Results from these statistics should be interpreted with great care. Depending on the genetic marker(s) employed, the target species, the sampling design, etc., interpretations may be not univocal. I would recommend you to study a little bit the theoratical bases of each statistics and read good articles with their applications, instead to search for shortcuts in RG. Good luck.

Negative Tajima's D signifies an excess of low frequency polymorphisms.

Arur is right.

It will tend to be negative under selective sweeps (and population growth) and positive under balancing selection (or population structure with sampling from many populations)

Fu &Li D 's negative value indicates an excess of singletons

Pls refer to Pg 54 of this DNASP doc :  http://www.ub.edu/dnasp/DnaSPHelp.pdf

Significant departure of these tests has been explained mainly to be due to an excess of new mutations as results of evolutionary forces, such as selective sweeps or population growth. Processes that produce an excess of old mutations also render significant but positive departures.

http://www.pnas.org/content/98/3/864.full.pdf

Hello Amit,

There was a similar question with several answers that may be helpful.

https://www.researchgate.net/post/Can_anybody_help_me_with_s_nucleotide_population_issue

Also below is a link from University of Connecticut that has a good explanation of these statistics.

http://darwin.eeb.uconn.edu/eeb348/lecturenotes/molevol-tajima.pdf

But the simple answer as everyone has mentioned is that there is a population expansion.

Best Regards