Question

Answer should show an understanding of the species concepts and a defensible use of evidence.
• Correct definition of species concept
• Selection of evidence that is relevant to the species concept and good explanation of how the evidence is used.
• Clear description of additional evidence that is relevant to the species concept

1. Two morphs of the marine snail Littorina saxatilis have been identified on rocky intertidal shores in Galacia, Spain. Johannesson et al (1995) made the following observations about these snails:

• One morph consists of snails with smooth, unbanded shells. These snails are found in the blue mussel zone, low in the intertidal. The other morph consists of snails with ridged, banded shells. These snails are found high in the intertidal, in the barnacle zone.
• All snails feed on algae, but algal species differ between the two zones.
• The two morphs overlap in a narrow midshore band at the border of the blue mussel and barnacle zones.
• Mating in the midshore band is not random. Females prefer to mate with males of the same morph, and only 8% of the snails in the midshore band are hybrids.
• Some female hybrids have been observed with embryos in their brood pouches, indicating that they are not sterile. However, the survival rates of hybrids relative to the survival rates of pure morphs are unknown.

Consider the information provided in this case study in addressing the questions below.

1. (2m) Does the case study describe sympatric or allopatric speciation? Explain your answer, including a definition of the mode of speciation that you have identified in this case.

2. (18m) Are the two morphs separate species:
   • According to the biological species concept?
   • According to the morphological species concept?
   • According to the ecological species concept?

For each species concept, define the species concept, explain what information you used to answer the question and suggest one piece of additional information that would be relevant in helping you to decide if the two morphs are separate species

Answer 1

Species are groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups. ... Thus, under the biological species concept, species are simultaneously a reproductive community, a gene pool, and a genetic system.

The major limitations of the biological species concept are that it is irrelevant to allopatric speciation and is inapplicable to: (1) fossil species; (2) organisms reproducing asexually or with extensive self-fertilization; and (3) sexual organisms with open mating systems (species that freely hybridize). On the other hand, the biological species concept is intuitively appealing to people because Homo sapiens fits in the narrow range of sexual reproduction that is the domain of the biological species concept.

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Evidence for evolution comes from many different areas of biology:

• Anatomy. Species might share physical features that are similar because of the presence of the same feature in a common ancestor (homologous structures).

• Molecular biology. DNA and the genetic code show the shared ancestry. DNA alignment can show how related species are.

• Biogeography. The global distribution of organisms and the unique features of island species reflect evolution and geological change.

• Fossils. Fossils document the existence of now-extinct past species that are related to present-day species.

• Direct observation. directly observe small-scale evolution in organisms with short lifecycles (e.g., insecticide-resistant insects).