Why two per couple?

[Coragyps]

I just finished the excellent What Evolution Is by Ernst Mayr - I should have gotton it the week it came out. Very clearly written, especially considering Mayr was 97 when he wrote it.

But he reminded me of a fact that now has me puzzled: on the average, all sexually reproducing organisms leave very nearly two offspring that also grow to be reproducers. Were that not true, obviously, we would be chest-deep in chuckwallas and houseflies would be extinct - or name your own organisms.

Elephants have one calf every three or four years, and clams shed a million eggs a year, but in either case, in the big picture, mommy and daddy just manage to replace themselves. I can cheerfully accept why things have to (no teleology implied) work this way, but how did this arrangement arise? What factors drive us all to such an equilibrium? It's easy enough to see if you think about simple systems with two predators and four prey species, but how many different species of clam are out there laying a million eggs per year each? Why hasn't just one of them crowded out all the others?

[pz]

Because successful monocultures are ripe for exploitation by parasites and predators. Because being really good at taking advantage of your environment unavoidably changes your environment.

[Nic Tamzek]

You are creating a problem where none exists. Even in situations where one species does more-or-less take over an ecosystem -- cheat grass in degraded western grassland comes to mind -- each individual will (on average) leave only one descendent. If they consistently leave more than one, then the population expands until environmental limitations restrict the growth, a negative feedback pushing growth back towards mere replacement.

If they leave less than one, then the population declines until competition becomes so low that reproduction improves. Or if the problem is not within-species competition, then species may well go extinct. Which is known to happen very regularly in geological history, so what's the problem?

Your question is a bit like "Why does the amount of water falling out of this bucket exactly equal to the amount of water the hose is putting in? Isn't that uncanny and amazing?" Well, no, not if the bucket is full.

There are lots of interesting questions about k-selection vs. r-selection, cyclical vs. stable populations, etc., perhaps you were trying to get at one of those?

nic

PS: I think it's more accurate to view individuals as being "designed" to overproduce, not replace themselves. All populations would grow exponentially if it weren't for external constraints, something proven regularly when the constraints are removed.

[theyeti]

Yeah, what Nic said.

There is no such thing as an organism that can only reproduce to replacement. Any such organism would quickly go extinct, because a single bad season would permanently degrade the population. Even the human female, probably one of the most infrequent reproducers on the planet, can produce up to 20 offspring in a lifetime. It's not hard to see how under favorable conditions, a small population will become a huge population within a tiny amount of time (geologically speaking), even at these puny rates. The important thing to remember is that environmental conditions -- from the perspective of a given individual -- are not independent of population.

theyeti

[Doubting Didymus]

Population sizes fluctuate around the place, do big exponential spikes and catastrophic die-offs and leap all over the map and only occasionally go into stasis. Most populations do a fairly large amplitude fluctuation offset against other species that they are linked to, such as predators or prey. Many species have characteristic regular population spikes. I'm not at all sure what Mayr means by 'on average' here.

[Risiko]

Lately I've been thinking about the evolutional advantage of producing lots of offspring.
Of course it gives a population the possibility to quickly regain after a temporary crisis, and to rapidly colonialize a new environment.

But my question is: Shouldn't lots of offspring also mean more (beneficial) mutants for selecion to act upon, leading to potentially "faster" evolution?

Edited to add:
This might explain why large animals goes extinct if environmental changes speeds up (dinosaurs, for instance), while smaller species with a large reproductional surplus(?) can stay abreast.

[Tom Sawyer]

But my question is: Shouldn't lots of offspring also mean more (beneficial) mutants for selecion to act upon, leading to potentially "faster" evolution?

That makes sense - the more children there are, the more likely that some of them will have beneficial mutations. If, to grab a random number, one out of every 1000 children have a beneficial mutation, and the population has an average of 100 children per generation, then you'd expect one beneficial mutation every ten generations. If the population grows so that there are 10,000 children every generation, then you'd expect ten beneficial mutations every generation, so the population would evolve that much faster.

[Peez]

Risiko:
Lately I've been thinking about the evolutional advantage of producing lots of offspring.
Of course it gives a population the possibility to quickly regain after a temporary crisis, and to rapidly colonialize a new environment.

But my question is: Shouldn't lots of offspring also mean more (beneficial) mutants for selecion to act upon, leading to potentially "faster" evolution?More individuals being born would mean more mutations, which would mean potentially more beneficial mutations. However, this would lead to faster evolution only if the availability of new alleles (genes) was limiting the rate of evolution. This may be the case in some instances, but is often not the case.Edited to add:
This might explain why large animals goes extinct if environmental changes speeds up (dinosaurs, for instance), while smaller species with a large reproductional surplus(?) can stay abreast.Keep in mind that it is not just the number of offspring, or even how close together they are produced, but also the time from birth to reproduction and how long the organism continues to reproduce. For example, if a tree continues to produce offspring for 500 years, it will continue to contribute its alleles to the population for half a millenium (obviously, those alleles could not be eliminated from the population). On the other hand, the alleles from an insect that lives only 6 months might easily have been lost in less than a decade.

Peez

[Risiko]

Originally posted by Peez
More individuals being born would mean more mutations, which would mean potentially more beneficial mutations. However, this would lead to faster evolution only if the availability of new alleles (genes) was limiting the rate of evolution. This may be the case in some instances, but is often not the case.Keep in mind that it is not just the number of offspring, or even how close together they are produced, but also the time from birth to reproduction and how long the organism continues to reproduce. For example, if a tree continues to produce offspring for 500 years, it will continue to contribute its alleles to the population for half a millenium (obviously, those alleles could not be eliminated from the population). On the other hand, the alleles from an insect that lives only 6 months might easily have been lost in less than a decade.

Thank you for the clarification, Peez.
I'm afraid I wasn't very clear in my post. I didn't meen to propose THE explanation to mass-extinctions. I was just thinking huge dinosaurs vs little furry animals.

PS I always forget to tell just what a wonderful resource this (the II/Secweb) is. There, now I did it.

[Will I Am]

Supplimentary question:

The 'idea' of sexual reproduction is to provide genetic diversity which will increase a species survivabiliy in a changing environment. Better than (effectively) cloning, right?

Well, why stop at two?

Why aren't there three sexes or more? More diversity! (No, really)

[liquid]

I can't recall a detailed answer to that one, but my memory is telling me the reason is something to do with the fact that you can acheive diversity perfecty well with 2 sexes and having 3 sexes just complicates breeding procedures at a cost without producing any significant benefit.

Then again, it might just be playing tricks on me!