Lesson 3: Modes of Evolution
Background Information
The term evolution refers to the cumulative change that occurs in populations of organisms over time. Sometimes evolutionary change is so dramatic that different populations of the same species diverge to become two or more distinct species. In the case of a group of birds called honeycreepers, for example, a single species that colonized the Hawaiʻian Islands about 5 million years ago ultimately diverged into 57 different species.
This process, which evolutionary biologists call speciation or adaptive radiation, can happen anywhere. However, it is most clearly demonstrated on geologically young land masses, such as newly formed islands or mountains. In these environments a population of organisms will typically find a set of environmental opportunities and pressures very different from the conditions they experienced in their place of origin. These environmental differences come in many forms and often cause sweeping evolutionary changes in a founding population.
Several environmental factors affect the process of speciation. The structural habitat of an area determines the ease with which creatures are able to move around and find shelter from weather and other organisms. Food, both the type and its availability, dictates the ease with which animals are able to acquire the energy they need to survive and reproduce.
Competition for various resources is another factor that can drive the process of speciation. Competitive pressure can come from organisms of the same species or from organisms of different species. Generally, in highly competitive environments, traits that minimize competition — traits that, for example, allow two different populations to feed on very different types of food — are advantageous.
Another factor that can influence speciation is predation. Predators typically reduce the rate of speciation because they limit other organisms' access to resources. On newly formed land masses, however, the number of predators is typically lower than on older continents. These younger environments, therefore, provide more opportunities for species to evolve into new and different species.
Allopatric speciation occurs after a population becomes separated geographically by a natural barrier. Sympatric speciation occurs without a geographical separation.
This process, which evolutionary biologists call speciation or adaptive radiation, can happen anywhere. However, it is most clearly demonstrated on geologically young land masses, such as newly formed islands or mountains. In these environments a population of organisms will typically find a set of environmental opportunities and pressures very different from the conditions they experienced in their place of origin. These environmental differences come in many forms and often cause sweeping evolutionary changes in a founding population.
Several environmental factors affect the process of speciation. The structural habitat of an area determines the ease with which creatures are able to move around and find shelter from weather and other organisms. Food, both the type and its availability, dictates the ease with which animals are able to acquire the energy they need to survive and reproduce.
Competition for various resources is another factor that can drive the process of speciation. Competitive pressure can come from organisms of the same species or from organisms of different species. Generally, in highly competitive environments, traits that minimize competition — traits that, for example, allow two different populations to feed on very different types of food — are advantageous.
Another factor that can influence speciation is predation. Predators typically reduce the rate of speciation because they limit other organisms' access to resources. On newly formed land masses, however, the number of predators is typically lower than on older continents. These younger environments, therefore, provide more opportunities for species to evolve into new and different species.
Allopatric speciation occurs after a population becomes separated geographically by a natural barrier. Sympatric speciation occurs without a geographical separation.
Allopatric speciation
...is an evolutionary process in which one species divides into two because the original homogenous population has become separated and both groups diverge from each other.
In their separate niches, the two groups go their own evolutionary ways, accumulating different gene mutations , being subjected to different selective pressures, experiencing different historical events, finally becoming incapable of interbreeding should they ever come together again. For many years this has been regarded as the main process by which new species arise.
Often this type of speciation occurs in three steps. First, the populations become physically separated, often by a long, slow geological process like an uplift of land, the movement of a glacier, or formation of a body of water. Next, the separated populations diverge, through changes in mating tactics or use of their habitat. Third, they become reproductively separated such that they cannot interbreed and exchange genes.
Under normal conditions, genes in a given population are exchanged through breeding, so that even if some variation occurs, it is limited by this "gene flow." But gene flow is interrupted if the population becomes divided into two groups. One way this happens is by "vicariance," geographical change that can be slow or rapid.
Bottleneck Effect
Genetic drift can cause big losses of genetic variation for small populations.
Population bottlenecks occur when a population's size is reduced for at least one generation. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population's genetic variation by a lot, even if the bottleneck doesn't last for very many generations. This is illustrated by the bags of marbles shown below, where, in generation 2, an unusually small draw creates a bottleneck.
In their separate niches, the two groups go their own evolutionary ways, accumulating different gene mutations , being subjected to different selective pressures, experiencing different historical events, finally becoming incapable of interbreeding should they ever come together again. For many years this has been regarded as the main process by which new species arise.
Often this type of speciation occurs in three steps. First, the populations become physically separated, often by a long, slow geological process like an uplift of land, the movement of a glacier, or formation of a body of water. Next, the separated populations diverge, through changes in mating tactics or use of their habitat. Third, they become reproductively separated such that they cannot interbreed and exchange genes.
Under normal conditions, genes in a given population are exchanged through breeding, so that even if some variation occurs, it is limited by this "gene flow." But gene flow is interrupted if the population becomes divided into two groups. One way this happens is by "vicariance," geographical change that can be slow or rapid.
Bottleneck Effect
Genetic drift can cause big losses of genetic variation for small populations.
Population bottlenecks occur when a population's size is reduced for at least one generation. Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population's genetic variation by a lot, even if the bottleneck doesn't last for very many generations. This is illustrated by the bags of marbles shown below, where, in generation 2, an unusually small draw creates a bottleneck.
Reduced genetic variation means that the population may not be able to adapt to new selection pressures, such as climatic change or a shift in available resources, because the genetic variation that selection would act on may have already drifted out of the population.
An example of a bottleneck
Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 — but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted. |
Founder Effect
A founder effect occurs when a new colony is started by a few members of the original population. This small population size means that the colony may have:
- reduced genetic variation from the original population.
- a non-random sample of the genes in the original population.
Hook: Video Introduction to Speciation
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Video: http://www.youtube.com/watch?feature=player_embedded&v=2oKlKmrbLoU
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Action
Allopatric Speciation
Have your students view the image to the right.
This is an example of vicariance; the separation of marine creatures on either side of Central America when the Isthmus of Panama closed about 3 million years ago, creating a land bridge between North and South America. Nancy Knowlton of the Smithsonian Tropical Research Institute in Panama has been studying this geological event and its effects on populations of snapping shrimp. She and her colleagues found that shrimp on one side of the isthmus appeared almost identical to those on the other side -- having once been members of the same population.
But when she put males and females from different sides of the isthmus together, they snapped aggressively instead of courting. They had become separate species, just as the theory would predict.
Discuss speciation, especially allopatric speciation, with your class:
Have your students view the image to the right.
This is an example of vicariance; the separation of marine creatures on either side of Central America when the Isthmus of Panama closed about 3 million years ago, creating a land bridge between North and South America. Nancy Knowlton of the Smithsonian Tropical Research Institute in Panama has been studying this geological event and its effects on populations of snapping shrimp. She and her colleagues found that shrimp on one side of the isthmus appeared almost identical to those on the other side -- having once been members of the same population.
But when she put males and females from different sides of the isthmus together, they snapped aggressively instead of courting. They had become separate species, just as the theory would predict.
Discuss speciation, especially allopatric speciation, with your class:
- Why does the geological history of the Isthmus of Panama make it a good place to look for evidence of allopatric speciation in marine organisms?
- What kind of life history strategy (mating patterns, number of offspring, degree of mobility of larvae and adults, etc) do you think might encourage allopatric speciation of populations along the isthmus?
- What life history strategies might encourage, not only allopatric speciation, but the kind of speciation that Schneider and his colleagues hypothesize among Ecuadorian hummingbirds?
- Assign students the numbers 1, 2, 3, or 4 by having them count off.
- Have students go to the Web activity An Origin of Species. The ones cover the mainland, the twos cover Windsor Island, the threes study Norcross Island, and the fours examine Warwick Archipelago. Students can work in groups, depending on the resources in the classroom. Have students try out the activity and view the Species Gallery. Ask them to take notes on what happens to their species over time.
- Once students have had a chance to view the Web feature, discuss the following questions:
- Over how long a time period do the pollenpeepers evolve in this Web feature?
- What influence do competition, habitat, food, and predators have on evolution?
- What is a biological species?
- How can one species, such as the hypothetical pollenpeeper, adaptively radiate into different species?
- What conditions contribute to speciation in a localized area?
- How might a species become reproductively isolated (unable to reproduce fertile offspring)?
Additional Teaching Resources
Hummingbird Species in the Transitional Zones
Have students watch the following video:
http://www.pbs.org/wgbh/evolution/library/05/2/quicktime/l_052_04_56.html
Distribute a handout containing the following information:
The hummingbird study that Tom Smith and Chris Schneider are conducting in Ecuador is part of a much larger research program spanning three continents. Evolutionary biologists are fanning out and tramping through varying ecosystems in Africa, Australia, and South America, catching and meticulously describing the animals that live there. It's a new venture aimed at answering an old question, one that underlies all of evolutionary science: What drives the formation of new species?
The prevailing theory goes back almost 60 years, to when biologist Ernst Mayr of Harvard University proposed the "reproductive isolation" theory. When a population of, say, lizards or birds becomes divided by geographical barriers, small changes over time will alter the genetic makeup of the separated groups. Eventually, they differ enough that, should they encounter each other again, they can no longer interbreed. The offshoot group has become a new species.
But in recent years an even older, contrasting view dating back to Darwin has been gaining ground. The globe-trotting biologists are discovering an intriguing pattern: In many places, species appear to have emerged at the transition zones between different ecosystems, without ever being geographically cut off from the parent stock. They are examples of how natural selection can act through ecological differences to spawn new species.
The Andean hummingbirds are not an isolated case. The force of ecology has also been studied in the leaf-litter skink, a small lizard found in Australia. Two populations living close to each other but in different ecosystems show in their DNA that they're genetically distinct. Chris Schneider figured out that the two populations had adapted to different ecosystems -- one an open forest and the adjacent one a closed rainforest.
The open forest lizards are smaller, have shorter limbs and bigger heads, and become sexually mature earlier. The reason: Predator birds more easily pick off lizards in the open forest, so the skinks there have evolved to reproduce earlier, generating offspring before they become a bird's dinner. The genetic differences, shaped by selection, have produced two distinct species living next to each other.
And the converse can be true: Populations separated by geography but living in similar environments may be almost indistinguishable. Says Schneider: "Time and isolation alone don't necessarily result in new morphologies -- whereas a new environment does."
Follow-up Discussion Questions:
http://www.pbs.org/wgbh/evolution/library/05/2/quicktime/l_052_04_56.html
Distribute a handout containing the following information:
The hummingbird study that Tom Smith and Chris Schneider are conducting in Ecuador is part of a much larger research program spanning three continents. Evolutionary biologists are fanning out and tramping through varying ecosystems in Africa, Australia, and South America, catching and meticulously describing the animals that live there. It's a new venture aimed at answering an old question, one that underlies all of evolutionary science: What drives the formation of new species?
The prevailing theory goes back almost 60 years, to when biologist Ernst Mayr of Harvard University proposed the "reproductive isolation" theory. When a population of, say, lizards or birds becomes divided by geographical barriers, small changes over time will alter the genetic makeup of the separated groups. Eventually, they differ enough that, should they encounter each other again, they can no longer interbreed. The offshoot group has become a new species.
But in recent years an even older, contrasting view dating back to Darwin has been gaining ground. The globe-trotting biologists are discovering an intriguing pattern: In many places, species appear to have emerged at the transition zones between different ecosystems, without ever being geographically cut off from the parent stock. They are examples of how natural selection can act through ecological differences to spawn new species.
The Andean hummingbirds are not an isolated case. The force of ecology has also been studied in the leaf-litter skink, a small lizard found in Australia. Two populations living close to each other but in different ecosystems show in their DNA that they're genetically distinct. Chris Schneider figured out that the two populations had adapted to different ecosystems -- one an open forest and the adjacent one a closed rainforest.
The open forest lizards are smaller, have shorter limbs and bigger heads, and become sexually mature earlier. The reason: Predator birds more easily pick off lizards in the open forest, so the skinks there have evolved to reproduce earlier, generating offspring before they become a bird's dinner. The genetic differences, shaped by selection, have produced two distinct species living next to each other.
And the converse can be true: Populations separated by geography but living in similar environments may be almost indistinguishable. Says Schneider: "Time and isolation alone don't necessarily result in new morphologies -- whereas a new environment does."
Follow-up Discussion Questions:
- Explain the hypothesis presented by the scientists profiled in this segment to explain the process of speciation in hummingbirds and possibly other species.
- How does this hypothesis differ from the traditional view that speciation often requires geographic separation of populations?
- Why were the researchers collecting blood from the populations they studied? Discuss at least two possible analyses that could be performed on those samples and, identify at least two different questions that might be answered with sufficient data.