EVOLUTION BY
NATURAL SELECTION
• Darwin realized that a process much like
artificial selection (domestication)
happens in nature
– Artificial Selection
Selecting for (intentional-controlled breeding) specific
traits
• His Theory of Evolution by Natural
Selection holds that evolution is the
logical outcome of four postulates, which
Darwin laid out in his introduction to
Origin
• He considered the rest of the book one
long argument in their support
• The 4 postulates, which apply to populations
of organisms, are…
1. Individuals within populations are variable.
2. The variations among individuals, are, at least
in part, passed from parents to offspring
(inheritable).
3. In every generation, some individuals are more
successful at surviving and reproducing than
others.
4. The survival and reproduction of individuals are
not random; instead they are tied to the variation
among individuals. The individuals with the most
favourable variations (adaptations), those who
are better at surviving and reproducing, are
naturally selected
• If these four postulates are true, then the
composition of the population changes from
one generation to the next
• The logic is clear:
– if there are differences among the individuals in a
population that can be passed on to offspring, and
if there is differential success among those
individuals in surviving and/or reproducing, then
some traits will be passed on more frequently
than others
• As a result, the characteristics of the
population will change slightly with each
succeeding generation
• This is Darwinian Evolution: gradual change in
populations over time
• Ex. Darwin’s Finches
• A few million years ago, one species of finch
migrated to the rocky Galapagos from the
mainland of Central or South America.
• From this one migrant species would come many
-- at least 13 species of finch evolving from the
single ancestor.
• The ecological niches exert the selection
pressures that push the populations in various
directions.
• On various islands, finch species have become
adapted for different diets: seeds, insects,
flowers, the blood of seabirds, and leaves.
• The ancestral finch was a ground-dwelling, seed-eating
finch.
• After the burst of speciation in the Galapagos, a total of 14
species would exist:
–
–
–
–
three species of ground-dwelling seed-eaters;
three species living on cactuses and eating seeds;
one living in trees and eating seeds;
7 species of tree-dwelling insect-eaters.
• The finches differed mainly in the size and shape of their
beaks.
• Most recently, Peter and Rosemary Grant have spent many
years in the Galapagos, seeing changing climatic conditions
from year to year dramatically altering the food supply.
• As a result, certain of the finches have lived or died
depending on which species' beak structure was best
adapted for the most abundant food -- just as Darwin
would have predicted.