by Marc Surtees, PhD
Evolution has been defined as a change with time. It is often assumed that observed small changes are proof of the theory that all living things evolved from a common ancestor that lived billions of years ago. Two examples of biological phenomena that are often claimed to provide examples of “evolution in action” are antibiotic resistance and warfarin resistance.
Antibiotics are chemicals that can prevent bacterial growth or kill bacteria. They work through several different means. Some inhibit the synthesis (or production) of bacterial cell walls; some inhibit the synthesis of proteins; still others inhibit the replication of bacterial DNA. All of these are, of course, detrimental to the bacteria that encounter the antibiotic. So detrimental, in fact, it often kills them.
However not all bacteria are susceptible to antibiotics. Some resistant bacteria inactivate the antibiotic by destroying or modifying the drug itself so that it is no longer toxic. Some resistant bacteria pump the drug out of the bacterial cell so that the concentration of the drug is too low to be effective. Still, other resistant species have an altered form of the target site of the drug (the place on the cell where the drug binds), so the antibiotic cannot “find” its target.
There are two mechanisms by which bacteria can become resistant to antibiotics; mutation and acquisition of resistance genes by conjugation and or transposition (transformation and transduction).
Bacteria reproduce by simple division but before this can occur the DNA inside the bacteria has to be copied. Sometimes errors can occur during the copying process and the result is a mutation. These mutations are usually harmful and the mutated bacteria will probably die or reproduce much more slowly that the normal type. However sometimes a mutation occurs that will allow the bacteria to resist the effects of the antibiotic. These resistance characters are often simple mutations (i.e. changes in a single gene) that result in the production of a modified target. The result is that resistant bacteria are slightly different from their susceptible ancestors.
So what happens if a bacterial cell has a mutation that allows it to resist the effect of an antibiotic? If that bacterium is in the presence of the antibiotic, then it will have an advantage: the drug will not kill it! It will be able to reproduce, while the susceptible bacteria (which are inhibited or killed by the antibiotic) will not. In the presence of the antibiotic, the resistant mutant has a selective (reproductive) advantage over normal cells.
Originally, most or all bacteria in the population would be susceptible to the antibiotic. Over many generations, the resistant type will make up a greater and greater percentage of the population. If the bacteria are growing in the presence of an antibiotic, most or all of the individuals in the bacterial population will be resistant to the antibiotic. The population has evolved resistance due to natural selection by antibiotics: the genetic structure of the population has changed, from susceptible to the antibiotic to resistant to the antibiotic. This process is completely reversible and if the antibiotic is removed then the numbers of resistant bacteria will decline and eventually the great majority will be susceptible to the antibiotic.
Conjugation and Transposition
There are genes that code for enzymes that destroy antibiotics and if the bacteria have these genes higher concentrations of antibiotic or longer treatment is necessary to kill the bacteria. Bacteria that do not have these genes can acquire them through one of two mechanisms that involve the exchange of genetic material.
Bacterial cells can join and exchange of genetic material in the process of conjugation. Inside many bacteria there is a somewhat circular piece of self-replicating DNA known as a plasmid, which codes for enzymes necessary for the bacteria’s survival. Certain of these enzymes, coincidentally, assist in the breakdown of antibiotics, thus making the bacteria resistant to antibiotics. During conjugation, plasmids in one organism that are responsible for resistance to antibiotics may be transferred to an organism that previously did not possess such resistance.
Transformation and transduction
In transformation, DNA from the environment (perhaps from the death of another bacterium) is absorbed into the bacterial cell. In transduction, a piece of DNA is transported into the cell by a virus. As a result of incorporating new genetic material, an organism can become resistant to antibiotics.
Warfarin is a substance that stops blood coagulation and is used to prevent strokes and heart attacks. Given in sufficient amounts warfarin will cause animals to bleed to death. For this reason is has been used as a poison to eliminate rats and mice. Warfarin works by binding to and interfering with the activity of an enzyme that produces some of the clotting factors that are essential for blood clotting.
When warfarin has been used as a rat poison it has been found that some rats are resistant. The more warfarin is used the more common resistant rats become. In rare cases, warfarin has been found not to work in some people who are given it for medical reasons.
The mechanism of warfarin resistance has been investigated in humans and rodents.
It is has been found that there are two possible mechanisms of warfarin resistance; increased metabolism and reduced receptor binding affinity.
As with all drugs there are enzymes that break down or metabolises warfarin. The activity of these enzymes varies from person to person. This natural variation in the enzyme activity means that people with a more efficient enzyme will get rid of the warfarin faster and prevent it from having an anticoagulant effect. In rats the animals with the more efficient enzymes break down the warfarin more quickly and will be less likely to be poisoned.
This is also the case for the enzyme that warfarin binds to. Individuals have slight differences in the enzymes they possess and some have a lower affinity for warfarin. These individuals will continue to produce clotting factors and will therefore be resistant to warfarin.
Discussion and Conclusions
Most types of antibiotic resistance were already in existence before antibiotics were discovered and used extensively to treat infectious diseases. In these cases the resistance has not occurred since antibiotics became common but rather the resistance genes that were already present have been selected for, and have therefore become more common. Similarly mutations that modify the target sites arise at a low frequency and in the presence of antibiotics, bacteria carrying these mutations can become more common. The same is true for warfarin resistance. There are always a few rare individuals who are resistant and these are selected for and become more common if warfarin is used. This is natural selection and shows that populations can adapt to changing conditions because of the nature of genetic information systems.
These changes in frequency of certain genes in a population have not resulted in the production of new species, even though enough time has elapsed for quite dramatic changes to be observed in bacteria. Assuming a conservative estimate of one hour for each “generation” of bacteria then, since the introduction of antibiotics, 500 000 generations have been produced. This is equivalent to about 10 million years for humans, during which time people have supposedly evolved from primate ancestors. Yet the antibiotic resistant bacteria are still the same species.
Both antibiotic resistance in bacteria and warfarin resistance in rodents provide examples of selection that occurs due to a change in the environment. Study of these phenomena shows us the nature and extent of the effects of differential survival. The increase in frequency of resistance is a good example of natural selection. But this study does not give evidence for macro-evolution, and does not prove that natural selection and random mutation could produce the living world as we know it from simple single-celled ancestors.
© Dr. Marc Surtees, 2006.
Dr. Surtees has a B.Sc. in Applied Biology and a Ph.D. in Zoology, having previously worked for a pharmaceutical company for 14 years.