Constraints
On Speciation In Human Populations: Phenotypic
Diversity
Matters
Clara
B. Jones1*
Director,
1Mammals
and Phenogroups (MaPs), Asheville, NC 28801, USA
ABSTRACT:
A
phenotype is an expression of a genotype interacting with a component
of an environment. Phenotypic diversity can be generated by
mutation, physiological mechanisms, developmental processes, or
learning (reinforcing and aversive stimulus-response effects).
Causes and consequences of lifetime reproductive success can be
partitioned into one or another of the previous mechanisms of
phenotypic diversity. This article highlights, in particular, the
ways in which behavioral diversity including cultural rules, enhances
a phenotype’s relative reproductive success. Expanding Frank’s
(2011) theoretical framework, it is argued that, while a diverse
(e.g., “modular”) human phenotype may broaden a phenotype’s
success in a given landscape, byproducts are produced that increase
gene flow between populations, limiting the potential for population
divergence and reproductive isolation. The mechanisms discussed
herein are not necessarily dependent upon conscious and aware
operations.
Key
words:
Homo sapiens; Behavioral flexibility; Collaboration; Cooperation;
Fitness landscape; Gene flow; Multilevel societies; Open groups;
Phenotypic diversity
INTRODUCTION:
In
the Order Primates, thirteen extant genera are represented by a
single species (Groves, 2001; Wilson & Reeder, 2005), indicating
that mechanisms and processes characteristic of those taxa have
delayed, interrupted, or prevented speciation events. Our own
species, Homo
sapiens,
is one of the thirteen. A review of each genus in the set of
thirteen reveals few commonalities. With the notable absence of
insectivores, virtually all dietary strategies are represented
(omnivore, frugivore-insectivore, folivore-frugivore, granivore). No
pattern is detected when the thirteen single-species genera are
compared for alpha- (α: within-habitat), beta- (β:
between-habitat), or, gamma- (γ: geographic)-diversity (Pimm &
Gittleman, 1992; Jones, 1997), the overwhelming ecological dominance
of humans is unique. Four of the thirteen genera (31%) are
nocturnal, and a mix of crepuscular, arboreal, and terrestrial habits
is exhibited. Similarly, a broad range of socio-sexual structures is
represented among these primate genera, for example, “solitary”
(Mirza,
giant mouse lemur), “monogamous” (Symphalangus,
siamang), polygynous (Erythrocebus,
patas monkey), multimale-multifemale (Oreonax,
yellow-tailed wooly monkey), and “multi-level” (Theropithecus,
gelada; humans).
Eight
of the thirteen species (62%) are typically found in one habitat type
or demonstrate a strong preference for same. The remaining taxa,
including, humans, have been observed in several habitat types,
making them good candidates for a number of comparative analyses
(genomics, physiology, and behavior, as well as, population,
community, and ecosystem ecology), . Significantly,
cooperatively-breeding primates are not represented among the subset
of thirteen (but, see, Allocebus,
hairy-eared dwarf lemur). On the other hand, several genera, are
distinguished by elaborate vocal repertoires (e.g., Lemur,
ring-tail lemur; siamang; Homo),
and all have one or more exaggerated anatomical or morphological
features (e.g., pelage, coloration, genital structures), suggesting
evolution by sexual selection, a controversial mechanism of
speciation (“macroevolution”: Servedio & Kopp, 2012).
Insufficient empirical data exist on the relative significance of
historical geographical barriers to gene flow that might have
facilitated the speciation process (Jones, 1987; Groves, 2001) or of
the roles played by habitat specificity (“habitat selection”:
Jones, 1997; but, see Erythrocebus)
in limiting a genus to a single species, a condition obscuring
patterns that may exist in Nature.
In
the present paper, humans are highlighted in an attempt to identify
both general and specific features constraining differentiation of
their populations into interbreeding, reproductively-isolated units
(“the biological species concept”: see Rundle & Boughman,
2010). Such analyses may contribute to our understanding of Homo
sapiens
as a “weedy”, invasive species, the most geographically and
ecologically successful taxon among terrestrial vertebrates. Though
many aspects of human biology are relatively well-known, the capacity
of technological societies to maintain high population densities
(high α-diversity), to successfully invade virtually all global
habitats (high β-diversity), to modify their areal ranges (high
γ-diversity), to utilize effective mechanisms of niche invasion and
expansion (e.g., cooperation, social learning, fire, tools,
migration, war), and to impose profound, deleterious effects on
global biogeochemistry demand systematic treatments of hominin
ecology, phylogeny, and evolution (Hill et
al.,
2011). Herein, a tentative attempt is made to identify selected
human characteristics associated with interruption, delay, or
prevention of reproductive (genetic) barriers (e.g., incompatible
habitats, “isolation by distance”, pre- or post-copulation mate
selection, or geographic barriers such as rivers, mountains, and soil
gradients) sufficient to transition from between-population gene
exchange, to (genetic) differentiation of populations (“population
divergence”), to the creation of genetic barriers and a completed
process of speciation. Behavior and social organization are likely
to interest a significant proportion of this journal’s readers.
Thus, the present discussion emphasizes phenotypic diversity and
population structure, as well as, learning to explain the systematic
status of Homo
sapiens.
This paper introduces a novel interpretation and application of the
single-species status of extant Homo
inferred from Frank’s (2013) treatment of the mechanisms
“smoothing” a “rugged” fitness landscape. Questions
regarding the nature of sub-species or racial identities in Homo
sapiens
are referred to Anthropologists.
Genetic
differentiation within and between human populations: incipient
speciation?
Genetic
differentiation and, possibly, incipient speciation of human
populations have been documented. Numerous studies exist identifying
clusters (“neighborhoods”) of “single-nucleotide polymorphisms”
(SNPs) in human populations, a pattern of results suggesting a past,
possibly, continuing, process of adaptation to local abiotic (e.g.,
soil gradient) or biotic (e.g., plant gradient) regimes (“local
adaptation”), a phenomenon similar to “habitat selection”. For
example, Xing et
al.
(2009; also, see ISWG, 2001) identified “shared [genetic]
variation” among 27 human populations in Africa, Asia, and Europe,
including, “caste and tribal samples” in India, demonstrating a
degree of genetic continuity across geographical regions. Further
statistical analyses of “SNP microarrays” (“haplotypes”:
closely-associated alleles on one chromosome), however, revealed
genetic structure between sampled sites, and notably, most individual
subjects were accurately assigned to the correct population. All
individuals were accurately mapped to continents, though genetic
structure was not detected for some “closely-related populations”.
Xing et
al.
(2009) concluded that their results confirmed a statistically
significant association between geography and genetics, including
social sub-groups (“caste and tribal” sub-populations). Despite
the strong patterns revealed by the previous study, it is important
to note that the authors’ findings pertain to differences in
genetic structure within and between populations, and do not specify
the functions (genotypes expressed as phenotypes) of those
discernible genotypes.
What
mechanisms might determine genetic structuring and differentiation of
human populations?
Fowler
et al. (2011; also, see Henry et
al.,
2011; Brent et
al.,
2013) considered “genetic stratification” within and between
human populations to be a function of mate selectivity or kin
preferences. These authors investigated whether or not variations in
specific genes were associated with social networks of “friends”,
where friendship was defined as “stable, non-reproductive
[non-sexual] unions”. Using microarray analyses, Fowler et
al.
(2011) demonstrated that one allele, DRD2, was associated with
homophily (assortment of similar types), while, another allele,
CYP2A6, was associated with heterophily (assortment of different
types). The aforementioned study assessed virtually every possible
interpretation and implication of the report, concluding, that
“phenotypic similarities between individuals connected in a social
network are reflected in their genotypes”. This hypothetical
construct, derived from empirical data, advanced the idea that some
social traits are correlated with genotype, an association requiring
some direct or indirect mechanism of individual recognition. A
straightforward extension of the Fowler et
al.
(2011; also, see Fu et
al.,
2012) report is that, where (genetically-correlated) homophily recurs
over time, reproductive isolation of similar genotypes is expected to
occur, that, left unimpeded, has potential to induce barriers to gene
flow decreasing likelihoods of genetic “mixing” within and
between populations. The latter scenario proposes a necessary,
though not sufficient, condition for speciation to occur. The
present paper addresses some of the behavioral mechanisms and
processes limiting reproductive isolation and preventing speciation
in Homo
sapiens,
emphasizing the ways in which human technology and other innovations
(e.g., tools, fire, language, ritualized warfare) have ameliorated
the potentially disruptive effects of “rugged” landscapes that
might enhance a process leading to speciation.
The
aforementioned extension of the research reported by Fowler et
al.
(2011) provides a plausible explanation for the latter authors’
findings as well as for the findings of Xing et
al.
(2009). The extension is amenable to quantitative (“individual”-
or “agent-based”) modeling as well as empirical testing with
opportunistic, “natural experiments” of concurrent mate
choice/genotype trait analyses using human subjects in natural
conditions. The “green beard effect” is a possible candidate as
a sexually-selected mechanism of homophily, including,
interindividual recognition (Brooks & Griffith 2010; Gardner &
West, 2010), possibly an element of a primate social “toolkit”.
For example, suggesting a mechanism for a “greenbeard effect”,
Mahajan et
al.
(2011) identified “inter-group bias” (homophily) in Rhesus
macaques (Macaca
mulatta).
These monkeys, residing in semi-natural conditions, discriminated
between in-group and out-group members, demonstrating a reliable
choice for particular individuals in their social groups.
Interindividual
recognition of the sort reported by Mahajan et
al.
(2011) probably characterizes all primates whose brains categorize
and compartmentalize information into simpler units (Sporns, 2011).
Thus, it is no surprise that environmental patterns are classified
systematically by a variety of rules, including similarity,
proximity, or other assortative features (e.g., psychophysical
operations: Matsuno & Fujita, 2009). Recent work by Yun et
al.
(2012) demonstrates another possible “green beard”
(interindividual recognition) mechanism: synchrony of motor patterns
between interacting individuals (e.g., gestures: Pollick &
DeWaal, 2007; also, see Brooks & Griffith, 2010, Nagasaka et
al.
2013). “Greenbeard” traits may be genetically correlated, and
the latter in addition to other features (e.g., skin color,
morphology) may have facilitated speciation in one genus (Macaca),
but interrupted the process in humans, depending upon differential
genotype x environment and phenotype x phenotype interactions..
For
example, human groups may be more permeable than non-human primate
groups, or humans may use a broader range of characters when making
decisions about who to associate with. Furthermore, on average,
humans may receive greater benefits from associating with different
types compared to speciose primate genera. The latter case might be
expected where intra-group competition is more intense than
inter-group competition (West et
al.,
2002). Peculiar features of our species, then, may have broadened
the areal effect of an individual’s reproductive success in
“rugged” landscapes (“fitness landscape”), and phenotypes
bearing these features are proposed to have directly or indirectly
promoted gene flow within- and between-groups, -populations, and
-regions limiting the potential for population divergence,
reproductive isolation, and speciation. Other primate genera
characterized by a single species are presumed to exhibit traits that
spread because of their success in managing thresholds of intra-group
competition, subsequently decreasing the likelihood of speciation
events by facilitating gene flow, preventing reproductive isolation.
Notwithstanding
evidence for clustering of genotypes within and between populations,
human behavioral diversity appears to enhance gene flow
Using
Frank’s (2011) theoretical framework, I posit that numerous
genetically correlated or uncorrelated behavioral and social traits
characteristic of human phenotypes mediated genotype-environment and
phenotype-phenotype interactions (“reaction norms”). Human
technological and other innovations (e.g., language, metacognition)
are proposed to have increased the proportional area on an idealized
(theoretical, multidimensional: Frank, 2011) or realized (a
3-dimensional abiotic and biotic environment: this paper) “landscape”
upon which a genotype, expressed as a phenotype, is more
reproductively successful relative to the mean lifetime reproductive
success of other genotypes in a population. This perspective can be
visualized by imagining a grid superimposed on a space subdivided
into areas defined by shared features (e.g., a habitat, a watering
hole, a grove of fruiting trees, other singular or clumped
resources).
Frank’s
(2011) treatment allows us to conceptualize a landscape on which
reproductively successful phenotypic innovations generated and spread
by mutation, developmental plasticity, or learning increased the
proportion of cells on the grid upon which a phenotype is effectively
successful. In other words, an individual’s “fitness landscape”
will be, proportionally, increased relative to the mean fitness of
others in a population not exhibiting the successful traits. In
Frank’s (2011) terminology, the aforementioned process is a
“smoothing” operation reflecting a phenotype’s capacities to
decrease stressful environmental events where degrees of stress can
be conceptualized as the extent to which the landscape approximates a
very rugged (challenging) or a relatively even (less challenging)
space in which to survive and reproduce.
Frank’s
(2011) treatment suggests that phenotypic diversity will be induced
by novel (e.g., disappearance of a limiting resource) or extreme
(e.g., severe drought) environmental events and that responses may be
genetic (mutation), cellular (physiological and developmental), or
learned (by trial-and-error or by “Hebbian” association).
Applied to humans, the present treatment posits that characteristics
such as cooperation, tool use, the application of fire for processing
food, the manufacture of clothing, language, long-distance dispersal,
social learning, and the like, effectively switched an environment
(“landscape”) from a stressful (difficult, dangerous, risky,
extreme, novel), “rugged” one, to a less stressful, more even, or
“smoother” one. Reproductively successful innovative human
phenotypes, it is proposed, extended networks within- and
between-groups and –populations, connecting networks to one or more
resource patches, including, other human individuals and groups,
thereby, broadening the effective spaces of phenotypes, decreasing
deleterious consequences of environmental challenges for (relative)
individual reproductive rates, growth rates of groups, and mean
fitness of populations..
Traits
characteristic of non-human primates and humans interrupt or prevent
population divergence
Empirical
examples drawn from the primate literature characterize Frank’s
(2011) concept of mechanisms functioning to “smooth” a
challenging (“rugged”) landscape. Analyzing species distribution
patterns of black howler monkeys (Alouatta
pigra)
and Central American spider monkeys (Ateles
geoffroyi)
in Belize, Jones & Jost (2007) showed that black howlers, but not
spider monkeys, had successfully traversed the Mayan
Mountains/Cockscombe Range in southern Belize. Howler monkeys are
adapted to a folivorous diet, an evenly distributed supply of food
compared to fruit upon which spider monkeys are heavily dependent.
As a consequence of the heterogeneous and often unpredictable
availability of their food supplies, Ateles
is expected to be more sensitive to environmental perpurbations
(Terborgh & Winter, 1980). The ability to consume old leaves is
thought to facilitate colonization (Jones & Jost, 2007),
providing a relatively accessible food resource in most habitats,
allowing flexible “switching” from howlers’ preferred diet (new
leaves, flowers, fluit) to less nutritious and physiologically
stressful foods (mature leaves) during periods when favored food
items are unavailable or scarce (Milton, 1980; Crockett, 1998;
Hamilton, 2010).
On
the other hand, a diet of fruit presents many challenges because of
its low nutritional value and patchy distribution (Terborgh &
Winter, 1980; Fleming et
al.,
1987), factors that may limit or retard the geographical spread of
species if appropriate food types or habitats are not encountered.
This comparison demonstrates one behavioral mechanism, enhanced niche
width, whereby the configuration of landscapes is modified by
spatiotemporal effects. The capacity to process old leaves
facilitated construction of a comparatively “smooth” landscape
for the widely distributed, speciose, hardy genus, Alouatta.
Another “smoothing” effect occasioned by a folivorous diet may
be reduction of costs from predation, since toxins ingested from
leaves may decrease the palatability of howler tissues, a hypothesis
supported by one study’s findings that human hunters considered
spider monkeys (frugivores) a tastier meat than that of howlers
(Jones & Jost, 2007). Differential attractiveness, then, may
“smooth” prey landscapes while increasing the ruggedness of
predators’. However, the speciose genus, Alouatta,
is considered to have differentiated via
a process of dietary and geographical partitioning, or, possibly,
hybridization (Bicca-Marques et
al.,
2008). Human adaptations, combined with learning capacities,
including cultural exchange, presumably avoided many dietary
challenges (e.g., fire, tools, weapons), outweighing deleterious
effects, including, tradeoffs, that might have been associated with
the innovations (e.g., increased inter-group competition).
Concepts
advanced by Frank (2011) are implicit in field research conducted in
Mexico by Chaves and his colleagues (2012; also, see Scherbaum &
Estrada, 2013). These authors studied Ateles
geoffroyi
in two conditions of rainforest habitat, continuous canopy and
fragmented patches, in order to compare and contrast utilization of
available food resources. Consistent with expectation, niche width
of monkeys inhabiting fragmented forest was wider than that for
monkeys in undisturbed forest, including a higher proportion of
leaves. Chaves et
al.
(2012, pp 109-111) concluded, “It is unlikely that [small fragment
size] can maintain viable populations in the long term, they may
function as stepping-stones
[italics added], facilitating inter-fragment movements and,
ultimately, enhancing seed dispersal in fragmented landscapes.”
Combined, where necessary, with descent from trees and ground
movement, increased niche breadth enhances the behavioral repertoire
of spider monkeys, facilitating “initial survival of a genotype in
response to novel or extreme environmental challenges, providing an
opportunity for subsequent adaptation.” (Frank, 2011, pp
2318-2319). Additionally, variations in other non-human primate
traits may function to “smooth” landscapes in feeding and
foraging contexts, for example, body size (Wheatley, 1982),
“time-energy [“fitness”] budgets” (Grueter et
al.,
2012), “decision and choice” (Scherbaum & Estrada 2012),
social behavior among females (Hanya et
al.,
2008), “co-residence patterns” and other hunter-gatherer features
(Hill et
al.,
2011), “egalitarian” and other prosocial tendencies (Gavrilets,
2012).
The
previous paragraphs in this section presage human habits serving
similar functions. Jones & Young (2004), for example, surveyed
hunters in Belize, demonstrating that, among non-volant terrestrial
or semi-terrestrial vertebrates, niche width varied with food
availability, implying an opportunistic (“utilitarian”) strategy
based on a hierarchy of preferences. Thirty-four hunters ranked
their favorite prey, yielding eight vertebrate species, with paca
(Agouti
paca)
reported to be the most favored bushmeat, “hicatee”, the Central
American river turtle (Dermatemys
mawii),
the least. Prey characteristics (predominantly medium-sized,
crepuscular or diurnal, and terrestrial) suggested that energetic
factors influenced hunting behavior by Creole men at this site,
possibly influenced by gustatory preferences, as suggested above.
Indeed, paca’s rich, non-“gamey”-tasting flesh, is considered a
national delicacy. Hunting practices of indigenous Belizeans are
strongly influenced by cultural practices, in addition to economic
ones (Jones & Young, 2004; also, see Wilkie & Godoy, 2001),
consistent with Frank’s (2011) emphasis on phenotypic variation
(e.g., niche breadth) and learning (e.g., imitation, observational
learning, cultural rules) as factors “smoothing a fitness landscape
with multiple peaks and valleys”. Combined with spatial
“concentration and dispersion” of human populations facilitating
the evolution of multilevel population structure, phenotypic
diversity in humans broadens a phenotype’s success in a given
landscape, while, concurrently, increasing gene flow between
populations, effects limiting the potential for population divergence
and reproductive isolation.
Humans
benefit from phenotypic diversity and learning
Following
Frank’s (2011) conceptual framework, the present article posits
that numerous traits characterizing Homo
sapiens served
to decrease environmental challenges deleterious to lifetime
reproductive success of individuals. These technological and other
innovations, once spread through groups, populations, and regions via
sex and social learning increased social and breeding networks,
mitigating environmental and social challenges. Tanaka’s (1976)
studies of the ≠Kade San (“bushmen”), hunter-gatherers in the
Kalahari (southern African desert) clearly demonstrate ways in which
a cultural innovation limits mortality and, by extension, enhances
reproductive success. The ≠Kade San, comprised of mobile and
mobile-subsistence units, inhabit a “marginal” environment
characterized by drought (Tanaka, 1976, Fig. 4.1, p 105) and seasonal
patterns of food availability (Tanaka, 1976, Fig. 4.2, p 108), a
spatiotemporal regime not unlike the heterogeneous environments in
which humans are thought to have evolved (Hill et al., 2011). On one
occasion, Tanaka (1976) observed chacma baboons (Papio
ursinus)
foraging in the Kalahari, noting that this primate’s home range was
limited by their inability to cross arid land. This researcher
compared the monkeys’ habits with those of the ≠Kade San, capable
of inhabiting the extreme desert environment as a result of digging
through the soil surface to locate and utilize the limiting resource.
This cultural practice permits a “band” to expand inherent
capacities, “smoothing” effects decreasing likelihoods of
sub-lethal or lethal outcomes, and increasing the likelihood of
contacts with other “bands” (see below). Such phenotypic
diversity is expected to impact individual life-histories (survival
and mortality), enhancing mean fitness of populations via increased
reproductive rates (Frank, 2011), with consequent effects on higher
levels of ecological organization (communities, ecosystems, biomes).
“Bands”
of “bushmen” from a variety of cultural groups share the desert
environment, sometimes interacting with one another (cf. Lee, 1976,
Map 3.2, p 85; Map 3.3, p 87; Map 3.4, p 93; also, see Tanaka, 1976;
Hill et
al.,
2011). These flexible land-use patterns (“spatial organization”),
limited by availability of water, are one component of a “rugged
landscape”, ensuring relatively frequent contact with other
cultural groups. As Tanaka’s (1976; also, see Lee’s chapter in
the same volume) chapter highlights, fluid patterns of interaction
increase potential for conflicts which the bands prevent or resolve
via cultural innovations such as reciprocity, cooperation, common
ceremonies, and the like, minimizing conflict and aggression,
permitting shared access to resources, cooperative manufacture of
tools and weapons, and overlapping ranges. Though Tanaka’s (1976)
report does not address the nature of intimate relations among
“bands” (see Lee, 1976), transfer of individuals between groups
and opportunities for sexual congress probably occurred, leading to
gene flow sufficient to prevent reproductive isolation and speciation
events. This scenario is consistent with the interpretations of
hunter-gatherer data reviewed by Hill et
al.
(2011).
The
evolution of human prosocial behaviors and constraints on speciation
Two
recent papers provided a detailed empirical review of “co-resident
patterns in hunter-gatherer societies” (Hill et
al.,
2011) and a preliminary quantitative (mathematical) treatment of “the
egalitarian syndrome” characterizing Homo
sapiens
(Gavrilets, 2012; see Crook, 1971). Hill et
al.
(2011) analyzed datasets for 32 extant hunter-gatherer societies with
a mean “band” size of 28.2 individuals. These authors documented
a profile including bisexual dispersal from natal groups, similar to
other apes and Neotropical Atelines. Though opposite-sex [adult]
siblings resided, with some frequency, in the same reproductive unit,
group membership comprising non-kin prevailed across “bands”.
Patterns of kinship and group architecture resulting from dispersal,
resulted in nested networks of relatives and non-relatives from
“bands” embedded in local (“patch”) contexts to higher levels
of sociosexual organization. These “multilevel” (“hierarchical”)
societies exhibited relatively “open” structures, permitting
selective immigration and emigration, and have been described for
other mammalian taxa (e.g., some cetaceans, elephants, geladas;
Hamadryas baboons, Papio
hamadryas).
In
“hierarchical” and other complex societies, problems associated
with temporal and spatial coordination and control must be managed,
and the theoretical literature on “scheduling” indicates that
such challenges are solved via within- and between-group “queuing”
(Andrews, 2004; also, see Alberts et
al.,
2003; Fruteau et
al.,
2013). Within- and between-levels, hunter-gatherers exhibit a broad
array of mechanisms, effectively, (1) increasing the similarity of
shared fitness optima (“fitness-sharing”: Sareni &
Krähenbühl, 1998) and (2) decreasing asymmetries (“egalitarian
syndrome”: Gavrilets, 2012). Hill et
al.
(2011), and most other students of human behavior and social
organization (e.g., Crook, 1971; 1972; Eibl-Eibesfeldt, 1989; West et
al.,
2006), characterize these mechanisms as one or another manifestation
of “cooperation” (and/or collaboration). However, despite the
benefits provided by cooperation, queuing, and similar features in
many conditions, limits on “prosocial” behavior in humans must,
also be addressed (Jones, 2005a, b; Burton-West et
al.,
2006; Chellew & West, 2013).
The
two aforementioned mechanisms are consistent with Frank’s (2011)
“smoothing” paradigm, operating to “solve” environmental
challenges, to repress selfishness and competition, to enhance access
to resources, and to decrease inter-individual and inter-group
conflicts. In these instances, social traits benefiting a
conspecific’s fitness are posited to limit morbidity and mortality,
as well as to enhance relative reproductive rates compared to
benefits that might accrue from alternative, selfish interactions
(e.g., “non-damaging” and “damaging” aggression). Discussing
hunter-gatherer “spatial organization”, Lee (1976) employed maps
to show how patterns of “concentration and dispersion” promote
inter-unit cooperation (“reciprocal access to resources”),
flexible access to abundant and scarce resources via
communication networks, and conflict-management via
“social” separation. Lee (1976) found that “concentration and
dispersion” increased unit size, on average, an effect that he
showed was correlated with higher rates of population increase.
Clustering
of “bands” at “patchy” sources of water and food may have
induced social competition, leading to social selection favoring the
evolution of collaboration, cooperation and behavioral diversity
(e.g., social learning, imitation, tool use). Increased
inter-individual contact with associated gene flow would be a
byproduct of this model, discussed using primate examples, by Crook
(1971, 1972; also, see Lee, 1976; Tanaka, 1976; Yellin, 1976). As a
result, likelihoods of gene flow between reproductive units (“bands”)
would increase, decreasing rates of population divergence and
opportunities for speciation events. The fitness strategies
discussed in this paragraph constitute adaptive mechanisms responding
to environmental challenges, transforming a rugged landscape to a
smoother one, enhancing lifetime reproductive success of individuals.
Interpretations of the literature advanced in this article are
testable empirically and quantitatively, and initial agent-based
treatments might be conducted employing the data presented in Hill et
al.
(2011). It would also be beneficial to compare populations and
regions exhibiting high, moderate, and low degrees of genetic
differentiation in an attempt to discern similarities and differences
among humans and their networks in each condition. For instance, is
network strength greater or lesser across these conditions, and do
these conditions and their features correlate with measures of
success (e.g., income, education, rules governing immigration and
emigration).
DISCUSSION
Frank’s
(2011) treatment of the ways in which phenotypic diversity and
phenotypic novelty serve individual interests by facilitating
lifetime reproductive success provides a schema that can be applied
to most human tactics and strategies. In particular, the model
permits researchers to evaluate the extent to which human responses
to environmental challenges promote problem-solving in a variety of
ways. The mechanisms addressed herein, as well as other responses
not discussed (altruism, spite, role-reversal, facultative
division-of-labor), are expected to facilitate the individual’s
avoidance, circumvention, delay, or confrontation with challenges
sufficiently severe, risky, rare, or difficult to compromise lifetime
reproductive success, including, the effects of morbidity and
mortality. Mortality records for extant hunter-gatherers require
quantitative treatments since humans are iteroparous breeders with a
typical litter-size of one, characteristics associated with
predictable environments in which adult survivorship is uncertain
(Stearns, 1982; Millar & Zammuto, 1983). Breeding positions of
individuals in mammal groups with the aforementioned characteristics
are generally precarious (Millar & Zammuto, 1983), and the
diverse phenotypic adaptations and novelties reviewed herein may
increase environmental predictability by increasing individuals’
abilities to cope with stressors.
Following
Hill (1976), humans appear to combine iteroparity with a high
fertility rate and notably high “reproductive effort”. This
combination of traits is not usually associated with mammals in
heterogeneous (“rugged”) regimes (Millar & Zammuto, 1983).
Similarly, most mammals are poor colonizers, and social mammals are
generally constrained by their dependence upon conspecifics and group
life (Cody, 1986), challenges that humans have overcome via the
“concentration and dispersion” spatiotemporal patterns and
multilevel societies described by Lee (1976), Tanaka (1976), Yellin
(1976), and others (Hill et
al.
1976), in combination with rule-governed repression of selfish
behavior (“culture”). Investigating patterns of juvenile and
female mortality should reveal relative survivorship, indicating
whether or not “bet-hedging” strategies were featured among early
Homo.
This information, once modeled, may expose in greater detail
thresholds of reproductive benefits that may have accrued to humans
from responses designed to solve problems presented in lethal or
sub-lethal regimes, mechanisms with byproducts decreasing likelihoods
of reproductive isolation and the potential for speciation. Finally,
students of mammalian taxa exhibiting noteworthy phenotypic diversity
(e.g., mammals exhibiting multilevel social organization) must bear
in mind that “plastic” traits will not yield the highest relative
fitness in many regimes (Jones, 2005a, 2005b; Pigliucci, 2010, Frank,
2011, pp 2312-2313). Thus, differential reproductive costs and
benefits of genotype x environment interactions require systematic
investigation for the human case.
ACKNOWLEDGMENTS:
I
am grateful to Steven A. Frank for commenting on an earlier version
of this paper. Jesse Marczyk’s extensive critique significantly
improved the manuscript.
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Citation:
Jones CB. 2013. Constraints On Speciation In Human Populations:
Phenotypic Diversity Matters. Hum Bio Rev, 2 (3), 263-279.
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