Final Answer:
The observation that the bacteria genus Chlamydia contains species that can only survive as intracellular parasites supports the b) Regressive hypothesis. This hypothesis posits that certain cellular organelles, such as mitochondria and plastids, originated from free-living bacteria that became endosymbionts within host cells. Therefore, the correct answer is b) Regressive hypothesis.
Step-by-step explanation:
The regressive hypothesis suggests that certain cellular organelles, like mitochondria and plastids, originated from free-living bacteria that evolved into endosymbionts within host cells. In the case of Chlamydia, which can only survive as intracellular parasites, the observation aligns with the regressive hypothesis. These bacteria exhibit reduced metabolic capabilities and share similarities with ancestral free-living bacteria. The regressive hypothesis explains the evolution of intracellular bacteria as a result of these organisms adapting to a parasitic lifestyle within host cells, supporting their common ancestry with free-living bacteria.
The Chlamydia genus lacks many essential metabolic pathways, further supporting the regressive hypothesis. This reduction in metabolic capabilities is consistent with the notion that these bacteria were once free-living organisms that progressively lost functions upon adopting an intracellular lifestyle. The regressive hypothesis provides a coherent explanation for the observed characteristics of Chlamydia, as it accounts for the reduction in cellular complexity associated with the transition from free-living bacteria to obligate intracellular parasites.
In summary, the regressive hypothesis is the most fitting explanation for the observed characteristics of Chlamydia, especially its obligate intracellular lifestyle and reduced metabolic capabilities. This hypothesis aligns with the idea that certain cellular organelles originated from free-living bacteria that evolved into endosymbionts, shedding light on the evolutionary path of intracellular parasites like Chlamydia.
Therefore, the correct answer is b) Regressive hypothesis.