Outbreeding depression is uncommon and predictable

Abstract Crosses between populations within species sometimes result in reduced fitness, especially in F2 and later generations (outbreeding depression). The primary mechanisms causing outbreeding depression in crosses between populations are fixed chromosomal differences and adaptive genetic differences, especially for long-isolated populations. Outbreeding depression is usually observed after crossing populations with ploidy differences or fixed differences for translocations, inversions or centric fusions: the magnitudes are usually ploidy > translocations and monobrachial centric fusions > inversions and simple centric fusions. Populations adapted to different environments (but with the same karyotype) often exhibit outbreeding depression when crossed, especially in the F2 and later generations. Even if outbreeding depression occurs, it is often only temporary, as natural selection acts to remove it, especially in large populations.