genes changing amount

[caharris]

Someone told me that evolution would show that the amount of genes would have changed the farther we go back, and that no one had proved it.

Do genes change in size or amount the farther we go back? And if so, is there proof of this, or is it an indirect observation? I know that they occasionally mutate, but his point was the size changes.

[CharonY]

Assessing the amount of genes in ancestors is not possible as most DNA is too degraded to get that information. Mammoth DNA was pretty bad to get already and in evolutionary terms that was just recently.

That being said there is little evidence that there should be a correlation. Maybe in the very early days after life arose there may have been less diversity. But then rapid duplication events and similar will lead to a sudden increase of genes with selective sweeps eliminating them.

 

The argument is based a often propagated fallacy that evolution should lead to an increase in genetic information. It does not. Also keep in mind that going back in evolution does not mean moving horizontally among extant species (e.g. from mammals to fishes) but back in time along the now extinct species.

[caharris]

So, are there any species today, or any that have been completely have gotten and haven't become too degraded that have different numbers? Or do all species have the same amount?

[CharonY]

What do you mean with degraded? There are species to species variatios, of course. The amount of genes they carry depends whether they live under constriction for genome size and the environmental conditions they live in. In protozoa the largest amount of genes have been found so far (around 60k, whereas humans are around 20-22k). In case you were wondering, complexity of organism does not scale with genome size, either. As a rule of thumb single celled organisms have to be much more flexible in terms of metabolic capabilities than more multicellular ones and require in relative terms a higher number of genes to respond accordingly. Exceptions are parasitic organisms, for instance that, similar to individual cells in our body, live in a relatively stable environment and hence are able to get by with less genes.

 

In other words, environmental constraints generally play a bigger role in determining genome size as well as number of genes. Again, there is no evolutionary scaling in terms of genome size. A plot of species vs. number of genes would be all over the place, precisely as we would expect under standard evolutionary models.

[caharris]

Thank you for the explanation

As far as when I said "degrading", I was referring to what you had put in your first response. Even as someone who's accepted evolution, this fills in some gaps of my (limited) knowledge.

[CharonY]

Ah, you meant old samples that have not yet degraded? There is little. A bit is discussed in another thread but paleogenomics is very challenging and there is little that has survived which is still intact enough to sequence. Even if one should find frozen samples most of the time one will only get fragments.

[caharris]

Thank you, I will check that thread out. I didn't know what it was called

[cypress]

 

In other words, environmental constraints generally play a bigger role in determining genome size as well as number of genes. Again, there is no evolutionary scaling in terms of genome size. A plot of species vs. number of genes would be all over the place, precisely as we would expect under standard evolutionary models.

 

Under standard evolutionary models, gradualism, mostly as a consequence of the assumption that changes occur primarily by random mutation (as opposed to environmental (and possibly other) factors as you have proposed) would indicate evolutionary scaling should be dominate based on phylogenetic distance so it is not precisely as we would expect under the most popular models. Clearly the darwinian model of gradualism by genetic error and selection doesn't match the evidence, as you so clearly illustrate.

[caharris]

Under standard evolutionary models, gradualism, mostly as a consequence of the assumption that changes occur primarily by random mutation (as opposed to environmental (and possibly other) factors as you have proposed) would indicate evolutionary scaling should be dominate based on phylogenetic distance so it is not precisely as we would expect under the most popular models. Clearly the darwinian model of gradualism by genetic error and selection doesn't match the evidence, as you so clearly illustrate.

 

Ehhhh... Can you dumb that down for me?

[CharonY]

Under standard evolutionary models, gradualism, mostly as a consequence of the assumption that changes occur primarily by random mutation (as opposed to environmental (and possibly other) factors as you have proposed) would indicate evolutionary scaling should be dominate based on phylogenetic distance so it is not precisely as we would expect under the most popular models.

 

This is incorrect. Even the simplest model would include selective sweeps. No model is based on mutation alone as it would amount to no net change but merely fluctuations in a population.

Moreover, even closely related species can inhabit different niches that put different constraints on genome size.

 

Ehhhh... Can you dumb that down for me?

He assumed that changes are gradual due to random accumulation of mutations. He followed that due to that closely related species should similar genomic contents, including size.

 

However this would exclude selective forces, one of the basic mechanisms of evolution. While the sequences of closely related organism would be similar (due to phylogentic relationship) selective forces can rapidly change the overall shape of the genome in terms of gene losses or duplications, depending on the presence or absence of given constraints. In higher organized organisms the rate tends to be a bit lower as dramatic changes strongly affects e.g. their body development. Single celled organisms on the other hand can allow for more flexibility in terms of changes (and still survive it), but they often live under energetically limited conditions and smaller genome sizes can allow for faster propagation, for instance.

And don't let me get started regarding horizontal gene transfer.

 

It has also to be noted that Darwin's model is obviously not the current one. What has survived the test of time are certain elements regarding the inheritance and variance of traits (now pinned down to genes and mutations) as well as the shaping forces of natural selection (though much more has added to the mix). Also Darwin's model was qualitative whereas current ones are quantitative (if sufficient data is available).

Edited June 26, 2010 by CharonY