those environments (Schluter 2000). The best-known example is that of Darwin’s
finches on the Galapagos Islands (Grant 1986; Schluter 2000). They were probably
founded by a finch species from South America. Subsequently they diverged into types
that feed on insects and others that feed on seeds; some species live on the ground,
others in trees, and still others among cacti. Adaptation follows the vagaries of unpre-
dictable environmental change, which can be detected in studies of 30 years or more
as Grant and Grant (2002) have done with the Galapagos finches.
Another example of adaptive radiation is seen in the endemic honeycreepers
(Drepanididae) of Hawaii (Fig. 3.1). Many of these species have become extinct
in the past 150 years. They appear to have evolved from a thin-billed insect eater.
From this type one group of species developed into long-billed nectar feeders
while another group evolved into long-billed bark-crevasse feeders. Yet another
group developed thick bills and feeds on fruit and seeds. This one family has filled
the niches normally filled on continents by many families of birds. Schluter (2000)
gives several other good examples of adaptive radiation.In discussing how animals fit into their environment we have considered the pro-
cess of adaptation through natural selection. We have noted that adaptation is not
perfect because conditions change and animals are constantly trying to catch up. Further,
the organisms are limited by the evolutionary pathways that their ancestors have
followed: both birds and mammals have evolved from reptiles but selection on
mammals today cannot produce feathers. The potential for growing feathers was lost
a long time ago.
Natural selection is constrained in what it can produce by what is currently avail-
able. The giant panda (Ailuropoda melanoleuca) of China is a large herbivore that eats
bamboo shoots almost exclusively. These bamboos provide low-quality food. Most
large mammal herbivores, such as horses, deer, and kangaroos, have long intestines
and special fermentation mechanisms in the gut to allow maximum digestive
efficiency (see Section 4.6); in contrast, carnivores such as cats and omnivores such
as bears have relatively short guts. The giant panda probably evolved from bears
in the Miocene about 20 million years ago (O’Brien et al. 1985a) and changed to a
herbivorous diet. Because of its carnivore ancestry it has a short gut and cannot now
make the evolutionary jump to the longer, more complex digestive system of
herbivores. Hence the giant panda has one of the least efficient digestive systems known
for a terrestrial vertebrate: only 18% of the food is digested compared with 50 –70%
for horses, antelopes, and deer. The giant panda compensates with other adaptations,
in particular by eating a very large amount of food and spending most of the day
doing so. This prolonged feeding in turn leads to behavioral adaptations: pandas are
solitary and spend little time in social and mating activities.Movement of the continents
Towards the beginning of last century, Alfred Wegener, a meteorologist, proposed
that the continents were at one time joined together and subsequently drifted apart
(Wegener 1924). Wegener’s idea was generally rejected. The discovery in the 1960s
that the earth’s surface is made up of plates, and that these move, proved that Wegener
was essentially correct. Volcanic activity and earthquakes along mid-oceanic ridges
produce prodigious amounts of submarine basalt and this spreads the sea floor. The
continents which float on these basaltic plates are thereby forced apart.ANIMALS AS INDIVIDUALS 23
3.5 The effects of history
3.5.1Phylogenetic
constraints
3.5.2Geographical
constraints