The formation of animal cleavage furrow ?

Animal cell cleavage furrow formation is caused by a ring of actin microfilaments called the contractile ring. which forms during early anaphase...The bridge is then broken and resealed to form two identical daughter cells during cytokinesis.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.