Introduction

 

Researchers in
evolutionary medicine set out to uncover the ways in which human susceptibility
to disease has been shaped by evolution (Vining and Nunn, 2016). The emerging
discipline is revolutionising our understanding of the cause of illness and has
recently become recognised within the domain of public health. The core
principles derive from life history theory, which analyses energy allocation
between the major competing functions of growth, maintenance and reproduction, each
affecting various health outcomes. By incorporating an evolutionary perspective,
alternative conclusions of the outcomes of health and disease can be drawn, and
subsequently used to make public health interventions more effective (Wells et
al., 2017).

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To
understand how life history theory has shaped human evolution and thus use
evolutionary theory to approach the issues posed by global public health, it is
necessary for both fields of research to understand the concepts of both
adaptation and constraint (Wells et al., 2017). In simple terms, adaptation is
the evolutionary process enabling organisms to adjust to the environment and
enhance evolutionary fitness (Stearns, 1986). In order to adapt, organisms must
overcome the various constraints posed by their surroundings which limit the
production of advantageous phenotypes (Garland, 2014). This essay will argue
that whilst both concepts are necessary to understand in EMPH, adaptation is
more useful for improving the effectiveness of global health interventions. The
argument will be justified by examples routed in human anatomy including human
growth variation, the “small but healthy” hypothesis and the Predicative
Adaptive Response hypothesis.

 

Background

 

The aim of
public health is to prolong life, prevent disease and promote health through
the organized efforts of society in human populations (Winslow, 1920). Many
interventions have been designed to benefit health and traditionally efforts were
focussed on pathogen related risk factors. The prevention of disease
transmission focussed on improving nutrition and living conditions to induce
resilience, and more recently lifestyle changes to reduce the risk in the rise
of non-communicable diseases have been encouraged (WHO, 2011).

 

Evolutionary
medicine on the other hand, aims to delve further into understanding the causes
of illness to uncover why humans have become susceptible to disease. The
current state of medicine is mainly pre-evolutionary, with explanations
remaining descriptive and mechanistic and only scratching the surface in
providing explanations for the variation in disease susceptibility in
individuals and societies (Wells et al., 2017). The integration of evolutionary
biology and medicine forms the foundation of the discipline, which recognises
that medical research can significantly benefit from a priori understanding of
the concept of adaptation by natural selection and how it affects health
outcomes. Evolutionary medicine uses an adaptionist perspective to explain why
some individuals become ill in different environments, which are subject to
different constraints. (Muehlenbein, 2010).

 

So far,
public health has greatly benefitted from including an integrated perspective
of evolutionary theory. An evolutionary perspective offers new understanding
into the resulting health consequences of changing environments and behaviour
patterns. This will inform the behavioural and physiological components of
interventions that will likely improve effectiveness, for example it may be
applied to improve non-communicable disease interventions. It has been argued
that without an understanding of adaptation, public health schemes may not
achieve the aims that have been set out. This is because adaptation by nature selection
has been engineered to maximise the genetic fitness (reproductive success) of
an organism under harsh environmental conditions which may cause the hindrance
of health and render organisms more susceptible to disease (Wells et al.,
2017).  This is can be overcome using an
evolutionary approach to highlight how human physiology and behaviour has
changed in response to the stresses of changing environments. Evolutionary
theory can be applied to explore the effects of biological, societal and
physical stresses and stimuli during the life-course (Kuh & Ben-Shlomo,
2004; Krieger & Smith, 2004, Marmot, 2005) which studies have already
supported in a range of species including humans (Stearns et al., 2000;
Winterhalder, 2000; Nettle, 2013).

 

Evolutionary Constraints and Genetic Fitness

 

An
understanding of how environmental constraints shape trait heritability and
evolution is necessary in evolutionary medicine to shed light on genetic
variability to shape health outcomes and susceptibility to disease (Wells et
al., 2017). For many years, tension has existed between researchers within the
evolutionary field who highlight the significance of adaptation by natural
selection and those who emphasize the role played by developmental and
phylogenetic constraints on the development of an organism (Fitch, 2012). Darwin
and Wallace’s theory of natural selection supplied the field of evolutionary
biology with new understandings of how the constraints of historical
environments shape biological variability (Darwin, 1859). This infers that an
understanding of adaptation is more vital to researchers in EMPH than
understanding simply observable constraints. The theory suggests that traits are
varied and that this variability is heritable by chance; the greater the number
of offspring produced by an organism, the greater the frequency of their traits
in subsequent generations. Over time, the phenotypes and genes of those
reproducing the most successfully will be continued throughout the lineage (Dennett, 1995). Genetic
fitness therefore, is shaped by natural selection to improve genetic fitness
which demonstrates that an understanding of adaptation is vital to provide
explanations of genetic variability.  Researchers
in evolutionary medicine can use this knowledge to treat disease in relation to
genetic variability. For example, personalised, gene-based medicine can be
developed to treat the disease risks prompted by genetic variation between
different ethnic groups (Ono et al., 2013).  

 

Two major concerns
exist within contemporary medicine that can be addressed using an understanding
of environmental constraints shape evolution. 
Firstly, the evolution of anti-biotic resistant or drug resistant
strains of pathogens, which develop when bacteria become resistant to treatment
(due to phenotypic adaptation, transfer and expression from resistant to
susceptible organisms or genetic mutation) (Arias & Murray, 2009; Händel et al., 2013). Secondly,
the emergence of infectious diseases, for example those caused by Ebola virus,
HIV and SARS (Jones et al., 2008). Using an understanding of constraint may
shed light on why treatment resistant pathogens and new infectious diseases
have evolved, which in turn may be of vital importance for developing methods
to overcome them.

 

Developmental Constraints and Human Growth Variation

 

Developmental
constraints are perhaps the most significant factor in the overall influence of
constraint on the evolution of complex traits (Fitch et al., 2012).  They are often interpreted as limitations on
phenotypic variability or constraints on the production of phenotypic variation
(Maynard-Smith et al., 1985). The recurrent laryngeal nerve in humans, which
stretches seven lengths longer than necessary to connect the brain and the
larynx, is an example demonstrating the effects of developmental constraints on
phenotype which can be used by researchers in EMPH to shed light on the
evolution of human anatomy (Bergman, 2010).

 

International
public health organizations are expected to use an understanding of constraint
to shape health outcomes and susceptibility to disease through the influence of
trade-offs (Wells et al., 2017). Trade-offs are a concept in life history
theory which occur when one trait cannot increase in fitness without a decrease
in fitness of another trait (Garland, 2014). They are a crucial concept for
public health organisations and researchers in evolutionary medicine because
they enforce recognition on the consequences of changing one trait. For example,
a major issue presented within global public health is human growth variation
which is constrained under poor environmental conditions. Growth is limited via
energetic constraints and acts as an indicator of how resources are allocated for
optimal reproductive value (McDade, 2003). Many bio-anthropological studies of human
growth trajectories have focused on the effects of nutritional stress as the
primary influence of constraints on growth (Stulp & Barrett, 2016). In
small-scales societies, a study by Walker et al. (2006) found that trade-offs
between growth and maintenance occurred in response to poor environmental
constraints, resulting in small adult stature, later ages of menarche and slow
growth rates compared to richer quality environments. Small stature therefore,
is an example where investment of energy favours maintenance over growth producing
an adaptive trade-off (Stulp & Barrett, 2016). This reinforces the argument
that whilst understanding constraint is necessary for researchers in EMPH, an
understanding of adaptations which stem from constraints is more useful for interpreting
the cause of phenotypic variability. For example, global health interventions
can use an understanding of adaptive trade-offs to develop supplementary food
programs in developing countries. By increasing energy available for growth
from better nutrition, improvements in growth rates and reduced stunting have
been made to improve health outcomes. This example shows that by influencing trade-offs
between growth and maintenance, researchers in EMPH can make significant
improvements in human health (Beaton & Ghassemi, 1982).

 

Evolutionary Adaptation and Plasticity

 

The concept
of adaptation offers the most significant insight into human vulnerabilities to
disease and is arguably the most important concept for researchers in EMPH to
understand (Muehlenbein, 2010). An adaptation can be described as a change or alteration in a
heritable trait which correlates to the improved reproductive success of an
organism. Adaptive traits, in the form of genetic or phenotypic alterations,
will thus increase in frequency within the population through the reproductive
success of adapted organisms which are better suited to the changing
environment (Stearns & Medzhitov, 2016). The way in which an organism adapts
is determined by the resources available and the constraints that have
accumulated in its lineage, which have in turn been altered by the process of
adaptation (Griffiths & Grey, 1994).

 

Adaptation a
complex concept which is vital for researchers in EMPH to understand, so that interventions
concerning global public health are as effective as possible in improving
health and preventing disease. The “small but healthy” hypothesis put forward
by David Seckler, is an example of an adaptive response to poor environmental
conditions constraining growth (Seckler, 1982). The hypothesis stated that
individuals short in stature, having faced malnutrition in childhood, are in
fact healthy and well-adapted to the constraints of the environment surrounding
them. This raises issues that challenge researchers in evolutionary medicine
and public health concerned with the characteristics and nature of human
adaptations. The “small but healthy” hypothesis argues the following: the body
reduces its rate of growth as an adaptation to low nutrient intake, in doing
so, the body retains a physiological equilibrium. This adaptation, Seckler
argues has no consequences for health other than short stature and since “small
but healthy” individuals are more likely to face nutritional constraints, populations
surviving impoverished intakes should no longer be considered malnourished,
even though classified as such by international standards such as WHO (Messer,
1986). International health interventions that focus on nutrition should
therefore direct attention to individuals who are not “small but healthy” yet still
faced with serious nutritional constraints leading to physical debilities, he
states (Seckler, 1982). If correct, this hypothesis suggests that public health
interventions could focus on a smaller and more manageable proportion of the
global population and reduce food aid and program budgets without harmful effects.
Adaptation therefore, lead the millions of individuals facing nutritional
constraints to be better suited to a lower nutrient intake (Pelto & Pelto,
1989). The hypothesis had potentially profound implications for public health
and evolutionary medicine taking action to combat nutritional constraints if it
is supported. The argument however, has been heavily criticised. Whilst in
life-history terms it remains relatively simple: being small under harsh
nutritional conditions is beneficial because energy can favour maintenance over
growth, there is substantial evidence against it (eg Pelto & Pelto, 1989; Rohmatullayaly et al., 2017). Seckler’s
argument suggests that researchers in EMPH should direct very little attention
to individuals that are in fact in need of vital health interventions.  This example demonstrates how an understanding
of adaptation is vital so that the appropriate measures are taken by
researchers in EMPH to improve global health and prevent disease.

 

Taking an
evolutionary perspective inclusive of developmental plasticity can also be used
to 

improve the effectiveness of health interventions (Rickard, 2016).
Plasticity involves responses to the environment by an organism from
potentially a single genotype (Wells et al., 2017). Responses occur either
momentarily or range across generations and are aimed at enhancing behaviour,
development or physiology to maximise fitness (Stearns & Medzhitov, 2016). Developmental
plasticity can evolve by natural selection in such a way that organisms which
have developed advantageous responses to improve their chances of survival will
produce a greater number off offspring with a higher evolutionary fitness
(Rickard, 2016). The importance of plasticity in EMPH is evident in the
adaptations of organisms, which have evolved in response to environmental
constraints. The Predicative Adaptive Response (PAR) hypothesis argues that
cues received during development influence the future development of a
phenotype in accordance to the conditions in the form of an adaptive response (Bateson
et al., 2014). This can be useful for researchers in EMPH to predict how an
individual will develop based on early life conditions and interventions can be
developed earlier in order to overcome future health outcomes. For example, if
a foetus experiences undernutrition it will adapt by altering its
insulin-glucose metabolism so that so that when faced with undernutrition in
adulthood, it is capable to survive. In this case, the PAR hypothesis is a
useful concept within adaptation because it holds important implications concerning
health and disease patterns which may prove useful for the development of
future public health interventions (Rickard, 2016). 

Conclusion

 

Understanding
the concepts that have shaped evolution are vital in the fields of evolutionary
medicine and public health. The concepts of adaptation and constraint provide
alternative conclusions to interpretations of patters of health and disease.
The concept of adaptation offers significant insight into human vulnerabilities
to both infectious and non-infectious disease (Muehlenbein, 2010). The concept
of constraint provides explanations on the bias and channelling of variation
that may explain and facilitate adaptation, thus providing the very framework
upon which natural selection acts (Fitch, 2012). This essay argues that
adaptation is a more vital concept to understand than constraint because it
offers the most significant insight into human vulnerabilities to disease and
has the most important influence on the effectiveness of health interventions.
The concept of constraint remains vitally important in evolutionary medicine
and public health to shape health outcomes and susceptibility to disease and
address the evolution of treatment resistant pathogens and emergent infectious
diseases. Finally, the concepts that shape evolution can be used in EMPH to
look towards the future of overcoming adverse health and potentially new
emergent diseases, and to shed light on the future implications of how humans
and organisms have evolved to what they are today.