EvoluTIonARy BIology 15
persists today when people speak of having Italian or Indian “blood.”) Blending
white and black paints produces gray, but mixing two gray paints yields more
gray, not black or white. Darwin never knew that Gregor Mendel had solved the
problem in a paper that was published in 1866, but not widely noticed until 1900.
Mendel’s theory of particulate inheritance proposed that inheritance is based
not on blending fluids, but on particles that pass unaltered from generation to
generation—so that variation can persist. The concept of “mutation” in such
particles (later called genes) was developed only after 1900 and was not clarified
until considerably later.
The Origin of Species is extraordinarily rich in insights and implications. Dar-
win supported his hypotheses with an astonishingly broad variety of informa-
tion, from variation in domesticated species to embryology to geographic pat-
terns in the distribution of species. And he showed, or at least glimpsed, how
research in every biological subject—taxonomy, paleontology, anatomy, embryol-
ogy, biogeography, physiology, behavior, ecology—could be advanced and rein-
terpreted in the light of evolution.Evolutionary biology after Darwin
Although The Origin of Species raised enormous controversy, by the 1870s most
scientists accepted the historical reality of evolution by descent, with modification,
from common ancestors. This theory provided a new frame-
work for exploring and interpreting the history and diversi-
fication of life, a project that was especially promoted by the
German zoologist Ernst Haeckel. Thus the late nineteenth
and early twentieth centuries were a “golden age” of paleon-
tology, comparative morphology, and comparative embryol-
ogy, during which a great deal of information on evolution
in the fossil record and on relationships among organisms
was amassed [2]. But the consensus did not extend to Dar-
win’s theory of the cause of evolution, natural selection. For
about 60 years after the publication of The Origin of Species,
all but a few faithful Darwinians rejected natural selection,
and numerous theories were proposed in its stead. These
theories included neo-Lamarckian, orthogenetic, and muta-
tionist theories [1].
Neo-Lamarckism includes several theories based on the
old idea of inheritance of modifications acquired during an
organism’s lifetime. In a famous experiment, the German
biologist August Weismann cut off the tails of mice for many
generations and showed that this mutilation had no effect
on the tail length of their descendants. Extensive subsequent
research has provided no evidence that specific mutations
can be induced by environmental conditions under which
they would be advantageous.
Theories of orthogenesis, or “straight-line evolution,”
held that the variation that arises is directed toward fixed
goals, so that a species evolves in a predetermined direction
by some kind of internal drive, without the aid of natural
selection. Some paleontologists held that such trends need
not be adaptive and could even drive species toward extinc-
tion (FIGURE 1.9). None of the proponents of orthogenesis
ever proposed a mechanism for it.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Evolution4e_01.09.ai Date 12-06-2016FIGURE 1.9 The extinct Irish elk (Megaloceros giganteus) had
such enormous antlers that it was cited as an example of orthoge-
netic “momentum” that drove the species to evolve a maladap-
tive feature that caused its extinction. Since the 1940s, evolution-
ary biologists have rejected this idea. The huge antlers probably
resulted from the animal’s overall large size and from natural
selection caused by competition among males for females.01_EVOL4E_CH01.indd 15 3/23/17 8:43 AM