The outer cells of a blastocyst become the , an organ that offers protection and nourishment. once the blastocyst has implanted in the uterus, it is called an . after eight more weeks of development, it is called a .

The image shows the evolution of a species of fish. a few fish from a population developed different social behaviors and evolved into different species. two fish according to the image, the fish underwent . the new species of fish had mating seasons that were different from that of the original fish. because of the differences in mating seasons, the fish underwent reproductive isolation. this mode of isolation would be .