Darwin and microorganisms

When Darwin developed the theory of natural selection, the scientific community would not like it at all.

The main reason was that his theory lacked an explanation for heredity.

According to Darwin, heredity worked as a blending of characteristics, but natural selection would have difficulty operating if it were a mixture.

Suppose new organisms are formed with characteristics from both parents and only through this mechanism would the phenotypic variation tend to disappear. Thus, organisms would not adapt or evolve (Box 1).

Upon further thought, Darwin considered this theory (pangenesis) great nonsense. Theories come and go until Mendel's theory of heredity was finally rediscovered by Fisher, Haldane, and Wright.

They managed to synthesize the theory of natural selection with Mendelian theory, establishing what became known as neo-Darwinism. From there, these ideas influenced all areas of biology, including microbiology.

Unfortunately, when Darwin published "On the Origin of Species" in 1859, microbiology was a new science*. If Darwin had explored microorganisms as a model system, he could have easily proven his theories because:

• They divide much faster than multicellular organisms.

• They reproduce through binary fission and not through complex events.

It was only 80 years after Darwin gifted us with his theory that Delbrück and Luria could explore microorganisms as a model system. They proved that bacteria could become resistant to bacteriophage T1 even before exposure to the virus.

This laboratory experiment coincided with the beginning of a significant phenomenon in the real world. It's a classic example of applying the theory of natural selection: drug-resistant pathogens.

How does resistance develop? In short, the increased use of antimicrobials in various areas and/or incorrect use provides highly selective conditions, and only the resistant ones can survive. They develop more easily without competition, increasing their frequency in the population.

Another example in the microbial world that depends on heritable variation and natural selection is the ability to infect multicellular organisms. These examples make microorganisms seem very clever. The truth is that evolution is a "blind" process that occurs by selecting genetic variations.**

However, it is a complex process; there is a balance between maintaining essential genetic information and generating genetic diversity to facilitate adaptation to environmental changes.

*At that time, there was still a debate between spontaneous generation and biogenesis. This year, the French Academy of Science held a competition for anyone who could "develop an experiment to shed light regarding spontaneous generation." With his experience in the wine industry and pasteurization, Pasteur showed the world that maggots do not arise from decaying flesh.

**Keep in mind that while the effect on genetic variation occurs randomly, an individual's survival and reproductive success is directly related to the function of inherited characteristics in a particular environmental context.

Box 1. Here comes the talk about "Aa."

To understand why Darwin's theory wouldn't work through mixing, imagine the following (I apologize in advance for the silly example. I thought it would be funnier than peas):

In a place far, far away, Princess Fiona decides to break up with Shrek because she is tired of the mess he left at home and then married a human being. Poor thing. Little did she know that human men are messy in the same way...

So, considering that:

• Fiona has an "AA" gene for dark green colour;

• The human, "aa," has a white colour;

• There is no dominance between the genes;

• The couple's child has the genotype "Aa," which means a mixture of the two colours: light green.

In the next generation, we have a problem. In the blending inheritance, "A" and "a" would physically mix to form a new type of gene, for example, gene "A`." And in the cross between two heterozygotes, all grandchildren would be light green.

Currently, we know that it doesn't happen this way. Mendelism efficiently preserves genetic variability because extreme genetic types are transmitted from generation to generation, even if disguised as heterozygotes.