|Why are Polynesians so BIG?
Finding thrifty genes in Polynesia
Why are Samoan people so big and huge? Is it the food or maybe the genetic structure and where does it comes from?
In this matter the thrifty gene is standing central. It is a extra gene which polynesian people have and it has a function of storage of extra fat.
Upon adoption of a more western lifestyle, Polynesians show a higher incidence of obesity and obesity-related disorders, such as type 2 diabetes, than most human populations. One possible explanation for these higher rates of diabetes in Polynesians invokes the thrifty gene hypothesis: the ancestors of present-day Polynesians experienced conditions (e.g. long open-ocean voyages) in which it was advantageous to have a thrifty metabolism, one that promotes the storage of fat. The allele(s) underlying this thrifty metabolism may therefore have been driven to high frequency by positive natural selection, thereby explaining the high incidence of diabetes in present-day Polynesians.
To test this thrifty gene hypothesis, we aim to obtain frequency data on diabetes-associated SNPs from 23 Han Chinese, 23 New Guineans and 23 Polynesians and compare Fst values from these SNPs to an empirical Fst distribution of 100,000 SNPs from these same three populations. High Fst values are a signature of the action of positive natural selection. Thus, diabetes-associated SNPs that show high Fst values may be better disease candidates than other SNPs.
Once putatively adaptive alleles have been identified, we plan on better characterizing the signature of selection by sequencing the surrounding genomic region and performing standard tests of selection based on the allele frequency spectrum and long-range haplotype structure. We suggest that these kinds of evolutionary considerations offer complimentary approaches to the identification of alleles that predispose to disease.
Opinion Box: Thrifty Genes in Polynesia?
Linguists, archaeologists, and geneticists disagree about the origin of the
Polynesians. However, most scholars agree that about 3,500 years ago pre-Polynesian ancestors carrying the Lapita cultural complex arrived in the Bismarck Archipelago, interbred with existing populations, and shortly thereafter achieved the sailing technology for substantial open ocean voyaging. Within a few hundred years these peoples surmounted one of the last great impediments to human settlement: the 500
miles of ocean between Vanuatu and Fiji, sailing against prevailing winds and currents.
Both cultural and biological adaptations were required to make this crossing possible. The cultural innovations likely included improvements in sailing technology and navigational expertise and enhanced food preservation techniques. The biological adaptations probably included the robust body build that facilitated paddling the vessels and metabolic adaptations to dietary and cold stress. All of these biological
adaptations may be the result of selection for a ‘thrifty phenotype’.
The Lapita sailing vessels had a large triangular sail that precluded sailing too close to the wind.
The canoes sat low to the water, making paddling possible when
winds were too low or from the wrong direction. Skeletal remains from Lapita sites and measurements on modern Polynesians depict a people of substantial stature with broad shoulders and hips, and robust, long limbs. The broad body, long limbs, and sizeable muscle mass are particularly well suited to the biomechanics of paddling—a voyaging strategy that only would have come into play in situations critical to survival.
Thus, this body build would have had a strong selective advantage.
Houghton (1996) has modeled the severe cold stress that early Pacific voyagers would have experienced. Maximum cold stress is achieved overnight when moderately low temperatures, high wind chill, and wet clothes and skin combine to produce substantial cold stress. Overnight voyages would have been very rare prior to the “break out” from the Solomon Islands. The same bodies that are well suited to paddling canoes also have a favorable low surface area to body mass ratio, excellent for
conservation of body heat in cold stress. Again, a large, robust body build would provided an advantage.
Traditionally the thrifty genotype argument has been used to explain adaptation to periodic famine. In the case of the Pacific it has been invoked as an adaptive response to caloric restriction associated with voyaging and settlement of the islands (Bindon and Baker, 1997). Metabolic efficiency in storage of excess calories is achieved
through over secretion of insulin which increases fat tissue formation and the accumulation of an energy store.
This would also increase subcutaneous fat tissue
which acts as an insulation against cold stress. It has been suggested, however, that a population as well adapted to a marine environment as the Lapita people were may not have suffered from extreme caloric deprivation during voyaging and settlement.
Even so, their diet would have been drastically altered: lower carbohydrate intake and an increase in protein intake. They would have eaten through their supply of the poi-like fermented crops (taro, breadfruit, banana) that they were carrying on their voyage and
then had to wait for new crops to grow before regaining their normal carbohydrate intake. Several people have argued that the thrifty genotype provides a metabolic adaptation to just such a high protein low carbohydrate diet through the metabolic shifts involved in hyperinsulinemia and insulin resistance.
Recent research into thrifty genes has provided some clues that the cold- and work-adapted body build and the metabolic shift to accommodate dietary stress may be related. These adaptations may be the result of mutations in the region of the insulin gene (INS), like the variable number tandem repeat (VNTR) polymorphism near INS
that causes increased transcription of both the INS gene and the close Insulin-like Growth Factor 2 (IGF2) gene. Increasing transcription of INS could generate high blood insulin levels (hyperinsulinemia) and decrease sensitivity to insulin binding in peripheral cells (insulin resistance). Meanwhile, high levels of IGF2 stimulate muscular and skeletal growth predisposing to a large, robust body.
I do not mean to imply that this particular VNTR polymorphism represents the thrifty gene, but it points to a possible area for exploration and integrates the biological adaptations found in modernday
Polynesians that appear to result from their voyaging history.