Populations change when individuals within the population acquire new characteristics that improve their reproductive success, or when changing environmental conditions favor one existing trait over another. Characteristics that are the most adaptive are more likely to be passed on to offspring. Gradually, members of the population with the advantageous trait become the majority. However, adaptations typically impose costs and well as benefits, and the evolution of such changes typically depends on the trade-off between those costs and benefits.
Researchers wishing to evaluate these trade-offs must develop techniques for studying just one characteristic of an organism at a time, a task made difficult by the fact that individuals in the wild can vary greatly.
Slow-moving salamanders make easy prey for garter snakes. However, one such creature, the rough-skinned newt, protects itself by secreting a neurotoxin that paralyzes nerves and muscles by blocking sodium channels. This is sufficient to deter most predators.
However, some populations of garter snake have evolved sodium channels that are resistant to the newt's toxin. These snakes have the advantage of being able to eat the newts and survive, but the toxin slows the snakes down for several hours. And, in fact, they never move as fast as nonresistant snakes. Thus, they sacrifice crawling speed for this advantage.
In an environment without rough-skin newts, the toxin-resistant snakes would not have an advantage. However, in an environment where rough-skin newts are present, the resistant snakes would have a clear advantage over their non-resistant peers.
While their reduced speed would make them vulnerable to predators, this would be outweighed by the advantage of being able to eat poisonous newts.
Because individuals in a population differ in many ways, scientists cannot easily assess the cost of a specific adaptation in nature. One way to investigate this question is by creating cloned or highly inbred populations, and then artificially manipulating the population to study the effects of changing a single gene.
Plasmids vectors can be used to transfer a specific allele to individual members of the inbred population. Using this technique, investigators measured the cost associated with resistance to an herbicide in the flowering plant Arabidopsis thaliana.
As a control, some plants received a plasmid that did not contain the herbicide resistance allele. Still other plants received no plasmid at all. Scientists then counted the number of seeds produced by each plant.
The plants that did not receive a plasmid produced about the same number of seeds as those that received a control plasmid, but the plants that received the resistance allele produced significantly fewer seeds.
Although the resistant plants will survive better in the presence of herbicide, the cost of maintaining resistance is fewer offspring. Therefore, nonresistant plants will outcompete resistant plants if no herbicide is present.
Let's look at another example. In some mammal species, one male controls reproductive access to many females. Such species tend to be sexually dimorphic—that is, the males look dramatically different than the females. Generally they are larger and bear weapons, such as horns, antlers, and large teeth.
There are metabolic costs associated with developing and maintaining these features. For instance, it turns out that males of these species generally are more susceptible to parasites than females are. However, the fact that these features are common in many species suggests that the benefits derived from possessing them must outweigh the costs.
Evolution takes place because individuals in a population that have a survival advantage typically produce more offspring and pass on that advantage to their offspring. Generally, however, these advantages impose a cost. Snakes that are able to eat poisonous newts and survive also move more slowly and are more susceptible to being eaten themselves. Flowers that can resist herbicide also produce fewer seeds than normal. Males in sexually dimorphic species carry a higher parasite load and have a higher mortality rate than males in monomorphic species, in which males and females look basically the same.
Scientists can study the costs of certain adaptations by creating a genetically identical population in the laboratory. Within this population, they can introduce one specific trait and compare individuals who carry the trait with individuals who don't. Another way to study the costs of adaptations is to compare entire populations in nature. By collecting data from a large number of individuals, scientists can draw conclusions about the effects of a particular adaptation.