Back To CourseCollege Biology: Help and Review
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Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.
Organisms have many different phenotypes, or observable characteristics. Your hair color and eye color are phenotypes, for instance. Many phenotypic variations are neutral, which means that they don't give organisms a benefit or disadvantage when it comes to survival and reproduction. People with blue eyes are not any likelier or unlikelier to survive and reproduce than people with other eye colors.
However, some phenotypes are either selected for or against by the conditions in which an organism lives. For example, people that live in places with strong sunlight, like near the equator, are likelier to survive and reproduce if they have dark skin to protect them from UV damage.
A selective pressure is any reason for organisms with certain phenotypes to have either a survival benefit or disadvantage. In the example above, strong sunlight is a selective pressure that favors darker-skinned people; lighter skin would be a disadvantage in these regions.
Selective pressures drive natural selection. Some members of the population will not survive and reproduce and, thus, will not pass on their genes into the next generation. Gradually, the population changes, and genes that improve survival and reproduction will become more common, while genes that are disadvantageous to survival and reproduction will become more rare. This change in the genetic makeup of a population is called evolution.
Selective pressures can take many forms, including environmental conditions, availability of food and energy sources, predators, diseases, and even direct human influence.
Let's take climate as an example. In a cold climate, animals need certain characteristics to survive, like a warm furry coat, the ability to make burrows to live in and the ability to collect and store food for the winter. The selective pressure of cold weather means that animals that don't have these characteristics are less likely to survive and reproduce. In a hot, dry climate, plants will have an advantage if they have phenotypes such as the ability to store water, large root systems to absorb what little water is in the soil and, perhaps, ways to prevent water loss even at high temperatures.
And what about food and energy sources as selective pressures? In a dense rainforest, plants on the forest floor will survive and reproduce better if they are able to gather as much light as possible, perhaps by having very large leaves. Thus, light availability can be a selective pressure for plants. Food acquisition is also a selective pressure. For example, sharp teeth and the ability to hunt prey are advantages for carnivores.
Of course, predation is its own kind of selective pressure - in other words, those very carnivores are a selective pressure for their prey. Animals that have sharp eyesight, are poisonous to their predators, can run very fast or can camouflage themselves or hide from predators will be likelier to survive and reproduce than animals without these phenotypes.
Diseases can also be selective pressures. One well-known example is the sickle cell trait in humans, caused by having one copy of the mutated hemoglobin allele that causes sickle cell anemia. People that have two copies of this mutated allele are likely to die of anemia at a young age, so you'd think that the allele would be strongly selected against, right? However, people with only one copy of the sickle cell allele have a survival advantage in malarial regions because they are more resistant to malaria. Scientists don't yet know exactly why they are more resistant, but in these regions, malaria is a selective pressure that keeps the sickle cell allele circulating in the population.
Now, let's put ourselves in the pathogens' shoes. For microbes such as the malaria parasite, as well as many other bacteria, viruses, and fungi, animals' immune systems are a major selective pressure. Pathogens that have phenotypes that increase their resistance to immune defenses are likelier to replicate and go on to infect another host. This leads pathogens to evolve very interesting abilities, such as disguising themselves from the immune system by changing their outer coats, or even hijacking our immune cells to make them into comfortable places to live.
Let's have one last example.What was that about direct human influence being a selective pressure? For hundreds and perhaps thousands of years, people have been domesticating and breeding animals and plants. How do they do this? By choosing organisms with specific phenotypes and allowing them to reproduce and pass on their genetic material. This means that human manipulation is a selective pressure that leads to so-called artificial selection.
One very important concept about selective pressure is that it must occur before reproduction in order to have any effect on the population. Many selective pressures are present throughout an organism's life, such as predators and other threats in the environment, diseases that are common in a population, and things that an organism must be able to do to survive and reproduce.
However, some selective pressures appear suddenly in an organism's life; for example, climate changes or pesticide treatment. What's more, some beneficial or detrimental phenotypes appear later in an organism's life. In order for genetic traits to be selected for or against, the selective pressure has to be there before the organism reproduces.
Let's imagine a disease that is caused by a mutation in a single gene and causes rabbits to become gradually paralyzed. One relevant selective pressure here could be predation. Clearly, a rabbit that is becoming paralyzed will be less able to run away from predators and will be likelier to be caught and eaten.
But the time of onset of this disease is crucial in determining whether it is selected against in the population. If the disease starts at a young age, affected rabbits will most likely be eaten before they have a chance to reproduce and pass on the mutated gene. However, if the disease starts in the rabbit's adulthood, it has probably already mated and passed on the mutated gene before it begins to have a survival disadvantage. In this case, the mutated gene won't be selected against.
In modern human societies, natural selection occurs to much less of an extent than it used to. Selective pressures, such as disease and predation, pose much less of a threat than they did thousands or even hundreds of years ago.
Modern medicine has made it possible to treat diseases that children used to die from, such as immunodeficiencies and juvenile diabetes. Nowadays, these conditions are less likely to prevent people from surviving until reproduction, so they are not as strongly selected against as they once were.
Similarly, living in developed areas keeps humans safer from predators, so predation isn't such a threat anymore. This means that phenotypes, such as sharp eyesight, quick reactions and the ability to run really fast, are not selected for in the way they used to be.
We've learned that selective pressures can come in many different forms, such as environmental conditions, threats from predators and diseases, and even human manipulation. Selective pressures drive selection which, in turn, leads to the evolution of a population, or changes in its genetic makeup over time. Importantly, in order for selection to occur, selective pressures must be present before the organism reproduces.
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Back To CourseCollege Biology: Help and Review
24 chapters | 426 lessons
Next LessonDarwin's Theory of Natural Selection: Overview