Evolution Questions (NOT a Creationist Thread)

Re changing chromosome number. The earlier reply seemed very informative, just a few comments (with a disclaimer that this is outside my field of knowledge (ie, I’m guessing), but Down’s Syndrome doesn’t really seem that bad from an evolutionary point of view. The baby is born and can live to reproductive age. Afaik Down’s Syndrome people can reproduce. I know that you can have a lot of oddities with the sex chromosomes and only single Y gets aborted. Some of them can reproduce with standard humans iirc. So you can have creatures with non-standard numbers of chromosomes mating with the current pool. Which should be enough given enough time for changes in chromosome number to be possible.

Smeghead, thank you for your informative post, but I think that you left my main question unanswered. Towards the beginning of my post, I conceded that a healthy animal could be born with a nonstandard number of chromosones, though I wasn’t sure how. You did an excellent job of explaining that part of it. The crux of my question, however, was that such an individual would most likely not be interfertile with “normals”, and, being surrounded by “normals”, would be effectively sterile. Is the predisposition for changes in chromosone number strongly enough genetic that there would be any reasonable likelihood that a full breeding population of such individuals would arise simultaneously?

surel: I never said that it was impossible for a person with Down’s to have children, but I stand by my statement that it’s a severe condition. In earlier (and more difficult) times, I suspect that a child with diminished mental capacity and physical coordination would be unlikely to survive to adulthood. Even today, I suspect that people with Down’s syndrome have a more difficult time finding mates than most of the population.

There is support for the idea of (relatively) rapid bursts of speciation, particularly following mass extinctions and colonization events. In an environment where many niches are “…unfilled (because prior occupants have either died off or never existed), a wider variety of mutations can prove beneficial. Mutations that tend to push an isolated group into a new niche are usually selected against if there’s
competition for that niche, and selected for if the niche is empty. Massive overgeneralization, of course, but you can see the logic in it. Anyone got hard data on this?”

We have changed the topic here about 6 different times but this area is one being explored via bacteria. Poor old E.coli has been dragged through the bushes and back in endless DNA experiments. A newly published study (PNAS, McKenzie June 2000 is probably close enough) shows an increase in mutations during periods of starvation. So, while we have long know that mutations occur in population expansions (sheer numbers) it now is beginning to look like the mutations that occur with starvation might be a way for the old population to make the changes that would allow better adaptation to the old enviornment as well as a carryover into the new. Interesting?

Chronos: Is the predisposition for changes in chromosone number strongly enough genetic that there would be any reasonable likelihood that a full breeding population of such individuals would arise simultaneously?

No, infact it is just the other way around, there are several “failsafe” mechanisms in place to prevent mutations and simultaneously among several individuals seems impossible to me.

See: http://www.uiowa.edu/~anthro/origins/campus/Lec3.htm

Geez, enough disclaimers, is it even possible to have a discussion here on some intelligent questions on evolution without a handful of disclaimers and boldy stating that yes ‘I’m a believer in evolution!’

Jois, that’s what I thought, too, hence the question. So, then, how DOES a breeding population with a different chromosone number arise? I’m not talking about polyploidy here; it seems to be pretty safe to say that most mammals are not polyploids (we have 23 pairs, which isn’t a multiple of anything), but there’s a good bit of variety in chromosone number.

There are several kinds of chromosome mutations. Some barely seem to make a difference and some are fatal, so we are looking for something inbetween.

Different species may have different number of chromosomes via fusions and unequal divisions and even a combination of both.

Donkey = diploid set of 62 chromosomes
horse = diploid set of 64 chromosomes
Thought to be due to “fragmentation” when a chromosome splits into two, increasing the number by one.
human = diploid set of 46 chromosomes
gorilla= diploid set of 48 chromosomes
chimp = diploid set of 48 chromosomes
Thought to be due to “fusion” when two or more almost whole chromosomes join end to end, reducing the number of separate chromosomes by one.

Those particular changes were survivable, it isn’t always so. From there is is just a matter of population size and dumb luck; there was no true increase or decrease in genetic material.

Is that better?

BTW: Down’s syndrome victims are sterile (males) and infertile (females).

Jois

Also see, “Why are we the only ones?” Last couple of posts starting to tip toe around population and time.

HorseloverFat said:

I think Padeye was just ribbing me and I was joking around back. Please notice the smiley face at the end.

Fascinating thread though!

Jois did a good job of clarifying my previous somewhat rambling post. Let me just add one or two things.

In plants, it’s a lot easier, especially if it’s a self-fertilizing type, for obvious reasons. Even if it doesn’t self-fertilize, it may reproduce asexually - through shoots, etc, allowing a population to arise.

A lot of the human c’some number abnormalities found are in the sex c’somes (XXX, XXY, XYY, XO, etc.) These are common because we’re set up to handle variations in amount of sex c’some material - both XX and XY are normal. These types of aberrations have been known to add up to a lot of material - XXXXXYYY or something like that is the record.

Jois also pointed out that changes in c’some number are often just rearrangements - same material is there, just organized differently - so reproduction isn’t too much of a problem. This is about the only type of change you’ll see in the “higher” animals. Once you get beyond a certain level of complexity, anything else is just too lethal.

As a way of adding one more c’some to the population, imagine this: somehow, an individual ends up with 4 copies of c’some A. I think it’s been well explained how this could happen. It would then have 2 “extra” c’somes, with material that can be fiddled with extensively without repurcussions. Mutations can run rampant without effect, because there are still two good copies of every gene present. It’s like adding a chunk of raw material for evolution to fiddle with. Eventually, the “extra” c’somes will diverge from their original form and will no longer even appear to be similar to the “mother” c’somes. Boom. New unrelated c’some added to the population.
This is oversimplified, of course. In reality, the two versions both diverge from each other, for instance. I’m sure other inaccuracies will be pointed out quickly :slight_smile: but the idea is good.

Something to think about, anyway.

Well, I was hoping someone else would take care of Chronos’ breeding population question. Population charts (I’ve only seen a couple) show that genetic changes start off small (one person or beast or plant) and then continue very slowly and then increase exponentially.

However, I like the reverse - removing a trait from a population - better. If we want to remove an incompletely dominant gene from 1::100 persons in a population to 1::1000 it would take 230 generations. And additional 230 generations would take the ratio from 1::1000 to 1::10,000. If we use the “score” or 20 years per generation, it’s nearly 5,000 years per 230 generations? If the gene is recessive the generations rises from 230 to 90,000. These figures were prepared for disadvantages at 1-2%.

In nature, new favorable mutations are rare, after all the existing species are products of past selection and should be close to the best possible adaptation to their surroundings as they are. Even a “good” mutation would have little more effect than a neutral mutation unless the enviornment were changing or even better, deteriorating.

Adding a truly dominant mutation would take 10,000 years to be present in 50% of the population, might reach 95% of the population in 35,000 years. Recessive is much much slower!

Like watching paint dry.

Source: Evolution, Colin Patterson, second edition, 1999, Cornell University Press, pp 40-47