It's so that fertilization will make a diploid zygote -- with the zygote having half its chromosomes of paternal origin and half its chromosomes of maternal origin. 

> 1) Frogs and toads are different but they are in the same group - amphibians. Are spider and mosquito related?How? 
Yes. Spider and mosquito are both arthropods -- segmented animals with a sheddable exoskeleton of chitin, and paired jointed limbs attached to segments. 

> 3) Taxonomy is the science of classification. Imagine you are a biologist and make a list of criteria how you would classify a pearl oyster. 

Has DNA, RNA, protein, and the genetic code. This places it in superdomain Biota. 
Has nucleated cells. This places it in domain Eukarya. 
Is multicellular, eukaryotic, and has no cell wall. This places it in kingdom Animalia. 
Has true tissues. This places it in subkingdom Eumetazoa. 
Is bilaterally symmetrical, or modified from this. This places it in clade Bilateria. 
The blastopore eventually forms the mouth. This places it in clade Protostomia. 
Has a trochophore larva stage in its life. This places it in superphylum Lophotrochozoa. 
Internal organs are enclosed within a mantle cavity. This places it in phylum Mollusca. 
Has a body enclosed within a hinged shell of two parts. This places it in class Bivalvia. 
The interior of the shell is lined with mother-of-pearl nacre. This places it in order Pterioida. 
The shell has a particular crystalline growth pattern (?) This places it in family Pteriidae. 
Produce pearls. This places it in genus Pinctada. 
All members of the genus Pinctada are called "pearl oysters."
It is because both sperm and ovum are formed by meiosis division which reduces the number of chromosomes  to half.
haploid no. of chromosome leads to recombination of DNA inorder to restore normal no. of chromosome in the cell. in this process certain feature of parent ar transfer to offspring