Previous sections have indicated the importance of homeostatic control, the requirement of a set environment and how evolution and natural selection adapts a species to favour its environment. In more extreme ecosystems, these laws of nature are essential. These types of evolutionary changes are as follows;
These types of changes in an organism affect its actual structure, for example, birds occupying islands further away from its natural habitat may evolve to grow longer wings over time due to its more remote nature.
Physiological changes are changes in the actual biological processes of an organism, for example, over millions of years, mammals, though diversifying, developed different limbs to suit the way they operate in an environment, such as the nimble fingers that a human possesses, for skills such as typing via the adapted pentadactyl limb that we possess.
Changes in the way an organism responds and interacts to its external environment. This is illustrated by the kangaroo water rat, an organism that hibernates during hot periods of the day to conserve water. This is an example of a behavioural adaptation in response to the prevailing conditions of the environment
This is called dormancy, where all unnecessary actions and reactions (i.e. moving) are restricted to preserve the limited resources and foodstuff available to them. In such ecosystems, organisms must pay close attention to how they direct the resource of water in their body. Here is why, the many possible ways we lose water from our bodies. Surprisingly, much of the water we lose from our daily intake is from respiration, as when we breath out, water is also accompanies the waste CO2.
All in all, these changes are necessary for an organism, and its species to survive in a particular ecosystem. Animals (and plants) that live in an aquatic environment over time have become suited to their habitat. To other animals, this would be a problem, you may notice if you stay in the bath or swim under water for a long period of time that your hands and feet become wrinkled. This is because osmosis is occurring between you and the aquatic environment.
The higher concentration in the external environment means that the concentration gradient of osmosis must even out, therefore the cells in our hands and feet become turgid (a higher than normal concentration of water). With this in hand, aquatic animals must perform osmoregulation to ensure that their bodies internal environment remains at its optimum level. Take note that osmosis will always occur until their is an equilibrium between the internal and external environment (isotonic concentration). Our genomes' are used to dry environments, so this deviation from our body's norm can indeed indicate that we are less suited to surviving in a water environment because we are not adapted to do so any more, unlike our distant ancestors.
In regards to organisms in a freshwater habitat, they face the problem of continuous osmosis of water in to their bodies, because their body is hypotonic in comparison to their external environment. This means that water will continue to diffuse into the fish until their is an isotonic concentration between the two environments. Since the fish would not be able to operate with such a high concentration of water in their system, evolutionary adaptations have overcome this problem. The kidneys which deal with the uptake of water are suited to their function, as the glomerulus' of the kidney has a large surface area to reduce the concentration of water in the blood. As these kidney tubules allow a large scale movements of ions out of the bloodstream, the organism needs to reabsorb some important ions such as salt back into the bloodstream via active transport.
You can relate this osmoregulation to the way glycogen is stored in the kidney of humans, as the homeostatic controls involved with this area are there primarily to ensure that the conditions inside the organism are suited to the demands put upon it. Excess water within such an organism would damage the internal organs of it beyond repair by damaging the cell membrane and organelles due to the pressure caused by the water.
The opposite problem exists in animals that live in the ocean, where the salty water means that water is at a lower concentration than most organisms. The result here is that the organism is hypertonic to its external environment (has a higher water concentration), and must re-absorb water to remain in a healthy state. Evolutionary adaptation has also favoured these animals, and the kidneys have been anatomically altered over time to ensure the long-term survival of such organisms. The glomeruli in such organisms have a much smaller surface area, because the need to excrete ions into its external environment is not as extreme, because most water needs to be retained for survival, unlike freshwater organisms. Little salt needs to be re-absorbed due to the fact the salt-water environment can be drunk, and therefore is not as scarce as it would be for a freshwater organism.
The key point is that these organisms need to maintain an optimum water concentration in their internal environment to continue to function normally and optimally.
In more extreme environments, such as the desert, water is scarce and water must be stored in times of need, hence a camels humps.
Much water can be lost via sweat, under extremely hot weather or strenuous exercise
Excretion via urine and faeces also accounts for much of our water loss
Water is also required by many of our biological processes, and is therefore an exhaustible resource for these continuing processes.
Behavioural adaptations come into play here, as some species, such as the kangaroo rat lay in long periods of dormancy when the weather is hot, which would result in the rat losing more water through respiration than it can afford. The rat lays dormant underground in moist conditions where the concentration in water in the air and the rats lungs is at its minimum, therefore the least amount of water is lost to the external environment.
All these adaptations are designed for the organisms to survive more tolerably in their present habitat, and without these evolutionary changes, some of the species on the planet may not have survived to this point.
A plant requires water as an essential ingredient of photolysis, the photochemical stage of photosynthesis where water is split using light energy. This is the part of the process in which a plant obtains its' energy, and thus illustrates the importance of water to a plant.
Water is absorbed from the root and is transported to all areas of the plant, this passage of water is called the transpiration stream. Water is absorbed by the roots of a plant, which possess many root hairs with large surface areas for extensive absorbing of water. We have already discussed how osmosis occurs across a concentration gradient in preceding tutorials, and this is the case here.
When water is required an the concentration of water in the roots is low, water is absorbed from the higher concentration of water found in the ground. This same occurrence goes for the concentration difference between cells, as the water rich root cells allow osmosis to occur up the plant until their is an equal concentration throughout the organism.
Scientists were previously perplexed by the nature in which water could defy the force of gravity and move up the plant, since water molecules are heavier than the surrounding atmosphere. Through extensive studies, the following factors are thought to play a role in pushing water up the transpiration stream.
This factor is brought about by osmosis and the unequal concentration of water across the plant. Osmosis will occur up the plant until their is an equal concentration
This is due to adhesion, the force of attraction between two different particles. When water passes up the thin xylem tubes, it adheres to its surface area, while the force of osmosis gently pushes the water molecules to their desired location
This is thought to be the major force that allows water to be transported throughout a plant. Water is transpired by a plant via stomata, which means water concentration in these areas will be especially low. Since osmosis occurs across a concentration gradient, water will 'rush' into these areas to even out the water concentration across the plant.
It is an eventuality that water is lost in some way, such as through the stomata, similar to how humans lose water through respiration when breathing out. Stomata are also the source of CO2 which a plant required for photosynthesis, and while stomata open in daylight for the intake of CO2 when most photosynthesis occurs, this allows an increased loss of water to the external environment.
These stomata structures alter in size according to their turgor, which is mostly determined to the water concentration in them. When water concentration drops in the stroma areas, the opening to the external environment closes. Stomata therefore play a regulating role in the homeostasis of water control. During a particular hot day, while the stomata is open for photosynthesis, transpiration can occur at a much higher rate due to the temperature. When this occurs, the water concentration drops in the stomata and therefore it loses turgor, and in turn the stomata opening closes. This ensures that water concentration is kept near its preferred level. This type of water transpiration occurs in mesophyte plants, which occupy climates that are of average rainfall and of average temperature.
Xerophytes, such as many species of cactus, live in more extreme environment. Evolutionary adaptations has allowed them to survive and prosper in such ecosystems as the deserts of the Earth. These evolutionary adaptations effectively reduce the transpiration rate of a plant, and promote water retention, and are as follows;
The leaf surface curves in on itself, meaning the water transpired remains in close contact with the leaf. The result is a high concentration of water adjacent to the leaf surface, which will move across the concentration gradient of water back into the plant. In this instance, less of the leaf surface area is exposed directly to the atmosphere.
The same effect is acquired in the concept of double insulated walls that are used in modern housing, where a cavity between inside and outside reduces the rate of diffusion between cold and hot air - meaning that heat retention is improved in a similar way to the water retention example above.
This has the same effect of curved leaves, preventing a clean break of water into the atmosphere from transpiration. Water molecules stick to these hairs, due to the adhesive attraction between the two structures.
Less stomata across a plant means less opportunity for water to escape.
Varying species of xerophytes also possess other various evolutionary adaptations suited to their existence. Areas that lack a steady flow of rainfall house plants with roots are spread across the grounds' surface, to absorb as much water as possible while they have the chance.
Hydrophytes are plants that live their lives in an aquatic environment, and face different problems from xerophytes. They receive oxygen they require from fluid-filled spaces that store oxygen for as and when it is in demand. One other problem (comparable to the wind) is the continuous water currents that barrage the exterior of the plant. In water, things weigh around two and a half times lighter than they would above ground. The plant must have a flexible nature to cope with the strains of this water current, and evolutionary adaptations in such hydrophytes has reduced the amount of xylem found in such plants. This is elementary as these plants, which are surrounded in water do not have to rely on the forces related to the transpiration as much as their above ground counterparts.
The water cycle, sometimes referred to as the hydrological cycle, is the continuous transfer of water from air, sea land and water in a continuous cycle. This continuous movement of water is sufficient to provide good living conditions on the planet while at the same time transferring the water to places where it is needed as a norm.
Water in the oceans evaporates with long, direct exposure to the heat from the sun, and with the warm air above the warming ocean, the droplets of water rise as cold air rushes in to fill the gap of the rising warm air. This elevation of water is essentially cloud formation.
The clouds are easily moved in the direction of the wind, and this driving mechanism pushes the cloud in any given direction, while more water evaporates from the ocean as well as transpiring from plants and animals. The clouds, which eventually rise to high altitude and discharge their water as they hit the cooler air.
This rainfall can end up in a variety of places in the short term future, many of which in some way promote advantageous conditions for life to take advantage of it; water being an essential compound for countless species.
The next tutorial looks at the freshwater ecosystem, which plays an important part in this water cycle but is also an important habitat for many species in which they depend on (and have adapted to) for survival.