The seedy mystery of why size matters
Bigger plants tend to have bigger seeds, a new study has confirmed, but scientists still don't know why.
Seeds vary enormously in size from dust-like orchid seeds to the enormous Maldive Island double coconut. And scientists are interested in understanding more about the evolutionary forces that determine seed size.
In the biggest study of its kind, Moles and an international team looked at a representative sample of 13,000 plants from across the world.
They mapped their findings against what scientists know about the evolutionary relationships between plants to identify the biggest changes in seed size over time.
They then compared this change in seed size to the change in other aspects of the plant, such as whether it grows near the tropics or is dispersed by wind or some other method.
Previously scientists thought these kinds of factors were responsible for determining seed size.
But to their surprise, the researchers found the strongest predictor of the evolution of seed size was the size of the plant: as plants got bigger seeds tended to get bigger and as plants got smaller their seeds tended to get smaller.
"It's more of a surprise than anything else because there is no obvious mechanical reason why that should be the case," says co-author Professor Mark Westoby, also of Macquarie University. "So in that sense it's a new question we need to focus on."
This is a question that evolutionary ecologists are now turning their minds to, says Westoby, using mathematical theory to model different combinations of plant traits to see which offer the best evolutionary fitness.
One of the curious facts about seed size is that although tiny seeds tend to come in large numbers this doesn't give their species an evolutionary advantage.
This is because there is a trade-off between size and fitness for survival with small seeds being not as well equipped for survival as their large counterparts.