SBI3U Evolution – 5
Variation Within Species
You and your classmates are all the same species, but clearly there is a great deal of
variety among the individual members of your species in your class. Why? How does this
variation arise? The answer is in your genes. Through sexual reproduction, parents pass
on genes to their offspring.
The number of possible combinations of genes that offspring can inherit from their
parents results in greater genetic variation among individuals within a population.
Consider a litter of kittens. Usually, all the kittens in a litter look different because,
through sexual reproduction, each one has inherited a different combination of genetic
information from its parents. The kittens have inherited different alleles. Some of this
genetic information is expressed in each kitten’s phenotype (physical appearance and
behaviour). Other genetic information has no visible effect but remains part of each
kitten’s genetic make-up and can be passed on to the next generation. Another important
source of genetic variation in a population is gene flow, which we will learn about later on.
Remember: Genetic variation in a population results from the variety of genetic
information in all individuals of the population.
Genetic Variation
How and why does genetic variation happen in a species or population? Changes in the
genetic material (DNA), called mutations, provide new alleles in a species and are the only
source of new genetic variation when inherited.
Mutations happen continuously in the DNA of any living organism. They can occur
spontaneously when DNA is copied during interphase before a cell divides. For example,
your DNA has about 175 mutations compared with your parents’ DNA because of
mutations that occurred as your DNA was copied. Mutagens, such as UV radiation, are
environmental agents that can also cause mutations in DNA. Mutations can be harmful but
they can sometimes be beneficial.
Mutations that occur in body cells disappear from the population when the organism dies.
If the mutation occurs in a sex cell, however, the mutation can be passed on to
succeeding generations as a new allele. Thus, mutations are the starting point of genetic
variation in populations.
Mutations Can Provide an Advantage
Mutations that were once no advantage, or perhaps were even a disadvantage, may
become favourable in a changing environment. In this situation, the mutation provides a
selective advantage – a genetic advantage of one organism over its competitors.
Over time, a selective advantage causes the organism to be favoured in terms of survival
and reproduction. In other words, a selective advantage helps an organism survive the
changing environmental conditions and reproduce.
Consider the water flea Daphnia, that normally lives in water that is around 20C and
cannot survive in water that is 27C or warmer. A mutation has enabled some populations
of Daphnia to survive in temperatures between 25C and 30C. Daphnia without this
mutation may not survive and reproduce when temperatures are high.
Rapid Reproduction and Selective Advantage
Some organisms reproduce very quickly, such as bacteria, viruses and many insects. The
reproduction times of these organisms are very short compared with the reproduction
times of most plants and animals. For example, some populations of bacteria can double in
under 10 minutes.
In populations that reproduce this quickly, a new allele that resulted from a random
mutation that was previously insignificant in the population may provide a selective
advantage to some individuals when the environment changes. As a result, the organisms
that have the new allele may survive long enough to reproduce and pass the genetic
information on to the next generation. In time, the gene that provided the selective
advantage becomes more prevalent in the population. One mutation can, in some cases,
mean the survival of the whole population.
Antibiotic-Resistant Bacteria:
Staphylococcus aureus, is a relatively common bacterium that can have minor or major
effects on human health, such as minor skin infections or more serious blood infections.
Individual bacteria reproduce asexually very rapidly and under optimal conditions, the
bacteria can reproduce every 30 minutes. Such rapid reproduction helps adaptation to
occur very quickly: the bacteria with an advantageous mutation may survive a changing
environment and reproduce, whereas others without the advantageous mutation may not.
The surviving bacteria reproduce rapidly, which leads to problems when doctors treat a
Staphylococcus aureus infection (or other bacterial infections) with antibiotics.
An antibiotic is a drug used to treat infections caused by bacteria and other microorganisms. Some examples of antibiotics are penicillin and tetracycline. The problem is
that some individual members of the species may have a new allele, from a random genetic
mutation, that makes them resistant to the antibiotic. Only the individual bacteria with
the new allele are able to survive and reproduce. They can then pass on the genetic
information that resulted in resistance to that particular antibiotic to their daughter
cells. Individual members of the population do not change during their lifetime. Rather,
over time, the population changes in its ability to resist certain antibiotics.
Remember: The population changes, not the individuals.
Review Questions for Evolution - 4 and Evolution – 5
1. How do adaptations develop?
2. What is the selective advantage for camouflage and mimicry?
3. Sexual reproduction and mutation are two sources of genetic variation. Describe
each type.
4. Using an example, distinguish between a mutation that provides a selective
advantage and a mutation that provides a selective disadvantage.
a) Explain selective advantage in terms of distribution of alleles.
b) Under what circumstances could a previous disadvantage become an advantage?
5. Choose a species of animal, and describe two traits. Explain the adaptive value of
both traits.
6. There are many antibacterial soaps and sprays currently available over the counter.
Why might your doctor suggest that you avoid using (or restrict your use of) these
products?
7. What is the source of new alleles?
8. Within a few weeks of a patient using an HIV drug called AZT, the patients’ HIV
population consists primarily of AZT-resistant viruses. Explain this result.