Name:__________________________Date:________________ Period:_____

Speciation Worksheet1. Do you think that tigers and lions are different species based on our scientific definition? Why or why not? Keep in mind that tigers and lions can mate to produce ligers, HOWEVER, ligers are not able to produce offspring.

2. Prezygotic isolation is where such factors as the isolation of habitats, a difference in breeding season or mating behavior, or an incompatibility of genitalia or gametes causes speciation.Postzygotic isolation is where barriers prevent the development of viable, fertile hybrids due to genetic incompatibility between the populations and hybrid sterility and breakdown.

For each of the following scenarios, write down whether the isolation is prezygotic or postzygotic.a) Ligers are unable to reproduce together because they are sterile:_____________________b) Eastern and western meadowlarks (a type of bird) do not mate because they use different songs to attract mates:____________________c) A large river separated the Albert squirrel which ended up producing two separate species:______________________d) Several species of orchids (a flowering plant) each release their pollen on different days and they are unable to fertilize each other because of that:______________________

3. Allopatric speciation is where speciation occurs in a much farther, remote location from the original species.Sympatric speciation is where speciation occurs within a local area.

Which reason(s) from the following list may explain why there are different species of Darwin’s finches: prezygotic and/or postzygotic, allopatric and/or sympatric speciation? Explain how each reason affects the finches (read the attached “Speciation in Darwin’s Finches”)

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4. Matching Match each of the examples to the type of prezygotic or postzygotic barrier below.

A. offspring die young when two species of salamanders mate – they do not make it to maturityB. two different species of birds have different mating calls so they cannot recognize each other as

matesC. one type of spotted skunk breeds in the spring while another breeds in the fallD. an elephant penis cannot fit into a frog vaginaE. garter snakes that live in water and one on land cannot mate although they live in the same

geographic areaF. mules are produced by mating a horse and a donkey, but mules are sterile and cannot breedG. if gametes of red and purple sea urchins do not fuse, there will be no zygote producedH. although some strains of cultivated rice can be cross-mated, after a few generations, sterile offspring

are created

_____ 1. mechanical isolation _____ 5. reduced hybrid viability

_____ 2. reduced hybrid fertility _____ 6. gametic isolation

_____ 3. habitat isolation _____ 7. behavioural isolation

_____ 4. temporal isolation _____ 8. hybrid breakdown

Name:__________________________Date:________________ Period:_____

5. Using your notes, classify each of the following situations as either a pre- or postzygotic isolating mechanism (first space) and then specifically which type of pre- or postzygotic isolating mechanism (second space):

a) In flowering plants, the pollen is often transferred from the pistil of one plant to the stigma of another by some type of animal (eg., bee, hummingbird). As a result, there are times when the pollen of one species ends up on the stigma of another. But of fertilization to occur, a pollen tube must form that allows the pollen to reach and fertilize the ovum. In most cases, this will not occur when the wrong pollen reaches the stigma due to differences in the structure of the pollen.

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b) A short hike through the Sandia Mountains from early spring to late summer will often reveal several different species of penstemon. The Rocky Mountain penstemon blooms in the spring while the very similar inflated penstemon blooms in late summer. As a result, hybrids are rarely encountered.

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c) Ligers are the result of a cross between a male lion and a female tiger. Almost all known male offspring of these crosses have been infertile.

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d) Norway rats and black rats are very similar. Attempts to cross the two species (why?) have led to pups being born, but they die within hours.

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e) When male fruit flies of one species (Drosophila melanogaster) attempt to mate with females of another (D. pseudoobscura), the act generally leads to severe injury or even death to both flies.

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6. Thinking in terms of natural selection, which isolating mechanism, prezygotic or postzygotic, which you expect to be most common in nature and why?

7. A research team studied two lakes in an area that sometimes experiences flooding. Each lake contained two types of similar fish: a dull brown form (B) and an iridescent gold form (G). The team wondered how all the fish were related, and they considered the two hypotheses diagrammed below:

a) Study the two diagrams. What does hypothesis A indicate about the ancestry of the fish in Lake 1 and Lake 2? What does hypothesis B indicate?

b) According to the two hypotheses, what is the key difference in the way the brown and gold fish populations might have formed?

c) A DNA analysis showed that the brown and gold fish from Lake 1 are the most closely related. Which hypothesis does this evidence support?

Name:__________________________Date:________________ Period:_____

d) What evidence could help determine whether the brown and gold fish are members of separate species?

Speciation in Darwin’s Finches(Adapted from Biology by Miller and levina, 2002)

Speciation in the Galápagos finches occurred by founding of a new population, geographic isolation, changes in the new population’s gene pool, reproductive isolation, and ecological competition.

1. Founders ArriveMany years ago, a few finches from the South American mainland – species A – flew or were blown to one of the Galápagos Islands. Finches are small birds that do not usually fly far over open water. These birds may have gotten lost, or they may have been blown off course by a storm. Once they arrived on one of the islands, they managed to survive and reproduce.

2. Separation of PopulationsLater on, some birds from species A crossed to another island in the Galápagos group. Because these birds do not usually fly over open water, they rarely move from island to island. Thus, finch population on the two islands were essentialy isolated from each other and no longer shared a common gene pool.

3. Changes in the Gene PoolOver time, populations on each island became adapted to their local environments. The plants growing on the first island may have produced small thin-shelled seeds, whereas the plants on the second island may have produced larger thick-shelled seeds. On the second island, directional selection would favor individuals with larger, heavier beaks. These birds could crack open and eat the large seeds more easily. Thus, birds with large beaks would be better able to survive on the second island. Over time, natural selection would have caused that population to evolve larger beaks, forming a separate population, B.

4. Reproductive IsolationNow, imagine that a few birds from the second island cross back to the first island. Will the population-A birds breed with the population-B birds? Probably not. These finches choose their mates carefully. As part of the courtship process, they inspect a potential partner’s beak very closely. Finches prefer to mate with birds that have the same-sized beak as they do. In other words, big-beaked birds prefer to mate with other big-beaked birds, and smaller-beaked birds prefer to mate with other smaller-beaked birds. Because the birds on the two islands have different-sized beaks, it is likely they would not choose to mate with each other. Thus, differences in beak size, combined with mating behavior, could lead to reproductive isolation. The gene pools of the two bird populations remain isolated from each other – even when individuals live together in the same place. The two populations have now become separate species.

5. Ecological CompetitionAs these two new species live together in the same environment (the first island), they compete with each other for available seeds. During the dry season, individuals that are most different from each other have the highest fitness. The more specialized birds have less competition for certain kinds of seeds and other foods, and the competition among individual finches is also reduced. Over time, species evolve in a way that increases the differences between them. The species-B birds on the first island may evolve into a new species, C.

6. Continued EvolutionThis process of isolation on different islands, genetic change, and reproductive isolation probably repeated itself time and time again across the entire Galápagos island chain. Over many generations, it produced the 13 different finch species found there today. The picture below shows how two species with larger and smaller beaks might have formed.

Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.

Name:__________________________Date:________________ Period:_____