What type of variation is necessary for natural selection to occur? (3 points) Group of answer choices

Artificial variation

Environmental variation

Genetic variation

Neutral variation

Select the correct answer. which term do biologists use to describe the average number of individuals of a species per unit area? a. carrying capacity b. population density c. minimal viable population d. survivability curve

Which is an advantage of having memory cells when a pathogen is encountered for a second time. a) the memory cells are what proliferate into clones of cells in response to the binding of an antigen. b) memory cells are vital to the primary immune response in that they immediately recognize pathogens even at the first encounter. c) memory cells are essentially effector cells that are short lived and attack antigens even without having a receptor specific to that antigen. d) it ensures that more lymphocytes with a receptor specific to a particular antigen will be present than in a host that had never encountered that pathogen.

The cluster of developing cells from conception until birth is called an