Natural selection in action

From Life and death on the wagon trail by Pat Shipman.

American folklore is rich with stories of the hardships encountered by those who explored and settled in the United States. These almost mythic tales speak of the better life that was to be found on the ever-beckoning frontier, of the chance of riches awaiting those with the courage and determination to seize the opportunity. Yet some of the episodes that have become near-legends are those of utter failure, rather than success. Few disasters have captured the public imagination like the dismal fate of the Donner party in 1847. Eighty-seven pioneers, led by George and Jacob Donner, attempted to cross the arid Great Basin of Utah and Nevada en route to Sacramento, California, using an untested trail known as the Hastings Cutoff.

In the end, barely half of the original party survived. The victims succumbed to various manifestations – natural and unnatural – of hunger and cold. The tragedy that befell them provided a perfect scientific test for Grayson: ‘a case study of demographically mediated natural selection in action’, as he said in a recent issue of the Journal of Anthropological Research. With an eye to history, Grayson has made a study of the records of the fate of the Donner Party, describing it as "a human group that was almost fully exposed to the vagaries of an exceedingly harsh and demanding environment." Under these circumstance, social and demographic factors such as gender and age would be expected to affect individuals' chances of surviving. Indeed, demographers have made survival predictions based on such factors. This was Grayson's aim: to test how well those predictions stood up in real life.

The research paper was Donner Party Deaths: A Demographic Assessment by Donald K. Grayson. 46% of the Donner Party perished in their ordeal.

Based on the literature, Grayson tested three accepted forecasts from prior disasters:

1) Mortality in the Donner Party should have struck particularly hard at the youngest and oldest members of the group. AFFIRMED: The oldest and the youngest members of the group had the highest mortality rates.

2) Mortality among the Donner Party should have been higher for males than it was for females and that this should have been the case for all age classes. AFFIRMED: Males died at a higher rate and they died earlier in the ordeal.

3) Mortality in the Donner Party should have been inversely scaled to the number of social contacts, and in particular to the degree of social connectivity as measured by the size of the available kin group. In other words, strong social networks reduce risk of mortality. AFFIRMED: The average kin group size of the survivors was 6.8 individuals, whereas the average kin group size of the nonsurvivors was only 2.3 individuals.

There is an interesting codicil to the familial network issue.

Probably, the women's high rate of survival can be partly explained by the size of their kin groups, too. Understandably, given the social mores of the era, all females traveling in the party were related by birth or marriage to other group members, whereas 15 males were "unattached". Nearly all these male loners (13 individuals or 87 per cent) died.

Guys - for your own good, get married. It helps you and it helps everyone else.