The Evolution of Evolution 123survived, because they could crack the toughest seeds and survive the shortage of food. The
next few years, all of the finches on that island were their descendants and had the stronger
nut-cracking bills found on other species of Galapagos finches. Since that time, the return of
wetter conditions has changed the finches yet again, so forms that have more normal beaks
for eating a wide variety of seeds could also survive. From this, it is easy to see how such
strong selection pressures could transform the ancestral finches (which still live in South
America) into a wide variety of specialized finches that perform the roles that other birds
play on the mainland. Instead of nuthatches, there are thick-billed finches; instead of wood-
peckers, there are finches with long bills for drilling wood and probing for grubs; instead
of warblers, there are finches with similar bills called warbler finches. One finch has even
learned to use a twig as a tool for fishing for insects in the hollows of trees! Recent research
has identified the genes that control beak shape in these finches and artificially duplicated
the pattern seen in nature by adding or subtracting those genes.
We don’t have to live in the Galapagos Islands to see evolution happen. We can see
evolution in action in our own backyards. The common European house sparrow is found
all over North America today, but it is an invader, brought from Europe in 1852. The initial
populations escaped and quickly spread all over North America, from the northern boreal
forests of Canada down to Costa Rica. We know that members of the ancestral population
were all very similar because they were introduced from a few escaped immigrants. Because
they have spread to the many diverse regions of North America, they are rapidly diverging
and on the way to becoming many new species. House sparrows now vary widely in body
size, with more northern populations being much larger than those that live in the south.
This common phenomenon, known as Bergmann’s rule, is due to the fact that larger, rounder
bodies conserve heat better than smaller bodies. House sparrows from the north are darker
in color than their southern cousins, perhaps because dark colors help absorb sunlight and
light colors are better at reflecting it in warm climates. Many other changes in wing length,
bill shape, and other features have been documented.
New species can arise even faster than people once thought. A study by Andre Hendry
at McGill University in Montreal analyzed the sockeye salmon near Seattle (fig. 4.14). These
salmon tend to breed either in lakes or in streams and have different shapes dependent on
the environment in which they breed. In the 1930s and 1940s, sockeye salmon were intro-
duced to Lake Washington east of Seattle and rapidly became established in the mouth of
the Cedar River. By 1957, they had also colonized a beach called Pleasure Point. In less than
40 years, these two populations have rapidly diverged. The males of populations that live
in the swift-flowing waters of Cedar River are more slender to fight the strong currents; the
females are bigger so they can dig deeper holes for their eggs to prevent the river from erod-
ing them away. The populations that live in the warmer, quieter waters of the lakeshore near
Pleasure Point have males with deeper, rounder bodies, which are better at fending off rivals
for mating privileges, and females with smaller bodies because they do not have to dig deep
holes for their eggs. These populations are genetically isolated and already show the differ-
ences that would be recognized as separate species in most organisms. Hendry was able to
show that this species split started in less than 40 years, and in just a few more generations,
they might be genetically isolated and become distinct species.
Another rapidly evolving fish is the three-spined stickleback (fig. 4.15). Sticklebacks that
live in the ocean have heavier body armor than those that live in lakes. In one pond near
Bergen, Norway, biologists have been able to document this change in less than 31 years.