What is it about the structure of the alveolar-capillary unit that makes diffusion possible? Select all that apply. The capillaries are large. The cell wall of the alveoli is thin. Many capillaries surround one alveoli sac. There is always low pressure in the alveolar-capillary unit. The alveolar-capillary unit provides a large surface area.

B. The cell wall of the alveoli is thin

E. The alveolar-capillary unit provides a large surface area

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: