In evolutionary time all possible phenotypic variables are in play. Phenotypic genetic malleability is limited only by physics.
An autopoietic system is a homeostat: a device for holding a critical systemic variable within physiological limits. In the case of autopoietic homeostasis, the critical variable is the system’s own organization. It does not matter, it seems, whether every measurable property of that organizational structure changes utterly in the system’s process of continuing adaptation. It survives. ~ English theorist Stafford Beer
Each habitat has its own difficulties: environmental demands and the trials of relations with one’s own species and others. These immediate requisites are common drivers of evolution. As a set of adaptations, tolerances to ecological changes also present challenges and opportunities to survival of a population.
Reptiles rose when drier times made amphibian life difficult. Mammals had a long wait under reptilian rule before their time came, a duration that might have been much longer but for a massive meteor strike. While supersized dinosaurs were wiped out, their smaller cousins, notably birds, survived; examples of evolutionary payback and payoff respectively.
Life-history variables invariably represent evolutionary trade-offs which murres and penguins exemplify.
Several seabirds fly and swim. Being able to do both means neither is optimally efficient.
Cormorants dive into the ocean, propelling themselves underwater with webbed feet. Their dips into the sea to snag fish are brief. Even so, cormorants must expend enormous amounts of energy to fly.
Murres also dive to hunt, flapping their wings underwater to swim. Because murre wings are built for flight, they create drag underwater.
Murres’ small bodies are just light enough to let them take off out of the water. Keeping their weight down means they cool quickly. In contrast, penguins stay comfortable in frigid waters.
Murres are on an evolutionary knife edge. In being able to employ their wings for propulsion underwater, they are the most inefficient fliers. To be better swimmers, murres would have to be bigger, or trim their wingspan. Either would sacrifice flight.
Like many marine birds, penguins have a considerable commute between feeding and breeding grounds. Rather than fly, penguins swim.
Penguin life-history variables favored the water as a way of life, forgoing flight. Penguins today are a long way from their ancestors 70 MYA who could soar through the sky.