View Full Version : Updated Marciano paper - Y-chromosome Haplogroups and Gene Mutations

12-09-2016, 02:25 AM
Maciamo has just published an updated version of his paper, available on academia.edu, "Major Y-chromosomal haplogroups are defined by gene-altering polymorphism affecting fertility and reproductory success".

(Auto correct changed the spelling of Maciamo's name in the thread title, and I didn't catch it before posting. My apologies to Maciamo. If moderators can correct the title, please do so. Thanks.)

This topic is important for a correct understanding and interpretation of modern and ancient haplogroup distributions, so I think it is worthy of discussion.

I often see misinterpretations leading to incorrect deductions with regard to the explanation of modern and ancient distributions of haplogroups. Those mistakes are often attributable to a lack of understanding of the mechanism of natural selection and its relevance to Y haplogroups.

Mutation of genes is the driver of reproductive success. Such mutations are the primary reason that various specific haplogroups dominate the modern population in various locations. This process happens very gradually over a long period of time. Haplogroups that dominate modern populations were not major haplogroups at the time of their appearance and early migrations. This fact is often overlooked in discussions, resulting in an incorrect interpretation of modern haplogroup distributions.

For example, this is the sort of concept that I often see proposed on this forum:

P312 had a handful of offspring somewhere around central Europe, including DF27, U152, and L21 which together made up the majority of the tribe. Each of these offspring grew their family to a significant size. Then, after a time, each of these families picked up camp and migrated, en masse, each moving separately as genetically homogeneous populations to different locations. The L21 group migrated to the Isles or northwest France and Belgium. The U152 group headed toward Italy. The DF27 group headed toward Spain.

This interpretation is wrong, because it relies on the assumption that DF27, U152, and L21 were major subclades of P312 at the time of the groups' migrations. A correct understanding of gradual haplogroup dominance via genetic fitness leads to a different interpretation.

In fact, DF27, U152, and L21 would have been only small minority of P312's many offspring. They and their ancestral lineages would have migrated along with many other lineages, most of which are now extinct. When first arriving at their destination, the MRCA of dominant modern haplogroups would have comprised only a small fraction of the migrating population. DF27, U152, and L21 MRCAs and their ancestors were only three lineages within a continuous migration of many P312 in all directions, who only later came to dominate by a lucky roll of the dice in their extraordinarily healthy genes.

The usual process of haplogroup dominance is one of gradual in situ expansion and eventual dominance, not mass migration of a homogenous population comprised of a single haplogroup.

So the concentration of a dominant haplogroup in a particular location will be something like a roughly linear and continuous growth over time, from its initial concentration of just over zero, all the way to its modern concentration. The concentration of a haplogroup at any particular time can be approximated by interpolating from zero at the time of the MRCA to the modern concentration. So, for example, the modern dominance of L21 in the Isles does not require that a large L21 tribe migrated to the Isles. If the L21 MRCA appeared in the Isles as a single individual among a small population of recent Bell Beaker arrivals circa 2500 BC, his progeny may have made up the major fraction of the population after a millennium of residence.

Another common misinterpretation is the idea that modern haplogroup dominance can be attributed to a single individual or family of high social status. For example, Niall of the Nine Hostages is often given credit for the success of M222. The problem with this idea is that advantages of social status are short lived, and quickly become diluted. Suppose Niall had 200 children. After a few generations, most of those children would be mixed with the greater population, no longer having any reproductive advantage. So, on average, the reproductive advantage of the lineage would only be marginally greater than that of the surrounding populace after a small number of generations. Furthermore, as soon as the lineage falls out of its advantageous social standing, the reproductive advantage would end.

On the other hand, an advantageous genetic mutation continues to spread with all of the lineage's offspring. If the MRCA has an advantageous Y mutation, all of his male offspring will carry the same advantageous mutation. The reproductive advantage will be permanently maintained for perpetuity, in the entire progeny, and the haplogroup will continue to grow until it is dominant.

So we should expect social status to have only a small role in the eventual dominance of haplogroups. Genetic fitness is the major source of haplogroup dominance. And the historical importance often attributed to supposed bottlenecks and explosions of branches, hypothetically associated with especially important and successful ancestors, is overstated, those tree features being mostly attributable to chance abnormal distributions of SNPs.

It is also worth noting that, while Maciamo's publication focuses on beneficial mutations, such beneficial mutations are not necessary in order for a particular haplogroup to dominate. Most genetic mutations are, in fact, detrimental rather than beneficial. At any given time, the majority of lineages within a population will contain at least one detrimental mutation. Most of those lineages will eventually die out. Thus, for a haplogroup to dominate, it is enough that it simply possess no detrimental mutations. More often than not, the healthiest lineages that dominate reproductively are the ones that simply maintain the status quo, while the detrimentally mutating lineages flounder and die. The lineages that contain an advantageous mutation are extraordinarily fortunate and rare.