At what point does DNA change enough to create a new species?

Question

And how is this (sterile?) new life form able to reproduce?

Answer 1

Evolution doesn't happen so much on an individual scale but on a population scale.  A mutation in one individual will spread throughout the population.  As mutations in one population accumulate, that population grows increasingly different from other groups of the same species.  Eventually, the two groups are no longer able to interbreed if they do come in contact again.  At that point, they are biologically distinct species.

Answer 2

The minimum number of mutation required for the formation of a new species is ONE. Even one mutation can create reproductive barrier.

Answer 3

This is a great question!  The answer is that it depends.  There is no set amount of DNA change that creates a new species.  The reason is that all DNA is not the same.  Some genes can have a drastic effects if changed while others have very little.

Changes in developmental genes (like Hox genes) can produce big changes in a species.  Also, any change that alters reproductive ability, like mating calls or timing of reproduction, can have a bigger effect.  However, if the individual with the mutation is just one within the population, it is unlikely that that individual will reproduce and therefore no speciation will occur.  

Usually speciation requires there be some separation of a population.  It can be physical (like a road is constructed that separates a forest into two habitats) or it could be temporal (like part of the population is mating at a different time than the other).  Then, the gene pool of these two populations are changing separately from each other which can eventually lead to genetic reproductive barriers if enough genetic changes are accumulated.

Often the genetic change that needs to happen is something major and at the chromosomal level.  For example, we have 23 pairs of chromosomes.  Chimpanzees have 24 pairs.  But, if you look at the chromosomes you'll see that one of our chromsomes is the result of two of the chimp chromosomes fusing together.  While the actual DNA sequence hasn't changed a whole lot (comparatively), this change in chromsome number would likely cause problems in chromosome pairing when it came time for a hybrid to make gametes.