Inbreeding and genetic diversity

[Mangetout]

@John Mace

They posit that species are fixed, and one species cannot change into another species. What they don’t realize, of course, is that the concept of a species is a human invention that only approximates the conditions in nature.

Actually (now it’s my turn) this is a slight mischaracterisation of the most common argument; they don’t argue that species are fixed; they argue that variation can occur ‘within kinds’ - the term ‘kind’ is never satisfactorily defined though and estimates of the total number of ‘kinds’ on the planet are coerced in the downward direction by those trying to explain how everything fit on the ark, and in the upward direction by those trying to explain the diversity we witness extant in the world today.

[John_Mace]

Yeah, you could replace “species” with “kind” in my last post. I chose “species” because “kind” isn’t well defined, and the closer you get to humans, it seems, the close “kind” and “species” begin to mean the same thing. Some creationists may be comfortable talking about cat-kind and dog-kind, for example, but if they talk about ape-kind it doesn’t include humans. Clearly a grouping called ape-kind that includes chimps, gorillas and orangs but not humans isn’t any meaningful “kind” at all. We’re a different kind of “kind”, I guess.

[Mangetout]

And that’s the whole problem with creationism; it isn’t consistent; a concept that is defined as essential right here needs to be negated over there. It’s a loose collection of ad-hoc arguments against science, with little concern given to coherence as a whole.

[Colibri]

@John Mace

This I don’t understand, but I’ll leave that for another thread.

(I shuffled the deck a bit on your post to group the sentences more clearly for what I did and didn’t understand.)

Sorry, I was being very brief. Evolution, in its simplest sense, is defined as the change of allele frequencies in a population over time. Such evolution can occur due to genetic drift or to selection without any new mutations occurring in the population at all. (Of course, the varient alleles present in the population at the start were generated by mutation at some earlier point in time.)

This site has some definitions of genetic drift and founder effects, as well as a couple of simple experiments in “bean bag genetics” which illustrate how evolution can take place due to genetic drift or selection in the absence of mutation.

Beneficial mutations - ones that are selected for - are quite rare and have relatively little effect on evolution over the short term compared to selection and other factors. Of course, over the longer term they are the main source of variation. It is thought that speciation typically takes place in small isolated populations. In such populations, founder effects, genetic drift and selection are all very important in changing allele frequencies. Beneficial mutations, being rare, have little chance of occuring in such small populations, and are relatively unimportant. Mutations generally play their role at a later stage in the speciation process, after an initially small founder population has grown to a larger size.

[Colibri]

@Smeghead

Actually, I would say macro-evolution has no scientific definition at all, as it’s a term that was coined by and is used almost exclusively by people attempting to argue that evolution doesn’t exist. I have never in my life heard a reputable scientist use the term macro-evolution. The supposed distinction between micro- and macro-evolution is completely arbitrary and has no scientific basis whatsoever.

Macroevolution is an entirely legitimate scientific concept, simply defined as evolution above the species level. It is commonly used in scientific articles and books; see the book cited in the link by Niles Eldredge, one of the founders of punctuated equilibrium theory.

It is true that it and microevolution are terms that have been seized on by creationists, without any real understanding of what they mean. However, in origin the terms are scientific, and continue to be used in the literature.

[John_Mace]

@Colibri

Sorry, I was being very brief.

No worries. I was being brief, too.

Beneficial mutations - ones that are selected for - are quite rare and have relatively little effect on evolution over the short term compared to selection and other factors. Of course, over the longer term they are the main source of variation.

OK. That makes more sense-- short term vs long term.

[Chronos]

That can’t be true, although you were right in the clarification about mtDNA Eve. You’d have to go back much further than “a few” thousand years before I (of European descent) share all my ancecstors with someone who is Native American, for example. Native Americans were pretty much isolated in the Americas for 10,000 years or more. I don’t remember how the estimates work for what you’re talking about, but that would only be true (IIRC) for people living on the same continents or within the same continuously interbreeding populations.

Depends on your standards of “few”, I guess. The results I’ve seen put the most recent common ancestor of all humans at around 4000 years ago (there’s been enough mingling since the Americans and Eurasafricans came back into contact for all of those populations to have been assimilated into the “family”), and the time of all common ancestors at about 15,000 years ago. I think I was partially conflating those two numbers, but even 15,000 years could arguably be called “a few thousand”. In any event, it’s a lot more recent than MitEve or Y-Adam.

[John_Mace]

@Chronos

Depends on your standards of “few”, I guess. The results I’ve seen put the most recent common ancestor of all humans at around 4000 years ago (there’s been enough mingling since the Americans and Eurasafricans came back into contact for all of those populations to have been assimilated into the “family”), and the time of all common ancestors at about 15,000 years ago. I think I was partially conflating those two numbers, but even 15,000 years could arguably be called “a few thousand”. In any event, it’s a lot more recent than MitEve or Y-Adam.

I’m still very dubious of either number. You’re saying that all humans alive today shared all their ancestors in common 15,000 years ago, right? And that evryone alive today has at least on ancestor in common from 4,000 yours ago? And of course if we use specific dates we can get around the confusion over what “a few” means. Think about it. Surely there are populations all over the world (South America, New Guinea, Australia, etc) that have no ancestors of Western European descent due to the fanning out of that population in the last few hundred years. And that doesn’t even touch on China, Japan, etc. I understand that the most recent common ancestor of all humans will be more recent than mtDNA-Eve or Y-Adam, but 4,000 years just doesn’t seem like enough time. I’m not even sure if there would be enough time if we couched that statement as something like: 95% of all humans today share at least one common ancestor from 4,000 years ago.

At any rate, I’d be interested in a cite that talks about this-- I remember reading about something similar, but can’t remember what or where.

The other thing to keep in mind is that these type of estimates are just that-- estimates based on a statistical analysis. They are generally expressed as a range of dates rather than one specific date. IOW, it’s not like counting tree rings to measure the age of tree, where you determine the exact answer. (I’m not saying you implied otherwise, but for those not familiar with the process used to arrive at these types of dates it can be a little confusing.)

[John_Mace]

Here’s what wikipedia says about the MRCA:

Depending on the survival of isolated lineages without admixture from Modern migrations and taking into account long-isolated peoples, such as historical tribes in central Africa, Australia and remote islands in the South Pacific, the human MRCA is generally assumed to have lived in the Paleolithic period [~2M years ago - ~ 10k years ago].

However, Rohde, Olson, and Chang (2004), using a non-genetic model, estimated that the MRCA of all living humans may have lived within historical times (3rd millennium BC to 1st millennium AD). Rhode (2005) refined the simulation with parameters from estimated historical human migrations and of population densities. For conservative parameters, he pushes back the date for the MRCA to the 6th millennium BC (p. 20), but still concludes with a “surprisingly recent” estimate of a MRCA living in the second or first millennium BC (p. 27). An explanation of this result is that, while humanity’s MRCA was indeed a Paleolithic individual up to Early Modern times, the European explorers of the 16th and 17th centuries would have fathered enough offspring so that some “mainland” ancestry by today pervades even remote habitats. The possibility remains, however, that a single isolated population with no recent “mainland” admixture persists somewhere, which would immediately push back the date of humanity’s MRCA by many millennia. While simulations help estimate probabilities, the question can only be resolved authoritatively by genetically testing every living human individual.

Other models reported in Rohde, Olson, and Chang (2004) suggest that the MRCA of Western Europeans lived as recently as AD 1000. The same article provides surprisingly recent estimates for the identical ancestors point, the most recent time when each person then living was either an ancestor of all the persons alive today or an ancestor of none of them. The estimates for this are similarly uncertain, but date to considerably earlier than the MRCA, according to Rohde (2005) roughly to between 15,000 and 5,000 years ago.

[John_Mace]

And here’s the abstract of the article in Nature that I’m sure you were referring to, Chronos. Modelling the recent common ancestry of all living humans. I’d be interested in seeing how other biologists critiqued that paper.

[lissener]

The OP seems to be under the impression that evolutionary changes are instantaneous. That the mere act of incest–wham!–causes, as an immediate effect, diversity [sic]. In fact, such concepts as diversity and inbreeding discuss changes that take place over long spans of time. They are the result of a pattern of inbreeding or crossbreeding, not merely of a single act. The changes are cumulative.

His understanding also seems to leave out other changes. The diversity of Homo sapiens not only arose over vast spans of time, but with innumerable opportunities for mutation: every child born is such an opportunity. Increasing diversity over time is a result of many such factors, not simply the single factor of the relationship of the parents.