Until quite recently, genome could only be studied indirectly, by using partial and sometimes unreprentative genomic sequences. this situation is changing rapidly as complate genomic sequences are becoming available.
How did this variation come into existance, and what mechanism can increase or decrease genome size to produce such variation ?. Do genomes contain mostly genic DNA, or is the genome made of mostly nongenic sequences ? does the nongenic fraction have a fuction , or is it merely "junk" ?.