In a recent study published in Biology Letters, Tobler and colleagues found that an indigenous religious ritual has caused genetic change in a local fish population. During an annual fertility ritual, the Zoque people of Mexico use a plant poison to stun and harvest fish from a section of nearby river; unsurprisingly, the fish in that section of river have developed higher tolerances to the poison than fish from other sections of the river. The authors see selection at work:
Furthermore, fish from sites exposed to the ceremony had a significantly higher tolerance. Consequently, the annual ceremony may not only affect population structure and gene flow among habitat types, but the increased tolerance in exposed fish may indicate adaptation to human cultural practices in a natural population on a very small spatial scale.
These results parallel those from another recent study, appearing in the journal Evolution, which found a strong correlation between urbanization history and human pathogen resistance:
A link between urban living and disease is seen in recent and historical records, but the presence of this association in prehistory has been difficult to assess. If the transition to urbanisation does result in an increase in disease-based mortality, we might expect to see evidence of increased disease resistance in longer-term urbanised populations, as the result of natural selection. To test this, we determined the frequency of an allele associated with natural resistance to pathogens such as tuberculosis and leprosy.
We found a highly significantly correlation with duration of urban settlement – populations with a long history of living in towns are better adapted to resisting these infections. Our results therefore support the interpretation that infectious disease loads became an increasingly important cause of human mortality after the advent of urbanisation.
Because the oldest urban environments arose in conjunction with the earliest forms of organized religions, we might expect certain religious groups to have higher pathogen resistance. But alas, these early religions — from Mesopotamia and Egypt — are no longer practiced or have been swamped by newer religions, making any such study impossible.
One thing is certain, the hunting and gathering groups who resisted urbanization (and its concomitant, agriculture) largely lacked resistance to many kinds of pathogens and paid a horrendous price for it.