Local extinction and recolonization, species effective population size, and modern human origins. 2004

Between-Group Competition Impacts Reproductive Success in Wild Chimpanzees

Between-group competition in social animals appears to be a prominent selective pressure shaping the evolution of territoriality and cooperation [1-4]. Evidence for an effect of between-group competition on fitness in territorial species, however, is mostly lacking because of difficulty in measuring between-group competition and its long-term impact [5]. Between-group competition corresponds to a complex set of interactions between neighboring groups, and its intensity seems to depend on the competitive abilities of each interacting group [6, 7]. We tested whether the competitive ability of groups and the pressure exerted by neighboring groups affected the reproductive success of wild female chimpanzees (Pan troglodytes verus). Using long-term data on four neighboring groups in the Taï National Park, Côte d'Ivoire, collected over the course of 54 observation years, we measured the competitive ability of habituated groups using the number of mature males and the pressure exerted by non-habituated neighbors with an index of neighbor pressure that combined the frequency of neighboring encounters and related spatial information. Importantly, we found that experiencing low neighbor pressure provides fitness benefits through increased offspring survival and shorter inter-birth intervals. Also, many males in a group are associated with shorter inter-birth intervals. We conclude that high between-group competition hampers fast reproduction and offspring survival when exposure is during the prenatal period. Our findings suggest that having many males in a group results in fitness benefits and that between-group competition should be considered as a potential selective pressure that shaped key social adaptations in the hominoid lineage.

Minimum founding populations for the first peopling of Sahul

The timing, context and nature of the first people to enter Sahul is still poorly understood owing to a fragmented archaeological record. However, quantifying the plausible demographic context of this founding population is essential to determine how and why the initial peopling of Sahul occurred. We developed a stochastic, age-structured model using demographic rates from hunter-gatherer societies, and relative carrying capacity hindcasted with LOVECLIM's net primary productivity for northern Sahul. We projected these populations to determine the resilience and minimum sizes required to avoid extinction. A census founding population of between 1,300 and 1,550 individuals was necessary to maintain a quasi-extinction threshold of ≲0.1. This minimum founding population could have arrived at a single point in time, or through multiple voyages of ≥130 people over ~700-900 years. This result shows that substantial population amalgamation in Sunda and Wallacea in Marine Isotope Stages 3-4 provided the conditions for the successful, large-scale and probably planned peopling of Sahul.

A parsimonious neutral model suggests Neanderthal replacement was determined by migration and random species drift

Most hypotheses in the heated debate about the Neanderthals’ replacement by modern humans highlight the role of environmental pressures or attribute the Neanderthals’ demise to competition with modern humans, who occupied the same ecological niche. The latter assume that modern humans benefited from some selective advantage over Neanderthals, which led to the their extinction. Here we show that a scenario of migration and selectively neutral species drift predicts the Neanderthals’ replacement. Our model offers a parsimonious alternative to those that invoke external factors or selective advantage, and represents a null hypothesis for assessing such alternatives. For a wide range of parameters, this hypothesis cannot be rejected. Moreover, we suggest that although selection and environmental factors may or may not have played a role in the inter-species dynamics of Neanderthals and modern humans, the eventual replacement of the Neanderthals was determined by the repeated migration of modern humans from Africa into Eurasia.

The replacement of Neanderthals by modern humans is thought to have been due to environmental factors, a selective advantage of modern humans, or both. Here, Kolodny and Feldman develop a neutral model of species drift showing that rapid Neanderthal replacement can be explained parsimoniously by simple migration dynamics.

Motion sickness: more than nausea and vomiting

Motion sickness is a complex syndrome that includes many features besides nausea and vomiting. This review describes some of these factors and points out that under normal circumstances, many cases of motion sickness go unrecognized. Motion sickness can occur during exposure to physical motion, visual motion, and virtual motion, and only those without a functioning vestibular system are fully immune. The range of vulnerability in the normal population varies about 10,000 to 1. Sleep deprivation can also enhance susceptibility. Systematic studies conducted in parabolic flight have identified velocity storage of semicircular canal signals-velocity integration-as being a key factor in both space motion sickness and terrestrial motion sickness. Adaptation procedures that have been developed to increase resistance to motion sickness reduce this time constant. A fully adequate theory of motion sickness is not presently available. Limitations of two popular theories, the evolutionary and the ecological, are described. A sensory conflict theory can explain many but not all aspects of motion sickness elicitation. However, extending the theory to include conflicts related to visceral afferent feedback elicited by voluntary and passive body motion greatly expands its explanatory range. Future goals should include determining why some conflicts are provocative and others are not but instead lead to perceptual reinterpretations of ongoing body motion. The contribution of visceral afferents in relation to vestibular and cerebellar signals in evoking sickness also deserves further exploration. Substantial progress is being made in identifying the physiological mechanisms underlying the evocation of nausea, vomiting, and anxiety, and a comprehensive understanding of motion sickness may soon be attainable. Adequate anti-motion sickness drugs without adverse side effects are not yet available.

If Body Fatness is Under Physiological Regulation, Then How Come We Have an Obesity Epidemic?

Life involves a continuous use of energy, but food intake, which supplies that energy, is episodic. Feeding is switched on and off by a complex array of predominantly gut-derived peptides (and potentially nutrients) that initiate and terminate feeding bouts. Energy is stored as glucose and glycogen to overcome the problem of the episodic nature of intake compared with the continuous demand. Intake is also adjusted to meet immediate changes in demands. Most animals also store energy as fat. In some cases, this serves the purpose of storing energy in anticipation of a known future shortfall (e.g., hibernation, migration, or reproduction). Other animals, however, store fat in the absence of such anticipated needs, and in this case the fat appears to be stored in preparation for unpredictable catastrophic shortfalls in supply. Fat storage, however, brings disadvantages as well as advantages, in particular an increased risk of predation. Hence, many animals seem to have evolved a dual intervention point system preventing them from storing too little or too much fat. The physiological basis of the lower intervention point is well established, but the upper intervention point is much less studied. Human obesity can potentially be understood in an evolutionary context as due to drift in the upper intervention point following release from predation 2 million years ago (the drifty gene hypothesis) combined with a stimulus in modern society to overconsume calories, possibly attempting to satisfy intake of a limiting micro- or macro-nutrient like protein (the protein leverage hypothesis).

Evolutionary perspectives on the obesity epidemic: adaptive, maladaptive, and neutral viewpoints

The prevalence of obesity in modern societies has two major contributory factors-an environmental change that has happened in historical times and a genetic predisposition that has its origins in our evolutionary history. Understanding both aspects is complex. From an evolutionary perspective, three different types of explanation have been proposed. The first is that obesity was once adaptive and enabled us to survive (or sustain fecundity) through periods of famine. People carrying so-called thrifty genes that enabled the efficient storage of energy as fat between famines would be at a selective advantage. In the modern world, however, people who have inherited these genes deposit fat in preparation for a famine that never comes, and the result is widespread obesity. The key problem with this, and any other adaptive scenario, is to understand why, if obesity was historically so advantageous, many people did not inherit these thrifty genes and in modern society are able to remain slim, despite the environmental change favoring fat storage. The second type of explanation is that obesity is not adaptive and may never even have existed in our evolutionary past, but it is favored today as a maladaptive by-product of positive selection on some other trait. An example of this type of explanation is the suggestion that obesity results from variation in brown adipose tissue thermogenesis. Finally, a third class of explanation is that most mutations in the genes that predispose us to obesity are neutral and have been drifting over evolutionary time--so-called drifty genes, leading some individuals to be obesity prone and others obesity resistant. In this article, I review the current evidence for and against these three different scenarios and conclude that the thrifty gene hypothesis is untenable but the other two ideas may provide a cogent explanation of the modern obesity phenomenon.