B.B.C. Article
The animals in the ocean have been getting bigger, on average, since the Cambrian period - and not by chance.
That is the finding of a huge new survey of marine life past and present, published in the journal Science.


Breathing air helped marine reptiles to maintain their extra body size

It describes a pattern of increasing body size that cannot be explained by random "drift", but suggests bigger animals generally fare better at sea.
In the past 542 million years, the average size of a marine animal has gone up by a factor of 150.
It appears that the explosion of different life forms near the start of that time window eventually skewed decisively towards bulkier animals.
Measured by volume, today's tiniest sea critter is less than 10 times smaller than its Cambrian counterpart; both are minuscule, sub-millimetre crustaceans. But at the other end of the scale, the mighty blue whale is more than 100,000 times the size of the largest animal the Cambrian could offer: a trilobite less than half a metre long.

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Rumraket wrote:There already existed microscopic organisms that couldn't possibly get any smaller in the precambrian, so the only way to go in size is up until you reach the physiological limit there too(and afaik the Blue Whale is very close to that limit). So it's not that evolution "favors" big over small, it's just that the only direction to go for a random walk that starts at the bottom is up.

jamest wrote:

Rumraket wrote:There already existed microscopic organisms that couldn't possibly get any smaller in the precambrian, so the only way to go in size is up until you reach the physiological limit there too(and afaik the Blue Whale is very close to that limit). So it's not that evolution "favors" big over small, it's just that the only direction to go for a random walk that starts at the bottom is up.

Seems odd that microscopic organisms would get larger, given that increased size requires an increase in energy input. I would of thought that those organisms requiring less food would have been selected for. Though I guess there are other factors at play.

Rumraket wrote:There already existed microscopic organisms that couldn't possibly get any smaller in the precambrian, so the only way to go in size is up until you reach the physiological limit there too(and afaik the Blue Whale is very close to that limit). So it's not that evolution "favors" big over small, it's just that the only direction to go for a random walk that starts at the bottom is up.

susu.exp wrote:

Rumraket wrote:There already existed microscopic organisms that couldn't possibly get any smaller in the precambrian, so the only way to go in size is up until you reach the physiological limit there too(and afaik the Blue Whale is very close to that limit). So it's not that evolution "favors" big over small, it's just that the only direction to go for a random walk that starts at the bottom is up.

This would be a good objection if that wasn't precisely what the study tested. If you have a passive trend as you describe it you get a somewhat different pattern than if you have a driven trend. If there's one thing to criticize it's that the BBC article keeps talking about drift, where it should say passive trend - a passive trend also arises when you have significant selection, but the direction of selection is not consistent among the species in question.
So what they detected was that not only was there selection on size in at least some portion of their dataset, but selection favoured an increase in size significantly more often than a decrease.