Chapter 12. Chapter 12: Evolution and Extinction
What is evolution...?
Guiding Question 12.1
What is evolution and how does natural selection allow populations to adapt to changing environments?
Why You Should Care
Evolution is the founding principle for the study of modern biology because it explains how every living thing you encounter came to be the way it is. An example of evolution that has many people concerned is methicillin-resistant Staphylococcus aureus (MRSA). Staphylococcus aureus is a common, normally harmless bacterium that is found on many people’s skin and nostrils. A break in the skin can lead to the bacteria becoming infectious and cause lesions or deep-tissue infections. Normally, such an infection is treatable with antibiotics, but MRSA is resistant to all known antibiotics.
MRSA became resistant through evolution and natural selection. In a patient with a Staphylococcus infection, even if the bacterium developed antibiotic resistance, the rest of the bacteria in a patient’s body would be killed by the antibiotics, and the resistant bacterium would be killed by the patient’s immune system. If the patient only takes a partial course of antibiotics, the non-resistant bacteria may be killed back, but not enough to make the patient well. The resistant bacteria will have an advantage by being resistant and proliferate. The indiscriminate and improper use of antibiotics, therefore, is making antibiotics less useful.
Test Your Vocabulary
Choose the correct term for each of the following definitions:
| Term | Definition |
|---|---|
| A trait that helps an individual survive or reproduce. | |
| Two species each provide the selective pressure that determines which traits are favored by natural selection in the other. | |
| The heritable variation among individuals of a single population or within the species as a whole. | |
| Humans decide which individuals breed and which do not in an attempt to produce a population with desired traits. | |
| Stretches of DNA, the cell’s hereditary material, that each direct the production of a particular protein and influence an individual’s traits. | |
| Differences in the gene frequencies within a population from one generation to the next. | |
| The process by which organisms best adapted to the environment (the fittest) survive to reproduce, leaving more offspring than less well-adapted individuals. | |
| A nonrandom influence affecting who survives or reproduces. | |
| The assortment and abundance of particular variants of genes relative to each other within a population. |
1.
For each of the scenarios described below, decide whether it describes stabilizing selection, directional selection, or disruptive selection.
| Type of Selection | Scenario |
|---|---|
| Dandelions in a field grow flowering stalks to various heights; they will not reproduce if their flowers are cut off. After the field is mowed routinely for a few years, most of the dandelions only produce flowering stalks that are close to the ground. | |
| A species of tree produces seeds that cannot germinate and grow in the shade. The seeds have flaps on them that are of varying size and shapes. Flaps of a certain size will allow the seeds to glide away and germinate. If the flaps are too small or narrow, they will not catch enough air and the seed will drop. If the flaps are too wide or heavy, the seeds will be too heavy to travel far. | |
| A breeding pair of seed-eating sparrows gets stranded on an isolated island. Their offspring have variously sized beaks; the larger the beak the larger the seeds the sparrow can eat. Trees on the island either produce large or small seeds, but not many medium-sized seeds. Although the large-beaked birds could eat small seeds, it’s more efficient for them to eat large seeds. |
Take it further: For an interactive look at natural selection, go to this website http://www.techapps.net/interactives/pepperMoths.swf and click on A Bird’s Eye View of Natural Selection.
4.
Coevolution is the process by which two species evolve in response to selective pressures from each other, and therefore adapt to each other. Consider a hypothetical relationship between a species of tree beetle and a species of bird that preys on the beetles. The beetles have naturally occurring variation in color; some colors more closely match the colors of the trees they live on than others. The birds vary in their abilities to see the beetles on the trees. The following three diagrams represent steps in a coevolutionary "arms race."
Select the proper order of events:
| A. |
| B. |
| C. |
| D. |
8.
A classic example of coevolution is the case of Heliconius butterflies and passionflowers. Heliconius butterflies of several species all lay their eggs only on the leaves of different species of passionflower. Passionflowers produce toxins that are meant to deter insect predation, but the butterflies have evolved immunity to this toxin. Passionflowers produce nectaries (nectar-secreting glands) on their leaves that encourage visits from ants and wasps, which come for the nectar but stay to eat any butterfly eggs on the leaves. To prevent intraspecific competition, the butterflies will not lay eggs on leaves that already have eggs on them. Passionflowers produce nectaries and leaf spots that are shaped like butterfly eggs.
A) Thought Question: Predict the next step in this coevolutionary "tennis match."
B) Thought Question: The Butterfly or the Egg? It’s possible that passionflowers were already toxic before Heliconius butterflies began to specialize on them. How could that be?
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