Figure 7.27: Abert Squirrel on the southern rim of the Grand Canyon ( 26 )signals. In this case, members of each group select mates according to different beak struc-
tures and bird calls. They do not need physical barriers, because behavioral differences do
enough to keep the groups separated.
Allopatric speciation and sympatric speciation are both forms ofreproductive isolation.
Allopatric speciation is due togeographic isolation. Sympatric speciation is due
tobehavioral isolation, or isolation due to different mating seasons, which is also
known astemporal isolation.Rates of Evolution
How fast is evolution? How long did it take for the giraffe to develop a long neck? How long
did it take for the Galápagos finches to evolve? How long did it take for whales to evolve
from land mammals? These and other questions about the rate of evolution are difficult to
answer, but evidence does exist in the fossil record.
The rate of evolution is a measurement of the speed of evolution. Genetically speaking,
evolution is how much an organism’s genotype (the genes that make up an individual)
changes over a set period of time. Evolution is usually so gradual that we do not see the
change for many, many generations. Humans took millions of years to evolve from a mammal
that is now extinct.
Not all organisms evolve at the same rate. It would be difficult to measure evolution on your
family because you are only looking at a small population over a few generations. However