Life, drift, sex and so on:
In that paper (M.L. Herbert & M.G. Lewis “Fluctuation of fertility with number in a real insect population and a virtual population” African Entomology vol. 21 no. 1 March 2013 page 119) among other things we explain that there is evidence for populations being restricted by a post-zygotic mechanism in fruit flies and by a combination of pre-zygotic and post-zygotic mechanisms in humans and mice.
There is also evidence for a post-zygotic mechanism in plants I have described elsewhere.
Drawing on that much I shall go on to generalize and then make a prediction. I won’t bet the house on this prediction, but I think it is reasonable. First let us establish a timeline.
Before you can have life on the earth you must have an earth that can support life. I think that can be accepted and although the issue leads to things that do interest me, we shall not pursue them just now.
Life must have started to drift; not all of it was identical. So there had to be some diversity. This had to happen after the first life appeared, so we have one, two, three events and we know their sequence but not really that much about the dates. We shall try to do better soon.
At some point the variety of life turned up sex of one form or another. Before sex was – as it were – formalized there was already genetic exchange. I say that because it goes on to this day. Bacteria will exchange genetic information. In a stewing mess of mixed bacteria genetic information will be released into the environment by one bacterium and get picked up and incorporated buy another. The most notorious example is antibiotic resistance. If one bacterium develops it it can pass it to another, not necessarily of the same type. So in the primordial sea genetic information was being passed around. Then some microorganism came up with the innovation of passing genetic information directly to another, not just dumping it in the sea. This was sex. It had an obvious advantage. Since it was not the only way genetic information got around sex was able to become widespread.
The sequence seems inescapable. After, and only after, there was sex there could be species. Sex was largely successful only with similar organisms, although again it still was not the only way genetic information could get around. But now speciation was in the circus act.
Given species one could have natural selection among species. Some species did better than others.
There must have always been some degree of diversity in the environment: water depth, chemical composition, contact with the bottom, which itself varied. So now there would be a race for speciation as outlined by Alfred Russel Wallace; there was accelerated speciation. Things must have gone in that order.
With the speciation race on successful forms were generally the ones that underwent speciation at a favorable rate; those two slow got outflanked while those two fast were eliminated by speciation effects causing infertility as I have explained in
http://nobabies.net/A%20December%20summary.html
After all these things had happened in this order then some mechanism appeared which caused infertility when the gene pool got too big; else the species was going to be wiped out by speciation effects. And, this is the critical point, this mechanism was able to spread widely throughout the mass of living things. That’s because it was still early times; there were other ways for genetic information to get around. Thus we find post-zygotic population-limiting infertility in plants as well as insects and mammals.
Given the infertility mechanism, now speciation was set to take off at a gallop. And so it did. It is called the Cambrian explosion. Sometimes it’s called the pre-Cambrian explosion. But by any measure Cambrian rocks are rich in fossils while those that went before are bare or have very primitive seeming life forms.
So in our story of how things got to be the way they are, this is the one step that seems to be the one where at least an approximate date can be assigned. There is one more event to place in order and look for the appropriate evidence for its time.
First let me do an aside. I must return to my big brother, one whom I have always regarded with awe bordering on idolatry. He taught me how to clean fish. Cleaning a fish can be done deftly or crudely. For the professionals, fast is good. The fish is examined. The best hunks of meat are hacked away and the rest is discarded. I cringe when I watch. But when it takes a whole afternoon to bring in a single fish nothing is going to be wasted.
A fish is a slab of bone and muscle with a head and a body cavity. Step one is to define precisely where the head is. That is removed with as little else as humanly possible. Then the body cavity is opened and emptied. The fins are addressed. Each fin has its attendant mussels and local bones. These are removed with great care. The big fin at the back is left attached. You can eat some of that when it has been cooked. Then you scale the fish.
There is time pressure. The fresher the better so you move as fast as you can without removing bits of yourself along with bits of fish. Then you do some serious washing up.
The head is fascinating. The structure is elegant. Its parts are many and highly adapted to each other. The body cavity contents are not appealing but there are a lot of them, all different looking. The bottom line is that a fish is a very complex organism.
Then there was the day we dissected a grasshopper in class. I opened the workbook and carefully snipped the exoskeleton as directed. Then I identified the parts according to the workbook. That was it. It was the crudest thing you can imagine. It looked like it had been slapped together on an assembly line; the fish looked like it had been crafted by Vermeer.
So don’t call me a vertebrate chauvinist when I say vertebrates are more complex than insects. It would hurt my feelings.
Now the post-zygotic population-limiting infertility of the fruit fly is very forgiving. You can let that population get very big, tens of thousands of flies, and it will simply go into gradual decline and then recover. The up side of that is that fruit flies are probably not going to go extinct no matter how we ravage ourselves and the environment. The down side is that the population is at constant risk of being drawn into a death spiral such that millions of flies are getting mixed and none are ever going to find a suitable mate. That whole species could go. But that would be no big deal. Insects are simple. Evolution can slap together a new species in less time than it has taken humans to go from stone axe to taking moon walks, to losing the technology that let us take moon walks.
But a fish? Now you are talking a masterwork. The number of generations to speciation might be the same, but a vertebrate takes a lot more tweaking and has to make do with a lower reproductive rate, which might be in part a defense against gene pool sizes getting too big too fast. It will take a long time to replace primates after we prove ourselves to be a really punk idea.
In order to achieve this higher order of complexity it was necessary to implement a better population-controlling mechanism. Enter pre-zygotic population-limiting infertility. Alone it is ineffective. On paper it is very effective but in real life it is not. Species can be crossbred. Occasionally a crossbreed can initiate a line that brings a lot of new genetic information together, so that is not a bad thing. But since every sexually reproducing organism already has post-zygotic population-limiting infertility, pre-zygotic infertility just has to add that last straw. And this, on present evidence, it does splendidly. The erring lineage is wiped out without a trace. OK, that may be overstating it. There is not quite enough evidence to make that claim. But it looks like it. At all events it is very unforgiving.
Mammals, apparently, all have it. So is it a vertebrate trait or a chordate trait (chordates being a larger group that includes vertebrates) or is it a mammalian trait? The chordates that are not vertebrates, things like sea squirts, are pretty primitive. I’m going to exclude them. So is it vertebrates or mammals?
My guess is that it is a vertebrate trait. It developed in a chordate that went on to become a vertebrate or developed in a vertebrate. I don’t know how you could tell which happened. So either the vertebrates or the mammals must have undergone a sudden burst of innovation and speciation into new and complex forms. Actually they both did. The vertebrate explosion happened – if my interpretation is correct – at the end of the Mississippian era while the mammalian explosion occurred after the cretaceous era. Since the end of the cretaceous is dated to a comet impact, there is already an explanation for that one. That makes it a vertebrate trait.
And that now is something you can test. Check out lizards. Given unlimited, or at least unlimiting, food and other environmental advantages, watch the time course of the population. If it follows the curve of mammals, then we are speaking of a vertebrate phenomenon. If it goes like fruit flies it is a mammalian one.
In fact I have a lot of lizards out by the pool. Given plenty of flies, which of course I already have, and a big enough cage, which I don’t, and a way to do a census, and I can’t think of one, I could do it myself. But I shan’t. I wouldn’t want you to think me strange.
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