It’s often said in evolution threads that one organ or another is expensive to maintain eveolutionarily speaking. Here is a recent example from this thread:
What got me to thinking about this was a question here about cave fish losing (devolving?) their eyes because they were not needed and were expensive to maintain. I’m guessing, referencing John’s post, that it means ‘needs more fuel’. Is this right? Is the idea that in times of low food availability fish with the birth defect of non-functioning eyes needed less food than sighted fish and so survived to reproduce? Because it seems like that would be an infinitesimal amount of food.
Our brains consume a lot of energy - if our brains were half their current size, our caloric needs would be about 12% lower. Needing 10% less food is a huge difference over hundreds of generations. Brains need even more oxygen. A human with a brain 50% smaller would have 15% more oxygen to power other organs (or could get by with smaller cardiovascular capacity). Because of this, we know big brains are really useful to us.
If we lived in a world where food was scarce but being intelligent didn’t give you an advantage in obtaining it, our brains would almost certainly get smaller rapidly. There is some evidence this is already happening, as our brains are considerably smaller on average than the brains of our ancestors 25000 years ago.
Big brains also need big skulls, which make giving birth all that much harder. That’s one of the (many) reasons so many women did and still do die in childbirth. To compensate, we started having smaller and more helpless infants, which in turn need more care. Our cousins the chimps have babies that can hang on to their mothers and walk after about six months. Our babies will be a year old before they can walk, and more like 4-5 years old before they can care for themselves in even the most basic ways, such as feeding, dressing, washing themselves, etc. So in that way having such huge brains can be a major drain on a species unless they got something back out of it – such as the ability to comprehend calculus and work childproof bottles.
IANA biologist, but I think it’s more along the lines that a species that is not using it’s eyes will not ‘‘notice’’ the accumulation of bad code that causes them to go blind, where as a species in which sight offers an advantage will weed out mutations that limit it.
In that particular comment, I meant that brains require tremendous amounts of calories compared to other parts of the body-- about 20% of what you take in, even when you’re doing nothing. The average brain weighs about 3 lbs, so for a 150 lb person, that’s 20% of your calories going to about 2% of your body mass.
I’m not sure what John meant, but that is only one of many possible reasons why eyes are expensive to maintain. Eyes are soft tissues with thin mucous membranes exposed to the environment and located at the first point of contact with sharp objects. That makes them highly prone to infection and injury. Any fish that simply had patches of scale-covered skin rather than eyes would suffer far less injury and infection and that reason alone is a major reason why eyes are very expensive.
Eyes are also a problem with streamlining, and this is particularly so in low light environments where eyes need to be large and bulging to gather the little light available. Added to that eyes can’t be covered in scales and so produce excess turbulence. Even if eyes required no energy at all themselves they would still represent an energy loss to the fish simply because a fish with eyes needs more energy to move.
So, in summary, something is “evolutionarily expensive” if it carries a lot of disadvantages, whether they be energy-based or something else? And if something has such disadvantages, we can tell that it must have other, very valuable, benefits that allowed it to be selected for?
It’s not always the case that the trait under examination was that which was specifically selected for. In the case of cave fish, again, for example, the most likely hypothesis is that other traits were positively selected for, but developmental pathways linked the development of those traits with the development of eyes. Thus, eye degeneration could merely be “along for the ride”, and not being selected for in and of itself.
That’s close enough for the layman, though I would phrase it in the past tense: If something is expensive it must have had benefits that allowed it to be selected for.
Things can often be retained even if they are expensive and useless.
That may be simply because the right mutations haven’t appeared to remove them yet. Evolution relies a lot on random chance, and just because eyes are expensive doesn’t mean that an eyeless mutant is guaranteed to appear.
Or it may be the result of trait linkage. Hind limbs, for example, are genetically linked to forelimbs. A bipedal organism may well retain a set of useless fore limbs even if they are expensive simply because any mutation that removes them will also remove the hind limbs. In this case the evolutionary route for total removal is to costly to to be justified, so the animal drags around useless and costly atavisms.
The authors are saying that because the improvement doesn’t result in perfect efficiency it can’t have been the result of selection for efficiency. It makes no sense. They are saying that just because the process results in the inefficient generation of useless retinal cells it doesn’t improve efficiency, despite the fact that it does eliminate a plethora of other inefficient processes.
And of course keeping the eyes still results inthe inefficient generation of useless retinal cells, it just means that they are followed by useless neurons and useless mucous production and useless pigments and so forth. But somehow the authors have concluded that because one single step is no more efficient the whole process is less efficient, it no justification whatsoever that I can see.
T. rex’s arms might not have been useless… They seem to have had considerably musculature on them, and one would expect that muscular atrophy would be relatively “easy” to evolve. One hypothesis I’ve heard is that they used them to hold on to each other while mating.
Also bear in mind that our bodies have been getting smaller, too, on average. The trend towards increasing body masses in the developed world is just an artifact of the last century or so, when advances in agriculture ensured us a reliable food supply. Before then, people were small due to poor childhood nutrition. Look at the skeletons of the earliest anatomically-modern human beings — those guys were built. Compare it to, say, a Middle Eastern peasant from about 4,000 years ago, and the difference is astounding.
Even the elites weren’t all that big — go to a museum and look at a suit of armor. They weren’t all that big, even though they were fed very well compared to their serfs.
Brain size and body mass are linked. It’s the ratio that counts, and we’re holding pretty steady compared to our gronking forebears.
People in the middle ages were short, but looking at armour won’t tell you anything for two very good reaosns.
First off armour has joints and is made to be worn. When put on a stand it collapses, just like any other suit. Even armour for a very tall man woudl look short on a stand.
Secondly armour was expensive and it was worn until it collapsed, and was then recycled into something else. The only armour that commonly survived was ceremonial armour, and an over-representaion of that was from young nobles at their investiture. IOW suits made for young men and boys. We do have some suits of ceremonial armour from kings from various events and they are much taller than the average suit.