What is evolution? What exactly does it mean when biologists say that humans and chimps evolved from a common ancestor, or observed evolution in some bacterial or viral strain? These questions must be addressed in order to understand the various theories behind the mechanisms that drive evolution. From the moment many people hear the word “evolution,” the first description that comes into their heads is a gradual transformation of humans branching off from primates. This happened billions of years ago, so in their perception of evolution seems to be more related to historical events than biological developments. Such was the case with my friend Theresa. The idea to investigate whether evolution is ongoing at present resulted from a conversation I had with Theresa, who did not believe evolution was still occurring. I also wanted to know the evidence that supports ongoing evolutionary claims.
It also does not help that the common definition of evolution in dictionaries is not accurate. For example, the Oxford Concise Science Dictionary defines evolution as:
“The gradual process by which the present diversity of plant and animal life arose from the earliest and most primitive organisms, which is believed to have been continuing for the past 3000 million years (macroevolution).”1
The definition, although not necessarily wrong, refers more to the history of evolution than to evolution itself. This makes it possible to dispute whether evolution is still occurring. It does not take into account microevolution, the change in genetic makeup of a population over small period of time.5
Most biologists will agree that evolution can be simply defined as “a process that results in genetic changes in a population spread over many generations.”1
The fuel of evolution is variation amongst a population. When offspring inherit variations of a gene that was successful in their parents, the next generation has a greater chance of survival if the environment favors the inherited features. The environment naturally selects organisms with inherited genes able to carry out reproduction. Therefore, evolution by natural selection must then operate on three factors: variation, differences in reproduction, and inheritance.2
With the definition of evolution and its mechanisms established, we can move on to macroevolution. This means that all organisms descended from the Last Universal Common Ancestor (LUCA)6 and over millions of years diverged into the different species that exist today such as plants, vertebrates, prokaryotes, etc. There is much evidence to prove macroevolution. Such evidence is gathered from fossil records, analyzing evolutionary trees, phylogram, cladistics, and many other modern biotechnological tools.5
Microevolution works in a different manner. It says an organism can change over time but it remains the same organism – the only change is in its genome. For example, a lizard can adapt to the changes that occur in its environment but it remains a lizard. If microevolution is still ongoing, then what proof is there to show for it? And even more complicated, are we humans still evolving as well?
Unicellular Organisms
The simplest organisms, bacteria and viruses, can be used to demonstrate examples of microevolution. In 2006, the Food and Drug Administration announced the outbreak of the foodbourne bacteria strain E.coli 0157:H7, a potentially deadly bacterium that causes diarrhea and dehydration.8 Over six states were affected, and about 160 people became infected. The strain had evolved from an original form that had been treated with antibiotics. The chart below shows dates of drug discovery and resistance for many drugs used to treat bacterial infection at one point but, as the years went on, developed resistance.2
| Drug | Discovery/Introduction | Resistance |
|---|---|---|
| Penicillin | 1928/1943 | 1946 |
| Sulfonamides | 1930s | 1940s |
| Streptomycin | 1943/1945 | 1959 |
| Cephalosporins | 1945/1964 | Late 1960s |
| Chloramphenicol | 1947 | 1959 |
| Tetracycline | 1948 | 1953 |
| Erythromycin | 1952 | 1988 |
| Vancomycin | 1956 | 1988/1993 |
| Methicillin | 1960 | 1961 |
| Ampicillin | 1961 | 1973 |
| Cefotaxime/ceftazidime | 1981/1985 | 1983/1984/1988 |
Bacteria can be treated with antibiotics, chemical substances toxic to the growth of microorganisms.9 They work by preventing bacteria from building membranes or cell walls needed to allow their genetic materials to reproduce in the host. However, if resistance to many antibiotics has occurred over the years, how is that possible? The bacteria can evolve in a surrounding where there is some variation in the gene pool and inheritance if a resistance gene is passed on to subsequent generations. For example, bacterial resistance to penicillin occurred when the bacteria was able to rebuild its cell walls in two ways. First, the proteins used to build its cell wall had been altered to bind to penicillin poorly or not at all; in this case the only way to kill off such bacteria is to bombard it with high doses of the antibiotics. Second, some bacteria break down the protein betalactamases for metabolic processes. Penicillin also contains a structure of atoms called betalactam ring analogous to the protein betalactamases. Therefore, the bacteria’s enzymes are able to recognize the antibiotic and break it down as a byproduct.
Other antibiotics use different mechanisms to stop bacterial growth. Tetracycline works by attaching to the ribosome of the bacteria to prevent protein synthesis. But bacteria impedes the drug over time by making changes in its ribosomal genes, so that the cells no longer bind to the drug.3
Conversely, viruses are forms of microorganisms that cannot be treated with antibiotics, but can be suppressed with vaccines. The virus that causes influenza has two common strains called H3N2 and H1N112. The virus has been able to evolve by modifying its surface protein, hemagglutinin (HA) to escape its host immune response.12 This results in a new dominant strain that the host cell must make new antigens to fight and recognize, or else it will weaken the immune system of the host, and eventually lead to death.
Other viruses can infect host cells and continually give rise to progenies that have different strains. One such example is the deadly Human Immunodeficiency Virus (HIV). The virus enters its host by binding to
CD4+ cells by the adsorption of glycoprotein on its surface to receptors on the target cell, followed by the insertion of its viral envelope with the cell membrane and genome into the host cell. The host cell begins to produce antigens, killer T cells and macrophages to attack the foreign cell. Some of the viruses might be killed off, but the ones that escape the first immune attack will mutate. They will pass on different molecular proteins to their offspring to evade the next batch of antibodies that will be produced.3
After years of producing different types of antibodies to fight off each evolved strain of the virus from the original one, the immune system becomes weak. The virus has at this point drastically depleted helper T cells, and the immune system can no longer recognize an HIV infection to fight it. Eventually, the weakened immune system becomes susceptible to various kinds of viruses which proliferate uncontrollably, and the host inevitably gets Acquired Immunodeficiency Syndrome (AIDS).3
In almost all cases, by introducing a new antibiotic or vaccine to prevent microorganism growth, the organism’s response is to evolve. Some people may argue that the lifespan of microorganisms are relatively short; therefore, it is possible to observe evolutionary changes in a short period of time. However, microevolution is not only limited to microorganisms.
Multicellular Organisms
Recent cases of microevolutionary developments have been observed in multicellular organisms. As recently as July 13, 2007, researchers noted that the male population of the Hypolimnas bolina (blue moon butterfly), which only accounted for 1% of the species, had recovered to 39%.4 The species lives in the South Pacific Island of Savaii, and the male embryos were at risk of being infected with the bacteria Wolbachia. Researchers are not sure whether suppressor genes in the host emerged from a mutation in the local population, or were introduced by migratory Southeast Asian butterflies that already had the mutation to suppress Wolbachia. What is certain is that the butterflies had been naturally selected for the mutated gene in order to have a higher rate of male offspring over a short period of time.
In humans, microevolution is more or less related to having a gene that benefits the population, and allows it to survive a fatal disease in that region. An example known for many years is sickle-cell anemia. Individuals with the dominant gene (SS) produce normal red blood cells, but are vulnerable to contracting malaria. Individuals with the recessive gene (ss) have the serious genetic disease sickle-cell anemia, and their red blood cells are sickle-shaped.3
The individual with the heterogynous gene (Ss) produces red blood cells that carry oxygen well, but that change to an abnormal shape and are destroyed by the body when they are invaded by malaria-carrying mosquitos. Therefore, individuals who are heterozygote for the disease are resistant to malaria.11 In areas where malaria is a common disease, such as many parts of Africa, the populations suffer from high rates of sickle-cell anemia. Variation in the population’s gene pool has contributed to heterozygote individuals’ natural selection and over time they have developed the benefit of not contracting malaria.
Another example is the disease cystic fibrosis. So far, studies have been done with mice which suggests that heterozygotes at the cystic fibrosis gene may not contract cholera – they differ in fluid secretion properties and as a result do not get the diarrhea that can cause severe dehydration and death in people with cholera.11 This could be linked to certain populations in Europe and of European descent who suffer from high levels of cystic fibrosis.
Conclusion
All the examples of the evolutionary changes in both single cell and multicell organisms are undergoing the biologist-termed process called the “Red Queen Principle.” The phrase is taken from the Lewis Carroll book Through the Looking Glass, in which Alice and the Red Queen run faster and faster but remain at the same place.7 In evolution, the concept means that the relationship between the host and parasite is in a sense an “arms race,” where the parasite drives the evolution of the host in order to survive, but then the host drives the evolution of the parasite.
To answer the question as to whether evolution is still ongoing, the answer is yes. However, we are not witnessing the large-scale evolutionary changes described by Charles Darwin, where different species derive from a common ancestor over a long period of time, called macroevolution. The evolutionary changes that are still ongoing occur on a small scale and over a short period of time, thus termed microevolution.
Works Cited
1 TalkOrigins Archive
2 Moran, Laurence, “What is Evolution,”
3 Palumbi, Stephen, “The Evolution Explosion”: How Humans Cuase Rapid Evolutionary Changes, New York, Norton, 2001, pages 37, 70
4 BBC NEWS, “ Butterfly shows evolution at work,”
5 Campbell, Neill A, Biology/Neil A, Campbell, Jane B, Reece-7th Edition, Pearson, CA, 2005.
6 Poole, Anthony “My name is LUCA”: Last Universal Common Ancestor,
7 C/Net, News.com, “Male butterflies swiftly out-evolve killer bacteria, viewed 12/05/2007
8 US Food and Drug administration News , viewed 12/13/2007,http://www.fda.gov/bbs/topics/NEWS/2006/NEW01450.html
9 Wikipedia, Antibiotic, viewed 10/12/2007,
10 Hardman, Heidi, “Recent evolution at single gene may have brought down heart disease risk in some human groups”, viewed 12/13/2007 http://www.bio.indiana.edu/~hahnlab/MediaFiles/MMP3/MedicalNewsToday.html
11 Freemen and Herron, 2001,chap.16, Lecture
12 Medical News Today, “Vaccine Formulation Research Shines Light On Flu Virus’s Evolution”,viewed 12/12/2007