Biodiversity, Evolution, and the One Health Approach

Biodiversity and its Measurement Techniques

Biodiversity is categorized across multiple scales, including genetic diversity, specific diversity (the number of species in a given environment), and ecosystem diversity within the biosphere. While millions of species exist, only a small percentage are scientifically known through sampling methods. One primary statistical technique for estimating population abundance is the Capture-Mark-Recapture (CMR) method. In this process, a first sample of $M$ individuals is captured, marked, and released. Later, a second sample of $n$ individuals is captured, where $m$ represents the number of previously marked individuals recaptured. The total population size $N$ is estimated using the formula: $N = \frac{M \times n}{m}$.

Beyond abundance, researchers estimate the proportion of individuals carrying specific phenotypic traits within a population. Starting from a sample of size $n$ with a calculated frequency $f$, an interval of confidence is established to estimate the character's proportion in the total population. There is a $95\%$ probability that the true proportion lies within the interval defined by: $[f - \frac{1}{\sqrt{n}}, f + \frac{1}{\sqrt{n}}]$. The precision of this estimation increases as the sample size $n$ grows larger.

Genetic Evolution of Populations and the Hardy-Weinberg Model

The Hardy-Weinberg model provides a mathematical framework to predict the genetic structure of a large population. Under specific "ideal" conditions—including infinite population size, absence of migration, absence of mutation, absence of natural selection, and panmixia (random mating)—the genotypic structure remains stable over generations. For a gene with two alleles, $A$ and $a$, where the frequencies are defined as $f(A) = p$ and $f(a) = q$, the following relationships hold: $p + q = 1$.

The distribution of genotypes remains constant at:

• Frequency of homozygous dominant $f(A//A) = p^2$
• Frequency of homozygous recessive $f(a//a) = q^2$
• Frequency of heterozygotes $f(A//a) = 2pq$

In reality, natural populations often show a discrepancy between observed frequencies and those predicted by the Hardy-Weinberg equilibrium. These gaps are explained by evolutionary forces, which include mutation, natural selection, genetic drift (random variations in allele frequencies), and migration. These processes modify the genetic composition over time. While the model assumes an infinite population, it is practically applicable to finite populations that are sufficiently large.

Human Impact on Biodiversity and the One Health Approach

Human activities, such as overexploitation and the fragmentation of ecosystems, significantly influence biodiversity and population abundance. Fragmentation breaks ecosystems into smaller, isolated patches, resulting in smaller populations that suffer from reduced genetic diversity, making them more vulnerable to environmental changes. It is estimated that approximately ten million species (or more) exist on Earth, but currently, one million plant and animal species are threatened with extinction. Human-driven extinction rates are extremely high, with hundreds of species disappearing daily, potentially signaling the sixth mass extinction in history.

To address these challenges, the "One Health" (Une seule santé) approach recognizes that human health, animal health, and ecosystem health are inextricably linked. This global perspective involves coordinated actions across three sectors:

• Human Health Indicators: Life expectancy, infant mortality rates, disease prevalence, and level of development. Levers for improvement include healthcare infrastructure, health education, hygiene promotion, and vaccination.
• Animal Health Indicators: Life expectancy, mortality rates, disease prevalence, intraspecific genetic diversity, and behavior. Levers include price control regulations, promotion of sustainable farming practices, and technological innovation.
• Ecosystem Health Indicators: Biodiversity levels, total surface area, degree of fragmentation, and biotope quality. Levers include protection through natural parks, sustainable resource exploitation, restoration projects, and public awareness.

Anatomy as a Product of Evolutionary History

Human and animal anatomy is the result of a long evolutionary process rather than a perfected design. The evolution of the eye serves as a primary example. Rudimentary eyes, such as those in snails, consist of simple photoreceptor cells and pigmented cells that provide a selective advantage by detecting light. Through a series of small anatomical changes driven by mutations and retained by natural selection, complex eyes emerged. These complex structures, featuring lenses for accommodation, an iris to modulate light, and a retina, appeared independently in distant groups—a process seen in both vertebrates (mammals) and certain mollusks (octopuses).

However, these structures are often examples of "evolutionary tinkering" (bricolage évolutif). For instance, humans possess an "inverted retina" where light must pass through several cellular layers before being detected, while the octopus has a "direct retina." Furthermore, anatomical traits are often compromises influenced by various constraints:

• Historical Constraints: The path of the aortic arch in humans is derived from an ancestral branchial arc.
• Developmental Constraints: Some traits are maintained despite having no clear function, such as male nipples, while others regress, such as the coccyx (a remnant of a tail).
• Structural/Selective Compromises: Features like the pelvic bone reflect a compromise between different biological needs (e.g., bipedalism vs. childbirth difficulties).

Evolutionary Concepts in Medicine and Agronomy

Evolutionary biology has critical applications in medical and agricultural fields. Microorganisms (bacteria, viruses) and insects have very short reproductive cycles, allowing mutants to emerge rapidly. When humans apply strong selection pressures through the massive use of antibiotics or pesticides, individuals with resistant traits have a significant selective advantage.

In medicine, the spread of multi-drug resistance in bacteria is a major health risk. Antibiotics do not create mutations; instead, they act as a selection filter that allows resistant variants to survive and multiply. To combat this, medical practices must shift toward prophylactic strategies (such as vaccines) and the development of new treatments. Similarly, in agriculture, the proliferation of pesticide-resistant pests requires new management modes, such as agroecology and integrated pest management, to adapt to the rapid evolution of these organisms.

Essential Vocabulary and Key Figures

• Abundance: The total number of organisms of one or more species in a given space.
• Specific Biodiversity: The diversity of living species in a given area.
• Genetic Drift: Random variation in allele frequencies in the absence of selective advantage.
• Sampling: The act of taking a small representative portion to understand a whole population.
• Hardy-Weinberg Equilibrium: A theoretical state where genotypic and allelic frequencies remain stable across generations under specific conditions.
• Genotype: The allelic composition of all genes in an individual.
• Panmixia: Random reproduction between individuals in a population.
• Phenotype: The set of observable characteristics resulting from the interaction between genome and environment.
• Antibiotics: Molecules that destroy or limit bacterial growth.
• Selective Advantage: A trait that favors an organism's survival.
• Evolutionary Constraints: Limitations on the course of adaptive evolution.
• Prophylaxis: Measures taken to prevent diseases.