Animals in the wild adapt to their specific environment so that they can survive and thrive. Those that are successful will be able to reproduce and pass on their traits to successive generations. This web page graphically demonstrates the interaction between environmental or natural selection, fitness of individual breeds of dogs and population sizes as generations are produced. To see this interaction select an environment, and observe what happens to the population sizes with each successive generation. The number of individuals in each generation is indicated by the height of the bar and the number with each bar.
There are two major forces at play in the production and selection of the adaptations. On the one hand, there is genetic variation. There are animals with different size legs, different thicknesses in coat, different shape fins or teeth, etc. Mutations on genes mainly cause the genetic variation. Not all mutations translate into traits that are appropriate for a given environment. Natural selection, which describes the whole of the elements, physical (e.g. temperature, humidity, elevation, etc.) or biological (e.g. sources of food, predators, etc.) with which animals interact with, comes into play as the second force so that fit traits are favored and unfit traits are eventually eliminated from the population. This process is constantly going on in the wild, however, it involves a progressive change that spans many generations.
We have taken 10 different dog breeds and ranked them on a 1-10 scale according to the thickness of their coat. The Mexican Hairless dog is 1 and the Old English Sheepdog is 10. We modeled release of random numbers of dogs per breed in three different environments:
The dogs we released are called Parents. We have assumed that the thicker the coat the better fit the dog is for the frigid Alaskan climate and we have set 7 (Husky) as the optimal fitness value. The optimal fitness value for the temperate zone was set equal to 5 (Golden Retriever) and for the desert, the optimal fitness value was set equal to 2 (Greyhound). The dogs were released so that geographical barriers (e.g. a deep river, a tall mountain) prevent crosses between breeds and have assumed that the population of each breed doubles with every generation.
The effect of natural selection can be quantified as the fraction of animals that die before reproduction per each generation. This number will depend on how close to the optimal fitness value a particular breed is. We have incorporated all this information into a mathematical formula and written a computer program that calculates the number of surviving dogs after each generation and plotted this data on the bar graphs. Note that as natural selection acts, the dogs with fitness values closer to the optimal fitness value for each environment slowly become a bigger fraction of all the dogs and that, eventually, after 9 generations, they become the majority even though they may have made up a minority of the 'Parents.'
Credits: the computer model of the Hardy Weinberg equation was developed and programmed for web delivery by T. Wei, M. Rimer, and J. Horton. This work was supported in part by US ARO under grant number DAAH04-96-1-0415, NIH grant S06 GM 08136 and the NM Agricultural Experiment Station.
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Last Updated: July 10, 1998