214 CHAPTER 9What caused this explosion of diversity? Do the number and ecological variety
of species depend only on current ecological conditions, such as how many differ-
ent kinds of resources can sustain different species? Or do they reflect the rate at
which new species have arisen? Why should the rate of speciation have been so
high in this fish family, and only in these lakes? Has speciation been caused by the
fishes’ mating patterns? By sexual selection on coloration? By adaptation to differ-
ent ecological niches? By genetic drift? Fundamentally, what we want to know is:
How do new species form?
Darwin first came to believe in evolution when he realized that different islands
in the Galápagos archipelago harbor different forms of mockingbirds and a variety
of similar finches. That these forms were similar, yet subtly different, could most
plausibly be explained by supposing that they had descended, with slight modifica-
tions, from a common ancestor. Pursuing this reasoning, Darwin concluded that all
species of birds—indeed all species of animals, and finally all living things—may
have originated by successive branching of lineages throughout the history of life,
from a single common ancestor. Modern research has affirmed that this is indeed
how the enormous diversity of organisms arose. The forks in the great Tree of Life
were caused by speciation, the process by which one species gives rise to two.FIGURE 9.1 Examples of the diversity of
cichlid fishes in Lakes Tanganyika (at left) and
Malawi (at right). Ecologically and morpho-
logically similar forms have evolved inde-
pendently in both lakes. (A) Rock-dwelling
species with rasping jaws. (B) Open-water
fish-eaters. (C) Fleshy-lipped species that
suck prey from crevices. (D) Rock-dwellers.
(E) Hump-headed species. (F) Slender,
striped species. (From [2]).Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_0901.ai Date 11-02-2016(A)(B)(C)(D)(E)(F)Petrochromis PetrotilapiaBathybates RhamphochromisPlacidochromis
LobochilotesTropheus PseudotropheusCyphotilapia CyrtocaraJulidochromis Melanochromis09_EVOL4E_CH09.indd 214 3/23/17 9:36 AM