Final answer:
Selection in yeast cells likely favors mitochondria that respire, as they allow for efficient ATP production through aerobic cellular respiration, offering a survival and reproductive advantage in oxygen-rich environments.
Step-by-step explanation:
At the level of mitochondria inside of a yeast cell, selection most likely favors mitochondria that respire. This is because yeast are eukaryotic organisms that contain mitochondria, which are specialized for aerobic cellular respiration. The endosymbiotic theory suggests that mitochondria evolved from a free-living, aerobic prokaryote that was phagocytized by an ancestral eukaryotic cell. This symbiotic relationship provided the host cell with the ability to use oxygen to release energy stored in nutrients, giving it a survival advantage.
Mitochondria are essential for energy production in eukaryotic cells, including yeast. They contain their own genome, which is indicative of their bacterial origins, as well as 70S ribosomes. The primary role of mitochondria is to produce adenosine triphosphate (ATP), the energy currency of the cell, through oxidative phosphorylation. Therefore, mitochondria that are proficient at respiration would typically be selected for in an oxygen-rich environment, as they would contribute to the yeast's survival and reproductive success by efficiently producing ATP.
While yeasts can also undergo fermentation, this is typically a response to low oxygen conditions and is less efficient in terms of ATP production compared to respiration. Hence, in the presence of oxygen, respiratory mitochondria would likely be favored by natural selection.