Tuesday, March 18, 2014

The Coevolutionary Process


What Is Coevolution?
Coevolution is the process of two species putting selective pressures on one another. The reciprocal selective pressures allow both species to evolve, while not allowing one to get ahead of the other [4]. Coevolution is most likely to occur between species with a persistent relationship that are in close ecological interaction with each other. There are three types of symbiotic relationships that are formed through coevolution. These relationships include: predator/prey, parasite/host, and mutualistic [2].

Cuckoos and Reed Warblers
http://s-f-walker.org.uk/2007intranet/2ndwk/cuckoo-eggon-edge.jpg [6]
The relationship between Cuculus canorus (cuckoo) and Acrocephalus arundinaceus (reed warbler) is an excellent example of coevolution between a parasite and its host. The cuckoo is a parasitic bird that tricks other bird species into raising their own young [4]. While the reed warblers are absent from their nest, the female cuckoo swallows the egg of the host species and then replaces the egg with her own young. The tricked reed warbler parents then proceed to raise the egg as their own. Once the cuckoo egg hatches, the cuckoo tosses the host eggs to their death [4]. Once parasitized, the reed warbler’s reproductive success is zero. Therefore, genetic variation that allows the reed warblers to stop the cuckoos will be naturally selected for and passed on to future generations [4]. The cuckoos then reciprocate and evolve traits which allow them to persist longer in the host nest. Both species are in a constant arms race to outsmart the other species [3].
The female cuckoo will lay eggs that appear to mimic the appearance of the eggs of their hosts, which hinders discrimination and removal of their eggs by the reed warblers. In the first stage of coevolution between C. canorus and A. arundinaceus, natural selection favors the birds that better discriminate cuckoo eggs from their own [4]. This rejection behavior then puts selective pressure of the cuckoo to lay eggs that resemble the host egg. This mimicry puts more pressure on the reed warbler to rejection a foreign egg even if it greatly resembles its own [3]. This cycle of better detection and rejection continues as long as there is continual renewal of genetic diversity in both populations.

Acacia Tree Ants
The relationship between the Pseudomyrmex ferruginea (acacia ant) and the Acacia cornigera (bullhorn acacia) is another example of coevolution. Unlike the cuckoo and reed warbler, this is a mutualistic relationship. The acacia ant no only depends on the plant for food and shelter but it also protects the bullhorn acacia from preying insects and other plants [1]. The acacia have evolved traits in order to support this mutualistic relationship. The tree has swollen and hollow thorns that serve as both the ants’ home as well as their protection. The tree also provides the ants with food both as nectar and Beltian bodies [1]. The ant has also evolved specific characters to aid in maintaining this mutualism. The ants serve as a defense against herbivores and they also remove fungal spores in order to prevent fungal pathogens from entering the plant. The characters of both the ant and the acacia are mutualistic traits that have evolved for the interaction in reciprocal fashion [1].

The Cost of Coevolution
Coevolution plays a critical role in generating genetic diversity. However, coevolution can come at a cost, especially between parasite and host. In order for two species to both evolve, each much exert energy to acquire new traits. Investment in certain traits can be costly, and can lead to a decrease in fitness. For example, the male reed warbler increases surveillance of the nest to decrease the chance of parasitism [4]. This leaves the female reed warbler susceptible to fertilization by another male. The cuckoo birds also have costs to developing new traits. The smaller size egg more closely resembles the host egg but it is also an easier prey [4]. Both species have costs to pay if they are going to be successful enough to pass their genes to the next generation.

References
[1] http://fission.sas.upenn.edu/caterpillar/index.php?action=retrieve&article=Janzen%2C1966coevolution.pdf
[2] http://evolution.berkeley.edu/evosite/evo101/IIIFCoevolution.shtml
[3] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2603233/
[4] http://books.google.com/books?id=bEtEL2z3hXcC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
[5] http://www.youtube.com/watch?v=Xm2qdxVVRm4
[6] http://s-f-walker.org.uk/2007intranet/2ndwk/cuckoo.htm

 

10 comments:

  1. This comment has been removed by the author.

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  2. I find the idea of co-evolution very interesting because it shows a constant struggle to survive. As discussed on here and in the book "The Art of Being a Parasite" the cuckoo bird and reed warbler have a fascinating relationship involving raising of their young. Both species continuously evolve in order to change the eggs and stay one step ahead of the each other. Another good example of co-evolution to check out would be the garter snake and the rough skinned newt. http://news.stanford.edu/news/2008/march12/newts-031208.html

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  3. Your post discussed two very good examples of co-evolution and they both had different outcomes. Co-evolution is such an interesting topic because there is a wide array of relationships that can define its meaning. Another example that I find interesting are the Labroides, which are cleaning fish. These fish are able to survive by eating the parasites that are on the outside of their "clients" (larger fish) because they are immune to the parasites. The client then benefits by having their parasites removed. Together the cleaner fish and the client are able to benefit. The unfortunate part of this relationship is the cost of being eaten, but I guess it's a risk these organisms are willing to take in order to survive.

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  4. It is interesting how reciprocal selective pressures cause 2 species to evolve in unison. One species does not become highly adapted while the other species stays exactly the same. For example, the reed warbler and the cuckoo bird evolve adaptations for each other at similar speeds. If one of the birds lagged at developing new traits, the other bird species could die off quickly. We do not hear of many species that can not keep up with the evolution of another species. However evolutionary lags do exist in nature. Stenseth and Maynard Smith compare an evolutionary lag between species to a lagging runner who sees his competitor race away. Natural selection can be very quick, but at times it can also be slow.

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  5. The example of the cuckoo and reed warbler is intriguing because it puts into prospect how intelligent animals are and how nature gives both sides certain advantages. On one hand, the cuckoo places its eggs into the warbler’s nest and the newborn cuckoo just takes over the host’s nest. On the other hand, to balance that, the warbler develops ways to counteract the trickery by having its own ways of discriminating between eggs. An example of co evolution that we see everyday is between beetles and flowers. This is an important relationship because the two rely on each other to get their jobs done. Plants make colorful flowers and the pollinators come and pollinate the flower where as the pollinator gets food from the flower. It is interesting to see the different relationships that exist between species and how they exist and work perfectly together.

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  6. Your blog explained co-evolution very well with defining co-evolution and also pointing out the 3 types of symbiotic relationships that are formed through co-evolution. Apoptosis is also another example of co-evolution in which the cell is now able to program itself to commit suicide when affected by a parasite. Specifically to how it explains co-evolution is that the parasite has now been able to counteract that through natural selection in which it is able to send signals to actually block apoptosis from killing the both of them. These examples that you have provide along with the one I just gave of co-evolution show and explains also how the Red Queen Hypothesis was able to come about. Even going back to your example of Cuckoos and Reed Warblers, with the Cuckoos evolving to produce better mimicry of the Reed Warblers eggs and with the Reed Warblers evolving to better recognize the parasitic eggs, you can see how this can be associated with the Red Queen Effect. Even though both of these species are evolving, they “keep in the same place.” It is very difficult to separate the Red Queen from co-evolution because as Leigh Van Valen reported, it is “what makes evolution proceed.”

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  7. Co-evolution is an intriguing topic. I like the examples that you used such as the cuckoo bird and the Acacia ant. These are two very good examples. Another example comes to my mind as well when I think about co-evolution is the example between figs and agaonids. This relationship is more of a mutualistic relationship. They are still in coevolution because they are two species with a specific relationship in close ecological interaction with each other, as you have stated. The fig must rely on the wasp to carry its pollen to another fig and the wasp must rely on the fig to provide a safe area to reproduce. These two completely different organisms rely heavily on each other for reproduction. Without one or the other, neither of them would be able to reproduce efficiently. Although this relationship involves two species that are very dependent on one another, it works perfectly because they have co-evolved for these purposes.

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  8. You're topic of co-evolution is very interesting and also very evident in the animal world, but I find it even more intriguing in the human world. A very quick example of humans using coevolution while fighting against one another can be seen through the cold war. While the US and the USSR were competing with each other in the space race, espionage, and nuclear races, they were also teaching us all a lesson in coevolution. For example, every time that USSR sent a more experience spy to the US, the US learned a better way to detect these spies. Also this human example hints at the Red Queen Hypothesis, since whenever the US made a better nuclear missile, the USSR did the same, and the arms race (literally) continued, but in the end the two had equal power even though the new missiles were 10x better than the first batch each made.

    Even though we focus on animals and parasites in our class, it is also interesting to see how humans are also related to these topics.

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  9. Another relationship that I inferred could be an example of parasite/host co-evolution, which as you mentioned, is one of the major relationships demonstrating co-evolutionary strategies, can be seen in our own species. Human Immunodeficiency Virus (HIV) devastates millions around the world; one of the reasons the virus is so hard to treat is due to its camouflage nature. For instance, the virus uses the enzyme reverse transcriptase to turn its RNA genetic material into DNA. It can then insert itself into the host genome, virtually hiding itself. The host immune system then ends up destroying their own cells, as it cannot decipher between “self” and “non-self.” To combat HIV’s camouflage nature, such as through increasing the potency of white blood cells for example, a stronger host immune response is selected for. Countering this, HIV strains that have increased abilities to remain hidden in our genome will be selected for. Without any permanent solutions (medications or vaccinations) to the disease, the back and forth conversation continues between the two players of the relationship.

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