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Tall or short, thick or thin what affects limb size?

For over 60 years, scientists have theorized that a person's body shape and size can be influenced by the climate where they live. Now a new study from the University of Tennessee, Knoxville, suggests there may be more to the equation.

The paper, co-authored by Kristen Savell, a third-year doctoral student in the Department of Anthropology at the University of Tennessee (UT), is among the first to document how evolutionary selection has shaped variation in human limbs around the globe.

The study was published this week in the Proceedings of the National Academy of Sciences. Savell co-authored the work with Benjamin Auerbach, University of Tennessee (UT) Associate Professor of Anthropology, and Charles Roseman, Associate Professor of Anthropology at the University of Illinois at Urbana-Champaign.

The study indicates that long bones of the arms and legs do not evolve independently as scientists had long assumed. But,adds Kristen Savell,limbs "are all evolving together in ways we didn't necessarily understand."

Since the mid-to-late 1900s, scientists have speculated that humans have adapted to their immediate environment based on geographic latitude — the distance of a place north or south of the earth's equator, typically measured in degrees and minutes — in order to maximize the heat loss efficiency of the body. Or more simply, people in warmer climates tend to have longer limbs and thinner bodies, while those in colder climates have shorter limbs and wider bodies.

(A) Allen's Rule: The principle is that in warm-blooded animal species having distinct geographic populations, the limbs, ears, and other appendages of these animals living in cold climates tend to be shorter than in animals of the same species living in warm climates.

(B) Bergmann's Rule: an eco-geographical rule stating that within a broadly distributed taxonomic group, populations and species of larger size are found in colder environments, and species of smaller size are found in warmer regions.

Now, researchers have more intensely studied the relationship between body proportions and latitude — in four geographic regions: sub-Saharan Africa, North Africa, Temperate Europe and the Arctic.

Savell and colleagues, using the same data, compared variation in lengths of four bones: (1) humerus (upper arm), (2) radius (forearm), (3) femur (upper leg) and (4) tibia (lower leg), as well as the diameter of the femur (the highest part of the thigh bone) and pelvic width, among 14 human populations, and including more than 400 individuals.

They found that the forearm (radius) and lower leg (tibia), as well as overall body size, all appear to evolve in direct response to natural selection as predicted — becoming shorter at higher latitudes. However, the growth of the arm (humerus) and thigh (femur), are influenced by more than just natural selection. Femurs do not change length in response to natural selection — at all.

Savell and colleagues found that natural selection alone would drive the humerus to lengthen at higher latitudes within colder climates. This is contrary to Allen's and Bergmann's rules, which suggest natural selection would push the arm to shorten with higher latitudes. Savell and colleagues found that because of genetic links between the tibia and radius — which shorten in higher latitudes — the humerus also shortens.

The study supports that the interaction of traits through genetic connections, influences human shape and size in response to natural selection as dictated by environment.

Savell: "Even when evolutionary patterns match expectations, the processes underlying them might not. It is important for all biologists and biological anthropologists to be aware how different measurements co-vary with each other." Savell noted that she and her collaborators are referring to changes in limb bone length on an evolutionary scale, so their data doesn't apply to differences between people that occurs in an individual lifetime.

"What we're looking at is how adaptation to latitude, often interpreted as a substitute for climate, has affected limb proportions over many, many, many generations."

Kristen Savell, third-year doctoral student, Department of Anthropology, University of Tennessee, USA

Human morphological variation is thought to have been partially shaped by natural selection associated with environmental factors like climate. Patterns of variation in body form correspond with latitude, but evolutionary processes that yielded this variation are not yet established. Examining the traits used in these studies (e.g., limb lengths) independently ignores their genetic covariation, which affects their responses to evolutionary forces. To address this relationship, we estimated the directional selection necessary to evolve correlated traits reflecting body shape across latitudes and examined trait-specific responses. Although most traits appear to be under directional selection, their response is constrained by between-trait covariance. This finding suggests that trait differences among human groups may not directly reflect the forces of selection that shaped them.

Variation in body form among human groups is structured by a blend of natural selection driven by local climatic conditions and random genetic drift. However, attempts to test ecogeographic hypotheses have not distinguished between adaptive traits (i.e., those that evolved as a result of selection) and those that evolved as a correlated response to selection on other traits (i.e., nonadaptive traits), complicating our understanding of the relationship between climate and morphological distinctions among populations. Here, we use evolutionary quantitative methods to test if traits previously identified as supporting ecogeographic hypotheses were actually adaptive by estimating the force of selection on individual traits needed to drive among-group differentiation. Our results show that not all associations between trait means and latitude were caused by selection acting directly on each individual trait. Although radial and tibial length and biiliac and femoral head breadth show signs of responses to directional selection matching ecogeographic hypotheses, the femur was subject to little or no directional selection despite having shorter values by latitude. Additionally, in contradiction to ecogeographic hypotheses, the humerus was under directional selection for longer values by latitude. Responses to directional selection in the tibia and radius induced a nonadaptive correlated response in the humerus that overwhelmed its own trait-specific response to selection. This result emphasizes that mean differences between groups are not good indicators of which traits are adaptations in the absence of information about covariation among characteristics.
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Aug 10, 2016   Fetal Timeline   Maternal Timeline   News   News Archive   

Leonardo DaVinci

In 1510, Leonardo DaVinci, now more scientist than artist, completes his study of the human body.
Image Credit: The Royal Collection



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