20. Occasionally cells stop dividing and enter another phase, G. If you damage your liver, new liver cells can be produced to replace up to 75% of the liver. However, if you sustain brain dam-
age, your body does not produce new brain cells. Explain this observation using what you have
learned about the cell cycle.

d i think its   answer d

1) gene flow

- it is described as the transfer of genes from one population to another population so that frequency of gene is increased.

-a zebra migrates to join a different herd of zebras.

2) natural selection

- a mechanism of evolutionary change in which individuals with advantageous traits survive and reproduce whereas the others ( those that can not adapt) eliminate from the population.

-competition for sunlight leads to taller trees.

3) mutation

- permanent change in the nucleotide sequence of dna ( genetic material) that could have harmful or useful consequences.

-the dna of a snake changes to make its venom stronger.

4) genetic drift - it is a random phenomenon ( that occurs by chance such as fire) that results in change in the gene frequency in a small population.    

-a grassfire randomly sweeps through a population of buffalo and kills most of the animals.

explanation:

true, i don't really know what you're asking but from the info you provided im guessing the answer is true.