How can reproductive isolation lead to speciation<span>? If populations cannot mate successfully with one another, genetic differences may accumulate in the populations. Over time they become very different and give rise to new species.</span>
        
             
        
        
        
ATP, mitocondria 
the stages are the citric acid cycle and the electron transport chain 
        
             
        
        
        
Answer:
Those organisms are fit which has the ability to survive in harsh climatic condition and transfer their traits to the next generation through the process of reproduction. 
Explanation: Fit means they are strong and compete with other organisms for food, shelter and other survival items. Those organisms which are not fit could not survive in harsh environmental conditions and removed from that ecosystem.