Evolution__3rd_Edition

266 PART 3 / Adaptation and Natural Selection
Evolution can proceed by symbiosis
Adaptations have been suggested
to evolve in few, large genetic
steps, or many, small ones
evolutionary novelty resulted from the combination of two pre-existing parts with
unrelated functions. A lactose-manufacturing enzyme evolved by combining a golgi
enzyme and an antibacterial enzyme.
The evolution of milk digestion is a molecular example in which a new enzyme
evolved by combining two pre-existing enzymes. A related process operates at a higher
level when two whole species merge by symbiosis and evolve into a new species with a new
combined physiology. For example, the mitochondria and chloroplasts in eukaryotic
cells each originated when one bacterial cell engulfed another bacterial cell. In the case
of mitochondria, the combined cell was capable (or soon evolved to be capable) of
burning carbohydrates in oxygen a a process that releases more energy than anerobic
respiration. The new cell had a more complex metabolism than either ancestral cell by
itself.
Evolution by symbiosis, or combining several genes into new composite genes, can
violate the letter, but not the spirit, of Darwinian gradualism. According to the gradualist
requirement, new adaptations evolved in many small, continuous stages. When two
cells merge, there may be a relatively sudden transition to a new adaptation in one big
step. However, no deep principle in Darwinism has been violated because the adaptive
information within each ancestral cell was built up in gradual stages.
10.5 Genetics of adaptation
10.5.1 Fisher proposed a model, and microscope analogy, to explain
why the genetic changes in adaptive evolution will be small
Evolutionary biologists distinguish between a “Fisherian” and a “Goldschmidtian”
view of the genetic steps by which adaptations evolve. Goldschmidt (1940) argued that
new adaptations, and new species, evolve by macromutations (or “hopeful masters”).
A macromutation is a mutation with a large phenotypic effect, such that the individual
carrying the mutation is outside the normal range of variation for its population
(Figure 1.7, p. 14). Fisher doubted whether macromutations contribute much to evolution,
and argued that adaptive evolution mainly proceeds in many small steps. The
mutations that contribute to adaptive evolution have small phenotypic effects.
Fisher’s argument begins by noting that living things are fairly well adapted to their
environments. They must be at least reasonably well adjusted, or they would be dead.
Next, Fisher assumes that most characters are in an optimally adapted state. If the character
is larger or small than the optimum, the organism’s fitness declines (Figure 10.4a).
Because living organisms are at least fairly well adapted, they are somewhere near the
peak in Figure 10.4a. We now assume that the direction of mutations is random with a
mutation having a 50% chance of increasing the character state, and a 50% chance of
decreasing it. A small mutation therefore has a 50% chance of improving the adaptation.
But a large mutation would make things worse either way. It either is directed away from
the optimum, or shoots past the optimum down the slope on the other side (Figure 10.4a).
Fisher calculated, on the assumption that the organism is near the adaptive peak, that
an indefinitely small mutation has a half chance of improving the adaptation, and the
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