Proximate & Ultimate Explanations
In 1951, biologist Ernest Mayr suggested that biologists split up explanations into two categories: proximate and ultimate. Proximate explanations describe the mechanism underlying a trait or condition. For example, describing the physiology, hormones, genes, or development underlying a biological trait is a proximate explanation. In other words, they explain HOW a mechanism functions. On the other hand, ultimate explanations describe the evolutionary explanation of a trait or condition, such as its adaptive function or phylogenetic history. They explain WHY a mechanism exists.
Today, using multiple types of explanations to provide a full biological understanding of traits that increase vulnerability to disease is the backbone of evolutionary medicine.
Before the Dawn of Darwinian Medicine, the Dutch ethologist Niko Tinbergen famously applied this approach to the field of animal behavior. While studying the behaviour of Herring Gull’s, Tinbergen noticed that when Herring Gull chicks peck at the beak of their parents, the parents provide them with food. After a series of experiments, he concluded that this is an innate behavior that newly hatched chicks will perform when exposed to a red dot – even if it was from an artificial stimulus! Innate basically means instinctual. The fact that the behavior occurs without any prior experience suggests that it is adaptive, or has been shaped by natural selection.
A proximate explanation would be: Herring Gull chicks peck at their parent’s beak when they see a red spot,
And an ultimate explanation would be: Herring Gull chicks peck at their parent’s beak because it provides them with food required for growth and survival
Today, like evolutionary medicine, animal behavior is a highly integrative field that heavily relies on evolutionary perspectives.
In 1963, Tinbergen further refined They are Causation, Development, Functional, and Phylogenetic Let’s take a look at each of these types of explanations in detail.
Number 1: Causation. Also known as Mechanism, causation questions ask What biological mechanisms are responsible for the characteristic? This includes factors such as hormones, neurotransmitters and genes
For example, different areas of the human brain are responsible for different functions. The function of Broca’s area, found on the frontal lobel of the cortex, acts as a causal explanation for language production. This was discoved when the physician Paul Broca noted two patients lost the ability to produce speech after damage to this area of the brain.
Number 2: Development. Also referred to as Ontology, development question asks What environmental factors that influence development are responsible for the characteristic? This includes factors such exposure to certain stimuli during critical periods, and early life nutriation.
For example, an experiment showed that depriving cats of vision during different development periods results in lasting changes to their vision later in life. When kittens were deprived of vision for the first month of their life, a critical period for visual development, they suffered impaired vision for the rest of their lives. However, kittens deprived of vision for the same amount of time, but in the second month of their life, went on to develop completely normal visual capabilities.
Number 3: Adaptive. Also called functional, adaptive questions ask How does the characteristic influence the individual’s ability to survive and reproduce? Elephant seals have a polygynous mating system, which means a single male will mate with multiple females by obtaining harems, which is a group of reproductive females. The male that gains access to these harems is determined by aggressive male-male conflicts that often result in severe injury and sometimes death. So, even though engaging male-male competiton can result in mortal costs, this aggressive behaviour can be explained as an adaptation – the male with highest competitive success gains access to the mating rights in the group for an entire breeding season – meaning his alleles will be well represented in the next generation.
Number 4: Phylogeny. Also known as evolutionary, phylogenetic questions ask How did evolutionary processes over many generations result in the evolution of the characteristic? For example, take the human eye – the complexity of the human eye makes it a common target for arguments of intelligent design. But evolution is gradual process over many generations. Understanding the evolution of the human eye not only requires an analysis of our own evolutionary history, but that of ancesteral species that had earlier forms of the modern eye. So, it’s not like species when from having no eyes to a complex modern eye… but rather many years ago, a species existed that could perceive light existed…
Now lets take a look at an example of how these four questions can be applied to explain a trait related to human health – Fevers!
From a perspective of causation, When our immune system detects a foreign pathogen that may be harmful, a fever may be triggered.
In terms of Development, Previous exposure to the pathogen may influence the immune system’s ability to eliminate the pathogen without triggering a fever.
A Functional explanation is fevers may make it more difficult for the invading pathogen to survive and replicate. It also slightly increases metabolism, which can improve immune function.
Finally, a Phylogeny: All warm-blooded animals have a fever response. Cold-blooded animals display what is called a behavioural fever: When infected, they seek sources of heat.
if we only focus on the proximate explanations of a fever it may seem like good idea to always take anti-pyretic medication such as Tylenol when experiencing symptoms of a mild fever? But With these ultimate explanations in mind, do you think it’s always the best idea? As it turns out Studies have demonstrated that antipyretics can have detrimental effects: increasing mortality when used in critical care and in influenza infections, and increasing viral shedding resulting in increased transmission and illness. The hypotheses that drove these studies could only have been generated when considering the ultimate explanations along with the proximate ones!
