The Elements of Evolution (42-11) Adaptation in Disguise

Adaptation in Disguise

A trait that’s maladaptive in one environment can be adaptive in another. ~ American marine molecular biologist William Detrich

Adaptation may be disguised in a broader context not readily recognized. Human resistance to malaria, prevalent in sub-Saharan Africa, has been a dual-track genetic adaptation: immune system modifications and alterations in human red blood cells (erythrocytes) that hinder the ability of the malaria parasite to invade and replicate there.

The altered erythrocytes may polymerize into a sickle shape when deoxygenated. The cell’s hemoglobin – the protein, iron-bound transport for oxygen and other gases – stops working properly.

The upshot of sickle-cell anemia is a shortened life span, to an average of 42 years in males and 48 in females. That is a better fate than succumbing to malaria in childhood.

Like blood type, eye color, and other traits, sickle-cell is passed down genetically. Out of context, it appears maladaptive. In context, it may be lifesaving.

 Hemoglobin

The protein hemoglobin tells a tale of adaptive evolution. Hemoglobin and hemoglobin-like proteins are found in bacteria, fungi, plants, and invertebrates, as well as all vertebrates.

One reason is that hemoglobin is a model of flexibility. The protein can work in multiple conformations. Each shape offers slightly different functionality. Hemoglobin adapted to meet the needs of its many customers. A plant variant of the molecule, leghemoglobin, carries nitrogen and scavenges oxygen, which is a poison to anaerobic systems.

The ancestral globin gene duplicated itself and diverged in sequence some 450 MYA in fish. Such genome duplication yielded core conservation while affording adaptation. Legumes created a symbiotic relationship with nitrogen-fixing bacteria via the same trick.

For fetal use several animals independently evolved a 2nd hemoglobin type – a genetic variant of the 1st – with a modified amino acid sequence. Fetal hemoglobin can rob oxygen from maternal circulation, thus improving healthy growth prospects for the little one.

The bar-headed goose flies over the Himalayan mountains at 9,200 meters, an altitude with 29% of the oxygen available at sea level. A single amino acid alteration lets the goose make better use of available oxygen compared to its lowland relative.

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Conceptually similar to the hemoglobin adaptation of bar-headed geese, octopi adapt to frigid water by a simple genetic expression tweak that speeds neuron processing which otherwise slows from the chill.

Organisms that sexually reproduce are granted the great advantage of genetic shuffle during meiosis. But even mitotic haploids manage to coherently adapt.