Darwin's 4 Postulates
Natural selection is the outcome of the four attributes
Postulate #1 - Variation
There is variation between the individuals and species. For instance, in Darwin's experiment with the white and yellow plants. 3/4 of the plants had white flowers with yellow spots and about 1/4 of the plants had yellow flowers. This concludes that there must be variation between individuals, this is why siblings are not identical, but share similar traits and characteristics. For instance, if you look at ladybugs or at zebras, both they all are different from each other organisms of their species. There are different colored ladybugs with different black dots, as well as there are zebras with different stripes.
Postulate #2 - Variation is Heritable
This is also known as Inheritance. This is when the offspring are born with the same or similar characteristics as their parents and their older generations. Inheritance can pass down traits such as eyecolor as well as diseases such as Tyrosinemia which is an error of metabolism. If a child has both recessive parents of the disease, he has 1/4 probability of being unaffected, 2/4 probability of being a carrier and 1/4 probability of having the disease. In Darwin's experiment with the flower, the "S" dominant allele reflected the white flower with yellow spots, while the "s" recessive allele represented the yellow flower. Therefore, the genotypes of the 48 flowers were: SS = 12 flowers, Ss = 24 flowers, ss = 12 flowers, reflecting that variation is inheritable and it can be passed down to younger generations.
Postulate #3 - Reproductive Success
In each generation, more offsprings are produced than can survive. This is where natural selection takes places and those who are well and best suited for the environment survive, those who fail to adapt to the environment, die. For instance, bears and fishes both struggle to survive. When a bear catches a fish, the bear is succeeding and therefore, is surviving. The fish, unfortunately, died, but left more "fitted" fish behind who were able to swim faster and escape away from the bear.
Postulate #4 - Extinction
Darwin's last postulate states that the traits that do not help an organisms survive and reproduce will eventually die out and become extinct. If all organisms with a defecting trait die, then eventually that trait will disappear. This is a "non-random survival and reproduction method" where the "individuals with the most favorable traits survive and reproduce more than do individuals with less favorable traits. These traits are passed down to the next generation, thus resulting in evolutionary change." (Buffalo State College)
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