Which term refers to the collective evidence that links the origin of some eukaryotic organelles to prokaryotes?

• A. The endosymbiotic theory
• B. Heterozygote advantage
• C. The germ theory of disease
• D. Hybrid inviability

Answer: (A)

The idea being tested is endosymbiotic theory, which explains how some eukaryotic organelles originated from free-living prokaryotes that were taken inside a host cell and formed a mutual relationship. The strongest evidence includes the fact that mitochondria and chloroplasts have their own circular DNA and ribosomes that resemble bacterial 70S ribosomes, not eukaryotic 80S ones. They replicate inside the cell by a division method similar to binary fission, not by mitosis, and they’re enclosed by double membranes, consistent with an engulfing event. Their genetic sequences and many of their proteins more closely resemble certain bacteria than the host cell’s nucleus do, and they carry features that reflect a bacterial ancestry, such as specific lipid membranes and transport systems. Altogether, this body of evidence supports the idea that these organelles originated from prokaryotes that entered a host cell and evolved a stable, mutual relationship. Other terms refer to different concepts entirely—holes in population genetics like heterozygote advantage, medical explanations for disease like the germ theory, or reproductive isolation phenomena like hybrid inviability.