Biology Reference
In-Depth Information
(b)
(a)
3.5
9
8
7
6
5
3
2.5
2
4
1.5
3
2
1
0.5
1
0
0
0
5
10
15
20
0
5
10
15
20
Phallus length (cm)
Phallus length (cm)
Fig. 7.23
Coevolution of male and female genitalia in waterfowl. (a) In species where the male has a longer
phallus, the female has a more elaborate vagina, with more spirals (right) and 'dead end pouches' (left). The vaginal
spirals are in the opposite direction to male phallus spirals, suggesting antagonistic rather than mutualistic 'lock and
key' coevolution. (b) Mallard duck,
Anas platyrhynchos
, a species with high levels of forced copulations in which the
male has a long phallus (bottom right) and the female has a long and elaborate vagina (top left). The white bar is
2 cm. From Brennan
et al
. (2007).
Caudal fin
Priapella
Xiphophorus
maculatus
Platyfishes
Platyfish
X. maculatus
X. xiphidium
X. nigrensis
X. cortezi
Swordtails
X. alvarezi
Swordtail
X. helleri
X. helleri
Past
Present
Fig. 7.24
The phylogeny of the swordtails suggests that the ancestor of the genus
Xiphophorus
lacked an
elongated tail, with the swordtail evolving in the lineage that diverged from the platyfish. Surprisingly, female
platyfish and
Priapella
prefer males of their species with experimentally elongated tails. This suggests that a
sensory bias in favour of tails evolved first and that this was then exploited by the swordtail species. (Basolo
1990, 1995). Photo of female and male swordfish
Xiphophorus helleri
. © Alexandra Basolo.