Page 70 - Environment: The Science Behind the Stories

Generalists
Foragers among leaves
Nectarivores

Seed and Bark pickers
fruit eaters

(a) Divergent evolution of Hawaiian honeycreepers
Figure 3.3 Natural selection can cause closely related spe-
cies to diverge in appearance or distantly related species to
converge in appearance. Hawaiian honeycreepers (a) diversi-
fied as they adapted to different food resources and habitats, as
indicated by the diversity of their plumage colors and bill shapes. In
contrast, cacti of the Americas and euphorbs of Africa (b) became
more similar to one another as they independently adapted to arid
environments through the evolution of tough succulent tissues to (b) Convergent evolution of a cactus in Arizona (top) and a
hold water, thorns to keep thirsty animals away, and photosynthetic euphorb (spurge) in the Canary Islands (bottom)
stems without leaves to reduce surface area and water loss.

generally requires a great deal of time, a species can- Selective pressures from the environment
not always adapt to environmental conditions that change influence adaptation
quickly. For instance, the warming of our global climate
today (Chapter 18) is occurring too rapidly for most species Environmental conditions determine what pressures natural
to adapt, and we may lose many species to extinction as a selection will exert, and these selective pressures affect which
result. members of a population will survive and reproduce. Over
However, genetic variation can sometimes help pro- many generations, this results in the evolution of traits that CHAPTER 3 • Ev ol u T i on, Bi odiv ER si T y, A nd Po P ul AT i on E C ology
tect a population against novel challenges. One of the enable success within the environment in question. Closely
honeycreeper species of the Hakalau Forest, the ‘amakihi, related species that live in very different environments and
has in recent years been discovered in ‘o¯ hi‘a trees at very thus experience very different selective pressures tend to
low elevations, well within the zone where avian malaria has diverge in their traits as the differing pressures drive the evo-
killed off all other honeycreepers. Researchers studying this lution of different adaptations (Figure 3.3a). Conversely, some-
population have determined that some of the ‘amakihis liv- times very unrelated species may acquire similar traits as they
ing here when malaria arrived had genes that by chance gave adapt to selective pressures from similar environments; this is
them a natural resistance to the disease. These resistant birds called convergent evolution (Figure 3.3b).
survived malaria’s onslaught, and their descendants rees- However, environments change over time, and organisms
tablished a population that continues to grow today. Simi- may move to new locations and encounter new conditions. In
larly, the ‘apapane (another Hawaiian honeycreeper) and the either case, a trait that promotes success at one time or place
‘o¯ ma‘o (a native Hawaiian thrush) also are showing some may not do so at another. Hawaiian honeycreepers such as the
degree of resistance to malaria. Scientists hope that perhaps ‘apapane and the ‘i‘iwi fly long distances in search of flower-
some individuals of the rarer native birds of Hakalau might ing trees. This behavior had long helped them to find the best
also harbor resistance genes that may help them persist in the resources across a diverse landscape. However, once malaria
face of malaria. arrived, the strategy backfired, as birds from malaria-free 69

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