Flexibility stems from phenotypic plasticity, the ability of an organism with a given genotype to change its phenotype in response to changes in its habitat, or to move to a different habitat. The degree of flexibility is inherited, and varies between individuals. A highly specialized animal or plant lives only in a well-defined habitat, eats a specific type of food, and cannot survive if its needs are not met. Many herbivores are like this; extreme examples are koalas which depend on Eucalyptus, and giant pandas which require bamboo. A generalist, on the other hand, eats a range of food, and can survive in many different conditions. Examples are humans, rats, crabs and many carnivores. The tendency to behave in a specialized or exploratory manner is inherited—it is an adaptation. Rather different is developmental flexibility: "An animal or plant is developmentally flexible if when it is raised in or transferred to new conditions, it changes in structure so that it is better fitted to survive in the new environment," writes evolutionary biologist John Maynard Smith.If humans move to a higher altitude, respiration and physical exertion become a problem, but after spending time in high altitude conditions they acclimatize to the reduced partial pressure of oxygen, such as by producing more red blood cells. The ability to acclimatize is an adaptation, but the acclimatization itself is not. Fecundity goes down, but deaths from some tropical diseases also go down. Over a longer period of time, some people are better able to reproduce at high altitudes than others. They contribute more heavily to later generations, and gradually by natural selection the whole population becomes adapted to the new conditions. This has demonstrably occurred, as the observed performance of long-term communities at higher altitude is significantly better than the performance of new arrivals, even when the new arrivals have had time to acclimatize.
Aaron has discovered two new species of birds living in the Amazon rainforest, Species A and Species B. He is curious as to how flexible these animals are in regards to living in different habitats. To study this, he has tagged two birds of each of the species: For Species A he tagged the birds as A1 and A2, and for Species B he tagged the birds as B1 and B2. He left Bird A1 and Bird B1 in their natural habitat and moved Bird A2 and Bird B2 to another habitat and then observed how well each bird thrived. He learned that while Bird A2 adapted to the new environment quickly and with no decrease in function, Bird B2 was unable to find suitable food and shelter to meet its basic needs.
Which bird species exhibits greater flexibility, Species A or Species B?