co-evolution

Co-evolution is concerned with the joint evolution of two or more species resulting from their ecological interaction (Kardong, 2005).

Organisms must continually change their genetic make-up (to adapt to a changing environment) if they are going to survive and reproduce. The competition for resources is fierce. Although organisms certainly must constantly adapt to a changing physical environment, uncommonly is the physical environment the reason for death or the cause of non-reproduction. Most often it is other organisms (parasites, predators, and competitors) that prevent survival and/or reproductive success (Ridley, 1993). Van Valen (1973) referred to the concept (he considered it to be a new evolutionary law) of the need for continual change of the genome of an organism , as the "Red Queen Hypothesis". The Red Queen is a reference to the Red Queen chess piece in Alice Through the Looking Glass (by Lewis Carroll, 1872), who told Alice that here (in this land) ".....it takes all the running you can do, to keep in the same place." (Prothero, 2004; Ridley, 1993). So, if species are going to avoid extinction, they must constantly improve their survival and reproductive fitness (Prothero, 2004). Thus, an evolutionary arms race can co-evolve between a predator and its prey, parasites and their host, or competitors for the same resources. For example, a newt develops a toxin within its flesh so that predators will avoid it. However, natural selection will favor the variants of the predator that are more resistant to the toxin and that can eat the newt and withstand the toxin. So, the newt becomes more toxic to the predator. The predator evolves more resistance to the toxin, and so on. There are many examples in nature of this evolutionary arms race.

Also, according to the Red Queen Hypothesis, sexual reproduction persists because it enables many species to rapidly evolve new genetic defenses against parasites that attempt to live off them. For example, if a certain bacterium species infects humans, some people are more resistant to the bacteria than others. The bacterium species may kill large numbers of people, but those exposed that were not killed have a higher degree of immunity to the effects of the bacteria, however, they still may get sick. But their defense mechanisms (like white blood cells) were better able to kill the bacteria that invaded the body. However, the variants of the bacterium species that were not killed by the body defenses of the more resistant humans, multiplied and became even more deadly. The humans that were not killed by the more deadly form of the bacterium species, produced offspring that inherited their higher degree of resistance to these bacteria. Any mutations in the bacteria to make them better able to cope with the defenses of the humans would be selected for. The same is true for the humans, any mutations or different variants of defense mechanisms that arose through sexual reproduction (and reshuffling of the genes) that favored resistance to the bacteria would be favored and passed on to offspring. Thus, the arms race would go, but in the end neither the bacterium species nor the human hosts would have gained much ground (a zero sum game).

The following statements fromWikipedia, the free encyclopedia on the internet at http://en.wikipedia.org/wiki/The_Red_Queen will help explain the Red Queen Hypothesis:

"The recognition of a simple example of a biological arms race (from Richard Dawkins) can be achieved by considering the contrast between two adaptations of the polar bear. This animal has a coat of hair which is thick to help the bear survive the cold of the arctic and white in order that the bear can stalk seals for food. For the first case the selection pressure is likely to be constant or subject to random change, in the second case the selection pressure is likely to increase steadily as selection for cautiousness in seals makes the average seal harder and harder for the bear to stalk successfully. As a result both the bear and the seal find themselves running a red queen's race over evolutionary time, each becoming better and better adapted (to stealth and caution respectively) but neither becoming any more successful (as they are engaged in a zero sum game).

Science writer Matt Ridley wrote a book The Red Queen in which he discussed the debate in theoretical biology over the adaptive benefit of sexual reproduction to those species in which it appears. The connection of the Red Queen to this debate arises from the fact that the traditionally accepted theory (The Vicar of Bray) only showed adaptive benefit at the level of the species or group, not at the level of the gene. By contrast, a Red-Queen-type theory that organisms are running cyclic arms races with their parasites can explain the utility of sexual reproduction at the level of the gene by positing that the role of sex is to preserve genes which are currently disadvantageous, but which will become advantageous against the background of a likely future population of parasites.

Sex is an evolutionary puzzle. In most sexual species, males make up half the population, yet they bear no offspring directly and generally contribute little to the survival of offspring. In addition, males and females must find each other to mate, and sexual selection often favors traits that reduce the survival of organisms. Thus, sex is highly inefficient.

One possible explanation for the fact that nearly all vertebrates are sexual is that sex increases the rate at which adaptation can occur. This is for two reasons. First, if an advantageous mutation occurs in an asexual line, it is impossible for that mutation to spread without wiping out all other lines, which may have different advantageous mutations of their own. Second, it mixes up genes. Some genes might be advantageous only when paired with other genes, and sex increases the likelihood that such pairings will occur.

For sex to be advantageous for these reasons requires constant selection for changing conditions. One factor that might cause this is the constant arms race between parasites and their hosts. Parasites generally evolve quickly, due to their short lifespans. As they evolve, they attack their hosts in a variety of ways. Two consecutive generations might be faced with very different selective pressures. If this change is rapid enough, it might explain the persistence of sex.".

Symbiosis (living together) is a co-evolutionary relationship between two organisms. The relationship may be good for both organisms, good for one and neutral for the other, or good for one at the detriment of the other.