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Allele Frequency Dynamics in a Pedigreed Natural Population Allele frequency dynamics in a pedigreed natural population Nancy Chena,b,c,1, Ivan Juricd, Elissa J. Cosgrovee, Reed Bowmanf, John W. Fitzpatrickg, Stephan J. Schoechh, Andrew G. Clarke, and Graham Coopa,b aCenter for Population Biology, University of California, Davis, CA 95616; bDepartment of Evolution & Ecology, University of California, Davis, CA 95616; cDepartment of Biology, University of Rochester, Rochester, NY 14627; dDepartment of Quantitative Health Sciences, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, OH 44195; eDepartment of Molecular Biology & Genetics, Cornell University, Ithaca, NY 14850; fAvian Ecology Program, Archbold Biological Station, Venus, FL 33960; gCornell Lab of Ornithology, Cornell University, Ithaca, NY 14850; and hDepartment of Biological Sciences, University of Memphis, Memphis, TN 38152 Edited by Nils C. Stenseth, University of Oslo, Oslo, Norway, and approved November 19, 2018 (received for review August 10, 2018) A central goal of population genetics is to understand how ductive value, a general measure of individual fitness (5–7). genetic drift, natural selection, and gene flow shape allele fre- Indeed, the reproductive value of an individual influences many quencies through time. However, the actual processes underlying aspects of the survival of an individual’s genotype, from the these changes—variation in individual survival, reproductive suc- probability of loss of a new, weakly beneficial mutation to the cess, and movement—are often difficult to quantify. Fully under- complex distribution of genomic blocks passed on to future standing these processes requires the population pedigree, the set generations (8). of relationships among all individuals in the population through Analyses of known pedigrees have been used to estimate indi- time. Here, we use extensive pedigree and genomic informa- vidual genetic contributions to assess founder effects in human tion from a long-studied natural population of Florida Scrub-Jays populations (1–3, 9–11) and to predict the probability of gene (Aphelocoma coerulescens) to directly characterize the relative loss in captive breeding populations (4, 12). Also, empirical pedi- roles of different evolutionary processes in shaping patterns of gree calculations have long been used to understand genetic genetic variation through time. We performed gene dropping models of human diseases (13) and are increasingly used in simulations to estimate individual genetic contributions to the natural populations to understand the genetic basis of quan- EVOLUTION population and model drift on the known pedigree. We found titative trait variation, fitness consequences of inbreeding, and that observed allele frequency changes are generally well pre- much more (14). To date, empirical pedigrees and gene dropping dicted by accounting for the different genetic contributions of approaches have been rarely used to study the temporal spread founders. Our results show that the genetic contribution of recent and loss of individual alleles (15–18). immigrants is substantial, with some large allele frequency shifts Here, we combine genomic data with a known population pedi- that otherwise may have been attributed to selection actually gree to describe and predict allele frequency change at many loci in due to gene flow. We identified a few SNPs under directional an exhaustively sampled free-living population of Florida Scrub- short-term selection after appropriately accounting for gene flow. Jays (Aphelocoma coerulescens) at Archbold Biological Station. Using models that account for changes in population size, we par- Intensive study since 1969 has resulted in lifetime fitness measures titioned the proportion of variance in allele frequency change for thousands of individuals on an extensive pedigree. Recently, through time. Observed allele frequency changes are primarily Chen et al. (19) generated genome-wide single-nucleotide due to variation in survival and reproductive success, with gene flow making a smaller contribution. This study provides one of the most complete descriptions of short-term evolutionary change in Significance allele frequencies in a natural population to date. Evolution is change in the genetic composition of populations. In nature, individuals reproduce, die, and move among pop- population genetics j pedigrees j fitness j gene flow j genetic drift ulations, leading to changes in the population frequency of the alleles they carry. Here we study an extensive family tree (pedigree) for an exhaustively sampled natural population n evolving natural population is essentially a vast pedi- of Florida Scrub-Jays to directly characterize the mechanisms gree, with genetic material transmitted down this pedigree A underlying how genetic material is transmitted to future gen- following the laws of Mendelian inheritance (except in rare erations. We link individual fitness with long-term genetic cases of meiotic drive). We often cannot directly observe the contributions and quantify the relative roles of evolution- actual processes underlying genetic change. Instead, population ary processes governing allele frequency change. This study genetic studies typically rely on current day patterns of genetic clearly illustrates how short-term evolutionary change occurs variation—or, if temporal samples are available, the variation in within a natural population. allele frequencies through time—to make inferences about the effects of genetic drift, natural selection, and gene flow in driving Author contributions: N.C., A.G.C., and G.C. designed research; N.C., E.J.C., R.B., J.W.F., evolutionary change. However, these evolutionary mechanisms and S.J.S. performed research; N.C. and I.J. contributed new reagents/analytic tools; N.C., can be precisely understood in terms of the differential genetic E.J.C., R.B., J.W.F., and S.J.S. performed data collection; N.C. and G.C. analyzed data; and contributions of individuals to the population pedigree over time, N.C. and G.C. wrote the paper.y combined with the stochasticity of Mendelian segregation. The authors declare no conflict of interest.y Knowledge of the population pedigree allows us to trace This article is a PNAS Direct Submission.y expected individual genetic contributions, i.e., the expected num- Published under the PNAS license.y ber of copies of a neutral allele contributed by a given individual, Data deposition: All data and code used in this study can be found at Figshare, to the population in future generations. Individual genetic contri- 10.6084/m9.figshare.7044368.y butions can be estimated analytically (1–3) or via gene dropping 1 To whom correspondence should be addressed. Email: [email protected] simulations, i.e., simulations of Mendelian transmission of alleles This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. down their pedigree of descendants (4). The long-term expected 1073/pnas.1813852116/-/DCSupplemental.y genetic contribution of an individual is an individual’s repro- www.pnas.org/cgi/doi/10.1073/pnas.1813852116 PNAS Latest Articles j 1 of 7 Downloaded by guest on September 30, 2021 polymorphism (SNP) data for nearly every individual in the Using the detailed population pedigree, we calculated both the population over the past two decades to demonstrate how genealogical and expected genetic contribution of each individ- decreased immigration into the population led to increased levels ual to the study population from 1990 to 2013. Fig. 1 A and B of inbreeding and decreased mean fitness over time. In this study, shows results for two illustrative males, both of whom first bred we link individual lifetime reproductive success with long-term in 1994. Male A lived until 2006 and had 41 offspring, whereas genetic contributions and allele frequency change. We show Male B only lived until 2000 and had 7 offspring. We define how the population pedigree presents a powerful opportunity an individual’s genealogical contribution to a given year as the to directly elucidate the relative roles of drift, gene flow, and proportion of nestlings in the birth cohort who are genealogi- selection in governing allele frequency dynamics over time. cally descended from the focal individual, while an individual’s expected genetic contribution is the expected proportion of alle- Results les at a locus in the nestling cohort that comes from the focal Individual Fitness and Genetic Contributions. First, we consider a individual. Beyond a few generations, few genealogical descen- series of inferences that can be made purely with the pedigree, dants are expected to inherit any genetic material, so the number ignoring the SNP genotypes for the moment. We estimated fit- of genealogical descendants should quickly outnumber the num- ness for all 926 individuals who bred in our study population ber of genetic descendants. Fig. 1 A–C nicely demonstrates this in 1990–2013 and were born before 2002 (the age cohorts who pattern in our data, providing empirical illustration for a sub- are all dead by the end of 2014). Lifetime reproductive success stantial body of theory on the relationship between genetic and was highly variable in our study population: The total num- genealogical ancestry (8, 22, 23). An individual’s genealogical ber of nestlings produced over an individual’s lifetime ranged contribution in 2013 is correlated with its expected genetic con- from 0 to 43, with 197 individuals (21%) producing no nestlings tribution in 2013 (Spearman’s ρ = 0.99, p < 2 × 10−16), but its despite having
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