What characteristics distinguish invertebrates from non vertebrates chordates

Why is it important to reduce the number of chromosomes in half when forming gametes for sexual reproduction?

Someone me with this: a mineral' is determined by its actomic structure.

Which of the following statements is true about the relationship between genetic variation and natural selection? a. genetic variation must be present in a population before natural selection can act on it. b. genetic variation arises in a population as a result of natural selection. c. natural selection can act on a population whether there is genetic variation within the population or not. d. after natural selection acts on a population, the amount of genetic variation in the population always increases.