NATuRAl SElECTioN ANd AdAPTATioN 59
with the largest horns has resulted in the evolution of smaller horns (FIGURE 3.5B).
In both instances, the very quality that adds value to the resource has been dimin-
ished by the response to selection.
Some species show adaptation to the ongoing climate change caused by human
production of CO 2 and other greenhouse gases. In many insects, the cue for enter-
ing diapause, a state of low metabolic activity that is necessary for surviving the
winter, is a critical photoperiod (day length). Northern populations are genetically
programmed to enter diapause at a longer day length than southern populations
because cold weather arrives at northern latitudes sooner, when days are still long.
William Bradshaw and Christina Holzapfel [3] sampled populations of the pitcher-
plant mosquito (Wyeomyia smithii) from southern Canada to Florida four times
between 1972 and 1996 and experimentally measured the day length at which the
insects entered diapause. They found that during this time, the critical photope-
riod became shorter: the insects became programmed to enter diapause later in
autumn (FIGURE 3.6). The change was greatest in the most northern populations,
as expected because the increase in temperature has been greater at higher lati-
tudes. The speed of evolution was amazing, having taken as little as 5 years.
These evolutionary changes can be so rapid because populations in altered
environments, especially those altered by human activities, can experience strin-
gent natural selection, and because they contain genetic variation in many charac-
teristics—a necessary ingredient of evolution.Natural Selection
The meaning of natural selection
In The Origin of Species, Darwin introduced natural selection with these words:Can it ...be thought improbable, seeing that variations useful to man have
undoubtedly occurred [in domesticated animals and plants], that other
variations useful in some way to each being in the great and complex battle of
life, should sometimes occur in the course of thousands of generations? If such
do occur, can we doubt (remembering that many more individuals are born than
can possibly survive), that individuals having any advantage, however slight,
over others, would have the best chance of surviving and procreating their kind?
On the other hand, we may feel sure that any variation in the least degree
injurious would be rigidly destroyed. This preservation of favoured variations
and the rejection of injurious variations, I call Natural Selection.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_03.06.ai Date 11-28-2016Note: Suggest using a zoom arrow to point from origin of water
in pitcher plant to zoon of mosquito to tie in the two images.Critical photoperiod (log hours)30 35 40 45 50 55
Altitude-corrected latitude1.151.181.211.241.271.1219881993FIGURE 3.6 In North America, the critical photope-
riod for entering diapause, in relation to latitude, has
decreased in the pitcher-plant mosquito (Wyeomyia
smithii), as shown by these data from 1988 and 1993.
The larvae of this mosquito develop only in the water
that collects in the tubular leaves of the pitcher plant
Sarracenia purpurea. Pitcher plants obtain nutrients
from the decaying bodies of other species of insects
that fall in and are trapped. (After [3].)03_EVOL4E_CH03.indd 59 3/22/17 1:19 PM