Why is phenotypic variation necessary for natural selection and sexual selection?

Natural selection only works when there are genetic variations between the individuals (within the population). It is because only those individuals who are best adapted to live in an environment will be favored by natural selection. Within a given population, there exists a normal amount of genetic variation that may or may not make an individual more adapted to the environment (changes in the environment). Conditions in environmental determine the success of certain individuals because they possess a trait that allows them an advantage in that environment (being faster, more colorful, bigger, or smarter than others in their species or have higher reproductive rates). Natural selection will favor the most adapted individuals.

The similar thing is with sexual selection where some individuals have better reproductive success than others (variation exist) either from being more attractive or preferring more attractive partners to produce offspring.

weathering - colorado river carves land in arizona to form the grand canyon. cracks form in a sandstone wall surrounding a house. a marble statue near the science museum is changing color due to rainfall

erosion - after a rainstorm, soil and rock piees litter the roadways. the sand dunes in the sahara desert are changing shape.

here's your answer.

hypoxemia refers to a below usual concentration of oxygen in the blood, mainly in the arteries. hypoxemia is an indication of an issue associated with circulation or breathing and may lead to various symptoms like shortness of breath.  

the condition is identified by determining the level of oxygen in the blood sample obtained from an artery. it can also be predicted by determining the oxygen saturation of the blood with the application of a pulse oximeter.  

the normal arterial oxygen is about 75 to 100 mm hg. the values underneath 60 mm hg is generally considered with the requirement for supplemental oxygen. the normal pulse oximeter readings generally range between 95 to 100 percent. the values under 90 percent are regarded as low.  

in the given case, the oxygen saturation is measured at 93 percent, hence, there is no need to administer oxygen, that is, one should withhold oxygen.