Which of the following statements is true?
a. all animals have a nervous system.
b. all animals sense and respond to the environment.
c. it is necessary to have a nervous system to sense and respond to the environment.
d. all animals have a nervous system, all animals sense and respond to the environment, and it is necessary to have a nervous system to sense and respond to the environment.

Examine the two squirrel populations in the accompanying figure. the populations are separated by a geographic barrier. if after a long period of time the two species are no longer separated, what evidence is needed to determine if speciation has occurred? the figure shows two populations of squirrels separated by a geographic barrier. examine the two squirrel populations in the accompanying figure. the populations are separated by a geographic barrier. if after a long period of time the two species are no longer separated, what evidence is needed to determine if speciation has occurred? the figure shows two populations of squirrels separated by a geographic barrier. polyploidy is creating new species. the two populations are not interbreeding freely. hybrid offspring of the two populations begin to appear. one species will increase into a population size twice as large as the other species.