting et al.’s wing

re-evolution hypothesis: (1) the topology in their stick insectphylogenetic tree, and (2) the distribution of wing/winglesstransformations on that tree. The latter issue, characteroptimization, is addressed here. Our reappraisal of theevidence convinces us that Whiting et al. have overstatedsignificantly the probability of wing re-evolution in stickinsects. When this is taken into account we see no groundsfor overturning the traditional view of stick insect evolution.

Was the ancestor of the stick insects really wingless?

Stick insects are viewed traditionally as having evolvedfrom a winged ancestor, wings being lost subsequently onseveral independent occasions. Whiting et al. (2003) showthat, given their tree, the ancestral stick insect is mostparsimoniously reconstructed as wingless, and this is said

to support their hypothesis that wings have been regained.But reconstruction of the phasmid ancestor is not the rele-vant issue. Instead the support for each hypothesis needs tobe compared when the alternative ancestral states areassumed to be true. The reconstruction of a particularstate at an ancestral node is not relevant to comparing thesupport for alternative hypotheses that one particular stateor another was present in the ancestor. Instead, we need toask the question: ‘How well does the model fit the data if theancestral state is winged, versus how well does the model fitthe data if the ancestral state is wingless?There are two possible states for the ancestor, winged and

wingless, which imply different sets of evolutionary changeswithin stick insects. If the Whiting et al. tree is correct andthe ancestor was wingless, wings re-evolved a minimum offour times, with three secondary losses. If, instead, theancestor was winged, wings have been lost a minimum of13 times. Because three losses are common to both scenariosthe relevant comparison is between four gains for onehypothesis and ten losses for the other, a ratio of just2.5 : 1. Moreover, some nodes on Whiting et al.’s tree arenot well supported in their analysis. If we allow that theseweak nodes (nodes with bootstrap values <70%) may bealternatively resolved, only eleven losses are required if theancestor was winged. The ratio for equivalency between thehypotheses falls to 2 : 1.Thus, despite appearances, the traditional viewpoint on

stick insect wing evolution can be maintained by assuming aprobability of wing loss not much greater than twice winggain. It is known that in other insects, wing loss is very muchmore likely than wing gain, as Whiting et al. (p. 266)themselves explain. Wing loss has occurred many thousandsof times and is a common response to a wide variety ofselective pressures (Roff, 1990; Wagner & Liebherr, 1992),whereas secondary wing gain has never been demonstratedconvincingly. At any ratio of probabilities greater than2.5 : 1, multiple wing losses without gains is the preferredexplanation on this stick insect tree.Reconstruction of the ancestor of stick insects depends

very much on the method used. Whiting et al. show that, ontheir tree, parsimony reconstructs the ancestor as winglessprovided that the cost (in number of steps on the tree) of a

Correspondence: J. W. H. Trueman. E-mail: john.trueman@anu.edu.au*Present address: Department Plant Biology, 228 Plant ScienceBuilding, Cornell University, Ithaca, NY 14853, U.S.A.

Systematic Entomology (2004) 29, 138–139

138 # 2004 The Royal Entomological Society

Hennig の補助原則

•対立する証拠が無い限り，常に相同性を仮定し， 収斂や平行を仮定しては ならない