Natural selection is defined as differential survival and reproduction of individuals in a population as a result of environmental influences.

• Differential means unequal.  Some individuals survive and reproduce at higher rates than others.

• Survival is not dying.  Not dying from the cold or the heat or lack of water.  Not being eaten by a predator or killed by a disease or parasite. Etc.

• Reproduction is producing offspring.

• Population is all the individuals of the same species living in the same area. Populations are linked together through reproduction through time.  Individuals in a population can breed with each other and have offspring.  Their offspring can breed with others in that group and so on, generation after generation.

• Environment is what surrounds an organism.  It includes both abiotic and biotic components.  Abiotic being the non-living things, such at sunlight, temperature, water, gasses, nutrients, etc.  Biotic being the living things, such as predators, herbivores, competitors, parasites, diseases, etc.

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Differences exist among individuals in a population in physical, biochemical, and behavioral traits as a result of the alleles that they inherited.  These alleles affect the traits that an organism displays and, therefore, their chances of survival and reproduction as they interact with their environment.

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Limited resources place selective pressures on a population as more offspring are produced each generation than can survive.  So which offspring will survive and reproduce?  Those that are better at acquiring food, water, and shelter. Those that are better at avoiding predators, diseases, and parasites. Etc.  Those that have the combination of physical, behavioral, and biochemical characteristics that are best suited to overcoming all the stresses the individual faces in its environment.  

 

Those individuals that survive and reproduce at higher rates have more offspring than those that reproduce at lower rates.  The offspring inherit the alleles that code for the traits that made their parents successful, so those alleles and traits become more common in the population over time. 

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Over may generations, natural selection results in adaptations: characteristics that help an organism to survive in the environment that it lives in. A trait has become widespread in the population because it has made generation after generation of individuals in the population more successful at growing, surviving, and reproducing than those that did not have it.

 

Natural selection (and sexual selection and genetic drift) acts on pre-existing genetic diversity.  It favors or disfavors alleles that already exist in the population as a product of mutations that occurred through errors in the DNA replication process sometime in the history of the population.  Natural selection does not create new alleles or new traits. Organisms do not develop new alleles and the traits they code for in response to some environmental stress or because they need them.  If they did, species would never go extinct, but the history of life on our planet is riddled with extinctions.  Pre-existing genetic diversity places a limit on the evolutionary potential of a species.

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Sexual Selection:  Differential reproduction among individuals in a population due to differences in mating success results in the elaboration of traits that favor mating success

• It differs from natural selection in that mating success determines which traits become more common in a population rather than environmental influences.

• It is similar to natural selection in that it operates on pre-existing genetic diversity that has built up over time in a population through random mutations, and it is a multigenerational process.

Sometimes natural selection and sexual selection work at odds to each other. 

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Watch the Two Videos Below on Intrasexual Selection in Rhinoceros Beetles

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Watch the Video Below on Intersexual Selection in Birds of Paradise

 

Watch the Video Below on Intersexual Selection in Peacock Spiders

 

Watch the Video Below on Fiddler Crabs in which the Large Claws of Males Are Used to Defend Territories (Intrasexual Selection) and Attract Females (Intersexual Selection): 

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Watch the Video Below for a Recap of Natural Selection and Sexual Selection

 

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Genetic Drift: Random changes in the frequencies of two or more alleles within a population due to sampling error.

• Random events can result in outcomes that differ from what is predicted by probability when the sample sizes are too small.

  • Think about flipping a coin.  The probability of getting heads is 50%.  If I flip the coin once and get heads, the probabilities of the next flip are not affected, so when I flip it a second time, I could get another heads. If this happens, I now have 100% heads and 0% tails, instead of 50% heads and 50% tails.

Endangered species have small populations so they are susceptible to losing alleles through genetic drift.  Because it is a matter of chance which alleles get lost and which get fixed, it is possible a harmful allele could become more common over time just by chance.

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