23.1Biosphere and Adaptation to the Environment


Environmental factors

The organisms that inhabit the planet are distributed across a vast array of environments, ranging from the abyssal ocean and within bedrocks deep underground, to the stratosphere of the atmosphere; and from arid deserts to sulfurous springs. The entire area of the earth that is home to organisms is called the biosphere. Organisms are influenced by inorganic factors such as light, temperature, moisture, soil, and atmosphere, as well as the actions of a variety of other organisms. On the other way round, the life activities of organisms can alter environmental conditions (reactions on the environment). For example, the growing of grass on bare land leads to the formation of bright forests, which will change the illumination intensity and temperature conditions in the woods, resulting in piles of dead leaves in the area. This will increase the organic content in the soil. Interactions between living organisms also include competition for the same foods and predation (in which predators feed on prey) (see Chapter 3).

In this way, organisms live in varying and diverse environments, where they have to survive by living in a functional manner. This is called adaptation, which is discussed in the next section.


Adaptation to the environment: the effect of natural selection

Organisms have the morphology and traits that are physiologically, and ecologically suited to their way of lifestyles in their living environment. This is called adaptation. Their traits are so well adapted and function so effectively that it seems as if they were purposely designed to suit their lifestyles and environment. But of course, they are not the results of the efforts of individual organisms to flexibly respond to the environment. While generations continue to live in a certain environment for a long time, individuals with more adapted and advantageous genetic traits are likely to produce more offspring because of natural selection. Through the repetition of this process, and the long-term progress of evolution, organisms grow to develop the adapted traits that they exhibit today.

The conditions for natural selection are very simple: (1) the occurrence of mutation among individuals; (2) inheritance of the mutation; and (3) advantages and disadvantages of survival and reproduction that occur as a result of the mutation. When these three conditions are met, adaptational evolution by natural selection progresses autonomously. Adaptation is particularly evident in organisms that live in extreme environments such as dry, hot, or cold climates. Nonetheless, adaptation can be universally seen in mild climates as well, as long as the three conditions of natural selection are met.


Life-history adaptation according to environmental changes

Adaptation can also be seen in life-history traits (traits related to reproduction and survival) corresponding to changes in the environment. An example is the comparison of two closely related species of the Typha plant found in North America (Table 23-1). The two species—T. angustifolia from North Dakota (close to the borderline between the US and Canada) and T. domingensis from Texas (near the Mexican border)—were investigated by planting them in an experimental environment in New York. In comparison with Texas, North Dakota is an unstable environment, with fewer days without frost and a large variation coefficient. T. angustifolia is a short plant, which matures quickly, and forms numerous small rhizomes and many small spikes. The purpose is to distribute as much energy as possible toward vegetative and seed reproduction, in order to adapt to the instability of its environment through high proliferation potency. On the other hand, T. domingensis exhibits a contrasting trait, in that it adapts to the environment through competitiveness against a high density (Table 23-1).

Generally, for species that adapt to environmental conditions such as intense changes in weather or food supply which causes a high death rate in the larval stage, it is more advantageous to adopt the small and prolific strategy of reproduction whereby organisms produce eggs, seeds or offspring that are smaller in size but larger in number and distribute these widely to increase the chance of at least some of their offspring to survive (r-selected species). This type of species has a set of traits that include rapid larval growth, short adult lifespan, and explosive proliferation followed by the parent generation’s death, as typically seen with small insect pests and naturalized plants that are spreading across the world. In contrast, species that adapt to environmental conditions with stable climate and food supply face intense competition for food and habitat. For these organisms, it is more advantageous to adopt the large and less prolific strategy of reproduction whereby organisms produce eggs, seeds or offspring that are smaller in number but larger in size so that their offspring can grow into larger individuals with the competitive strength to reliably acquire resources (Κ-selected species). When comparing r-selected species and K-selected species, it is appropriate to compare species that are phylogenetically close to each other. However, some studies have compared vastly different species, such as the fly and elephant.

Table 23-1 Comparison of the life-history traits of two species of Typha plant found in North America


Bar-headed geese migrate over the Himalayas

Column Figure 23-1 Oxygen saturation in respect to oxygen partial pressure (PO2, unit: Torr)

The bar-headed goose, which flies over the almost 8000-m high Himalayas to migrate from India to Tibet, is an example of adaptation to severe environments through natural selection. Bar-headed geese possess a hemoglobin α-chain in which the 119th amino acid is changed from Proline to Alanine. This results in a gap between the 55th leucine in the β-chain, thereby altering the structure of the heme domain, and markedly increasing the affinity for oxygen molecules. Such a mutant gene must have been generated in the ancestors of bar-headed geese, and the allelic mutant gene spread among a group of geese. This group is thought to have eventually formed a separate species through reproductive isolation (see Selection 6 of Chapter 1). This is how the bar-headed goose became the only species that inherited this genetic trait allowing the birds to fly at a very high altitude for migration.

The terms r-selected species and K-selected species are taken from intrinsic growth rate (r) and carrying capacity (K), which are two parameters of the logistics equation (see Chapter 3). The method of adaptation to the environment through a set of traits under certain environmental conditions is called an adaptive strategy.

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