Evolutionary definition of disease phenomenon

1
Evolutionary definition of disease
phenomenon Libertini G. (M.D., Independent
Researcher)
Disease is commonly defined as the alteration
of physiologic conditions. If we accept that
evolutionary mechanisms are indispensable for the
full understanding of any biological phenomenon
1, it is necessary to investigate if and how
disease phenomenon is explainable and
classifiable in evolutionary terms and if this
approach could give us useful indications
2-4. Four different evolutionary mechanisms can
cause disease 1) Alterations of the genotype
The transfer of genetic information from a
generation to the next is imperfect, a fact that
is fundamental for the whole evolutionary theory.
As these modifications are a change in a very
complex and ordinate system, they are, when not
neutral, a possible cause of physiologic
dysfunctions 2) Alterations of the ecological
niche The modification of the ecological
conditions to which a species is adapted is a
change in a very complex and ordinate system and,
when this change is not neutral, is a possible
cause of disease (Fig. 1 and Table I) 3)
Interaction with other species There is a
continuous competition among the species with
conflictual evolutionary exigencies and
specifically between an organism and its
parasites (bacteria, virus, fungi, worms, etc.).
In particular, since the relationship between an
organism and its parasites is analogous to that
between a prey and its predators, it is
predictable that, such as it happens in
prey-predators case, parasites will damage more
very young and old individuals and less
individuals of intermediate ages, to minimise
disadvantages and maximise advantages both for
host and parasite 4) Conditions beyond
adaptation range of the species. Moreover,
age-related fitness decline (Fig. 2), usually
called ageing in its more advanced expression
at ages not existing in the wild, is not a
disease in evolutionary terms but a specific
biological function 10, advantageous in
certain conditions in terms of inclusive fitness
11-14 and not caused by the combined action of
many harmful genes (Fig. 3). The simple fact that
individuals in protected conditions
(civilisation, captivity) survive at ages not
existing in the wild is a predictable cause of
greater expression of alterations underlying
age-related fitness decline 13. Evolutionary
interactions between fitness decline and other
category of diseases are important. In
particular, diseases of categories 1 and 3 will
increase their frequencies in relation with age
and, if protected conditions increase life span,
frequency and severity of these diseases will
increase in proportion (Fig. 4). Interactions
between diseases of categories 2 and 3 are very
important too (Table II). Diseases classified in
evolutionary terms are comparable to analogous
categories of car breakdowns (Fig. 5). The
evolutionary arrangement and classification of
the diseases is not an useless theoretical
exercise. On the contrary, it is essential for a
better understanding of disease epidemiology and
to improve their prevention and control. The
knowledge of evolutionary mechanisms explaining
disease origins should be an indispensable
component of the medical education and of the
definition of sanitary politics (Fig. 6).
Table I - Alterations of the ecological niche ? Diseases (2-9 other 23 references)
Excessive ingestion of salt ? Hypertension (? heart hypertrophy, congestive heart failure, arrhythmia and sudden death)
Excessive time spent focusing close up or in improper conditions of vision ? Myopia (till 70-90 of a population affected) and other refractive defects (astigmatism, hyperopia)
Excessive ingestion of unsaturated fats, caloric foods, meat with high fat content ? Obesity (? renal cell carcinoma, heart hypertrophy, congestive heart failure, arrhythmia and sudden death), type 2-diabetes and increased vascular risk (? myocardial infarct, cerebral ischemia, infarcts in all the vascular districts, heart hypertrophy and failure, etc.)
Excessive exposure to noise, smoking, high Body Mass Index ? Hearing loss
Smoking and/or air pollution ? Chronic bronchitis and respiratory diseases, emphysema, coronary heart and other cardiovascular disease, pregnancy complications, lung / lariynx / bladder / kidney / pancreas carcinoma, peptic ulcer
Excessive ingestion of simple and refined carbohydrates (in particular sugar) and other dietary modifications ? Dental caries, pyorrhoea, crowded teeth
Scarce ingestion of fibre ? Constipation, colon diverticulosis, colon carcinoma, stomach carcinoma, type 2-diabetes, metabolic syndrome and cardiovascular disease, appendicitis
Scarce ingestion of calcium and reduced physical activity ? Osteoporosis, back pain
Altered conditions of sociality, stress of civilised condition ? Mental and psychiatric disorders, headache
Reduced exposure to natural allergens in the childhood ? Allergies
Exposure to chemical substances artificially synthesised ? Allergic diseases
Alcoholism ? hepatic steatosis, steatohepatitis, cirrhosis, larynx carcinoma
Various factors ? Increased incidence of many types of cancer
Fig. 1 Upper side ancestral dietary habits and
teeth ... excellent and free from dental
caries. Lower side modern diets and multiple
dental caries, crowding of the teeth, changes
in facial form, pyorrhoea 5.
Table II - Alterations of the ecological niche ? Diseases related to category 3 (20-22 other 24 refer.)
Extraordinary growth of human population and of its demographic density, cohabitation or proximity with bred animals, dangerous hygienic habits ? dreadful and non-dreadful epidemics (black death, bubonic plague, smallpox, typhus, cholera, influenza, hepatitis A, tuberculosis, measles, parotitis, HIV, etc.)
Hygienic or iper-hygienic habits that restrict and delay first exposure to microbes and parasitic worms or make impossible infections or infestations ? Gravity of first infection in adulthood (e.g., polyomielitis), anomalous maturation of immunologic capacities (? allergies and atopic diseases, autoimmune diseases as Crohn's disease, ulcerative colitis, multiple sclerosis, etc.)
Use and abuse of antibiotics ? deadly infections by antibiotic resistant bacteria
Abuse of topic disinfectants ? Alteration of normal microbial flora of epidermis and external mucosae (especially of armpits, genitals and hands) and consequent infections by pathogens, in particular fungi.
Fig. 3 Curve A is the life table in the wild of
a real species showing an age-related fitness
decline. B is a hypothetical life table of the
same species with only the extrinsic mortality at
its lowest value and without the age-related
fitness decline. C is a hypothetical life table
with the same mortality of B plus the effects of
a great number (500/year) of noxious genes acting
at years t1, t2, ... Curve A is quite different
from C and, so, it is unjustifiable as an effect
of noxious genes insufficiently eliminated by
natural selection 11-13.
Fig. 2 Age-related fitness decline. Source of
data for age group lt 35 (world records)
http//en.wikipedia.org/ wiki/World_records_in_ath
letics for other age groups http//www.world-mast
ers-athletics.org/records_output/
rec_list_outdoor_m.php.
Evolutionary Medicine is scarcely considered by
Physicians and even by Evolutionary Biologists
while should be a pivotal topic for both

Fig. 4 Risk factors 15,16 (and some genetic
diseases 17) increase apoptosis rate and cell
turnover. Protective drugs contrast the effect of
risk factors 15,18,19 but it is not documented
the capacity of reducing physiological cell
turnover rate.
Fig. 6 Life tables of human species
illustrating a historical progressive increase of
life span while longevity appears unchanged
(curves A-E). Actual condition in developed
countries is roughly indicated by curve E. With
good preventive measures (due consideration of
ancestral lifestyle to which our species id
adapted!) and better health treatment, curve F is
a likely outcome, with a little further increase
of life span (dashed area) but not of longevity.
Only with a modification of the progressive
increase of mortality caused by intrinsic factors
(ageing) a drastic increase of life span and
longevity could be possible (curve G). Modified
from 23 and redrawn.
Fig. 5 Categories of car breakdowns, in analogy
with evolutionary classification of diseases and
the peculiar phenomenon of ageing.
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