effect hypothesis A model proposed in 1980 by the palaeontologist Elisabeth Vrba to account for evolutionary trends. She proposed that a species, occupying a restricted ecological niche, would continually give rise to daughter species by punctuated equilibrium. These new species would have a variety of characteristics, but because of the features of the particular ecological niche, only species that possessed a particular suite of characters would survive. The surviving species would speciate in their turn, with the same result, and at each level the lineage appears to be ‘pushed’ further and further in a given direction. Compare directed speciation.

effect hypothesis A model proposed in 1980 by the palaeontologist Elisabeth Vrba to account for evolutionary trends. She proposed that a species, occupying a restricted ecological niche, would continually give rise to daughter species by punctuated equilibrium. These new species would have a variety of characteristics; but because of the features of the particular ecological niche, only species that possessed a particular suite of characters would survive; the surviving species would speciate in their turn, with the same result, and at each level the lineage appears to be ‘pushed’ further and further in a given direction.