Phenotypic traits of primate hybrids: Recognizing admixture in the fossil record

Abstract

For many years, the likelihood that hybridization occurred in human evolution has been debated. Tattersall and Schwartz pointed out one of the core problems with resolving this debate, namely “that nobody has any idea what a Neanderthal/modern human hybrid might look like in theory, and few have dared to suggest in practice that any particular known fossil represents such a hybrid.”1:7117 Moreover, while molecular data is proving increasingly useful for characterising hybrid zones, the utility of the phenotype for this purpose is not clear.2 Here I address these issues, discussing both theoretical and empirically‐derived expectations for what hybrid morphology looks like, with an emphasis on the skeleton of hybrid primates, and consideration of the hominin fossil record.

Facial asymmetry and genetic ancestry inLatinAmerican admixed populations

“…This Whole model adjust R 2 : 9.184%, P < 0.00001, 100,000 rounds reinforces the idea that the novel genetic and environmental landscape of the admixed individuais in Latin America does not represent a case of developmental instahility enough to trigger the expression of asymmetric phenotypes. Furthermore, and given the negative relationship between heterozygosity and FFA, these results lend support to the idea that the genetic hasis of such asymmetries can he seen as a case of underdominance, rather than an additive, dominance, or epistatic scenario (see Ackermann, 2010;Ackermann et al, 2006). In terms of type of asymmetry, our data indicates that DA is more important than FA as a source of facial asymmetries, hringing support to previous works suggesting a greater component of DA on the asymmetric normal variation in the human cranium, face and dentition (Schaefer et al, 2006;DeLeon, 2007;Ercan et al, 2008;Barrett et al, 2012;Claes et al, 2012), as well as on dismorphological samples (Mclntyre and Mossey, 2002;Hennessy et al, 2004;Klingenberg et al, 2010h).…”

“…populations. As discussed previously (Ackermann, 2010;Ackerm.ann et al, 2006), admixed individuais can show a range of morphologies, resembling one parenta! group or the other, or displaying novel phenotypes, depending on dominance and epistatic interactions between alleles :fixed or predominant in either parenta!…”

“…On their comparison among Mexican colonial skull samples, Martínez-Abadías et al (2006) showed that the craniofacial phenotypes resulting from Spanish-Amerindian admixture departs from an intermediate position in the morphospace between parental groups, thus suggesting that heterosis phenomena underlie the behavior of craniofacial phenotypes in admixed populations. On the same line, in their study on baboon hybrids produced in captivity and on natural populations 1 Ackermann et al (2006) further verifi.ed the significant signals of craniofacial heterosis, suggesting caution against sim.ple assumptions that hybrids will display the average of parental phenotypes (Ackermann, 2010).…”

“…In such circumstances, F1 hybrids would typically have intermediate phenotypic trait values relative to the parenta! taxa (Ackermann, 2010). However, admixed individuais can deviate from the average due to many factors that can affect phenotypic variation in a hybrid population, for instance, when in a single gene one allele (e.g., coming from a parenta!…”

“…population) masks the effect of other allele in infl.uencing some trait (dominance), or when the action of one gene is modified by one or several other genes (epistasis). Variations produced in such ways often results in substantial variation (Falconer and Mackay, 1997;Ackermann, 2010), including hybrid morphology that is not intermediate (Martínez-Abadías et al, 2006) and/or asymmetries. In this context, the exploration of the phenotypic expression of asymmetries in an individual and the results of a population levei process like admixture would be useful to characterize individual facial asymmetries and population leveis of DA and FA.…”