Darwin s Theory of Natural Selection Postulate 1 Individuals within a species are variable Postulate 2 Some of an individual s variation is passed on to its offspring o Variation is heritable Postulate 3 In every generation more offspring are produced than can survive o the struggle for existence survival o Ocean Sunfish Mola mola the most fertile organism on the planet a female can produce as many as 300 million eggs at once Less than 1 of these eggs reach adulthood Postulate 4 Offspring that survive and reproduce may have inherited variation that gives them an advantage o If two beetles reproduce and create 10 off spring their offspring must be struggling with the environment Limited food there may only be food for 5 beetles to survive Imagine that the 5 survivors had longer legs than their siblings that died How did their longer legs create an advantage o Possibilities include Increase in quickness therefore reduced the likelihood of getting consumed by predators Get food faster o Benefits of this variation that they inherited increased chance of survival and growth Increased likelihood of reproducing Variation increased their fitness Fitness does not equal to quickness it is an index of reproductive success Fitness increases with increased reproduction output This is important so they survive and reproduce Their fitness is zero if no offspring are reproduced o Survival of the fittest o Imagine that the same struggle to survive and reproduce is repeated over 1000s of generation then each successive generation of beetles will be slightly quicker and long legged May change so much that speciation occurs Evolution by Natural Selection One mechanism by which evolution occurs Conditions that must be met o Requires that the variation is adaptive and the results in selection survival reproduction for against organisms Variation must exist for given trait Without variation selection does not affect survival in any way o All individuals respond the same to a given selective pressure o Also there are no advantages to be passed along to future generations o Variation differential survival reproduction success fitness Classic peppered moth example Medium ground finch In the beginning of time the light morph moths had selective advantage Then tree bark began to darken related to the Industrial Revolution then the dark morph moths had the advantage Once people realized that putting pollution in the air was bad they changed their ways to lighten up the pollution Once bark lightened up the light morph moth now has the selective advantage Has variability in its beak bill characteristics o Some individuals in population have a narrow thin beak that is elongated thicker beak o Other members of the population have a short blunt Is there an advantage to a particular beak type It is all built on food availability If the bird has a pointed beak it is beneficial for smaller seeds However if the bird has a blunt beak then it is more effective for breaking thick walled seeds Food availability o Pointed beaks common following o Blunt beaks common following dry wet years years Food availability can drive natural selection o Variation must be heritable Can be passed on to offspring Jean Baptiste Lamarck 1744 1829 Evolution by acquired inheritance variation o Trait variation passed to offspring was based on use disuse by the parent Giraffes Theory that giraffes got their long necks from stretching their necks to reach food
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