Natural and sexual selection in a monogamous historical human population

Abstract

Whether and how human populations exposed to the agricultural revolution are still affected by Darwinian selection remains controversial among social scientists, biologists, and the general public. Although methods of studying selection in natural populations are well established, our understanding of selection in humans has been limited by the availability of suitable datasets. Here, we present a study comparing the maximum strengths of natural and sexual selection in humans that includes the effects of sex and wealth on different episodes of selection. Our dataset was compiled from church records of preindustrial Finnish populations characterized by socially imposed monogamy, and it contains a complete distribution of survival, mating, and reproductive success for 5,923 individuals born 1760-1849. Individual differences in early survival and fertility (natural selection) were responsible for most variation in fitness, even among wealthier individuals. Variance in mating success explained most of the higher variance in reproductive success in males compared with females, but mating success also influenced reproductive success in females, allowing for sexual selection to operate in both sexes. The detected opportunity for selection is in line with measurements for other species but higher than most previous reports for human samples. This disparity results from biological, demographic, economic, and social differences across populations as well as from failures by most previous studies to account for variation in fitness introduced by nonreproductive individuals. Our results emphasize that the demographic, cultural, and technological changes of the last 10,000 y did not preclude the potential for natural and sexual selection in our species.Bateman gradient | Bateman principles | selection differential | human evolution | mating system D isagreement about the strength of selection affecting humans has influenced how the theory of natural and sexual selection has been perceived since Darwin (1). In particular, humans exhibit a pronounced ecological flexibility through social and cultural adaptations (2) that is often invoked to cast doubt on the continued relevance of Darwinian selection in our species. Even among those applying evolutionary theory in humans, some scientists consider recent selective pressures weak and therefore only study human adaptations in the demographic, social, and physical environments of the Pleistocene, whereas others view selection as an effective force also operating in societies affected by the agricultural revolution (3).Darwinian selection results from variation in fitness between individuals of the same population and the evolutionary response of traits to this selection depends on the genetic covariation between traits and fitness. Such evolutionary change is traditionally approximated by multiplying heritability (the proportion of phenotypic variance in the trait attributable to genetic differences among individuals) and phenotypic selection (the covariance between the trait and ...

Lifetime reproduction and the second demographic transition: Stochasticity and individual variation

“…They measured variation by the standardized variance (Crow's I;Crow (1958)), which measures the maximum opportunity for selection. Courtiol et al (2012) found a value close to 2, which is much higher than the values (≈ 0.5) that we calculate for the countries in our dataset during the transition (Figure 9). Of course, the vital rates in Finland in the 18th and 19th century are different from those of Europe during the second demographic transition, but this crude comparison suggests that the opportunity for selection is to a large part determined by heterogeneity, with only a relatively small contribution from individual stochasticity.…”

“…This measured variance includes both individual stochasticity and heterogeneity, but such studies are rare, and many of them (e.g., Spuhler 1976) consider only surviving women and/or only married women (Courtiol et al 2012). In one exceptional study, however, Courtiol et al (2012) measured the standardized variance in LRO for an entire cohort of Finnish women living in the period 1760-1849, using data from the church books of four Finnish populations. They measured variation by the standardized variance (Crow's I;Crow (1958)), which measures the maximum opportunity for selection.…”

Culled males, infant mortality and reproductive success in a pre-industrial Finnish population

“…Since the eighteenth century in Finland, the Lutheran Church has required the submission of accurate registers of all births, inter-parish movements, marriages and deaths in the country. From these records, we compiled life histories for individuals from rural parishes previously used in analyses of life-history traits (see the electronic supplementary material for more details) [15].…”

Many ways to die, one way to arrive: how selection acts through pregnancy

“…Finnish women living 1760--1849 had an average of 4.27 babies, whereas landowning women had an average of 4.55 babies: a change in absolute fitness of this magnitude would cause a geometric rise in the number of descendants in a few generations [14] (Figure 2a). …”