THE GEoGRAPHy of EvoluTion 477
inhabit. Most of the fragmentation of Gondwana happened in the Mesozoic (see
Figure 17.19C), before the K/Pg mass extinction 66 Mya [39].
In many cases, disjunct distributions must be attributed to dispersal [7, 8], and it
appears likely that dispersal explains more disjunct distributions than vicariance.
Explaining the African/South American disjunction of the cichlid fishes would
require that the family arose more than 110 Mya, but both fossil and DNA evidence
indicates that they are at most 65 My old [14]. They must have dispersed, somehow,
across the Atlantic Ocean when it was considerably narrower than it is now. (These
freshwater fishes do not tolerate salt water, so they present a conundrum.) The
southern beeches (Nothofagus), distributed in southern South America, Australia,
New Zealand, and on the island of New Caledonia have a classic Gondwanan dis-
tribution, but the fossil-calibrated sequence divergence of several genes indicates
that two subgenera that are in both New Zealand and Australia evolved more than
30 My after these land masses separated (FIGURE 18.9) [4]. However, some clades
are indeed older than the continents on which they occur. These include many
groups of insects, and the marsupials—opossums in tropical America and kanga-
roos, koalas, and many others in Australia—which seem to have spread through
Antarctica between South America and Australia before these separated, about 50
Mya [39]. The cypresses (Cupressaceae) originated in the Triassic, when Pangaea
was intact; one of the two major clades is distributed mostly in Eurasia and North
America (formerly parts of Laurasia), and the other is distributed mostly in South
America and other parts of Gondwana (FIGURE 18.10) [26].Dispersal
The normal dispersal processes that occur every generation account for the
gradual spread of species via more or less suitable habitat into new areas, where
they may differentiate into distinct species (allopatric or parapatric speciation; Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_18.09.ai Date 12-08-2016Aust/SA riftNZ and N. Cal/
Aust and SA rift
NZ/N. Cal rift80 70 60 50 40 30 20 10 0
Time (Mya)N. Cal
NZ
SA
NZ
Aust
SA
NZ
Aust
SABrassosporaFuscosporaLophozoniaNothofagusNothofagusFIGURE 18.9 A simplified phylogeny of the four major lineages (subgenera) of south-
ern beeches (Nothofagus), showing branching dates estimated by DNA sequence
difference. In the subgenera Fuscospora and Lophozonia, closely related species
are found in New Zealand (NZ) and Australia (Aust), even though these land masses
separated long before the rift between Australia and South America (SA). The brown
bars show estimated times at which Gondwanan land masses separated, antedating
the divergence of Nothofagus lineages. Consequently, vicariance by continental drift
does not explain the disjunct distribution of these plants. N. Cal, New Caledonia. (After
[4]; photo by D. J. Futuyma.)18_EVOL4E_CH18.indd 477 3/22/17 1:39 PM