The size of a food chain can vary, but the number of levels that a food chain can reach is limited. Why does this limit exist?

Plants are not infinite in number.
Producers are limited by consumers.
Carnivores are too large to be eaten.
Energy is lost at each trophic level.

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:

ucd e e. rr. t t t t t t. to

and after water is usually warmed by the sun the water temperature raises and then evaporates .