Why do certain human genetic disorders, such as sickle-cell anemia, occur more frequently among one ethnic group than another?

Species can vary in several ways. which of the following is one of darwin's ideas that was accepted by most scientists of his time due to the large number of fossils being discovered at that time? a. species vary around the world. b. species have specific adaptations to their local environment. c. species that live in seawater can also live on mountains. d. species change over long periods of time.