  In this work we derive new analytic expressions for fixation time in
Wright-Fisher model with selection. The three standard cases for fixation are
considered: fixation to zero, to one or both. Second order differential
equations for fixation time are obtained by a simplified approach using only
the law of total probability and Taylor expansions. The obtained solutions are
given by a combination of exponential integral functions with elementary
functions. We then state approximate formulas involving only elementary
functions valid for small selection effects. The quality of our results are
explored throughout an extensive simulation study. We show that our results
approximate the discrete problem very accurately even for small population size
(a few hundreds) and large selection coefficients.
