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16.2 Natural Selection -- Part 1
The type of natural selection depends on the shape of the distribution of the trait.
Natural selection is considered in a genetic context.
Natural selection favors the variant that is the most adaptive when the variation is exposed to the environment.
A governing board decides which students will be admitted to a college.
Some students will be allowed to enter, while others will not.
The prize awarded in the case of natural selection is the chance to reproduce.
The investigators have defined three types of natural selection.
The intermediate phenotype is favored with stabilizing selection.
When Swiss starlings lay four or five eggs, more young survive than when the female lays fewer than this number.
The production of yolk and how long the female mates are involved in determining clutch size are some of the factors that are determined by genes.
Over time, natural selection changes the average value of a phenotype.
The human birth weight is an example of stabilizing selection.
Hospital data shows that human infants born with an intermediate birth weight have a better chance of survival than those born with an average birth weight.
A baby's systems may not be fully functional if it is large.
The birth weight is influenced by the mortality rate.
When an extreme phenotype is favored, the distribution curve shifts to one of the extremes.
Over time, a population's frequencies of a certain trait change.
When a population is adapting to a changing environment, there can be a shift.
Directional selection can be seen in the changes of a population.
Most of thebacteria are susceptible to the antibiotic drug.
The non resistantbacteria die or decline in numbers when they are exposed to the antibiotic in the petri dish.
Subsequent generations result inbacteria that are antibiotic resistant and thus have a greater ability to survive and reproduce.
A shift in the distribution curve occurs when antibiotic resistance is prevalent in the population.
A shift in the distribution curve is caused by directional selection when natural selection favors one extreme phenotype.
Two or more extreme Page 288 phenotypes are favored over the intermediate one.
British land snails have a wide range of habitats that include low-vegetation areas and forests.
The snails with dark shells are more prevalent in forested areas.
Light-banded snails are more prevalent in low-vegetation areas.
The population has two distinctly different phenotypes.
Light-banded snails are more prevalent in forested areas.
The ability to compete with other males for a mate and the ability to produce surviving offspring can be achieved by sexual selection in males.
The female increases her fitness.
Sexual selection is considered a form of natural selection because it affects fitness.
Choosing the best mate is important because females produce few eggs compared to males.
Two hypotheses regarding female choice were tested in a study of bowerbirds.
Females choose mates on the basis of the trait that improves the chance of survival.
Females choose mates based on their appearance.
Aggressive males were usually chosen as mates by females.
Aggressive males might be good at stealing blue feathers from other males because females prefer blue feathers as bower decorations.
The data did not support either hypothesis.
The males are larger than the females.
It is possible that the male has health and vigor in him.
It is possible that females' choice of flamboyant males gives their sons an increased chance of being selected.
The drab females like flamboyant males.
Some investigators think that male features might indicate that they are free of parasites.
In barn swallows, females choose males with the longest tails, and investigators have shown that males that are relatively free of parasites have longer tails.
Males can father a lot of offspring because they produce a lot of sperm.
In order to inseminate as many females as possible, we expect males to compete.
The Baboons live in a troop.
One animal giving way to the other is the result of confrontations.
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