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Adjunct Professor Mary Poffenroth teaches various topics in biology in a series of Mahalo-produced videos. In this installment, Professor Poffenroth explains natural selection. She makes the point that evolution is a process affecting populations of organisms, not individuals. Professor Poffenroth explains how mutations are chance events, not willed by the organism.
Natural selection is the process by which organisms that possess certain traits tend to survive longer and produce more offspring than organisms that lack these traits. Over many generations, those traits that confer fitness (that is, comparative success at survival and reproduction) are likely to increase in frequency in the population.
Since environmental conditions are always in flux, it is not always possible to predict beforehand which traits will confer a survival advantage. A trait that enhances fitness in one environment can decrease fitness if the environment changes or if the population migrates to a new environment.
Darwinian theory of random mutation as the engine of natural selection replaced the earlier Lamarckian theory of inheritance of acquired traits. In Lamarck's theory, organisms were thought to be capable of evolving by force of will, intentionally modifying their behavior, then passing these modifications on to their offspring. In Darwin's theory, the variations are chance mutations over which the organisms have no intentional control. According to the current evolutionary model, the variations in traits that the environment selects for or selects against are, in the first place, random accidents of birth.
Although it is tempting to see modern species as having "advanced" or "improved" over their ancestral forms along an upward trajectory, there is no proposed mechanism for this sort of directional change in Darwinian theory. There are only random changes that are likely to spread through a population over multiple generations if they prove beneficial to the organism, versus equally random mutations that are culled by the environment when they fail to provide any survival advantage.
Natural Selection and Environmental Change
Traits that have served as adaptations for many generations may become less adaptive as the environment changes.
Desertification of once lush areas or the formation of mountains in areas that were once flat are examples of gradual changes that may take hundreds or millions of years. Catastrophic changes, such as those brought by floods, volcanoes, fires, plagues, or the introduction of invasive species, are changes that can happen over short periods. Evolutionary change can also occur when a population migrates to a new environment.http://www.youtube.com/watch?v=kjgNT0rBy_8&feature=player_embedded
Speciation events, or the branching of a single species into two distinct species, occur when a breeding population is separated into two populations that no longer interbreed. The separation can be due to an environmental barrier. Two populations that share a common ancestry may eventually become distinct species over many generations, as they evolve in different environments.
Natural Selection and Random Mutation
Although factors in the environment influence which traits become more prevalent in a population over generations and which traits die out, the environment does not have a causal influence on which mutations occur in the first place.
In Professor Poffenroth's example of an animal being born with web feet to a population of animals that lack web feet, the feet would be maladaptive and less likely to be passed on to subsequent generations if the animals need to walk on dry land. Only when the territory is flooded do the web feet prove advantageous for swimming. What was a burden in the first environmental condition becomes an asset in the new environment.
Note that the creature did not will itself to grow web feet, nor was there any input from the environment to cause the mutation that it would later prove useful for swimming. Instead, the mutation was a random occurrence and merely proved useful...
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