Evolution - a simplififed view

The next time you’re outdoors, quietly observe the living world around you. Looks at the plants (flora) and animals (fauna) in your immediate surroundings. Now, imagine that, as you remain rooted to your spot, you are transported hundreds of thousands of years into the past. Or into the future. Will you recognise the lifeforms around you? According to scientists, the living world was a very different place in the past. It will also be very different in the future.

One scientific model that accounts for the change in the biosphere is evolution. The word ‘evolution’ simply means‘change’. It is important to remember that biological evolution does not refer to developmental changes (i.e. changes that occur to an individual as a result of growth and development). Biological evolution describes changes in populations (groups of organisms). Also, evolution occurs over a long period of time.

Examples of population-level changes that are explained by evolution

The following examples are data (observations) about population-level changes. Although these examples describe very different systems, the principles that explain those changes is the same in each case.

1. Rabbits and myxamatosis

In Australia, rabbits are an introduced species. Soon after their introduction, they became pests. In the 1950s, a number of rabbits were infected with the myxoma virus and released into the wild. The myxoma virus causes lethal infections of rabbits (called myxamatosis) and is highly contagious. Within a few years of the start of this program, 99% of rabbits had died. However, the rabbit population began to recover. Even today, the myxoma virus kills only about 50% of the animals it infects.

2. Rabbits and Calicivirus

Another type of lethal infection of rabbits is caused by calicivirus. Although this virus wasaccidentallyreleased in South Australia in the 1990s, it was initially very effective in controlling rabbit numbers. However, just as with the myxoma virus, rabbit populations began to recover.

3. Peppered moths in England

In England, a species of moths, commonly referred to as the peppered moths (Biston betularia), exist in two forms: a lighter form (called typica) and a darker form (calledcarbonaria). In pre-industrial England, the lighter form was more abundant. In the industrial era, the darker moths became more abundant. Since atmospheric pollution control came into place in modern times, the frequency of the lighter moths has increased.

4. Mosquitoes and DDT

DDT is a chemical that kills insects such as mosquitoes (hence, prevents the spread of mosquito-borne diseases). In the 1950s, low doses of DDT were sufficient to control mosquito populations. However, since then, higher doses of DDT wererequired to eradicate the mosquitoes.

5. Antibiotic resistance

Antibiotics kill many types of disease-causing bacteria. Since the introduction ofantibiotics in medicine, the control of infectious bacterial disease through the use of antibiotics has become more difficult. Generally, higher does of antibiotics are needed to control even relatively common infections. Newer antibiotics are needed, as those that have been used frequently in the past are now becoming less effective.

What do all these examples mean?

Well, they all describe changes to populations of organisms. For example, populations of rabbits, moths, mosquitoesand bacteria have changed over a period of time. How have they changed? They have all become resistant to the threats that affect their populations - rabbits to the myxoma viruses andCaliciviruses; mosquitoes to DDT; bacteria to antibiotics and the moths to predation. In going from susceptible to resistant, the populations have changed. This change is referred to as evolution.

How to these examples contribute to a description of evolution?

For simplicity, consider‘susceptibility’ or‘resistance’ to be a biological trait. In the initial population of rabbits, most of the animals were had no resistance to infection by myxoma virus. A few individuals, however, were naturally resistant to infection (purely by chance). When the rabbit population was free of the virus, all individuals had an equal chance of surviving and giving rise to young. However, when the virus infected the population, the susceptible rabbits (the majority of the population) succumbed to the infection and died. The few resistant rabbits survived and produced the next generation of rabbits. The individuals of this generation are resistant to infection by the virus. Thus, over several generations, the majority of the individuals in that rabbit population become resistant. This change in the rabbit population (i.e. evolution) was caused by virus infection epidemic. Such events that result in changes to populations are called natural selection. Natural selection are natural events such as disease, famine and resourceavailability that‘select’ for specific individuals that carry certain advantageous traits.

A working model of evolution

In most populations, individuals carry a variety of traits (e.g. variations in height or eye colour in humans). Usually, these variations do not influence the ability of those individuals to survive till reproductive maturity and given rise to the next generation of individuals. However, when something changes in the environment (e.g. disease, food availability), individuals with certain trait variants will not survive (natural selection), while those with other variations will not. The survivors will then produce the next generation of individuals (this is sometimes referred to as the’survival of the fittest’). Thus the biological structure of the population changes with time.

Sham Nair 2014