Hudson River Tomcod Evolution

Over a span of nearly thirty years, more than a million pounds of polychlorinated biphenyls (PCBs) were released into the Hudson River, severely polluting the water and harming the wildlife that inhabit it. These toxins, which are responsible for the death of many fish and birds and the cause of many serious health problems in humans, settle to the bottom of the river, as they are unable to break down. One species in particular that was affected by this extreme level of pollution was the Hudson River Tomcod. Now, nearly sixty years since the Hudson River Tomcod’s initial exposure to these harmful toxins, scientists have discovered that this organism’s exposure has allowed it to develop a resistance to the PCBs. The Hudson River Tomcod’s ability
But, those fish embryos, like the Hudson River Tomcod, that possess a resistance to PCBs, are able to develop normally. Recently, scientists have discovered the type of adaptation that has allowed the Hudson River Tomcod to develop a resistance to theses contaminants and it has increased our understanding of the process of evolution. The receptor proteins, AHRs (aryl hydrocarbon receptors), which are present in the cytoplasm of cells of all vertebrates, play a large role in the Tomcod’s PCB resistance. PCBs, when diffused into cells, bind to the AHR proteins, and are carried into the nucleus, attaching to the cells DNA and activating genes that would normally not be activated. But, because of a mutation in the gene that codes for AHR, this pathway is blocked in the Hudson River Tomcod, preventing the PCBs from binding to the AHR proteins and prematurely activating genes. The mutation in the gene which encodes the AHR protein is a simple six base pair deletion, resulting in a protein that in only two amino acids short of the normal protein, which is made up of roughly 1000 amino acids in total. This random mutation led to a tremendously beneficial result for those Tomcods that live in a toxic environment, such as the Hudson
Those species above the Hudson River Tomcod on the food chain consume the fish and then are unable to resist the PCB contamination in a way such as that of the Hudson River Tomcod. This is problematic because normally species, which consume Tomcods, such as birds, would not eat PCB-contaminated food because those organisms that are contaminated typically do not survive long enough. This is one way in which the evolution of one species also has an impact upon other species within a particular environment, something which is often referred to as coevolution. In regards to the Hudson River Tomcod, natural selection occurred because the environment chose the trait that was most suitable for their environment; PCB resistance. This resistance allowed them to thrive in an environment where many other species struggled to survive. Without the initial genetic mutation that was responsible for the development of this species PCB resistance, there is a high chance that the Hudson River Tomcod may have been extinct today. The manner in which the Hudson River Tomcod changed over time makes this species a prime example of present day