How does crossing over increase variation in a population?

Crossing over or the recombination, is the process, which involve the exchange of chromosomal segment between the non-sister chromatids in the meosis.

It creates a new combination of genes in the gametes, which are not found in the parents and contribute to the genetic diversity of the population.

Due the this the chance of survival of the offspring increases under diverse environment.

when it's short and dry.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.