Evolution is a process which involves gradual change in genetic make up of a population in consecutive generations. It results from natural selection which acts on the genetic distinction among individuals leading to development of new species. The new species formed have a higher guarantee for survival. On the other hand, co-evolution occurs when two or more species interact in a way that each exerts selective force on the other so that the two evolve together. Nature has endowed various species with mechanisms for increasing their chances of survival. As a result, these animals have evolved a number of adaptations that assist them in accessing food and protecting them from their predators. Therefore, co-evolution can benefit, harm, or do nothing to the participants. Basing on this background, this paper seeks to discuss the various forms of biological relationships. In addition, the paper will describe an evolutionary path for each interaction discussed.

2.0 Biological Adaptations

2.1 Mutualism
This is a form of interaction between individuals of different species in which they derive survival benefits from each other. Such forms of interactions play a vital role in evolutionary biology. According to Barbier (2007), in terrestrial ecosystems, more than 90 of natural plants depend on mycorrhizal relationship with fungi. This is to provide inorganic compounds and draw elements. Moreover, mutualism is the driving force behind the biological diversity witnessed in the world. This includes things like different flower varieties ensuing from mutualistic pollination. However, determining the exact survival benefits for the different individuals involved in mutualism is usually very difficult, but those organisms which interact successfully with a mutualistic practice gain more than those that do not (Barbier, 2007). Therefore, mutualistic behavior leads to direct benefits of the individuals involved because it does not require any concern for the welfare of the partner.

Even if mutualism is like cooperative relationship between two different individuals, a distinction can be made. The difference lies in how mutualism involves an exchange of three very different types of benefits (Levin, 2000). Indeed, mutualism can be viewed as an economic exchange taking place in a biological market place. This means that organisms offer essential nutrients, which they find cheap to produce. Consequently, they receive nutrients that they cannot produce. A good example is the relationship between rhizobium bacteria and legumes species. This type of bacteria lives in the root nodules of legumes and helps in transforming atmospheric nitrogen into (NH3), which can be absorbed by plants. In exchange, the bacteria get carbon that is fixed by plants during photosynthesis. Likewise, the carbon is fed to mycorrhizal fungi. On the same note, the benefit fungi give to plants is that they increase access to soil phosphorous, which is essential for growth. The primary benefit of mutualism is very vital, and most researchers believe that such evolution is what permitted plants to occupy land about 400 million years ago (Levin, 2000).

In addition, mutualism can also provide transportation which is more common in pollination, where insects visit plants to get food in the form of nectar. The pollen is transported among the different plants that the organism visits. Interestingly, scientists argue that half the foods that living things consume result from such exchanges. Similarly, seeds of fruits dispersed by people are also an illustration of mutualism. It is a benefit because these seeds are carried way from their mother plant to other areas where they can properly germinate. The third form of benefit in mutualism is protection from natural enemies. For instance, ants protect a species of caterpillar called Lycaenidae and other plants. This interaction helps in attracting and ensuring a dedicated center of defendants.  Hermit crabs are also guarded by anemones such that in case of an attack, the anemones defend them using their stinging mechanism. The anemones are in return rewarded by being transported to richer feeding locations (Levin, 2000).

The above three benefits are categorized into the form of reward a partner obtains from a mutualism interaction. Moreover, mutualisms can also be differentiated by looking at whether they are symbiotic or not. Species are considered to be symbiotic if they are seen to be in close physical relationship for nearly all their lifespan. Although both mutualism and symbiosis are considered to be synonymous, a distinction can be made. In scientific terms, symbiosis means that one or none of the partners might benefit in case an interaction occurs. Only when it benefits at least one partner that it qualifies to be a symbiosis mutualistic. Therefore, not all mutualisms are symbiotic. According to Fox et al. (2004), the plant and Rhizobium relationship is an example of mutualism symbiotic. On many occasions, the symbionts are very similar that it is almost impossible to make a distinction. An example of this evolution is in eukaryotic cell. This type of cell is believed to have originated from a symbiosis between a primitive cell and bacteria that gave birth to the present day mitochondria (Fox et al., 2004).

Apart from the benefits associated with mutualism, there are costs that go with this form of interaction. The products and services provided by partners in mutualism constitute some form of investment. For instance, up to 20 percent of the total carbon of the plant can be fed to mycorrhizae and more than 40 percent is dedicated to producing nectar for pollinators (Bronstein, n.d.). In addition, mutualism may result into losses on partners. For instance, the ants that guard aphids sometimes consume them instead of defending them. Damages may also incur when organisms cannot survive in the absence of mutualists. This can be illustrated in cases where plants are unable to reproduce because their pollinators cannot persist in the sites of their location. Levin (2009) states that mutualism occurs only when the benefits received by each partner are higher than the costs experienced in the interaction. However, both the benefits and costs experienced vary with circumstances. A case in point, the interaction between a small hydroid and a hermit crab tends to be more beneficial to the hermit. This is because by placing the hydroid on its shell, other organisms are prevented from damaging the shells of the hermit. Conversely, the hermit also loses because hydroids attract marine worms which weaken the shell, making it possible for them to be crushed by the predators of the crab. In this case, the net impact of the relationship varies more for the crab. It will only be considered mutualistic if the worms and the predators are rare but hostile if the two are widespread. For this reason, mutualism is very complex and the outcomes depend largely on the context in which it occurs. This variation in turn affects the distribution and size of species population that depends on mutualisms. It is also seen to affect the manner in which the interaction evolves. Some biologists argue that the variation results are the raw material for the evolution of relations.

Furthermore, mutualism plays a key role in reproduction and survival of species in all environments. Therefore, it is important to work hard in defending the well-being of mutualists in such interactions. By doing this, these organisms will be conserved. One of the things that affect mutualism is habitat destruction, which results from land activities carried out by people. This form of disintegration forms a small number of species from the original population by weakening their connection due to dispersal. The decrease in number caused by breakup eventually reduces their mutualism interactions. In addition, alteration of habitat may lead to poor habitat quality for mutualists, and they may be so isolated that they cannot move in between their different locations. For instance, the forest fragmentation in Argentina has led to low seed manufacture of around three-quarters of plants in different fragments due to the loss of native bee pollination brought by forest disintegration. Despite the problems caused by loss of species in mutualism, additional of new species can similarly harm mutualists.

2.2 Commensalism
Commensalism is another form of interaction which benefits one organism without harming the other. The organism that benefits is called commensal, which is offered shelter, food, and transport by the host species. In this form of interaction, the host is usually larger than the commensal. In addition, the host remains unchanged, but the commensal is modified to suit the new environment.  Like mutualism, commensalism interaction is also considered as a symbiotic relationship. This is because the interaction involves different species. Similarly, commensalism can be categorized by the form of benefit acquired by the commensal. Therefore, there are three types of commensalism, namely, phoresy, inquilinism and metabiosis.

First, phoresy is as a result of transportation benefit in commensal association. This is a kind of biological hitch-hiking where one partner benefits through access to a form of movement, but the organism offering the service is not affected by doing so. A case in point, most plants have fruits that hold on to hair and as a result, they are dispersed by mammals as they move. Burdlock and stick-tight in North America are good examples of plants that are dispersed by animals. This is because their fruits contain anatomical adaptations for sticking to the hair. Rarely are there incidents where the organisms with hair are overloaded with the fruits causing damage. Therefore, this biological association is a good example of commensal interaction.

Secondly, epiphytic plants which grow on other plants are a form of commensalism called inquilinism. These plants benefit by living on larger plants because they gain access to a substrate upon which they do well in the canopy. Beside the benefits, they do not harm the host trees. Examples of epiphytic are orchids, ferns, and lichen as illustrated by Casagrande et al. (2004). Similarly, there are animals that are commensals like sea anemones. The anemones get access to food by growing on the upper carapace of crabs, and they do not cause any harm. In such forms of interaction, the commensals use hosts for housing.

Commensalism can also be described as metabiosis. This is an indirect dependency where the commensal uses something created by the host. However, this happens after the death of the host. For instance, hermit crabs use gastropod shells to protect their bodies. Poindexter and Leadbetter (1985) observe that commensalistic relationships may also be based on the modification of environment by one species. The modification is made in such a way that the growth of other species is permitted. For example, facultative anaerobes remove oxygen to allow growth of obligate anaerobes.

Although commensals do not affect their hosts, most biologists argue that a close interaction of two species may not be impartial for either partner. Therefore, many of the associations identified as commensal could also be mutualistic or parasitic in some ways that are not yet clear as seen in epiphytes. Epiphytes take large amounts of nutrients that are necessary for the host welfare. Consequently, these epiphytes might cause vegetation to break or darken, thus preventing photosynthesis from taking place (Poindexter  Leadbetter, 1985).

2.3 Camouflage and warning colors
Camouflage refers to the ability of an organism to blend with their surroundings so that other species may avoid them. However, this ability differs from species to species depending on several factors. For instance, camouflage depends on the behavior and physiology of an animal a case in point is in the face that hairy animals have a different form of camouflage from scaly animals or feathery. In addition, camouflage is also affected by the surroundings of the animal. The most common technique of camouflage is for the animal to match its background. In such cases the various objects in the environment are referred to as the model for the camouflage. It equally bears noting that camouflage also depends on the physiology and behavior of the predator of the animal. Animals develop only colors that will help them survive. Therefore, no animal will develop the color of the background, if this would not protect them from their predators.

Most animals, however, develop the color of their surrounding because it is the most effective approach. According to Gendernalik (2007), animals like hedgehogs and squirrels have brownish colors that blend with the brown color of the soil and trees in the forest. Also, sharks and dolphins have grayish blue color, which is crucial in helping them blend with the underwater coloring.

There are two ways in which an animal can develop different colors. Animals have microscopic normal pigments called Biochromes which develop colors chemically. This composition is in such a way that the chemicals absorb light colors and replicate others. The pigment color is a combination of many visible light wavelengths that reveal that pigment. In addition, animals use microscopic physical structures which act like prisms. In this manner, they are able to reflect and scatter visible light in order to reflect certain mixture of colors. Polar bears, for instance, though they contain black skin, emerge as white due to their clear hairs. The light waves shinning on the skin of polar bears are bent by each hair such that, some light goes to the surface of the skin while the rest is deflected back out. That is how the white coloration evolves. The two ways of coloration are genetically determined and are developed progressively by natural selection. Normally, an animal that blends with the background is more likely to be ignored by its predators, increasing its chances of survival. For this reason, an animal that looks like it environment has high chances of reproducing than the species that does not. Subsequently, the offspring of the species that camouflage will inherit the identical coloration and will also live long to hand it over to future generation.

Although camouflaging coloration is common in environment with most species, it is not common for species to change coloration to look like a changing surrounding. But there are animals that have formed special adaptations that allow them change their color as the environment changes. Good examples are animals and birds that are able to deal with the changing seasons. For instance, during summer, green and brown are the common colors and in winter the surrounding is covered with snow. Therefore, mammals and birds cope with this by developing various colors of fur and feathers which suit the time of the year. They either change their quantity of daylight or the temperature changes activate a hormone that helps the animals to produce various biochromes. The use of color for defense is called cryptic coloration

Many small animals have also evolved toxic chemicals that make the organism poisonous for predators (Montgomery, 2009). However, even with this powerful weapon for defense such animals need to notice predators before they are attacked. This can only be possible by the evolution of warning coloration also called aposematic. An example is the monarch butterfly larva which uses coloration to communicate about its presence. The larva feeds on leaves called milkweed which are poisonous to vertebrates. The toxins are then stored, making the larva distasteful to its vertebrate predators, and this continues even after metamorphosis. As such, the adults too are poisonous to vertebrates. In addition, most of these species are bright-colored, which makes it easier for the predators to see. Scientists are in agreement that the bright coloration has developed to help the predators because they have the ability to associate bright colors with toxic species.

2.4 Mimicry
Mimicry is another form of symbiotic relationship that is somehow indirect. Here, a species imitates the actions, mold, or even color of another species so as get close to the prey or  gain protection in case of any imminent danger. In 1963 at the International Zoological Congress, mimicry was defined as the close resemblance of a species to another, which is a result of its often unpleasant and conspicuous recognition by the predators (Tomar  Singh, 2003). Drawing from these definitions, one can say that mimicry is an apparent yet close match of one organism to another for protection or disguise. In addition, mimicry is a tool employed by nature to defend creatures against their natural enemies. The organism that mimics is known as the mimic and the one to which it imitates is the model. Mimicry complex is a result of the set of mimic and model species. As such, mimicry is an experience of living creatures and not of dead species.
Mimicry can take various forms. In most cases, it involves species which have evolved to look like inanimate things and thus, it is a form of camouflage. In fact, Paulton (1889) points out that there are five different types of mimetic colors in which the mimic resembles the model. Those colors that cause the organism to look like some part of its surroundings are called apatitic, and those that are changed for sexual experience are Epigamic colors. Similarly, mimics may acquire semantic colors so as to warn their enemies. There are also incidents where the mimics use colors which blend with their background to hide themselves the colors are called cryptic colors. For instance, desert insects look like sandy items in their surroundings. Moreover, mimics use pseudoaposemtic colors similarly as apatitic ones to resemble other organisms for confusion. The evolutionary benefit of such forms of mimicry is to enhance the ability to flee from recognition.

Several insects have also developed resistance mechanism against their predators by becoming unpleasant and poisonous to eat. This is only beneficial if the predator knows before they try to feed on the disgusting species. According to Gendernalik (2007), there are four types of insect mimicry, namely, Batesian, aggressive, Wasmannian and Tephritid mimicry. Batesian mimicry was first observed by Bates (1862) where he presented an assumption explaining the related color forms in some species of tropical butterflies in diverse families (as cited in Gendernalik, 2007). Bates (1862) thought that an edible butterfly class vulnerable to predation would evolve because of selection by a bird predator to resemble a disgusting model species (as cited in Gendernalik, 2007).

In his argument, Bates (1862) hypothesized that if the mimics were rare than the model, then the predator would get the unpleasant model more often (as cited in Gendernalik, 2007). As a result, the birds would learn to evade all butterflies that resembled the disgusting ones. In Batesian mimicry, the model is distasteful while the mimic is not and therefore, predators will learn to avoid the model. Consequently, the distasteful mimic is guarded since it looks like the unpleasant model (Gendernalik, 2007). A good example of Batesian mimicry can be illustrated by Harlequin Snake eel, which mimics the Banded sea snake called Laticauda colubrine, which is a very toxic species with noticeable black and white warning coloration (Abbott, 2000). In this form, predators are definitely going to avoid it.

Aggressive mimicry is mostly shown by carnivorous animals like fish species as well as spiders and can be divided into two, namely, concealing and alluring (Tomar  Singh, 2003). In concealing mimicry, the mimic develops cryptic coloration.  Examples are spiders which exist on golden rod and various flowers. These spiders have yellow color that looks like that of the flowers on which they are found. As such, the spiders become invisible to the predators. Moreover, there are other species of spiders which look like vegetation or compost of the birds and therefore cannot be seen by their predators. Alluring is the second form of aggressive mimicry, in which the mimics blend to their background and attract their preys. For instance, an angler fish blends with the bottom of the sea and its first finray of dorsal fin is modified. The fish carries a fleshy cutaneous appendages-bait with its open and as such, the fish can turn around at all directions without any obstruction. Therefore, if any fish moves at its free lure, the angler fish is in a position to eat it immediately. Furthermore, there are spiders known as Lasso that can produce one horizontal strand, which they hang themselves with. They also produce another strand with a sticky end bead, which they use for trapping, held up with one of the legs. In this manner, the spider is able to draw the flying insects (Tomar  Singh, 2003). In aggressive mimicry therefore, a mimic looks like another harmless model and uses its harmless look to either attract the prey to close distance or safely gets into web or territory of the prey to consume it.

Mimicry is termed as Wasmannian mimicry if the mimic imitates the model so as to enter the region of the model in order to hide among the full colony or feed on the resources of the model. This kind of mimicry was first described by Wasmann and named by Rettenmeyer in 1970. In this form of mimicry, the mimic may not be harmful to the host. An example of Wasmannian mimicry is with the flying spiders which closely look like ants so as to get lost in the colony and therefore avoid the predators.

There is yet another form of mimicry in which the mimic looks like the predator so as to run away from it. Such mimicry is called Tephritid and the mimic is usually a fruit fly and the model jumping spider. For instance, a tephritid fly mimics a jumping spider by raising its wings to show salticid leg look-alike patterns then the spots on the end of it abdomen imitate the eyes of the jumping spider. In addition, the fly closely mimics the behavior of waving the leg and jumpy movements of the jumping spider by moving its wings and dancing from side to side. Consequently, the jumping spider displays the same jumpy behavior when another salticid gets into its area. Similarly, the salticid that wanders into the territory of another salticid will show back before moving back. When a salticid finds another conspecific, it will move back because the first salticid inhabitant is dominant. Most flies employ wing flicking and markings behavior during courtship. Therefore, it is likely that the tephritid mimicry evolved from courtship behavior of salticid.

According to Montgomery (2009), mimicry is not only an animal behavior of animals but is also common in plants. A good example of mimicry in plant is with the flowers of Bee Orchid. In this case, the flowers rope in insects to aid in pollination. This is done by giving out some appetizing treat. The Bee orchid flowers have evolved to mimic female bees. By doing so, they trick male bees into thinking that they are being offered some sort of delicious treat. In some cases, the orchids release sex hormones to attract males. Consequently, the male bee will try to mate with the fake bee and in the process, it will pick up and drop the pollen, eventually helping to fertilize the orchids. This form of mimicry is quite impressive but can be explained as insect mimicry. At first, any form of alteration that is seen by a bee would be favored as the flower. This would increase chances of the flower being fertilized and its pollen dispersed. In addition, due to time and energy wastage by mating with a flower, the bee would avoid the enticing plants. As a result, the evolution of more bee-like flowers would result. Therefore, it can be stated that the modern day excellent mimicry evolved after some generations.

3.0 Conclusion
From the research, it is clear that many of the adaptations employed by organisms to suit in their environment are all forms of co-evolutionary processes. Co-evolutionary is therefore very crucial for biological diversification. For instance, eukaryotic cell, the founder of multicellular life, co-evolved by symbiosis among two early forms of existence, where one acted as the host and the other as the organelles. Today, the host and the symbiont are highly specific to each other. In addition, each cell depends on the other for some cellular functions and cannot function independently because of some lost genes. Similarly, plants evolved in the same manner. Moreover, animals rely on co-evolved symbiosis especially in nutrition. This is where animals rely on symbionts to break down complex chemicals or give out essential vitamins for growth. However, co-evolution depends on the manner in which species interact with one another as illustrated in the discussion above. Finally, the co-evolutionary process allows the history of life in the world, biodiversity, and arrangement of biological communities. For this reason, by learning how species interact and adapt to the environment, one learns more on co-evolution. No species can live without interacting and depending on the others and therefore, co-evolution will continue to be an important aspect in the continued life of adaptation and diversity.          


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