Module 8

Module 8—Populations, Individuals, and Gene Pools

 

Explore

 

The frog in the photo has a wonderful array of weaponry to protect itself from the many snakes that surround it. Its bright colour acts as a warning flag to predators. If eaten, the toxin in its skin and blood will poison its predator in minutes. Its sticky, finger-like digits attach to anything, allowing it to easily escape predators or catch a meal. 

 

Although you may be familiar with the defences animals have evolved, you probably don’t often think of plants as having structural defences, other than obvious spines or thorns. However, like snakes and insects that produce venom, a lot of producers have also evolved chemical defences against consumers. Some plants produce chemical toxins that can poison herbivores who feed on them. For example, Delphinium glaucum, a beautiful, tall, purple-flowered spike often seen in gardens, grows naturally on pasture land in the foothills of southern Alberta. This plant is responsible for the death of much livestock and wildlife every year.

 

Most poisonous substances and medically active drugs are extracted from plants, acting as reminders that these compounds are present to discourage consumers from eating them. The antibiotics extracted from plants and fungi are there to protect the plant from bacterial infection. Perhaps the best way to avoid being eaten is simply to taste terrible or, like a skunk, to smell terrible.

 

The strategy works as long as the consumer has a good sense of taste and smell, and a good memory. The roots of some plants secrete toxins that inhibit the growth of neighbouring plants. This mechansism reduces interspecific and intraspecific competition for water, minerals, and light. Similarly, when wolves, bears, or dogs mark their territory with their urine, they are warding off competition for food and mates—the chemical compounds in their urine giving clear warning that this territory has been claimed.

 

Behavioural defences are fascinating. Think of the arched back of a cat to create a large, threatening silhouette; a bear standing on its hind legs, or a dog or wolf with its hair standing on end. Sometimes aggression is not the best policy: adopting submissive behaviours (belly up, tail down, no eye contact) might be a better strategy for avoiding attack.

 

Protective colouration provides a selective advantage when bright colours signal clear warnings to would-be predators or herbivores. By looking dangerous, an organism can scare off predators. Cryptic colouration or camouflage is the opposite of protective colouration. Blending with the surroundings allows prey organisms to hide from predators or allows predators to sneak up on prey.

 

These defence mechanisms become important when an organism is competing with others in its own species (intraspecific competition) to escape from predators or to attack prey. Because most structural, chemical, and behavioural defensive traits are coded in genes, those members of the population who outlive the rest of their group are naturally selected to pass the successful alleles for these genes to the next generation.

 

Similarly, when two species with similar niches (e.g., coyote and wolf) are undergoing interspecific competition for scarce prey, it is their species-specific defence mechanisms that will determine whether it is coyote or wolf that is squeezed out of the niche. If the organism can outwit or outlive its competitors in its own species long enough to reproduce, then the alleles for those winning traits appear in the next generations. The alleles of the losers do not appear in future generations; if selection is strong enough, these alleles may be lost from the gene pool forever.

 

It is worth emphasizing that when food and habitat are plentiful, an organism doesn’t need to be particularly “talented” in defending itself or finding food. But, when resources are scarce, intraspecific competition starts and allele selection and microevolution occur. Sometimes the variation between members of the population is not even visible; however, when scarcity becomes extreme, even the tiniest structural, chemical, or behavioural advantage can make the difference between survival and death.

 

Those that have the alleles that provide an advantage will live to reproduce, changing the population’s allele frequencies and bringing about gene pool change or microevolution. In the same way, competition between organisms from different species in the same habitat, competing in the same trophic level, is not a problem as long as food and other resources are plentiful—both competitors can co-exist nicely. However, when scarcity occurs (e.g., due to drought, severe winters, disease, or overcrowding) interspecific competition will result in a winner and a loser, and competitive exclusion occurs.

 

Read

 

You may wish to review “Interspecific Competition” on pages 718 to 719, and “Defenses Against Consumers” on pages 722 to 723 in the textbook. A chart with terms, definitions, and examples is a good method of adding this information to your course folder.

 

Try This

 

Read the following two statements. Although both of these indicate chemical defences, what is the difference between the two strategies?

1. Some species of plants produce toxins that prevent the plants from being eaten by herbivores.
   
   
2. Some species of plants secrete chemicals into the ground that prevent other plants of its own species from growing near it.

Self-Check

 

Describe the following as being either: (a) chemical defence, (b) cryptic colouration, (c) protective colouration, or (d) mimicry.

1. When elephants begin to eat the leaves of the thorn tree, the tree releases a bitter-tasting substance into the leaves, which prevents elephants from consuming the whole tree.
   
   
2. The beautiful red, blue, and yellow tree frogs of Costa Rica produce some of the most poisonous venoms of the natural world.
   
   
3. The dusty, golden-brown coyote is difficult to spot against the dry grasses of the prairie.
   
   
4. A harmless sea snake has almost the identical bright markings as the highly venomous coral snake.

 

Module 8: Lesson 6 Assignment

 

Your Module 8: Lesson 6 Assignment is a practical investigation of plant competition—both intraspecific and interspecific. The assignment follows “Investigation 20.A: Interspecific and Intraspecific Competition Among Seedlings” on page 720 of your textbook. The purpose is to learn what happens when many individuals are trying to survive with the scarce or limited resources (growing conditions) that you will create.

 

Part 1 of the investigation is helpful in allowing you to see how slight, and perhaps invisible, variations between members of the same species allow one member to compete and survive better than its neighbours. It is a simple experiment in natural selection, where only those variants of the species that have the genes it takes to compete effectively will survive to pass on those genes. In Part 2 you will look at competition between members of different species for the scarce resources offered in the growing pot. You will see how one species is perhaps genetically far better adapted to compete under the particular growing conditions you have set.

 

Retrieve the copy of the assignment that you saved to your computer earlier in this lesson. Complete the assignment. When you finish, be sure to save your work in your course folder and submit your assignment to your teacher.

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