Evolution of High Altitude Athletes

How altitude shaped extreme performance

High on the Tibetan Plateau, where oxygen levels fall to half those at sea level and temperatures plunge far below freezing, evolution has produced one of the most remarkable athletes on Earth. Tibetan antelopes (Panthelops hodgsonii) live at altitudes up to 5,500 m, a zone where most mammals would struggle simply to remain conscious. Yet these animals run fast and continuously for tens of kilometres, and migrate across immense distances in an environment that is both physiologically punishing and ecologically sparse. The evolution of athletic performance in these animals is a good example of plasticity’s evolutionary mechanism.

Here we explore how plasticity in an ancestral phenotype was an evolutionary doorway into this extreme habitat, and how that plasticity then enabled a genetic change to transform a species’ physiology. Tracing the biological detail of this evolutionary story we see how plasticity affects natural selection, guides evolutionary direction, and ultimately produces new biological possibilities. (The excellent study upon which this page is largely based is cited below)¹.

Oxygen at altitude: a fundamental challenge

Life at high altitude is dominated by a single constraint: oxygen scarcity. Every step, every breath, every heartbeat must compensate for the thin air. Animals that thrive here must solve the same problem in several ways — increasing ventilation, altering blood chemistry, modifying muscle physiology, or changing behaviour. One of the most powerful tools available to mammals is haemoglobin variation. Haemoglobin is a four‑globin protein complex that binds and transports oxygen. By altering which globins are used, animals can tune how strongly haemoglobin attracts oxygen — its oxygen affinity.

Foetal vs adult haemoglobin: a built‑in contrast

In mammalian foetal blood adult globin is wholly or partially replaced with the specifically foetal type. This developmental switch is universal across mammals and provides a template for evolutionary innovation. And it is also through this structural flexibility that haemoglobin can enable survival at high altitudes.

In mammalian foetal blood adult globin is wholly or partially replaced with the specifically foetal type. This developmental switch is universal across mammals and provides a template for evolutionary innovation.

Caprine innovation: a third haemoglobin and the power of plasticity

Sheep and goats — the caprines — add a third option: a juvenile globin. It has high oxygen affinity like the foetal form, but is normally used only in lambs and kids. Having additional blood oxygen may help infant animals at a stage when they remain very delicate.

Additionally, adult caprines retain the ability to switch back to this juvenile globin when oxygen becomes scarce. This is another case of phenotypic plasticity: the ability of a single genotype to produce different phenotypes depending on environmental conditions.

Under hypoxia, adult caprines:
● reduce production of adult globins
● increase production of juvenile globin
● raise overall haemoglobin oxygen affinity
● improve oxygen uptake in thin air.

This plasticity will increase available grazing in high altitude terrain where it is scare. In addition, high altitude might initially have been restrictive to many other animals, consequently reducing competition for grazing as well as being beyond the reach of, or at least difficult for predators.

Why not use high‑affinity haemoglobin all the time?

The Miocene: a world of herbivorous opportunity

Between 23 and 5 million years ago, during the Miocene epoch, global grasslands expanded dramatically. This was a transformative ecological event. Herbivores diversified, predators adapted, and entire ecosystems reorganised around open, grassy landscapes.

A common ancestor of modern caprine animals, one possessing the haemoglobin plasticity we have described, lived prior to and within the Miocene, and this ancestral population began to split into several evolutionary caprine lines early in that latter period. One of these was a line that became the animal group Pantholopini, of which the Tibetan antelope is the sole modern member.

But predators evolved too. Wolves and snow leopards became altitude‑capable hunters, and the evolutionary arms race intensified.

In this context, the Tibetan antelope lineage began to evolve extraordinary athletic performance — speed, endurance, and the ability to migrate across vast, high‑altitude landscapes. To appreciate how they achieved this, we need to understand something called the Baldwin effect.

A genetic leap: the deletion that locked in performance

At some point after the lineage diverged, a major genetic event occurred: a large deletion in the ancestral globin gene cluster.

This deletion:
● removed the genetic region responsible for haemoglobin plasticity
● eliminated the ability to switch between adult and juvenile globins
● left adults producing exclusively the high‑affinity juvenile isoform.

Why this deletion mattered

In a low‑altitude animal, this mutation would have been harmful. High‑affinity haemoglobin at sea level risks oxygen toxicity and poor oxygen delivery to tissues. Such a mutation would probably have been eliminated by natural selection.

But in this lineage, the deletion occurred in animals already living at altitude, because plasticity had placed them there. In that environment:
● high‑affinity haemoglobin was beneficial
● the deletion improved athletic performance
● individuals carrying it had higher fitness
● the mutation spread through the population.

The plasticity therefore shaped the selective environment in which the mutation arose.

Plasticity as an evolutionary guide

The deletion itself was random — a developmental or reproductive error — but its evolutionary fate was not. Plasticity had already:
1. Enabled the ancestral population to occupy high altitude
2. Created the conditions under which the deletion was advantageous.

Thus, plasticity influenced which genetic variants were exposed to natural selection, making it more likely that genetic based haemoglobin change had fitness benefit. This is the essence of genetic assimilation, a process where:
● a plastic phenotype is expressed in a new environment
● a genetic change arises that stabilises or fixes that phenotype
● plasticity is reduced or lost
● the phenotype becomes hereditary.

Note that critical outcome of genetic assimilation: phenotypes shifted by plasticity in response to environmental change become hereditary.
Genetic assimilation is one of a family of similar, though mechanistically distinct processes generally known as the Baldwin effect.

The emergence of an extreme athlete

With haemoglobin plasticity removed and high‑affinity haemoglobin fixed, the Tibetan antelope lineage was now genetically committed to high‑altitude life. This set the stage for further adaptations:
● enhanced cardiovascular capacity
● specialised muscle physiology
● extreme endurance
● long‑distance migration
● predator evasion through sustained high‑speed running.

These traits did not arise all at once. They accumulated over time, but the haemoglobin deletion was a pivotal enabling step — a physiological foundation upon which further adaptations could build.

The connection between the original plasticity and the second step (the selective environment) caused the plasticity strongly to influence the evolutionary direction as the line that eventually led to Tibetan antelopes branched from the common caprine ancestor.

What this story reveals about evolution

The Tibetan antelope is not just an example of adaptation to altitude. It is a case study in how phenotypic plasticity interacts with natural selection to shape evolutionary outcomes in ways the Modern Evolutionary Synthesis (MES) does not fully capture.

Key lessons

1. Plasticity provides immediate, functional responses
When environments change or fluctuate, plasticity offers ready‑made phenotypes that match the new conditions. These phenotypes are not random; they are elicited by the environment itself.
2. Plasticity creates new ecological opportunities
By enabling ancestral caprines to function at high altitude, plasticity opened a new adaptive landscape that would otherwise have been inaccessible.
3. Plasticity shapes which mutations matter
The haemoglobin deletion was beneficial only because plasticity had already placed the population in a high‑altitude environment. In a low‑altitude context, the same mutation would have been harmful and eliminated.
4. Plasticity mediated phenotype shifts can be fixed by Baldwin effects
When a plastic response becomes consistently advantageous, natural selection can fix it genetically, reducing or eliminating the original plasticity.
5. Plasticity guides evolutionary direction
This is the crucial point for the website’s broader theme.

In MES-style evolution, random genetic variation arises first, and only afterwards does selection determine whether it is relevant.
 In plasticity-first evolution, the environment induces a phenotype that is already relevant to the environmental challenge. Selection then acts on genetic variants that stabilise or refine that phenotype.

Plasticity therefore enables evolution to respond directly to environmental change, and in ways that are specific to the nature of that change.
It does not replace mutation and selection — it structures the landscape in which they operate.

Conclusion – and a question

The Tibetan antelope’s extraordinary athleticism emerged through a two‑step evolutionary process:
1. Phenotypic plasticity allowed ancestral caprines to exploit high‑altitude environments, generating phenotypes that were directly relevant to the challenges of hypoxia.
2. A genetic deletion then fixed one of those plastic responses — high‑affinity haemoglobin — enabling extreme performance and driving the emergence of a new species.

This illustrates a central theme of the website: adaptation does not arise solely from random genetic change filtered by selection.
Plasticity allows organisms to produce functional phenotypes in response to environmental conditions, and these environmentally induced phenotypes can then become targets for natural selection.

The antelopes’ story shows that phenotypic plasticity did far more than help this lineage survive harsh conditions. It actively guided the direction of evolution, creating a phenotype that matched the demands of high altitude long before any genetic change occurred. The later deletion in the haemoglobin gene cluster did not initiate adaptation — it stabilised a plastic response that was already working.

This raises an important question. How does this two‑step process of plasticity and the Baldwin effect fit within the Modern Evolutionary Synthesis, which traditionally assumes that genetic variation appears first and phenotype follows? And why has the evolutionary role of plasticity only recently begun to gain recognition in the scientific literature?

The next page explores these questions, beginning with a clear summary of plasticity‑Baldwin processes and then examining how they have been understood — and often overlooked — within modern evolutionary theory.

Scroll to bottom of page to comment

References

1. Signore, Anthony V., and Jay F. Storz. ‘Biochemical Pedomorphosis and Genetic Assimilation in the Hypoxia Adaptation of Tibetan Antelope’. Science Advances 6, no. 25 (June 2020): eabb5447. https://doi.org/10.1126/sciadv.abb5447. [↩]