Speciation Questions
Bring on the tough stuff
1) Is the following statement true or false? Explain why.
According to the BSC, "true" species cannot produce hybrid offspring that are viable and fertile in the laboratory.
2) Imagine a series of closely related but morphologically distinct species whose geographic ranges are represented in the map below, with each occupying a distinct ecological niche around the perimeter of a large lake. The patterns of hybridization among these species suggest that they are ring species, such that Species 1 can hybridize with 2, 2 with 3, 3 with 4, and 4 with 5, but species 1 and 5 cannot hybridize. All hybrids are viable and fertile. According to each of the following species concepts—Biological, Ecological, and Phenetic—are these populations 5 distinct species or one single species?
3) Provide and explain two reasons why creating a universal species definition is difficult.
4) Briefly explain the difference between hybridization and hybrid speciation.
5) Would you expect to see greater intrinsic or extrinsic postzygotic isolation between two species that recently arose through sympatric speciation?
6) Briefly describe allopatric and sympatric speciation, and provide a real-world example of each.
7) What does it mean to be sister species? Identify the sister species from the tree below.
8) Describe the dominant evolutionary pattern according to the punctuated equilibrium view of evolution.
9) Define secondary contact and describe the mechanism thought to maintain distinct species identities when secondary contact occurs.
10) Identify whether assortative mating is a type of pre- or post-zygotic reproduction isolation and describe its importance in sympatric and allopatric speciation.
Possible Answers
1) Is the following statement true or false? Explain why.
According to the BSC, "true" species cannot produce hybrid offspring that are viable and fertile in the laboratory.
False. The BSC claims that "true" species cannot produce hybrid offspring under natural conditions; it makes no claims for the weird things that can take place in a laboratory or with biotechnology. The BSC's criterion for distinguishing between two species is that there must be some barrier that prevents gene flow between species. Under natural conditions, the reproductive barrier usually involves geographic separation (allopatry), though there are other isolating mechanisms that can be important, too. If two species that are reproductively isolated (that is, they share no gene flow) are brought together in a lab to hybridize, this does not violate the species definition of the BSC.
2) Imagine a series of closely related but morphologically distinct species whose geographic ranges are represented in the map below, with each occupying a distinct ecological niche around the perimeter of a large lake. The patterns of hybridization among these species suggest that they are ring species, such that Species 1 can hybridize with 2, 2 with 3, 3 with 4, and 4 with 5, but species 1 and 5 cannot hybridize. All hybrids are viable and fertile. According to each of the following species concepts—Biological, Ecological, and Phenetic—are these populations 5 distinct species or one single species?
The Biological Species Concept: They are one single species. Even though species 1 and 5 cannot hybridize, they can share genes via hybridization with intermediate species, therefore there is no reproductive isolation between them.
The Ecological Species Concept: They are five distinct species. The ESC makes no issue of hybridization, as species are distinguished according to their ecological niches. Since the 5 species all occupy distinct niches, they are distinct species according to the ESC.
The Phenetic Species Concept: Could go either way, depending on which morphological characters were used to distinguish between species. Assuming the morphological traits used to characterize species are distinct, then they are five distinct species according to the phenetic species concept.
3) Provide and explain two reasons why creating a universal species definition is difficult.
Asexual reproduction: scientists often rely on reproduction and gene flow to define species, but there are many organisms that do not reproduce sexually, and therefore traditional species concepts do not apply to them.
Misleading morphology: It is often unclear in nature which organisms are capable of mating with which others. It can be that morphologically distinct individuals—what we expect to be distinct species—are actually one species, or it can be that cryptic reproductive isolation prevents gene flow among morphologically similar organisms that we would assume are the same species. Ultimately, morphology can be misleading.
Hybridization: Many species have low levels of hybridization with other species. Even if hybrids have reduced fitness compared to parent species, as long as they are fertile it allows low levels of gene flow between what otherwise would be considered separate species.
Horizontal Gene Transfer: There is considerable evidence of shared genes between unrelated groups of organisms as a result of gene transfer in the absence of reproduction. While this is most common in prokaryotes (like bacteria), there are known cases in eukaryotes (like plants, animals, and fungi), as well.
4) Briefly explain the difference between hybridization and hybrid speciation.
Hybridization occurs when two parent species mate to form a hybrid offspring. If the hybrid offspring is inviable or infertile then the designation of parents as separate species is sound according to the BSC. However, the hybrid is an evolutionary dead end, so the end result is two species—the original parents.
In hybrid speciation, the hybrids are perfectly fit and they can mate with other hybrids to make fit offspring, but usually some mechanism isolates hybrids from backcrossing with parent species. The end result is 3 distinct species: the original parent species and the new hybrid species.
5) Would you expect to see greater intrinsic or extrinsic postzygotic isolation between two species that recently arose through sympatric speciation?
Extrinsic isolation. Because the species arose in sympatry, it is unlikely that intrinsic barriers are acting to obstruct gene flow. It is more common that niche partitioning and assortative mating prevent gene flow between sympatric species, which are both examples of extrinsic reproductive isolation.
6) Briefly describe allopatric and sympatric speciation, and provide a real-world example of each.
Allopatric speciation occurs when populations of a single species become reproductively isolated from one another by a geographic barrier. A classic example is the snapping shrimp that live around the Isthmus of Panama. When the isthmus closed several million years ago, it created a geographic barrier between a previously continuous species of shrimp, leading to the formation of distinct shrimp species on each side of the land bridge.
Sympatric speciation occurs when two populations that have overlapping geographical ranges become reproductively isolated. For example, the cichlid fishes in African rift lakes are believed to have sympatrically speciated. They took advantage of different niches and developing strong pre-zygotic isolating barriers between species.
7) What does it mean to be sister species? Identify the sister species from the tree below.
Sister species are two species that share a most recent common ancestor. They are the result of the most recent speciation event in their lineage. In the tree above, species A and C are sister species.
8) Describe the dominant evolutionary pattern according to the punctuated equilibrium view of evolution.
The dominant evolutionary pattern, as observed in the fossil record, is that there are long periods of relative evolutionary stasis, with intermittent short periods of relatively fast evolutionary change and speciation.
9) Define secondary contact and describe the mechanism thought to maintain distinct species identities when secondary contact occurs.
Secondary contact occurs when two species that arose via allopatric speciation are brought back into contact with each other.
If sufficient time has passed and evolutionary divergence has taken place during allopatry such that hybrids have reduced fitness compared to their parent species, reinforcement may occur upon secondary contact. This will maintain the distinction between the two species.
10) Identify whether assortative mating is a type of pre- or post-zygotic reproduction isolation and describe its importance in sympatric and allopatric speciation.
Assortative mating is a mechanism of prezygotic isolation, because it affects which individuals will mate with one another.
It plays a large role in sympatric speciation by maintaining reproductive isolation among species whose ranges overlap. In allopatry, however, where geography provides an adequate barrier to gene flow, assortative mating is not necessary at all for the maintenance of reproductive isolation.