Alexander Fleming kept finding mold contamination that was killing his bacterial cultures. This observation eventually lead to the development of the antibiotic penicillin. Why would a fungus produce a compound that is selectively toxic to bacteria?

Fungi compete with bacteria for resources

Explanation:

Alexander Fleming discovered the Fungi called Penicillium notatum. He discovered that his culture was contaminated by a Fungi mold and the Bacteria in the culture were dying.

He figured out the mold as the cause and found it to be Penicillium. He also discovered the death was caused by the competition of fungi and bacteria for available resources and the fungi’s interference with the cell wall(cell lysis) of the bacteria.Several years later after technological advancements and researches , the fungi was extracted and isolated and produced in large quantities as a potent antibiotic.

relative dating involves things like higher layers are most recent than rocks lower in the sequence. this can be extended to the known sequence of fossils. rocks containing rabbit fossils are later than those containing dinosaur fossils.  

absolute dating methods mainly use radioactive elements that have known rates of decay and produce recognizable products. for example potassium 40 decays to produce argon. once a rock has solidified the argon is trapped. by measuring the argon and potassium0 you can get an accurate number for the age.  

some other methods are good for archaeological ages. dendrochronology involves recognizing the sequence of tree rings. the orientation of magnetized particles in baked clay can be used to date a fire.

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