The Time Scale of Evolution

The average evolutionary rate for Viperidae cytochome b has been estimated to 1.4% sequence difference per 1 million year (Ursenbacher et al. 2006a). For Natrix, we calculated a similar rate of 1-1.35% per 1 million year (Guicking et al. 2006a). For L. agilis, we estimated a higher rate of up to 2.5% per 1 million year (Kalyabina et al. 2001), but for E. orbicularis, a lower level of 0.3-0.4% per 1 million year (Lenk et al, 1999). These differences are interpreted as reflecting the different metabolic activity in reptiles, which is lowest in turtles and highest in lizards. Life expectancy is reciprocal to metabolic rates.

If these estimates are correct, sequence differences of 1% or more in turtles, 3% or more in snakes, 6% or more in lizards are due to prePleistocene branching events and hence, cannot be associated with Ice Age refugia. All species groups investigated here originated in the Tertiary and speciation events are of Pliocene age (Table 1). This is in accordance with the fossil record, as remains of today's snake species were found in Pliocene sediments as old as 3.4 Million years (Szyndlar and Böhme 1993). A mid-Miocene record of Natrix was considered a direct ancestor of the extant N. natrix, suggesting that the three clades in Natrix might have already been separated in the middle Miocene (Ivanov 2001). According to our estimate, the main clades of Natrix species are of late Miocene age, the Southern Spanish clade of N. maura is dated Pliocene, and the differentiation between the lineages of the main

Table 1 Speciation, sequence differences and approximate time estimates

% sequence

Time estimate

difference

Time estimate

for major

between sister

for species

groups within

Group

species (cyt b)

(m.y.)

species (m.y.)

Reference

Emys

1.4—1.7a

3-4

1.4-3

Lenk et al. (1999)

Lacerta agilis

6.5-7.3b

2.6-2.9

0.7-2.4

Kalyabina et al. (2001)

L. viridis

6.6-8.4c

2.6-3.4

0.5-2.5

Böhme et al. (2006)

complex

Hierophis

10.5d

8-9

0.9-2.7

Nagy et al. (2002, 2004)

Zamenis

7e

5-7

0.5-1

Lenk et al. (2001)

(Elaphe)

Natrix

18f

13-22

2-8

Guicking et al. (2006a)

Vipera berus

5g

4

1.1-1.6

Kalyabina-Hauf et al.

group

(2004b)

"Distance between E. orbicularis und E. trinacris bDistance between L. agilis sspp. and L. (a.) boemica (assuming species status)

cDistance between L. viridis and L. bilineata dDistance between H. viridiflavus and H. gemonensis eDistance between Z. longissimus and Z. lineatus fDistance between N. natrix and N. tessellata gDistance between V. berus and V. seoanei

"Distance between E. orbicularis und E. trinacris bDistance between L. agilis sspp. and L. (a.) boemica (assuming species status)

cDistance between L. viridis and L. bilineata dDistance between H. viridiflavus and H. gemonensis eDistance between Z. longissimus and Z. lineatus fDistance between N. natrix and N. tessellata gDistance between V. berus and V. seoanei

European clade is Pleistocene (Guicking et al. 2006a). On the other hand, Nagy et al. (2003) estimated evolutionary rates for colubrid snake mitochondrial genes as twice the rate as we assume here, hence, of the same magnitude as in Lacerta. If we accept this alternative, Hierophis and Zamenis would show similar time estimates for sister species as Lacerta (Pliocene); but Natrix speciation would still be Miocene.

Apart from Natrix, the main intraspecific clades in our selection are of late Pliocene or Pleistocene age. As pronounced climatic oscillations started already in late Pliocene, it seems appropriate to consider these intraspecific radiations as effects of climate change and associated range restrictions.

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