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BILD 3 - Lecture 26

BILD 3 - Lecture 26 - Often if the two species share some...

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Often, if the two species share some but not all resources, there is no way in which one species can become so numerous that it drives out the second species. It is this fact that leads to the coexistence of different species in the same ecosystem, even though they share some components of their ecological niches. If the product of α 21 and α 12 is less than one, so that there are unique resources available to each species and the competition between the two species is low, then the total numbers of the two species that can live together in an ecosystem is larger than the numbers of either species that could occupy the ecosystem by itself. Such a situation is shown in the figure below, in which K 1 = 300, K 2 = 300, α 21 = 0.5, and α 12 = 0.5. The numbers of each species that can occupy the ecosystem alone are 300, but the total carrying capacity is 400 (200 of each species) when both species are present and the competitive interactions have led to an equilibrium. Provided that α 21 and α 12 do not change, the two species may persist indefinitely. 200 + 200 = 400 individuals of the two species can coexist at equilibrium Numbers of species 2 Numbers of species 1 K 1 / α 12 K 2 / α 21 If species 2 could reach 600, then species 1 would be driven to zero If species 1 could reach 600 then species 2 would be driven to zero Shaded area shows the total number of individuals possible
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Evolution and Ecology Note that it is to the advantage of two competing species to continue to evolve to reduce the amount of resources that they share, and to exploit new resources. If they do so, then the numbers of the two species that the ecosystem can support will increase. Such selective pressures can continue to drive speciation even after the gene pools of the species have become separated because of isolating mechanisms. The ecological resources that become available to a species can include resistance to disease. My colleague Ajit Varki has shown that about two million years ago our ancestors lost a gene that produces a particular sialic acid on the tips of chains of sugar molecules on the surface of our cells. This acid is an attachment point for a malaria parasite that affects chimpanzees. Our ancestors were now resistant to this parasite, and remained resistant for most of their subsequent history. It was during this time that our ancestors acquired larger brains and human-like hands, and some of them left Africa for Europe and Asia. It may be that we did so well in part because we were released from an environmental limitation that affected chimpanzees. Then, between 50,000 and 10,000 years ago, a mutant malaria parasite arose that could invade human cells. This was the dangerous parasite Plasmodium falciparum . Malaria has dramatically affected our recent history, but it may be that resistance to malaria reduced our ecological overlap with chimpanzees and helped to drive our earlier evolution.
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