Why do pesticides sometimes stop working?
Pesticides were designed to kill bothersome insects. However, sometimes these pesticides will stop working. A pesticide that has worked in the past may no longer kill a particular type of insect. This is due to the development of resistance in the population of insects. The development of resistance to pesticides is one example of microevolution, a small change in a population.
Microevolution and Macroevolution
Does evolution only happen gradually through small changes? Or is it possible that drastic environmental changes can cause new species to evolve? Or can both small and large changes occur?
Evolutionary changes can be both big and small. Some evolutionary changes do not create new species, but result in changes at the population level. A population is a group of organisms of the same species that live in the same area (Figure below). But what exactly is the definition of a species? A species is a group of organisms that have similar characteristics (they are genetically similar) and can mate with one another to produce fertile offspring.
This school of fish are considered members of the same species because they are able to mate with one another. They are also considered a population because they live in the same part of the ocean.
You already know that evolution is the change in species over time. Most evolutionary changes are small and do not lead to the creation of a new species. When populations change in small ways over time, the process is called microevolution. Microevolution results in changes within a species.
An example of microevolution is the evolution of mosquitoes that cannot be killed by pesticides, called pesticide-resistant mosquitoes. Imagine that you have a pesticide that kills most of the mosquitoes in your state. Through a random mutation, some of the mosquitoes have resistance to the pesticide. As a result of the widespread use of this pesticide, most of the remaining mosquitoes are the pesticide-resistant mosquitoes. When these mosquitoes reproduce the next year, they produce more mosquitoes with the pesticide-resistant trait. Soon, most of the mosquitoes in your state are resistant to the pesticide.
This is an example of microevolution because the number of mosquitoes with this trait changed. However, this evolutionary change did not create a new species of mosquito because the pesticide-resistant mosquitoes can still reproduce with other non-pesticide-resistant mosquitoes.
Macroevolution refers to much bigger evolutionary changes that result in new species. Macroevolution may happen:
- When microevolution occurs repeatedly over a long period of time and leads to the creation of a new species.
- As a result of a major environmental change, such as a volcanic eruption, earthquake, or an asteroid hitting Earth, which changes the environment so much that natural selection leads to large changes in the traits of a species.
After thousands of years of isolation from each other, Darwin's finch populations have experienced both microevolution and macroevolution. These finch populations cannot breed with other finch populations when they are brought together. Since they do not breed together, they are classified as separate species.
- Microevolution is the process by which organisms change in small ways over time.
- Macroevolution refers to larger evolutionary changes that result in new species.
Use the resources below to answer the questions that follow.
Explore More I
- Microevolution vs. Macroevolution at http://www.youtube.com/watch?v=jeza0g3E8wE (5:21)
- How does microevolution differ from macroevolution with regard to species?
- What are four factors believed to cause microevolution? How do these four factors relate to macroevolution?
- What happens to the allele frequency of genes in a population that has undergone microevolution?
- What is phyletic gradualism? How does this differ from "punctuated equilibrium"?
Explore More II
- Cambrian Explosion - Shape of Life at http://shapeoflife.org/video/other-topics/cambrian-explosion (13:07)
- What was the Cambrian Explosion? When did it occur?
- Do you think the Cambrian Explosion represents microevolution or macroevolution? Explain your reasoning.
- What are some of the ideas about the cause of the Cambrian Explosion?
- How much similarity can be seen between the current body plans of organisms and the body plans seen in fossils from the Cambrian Period?
- How do you know if two related organisms are members of the same species?
- Compare and contrast microevolution and macroevolution.
- Does microevolution lead to a new species? Why or why not?
- What is the outcome of many microevolution events?