According to the most common evolutionary stable strategy (ESS),...

According to Fisher's principle, the most common evolutionary stable strategy (ESS) for the sex ratio in a population is $1:1$.

If one sex becomes less common, it will have a reproductive advantage, leading to increased production of that sex until the ratio balances out again. Thus, the stable ratio of males to females is equal.

Among the given options, $1000 : 1000$ represents the $1:1$ ratio.

Answer: 1000 : 1000