Exercises: Phylogeny, Common Ancestry, and Speciation

Define clade and explain what a branch point (node) on a phylogenetic tree represents.

A phylogenetic tree is described as follows: the fish lineage branches off first; then the frog lineage branches off; then the lizard lineage branches off; the final branch point splits into mouse and human. (a) Which two organisms are most closely related? (b) Which organism could serve as an outgroup for the lizard, mouse, and human? (c) Is the human more closely related to the lizard or to the frog? Explain.

Classify each pair of structures as homologous or analogous, and name the kind of evidence each provides:

(a) a bat wing and a human arm;

(b) a bird wing and an insect wing;

(c) a dolphin flipper and a shark fin.

The table shows the number of amino acid differences in a protein compared with the human version (hypothetical values for illustration).

(a) Order the species from most to least closely related to humans. (b) Sketch in words how the tree would branch. (c) Explain why molecular data such as DNA and protein sequences are strong evidence of common ancestry.

Two related squirrel species live on opposite rims of the Grand Canyon. They are thought to have descended from a single ancestral population that was split by the canyon. (a) Name the mode of speciation. (b) Describe the steps from the initial population to two species. (c) Explain why the two populations may have become reproductively isolated even if no one prevented them from meeting later.

Classify each reproductive barrier as prezygotic or postzygotic:

(a) two frog species breed in different seasons;

(b) a horse and a donkey mate, but their offspring (a mule) is sterile;

(c) two bird species respond only to their own courtship song;

(d) a hybrid embryo forms but stops developing early.

Then explain why a sterile hybrid does not fit the biological species concept as part of one species.

The table shows traits present (+) or absent (−) in five animals.

(a) Construct a cladogram and indicate where each trait first appears. (b) A newly discovered fossil has a vertebral column, jaws, four limbs, an amniotic egg, and no hair. Where would you place it, and why? (c) Explain why the lamprey is the best outgroup here.

Two forms of stickleback fish live together in the same lake: one feeds near the surface and one feeds near the bottom. In mating trials, 90% of pairings within a form produce offspring, but only 5% of pairings between forms do, and hybrids have lower survival. (a) Evaluate whether the two forms should be considered separate species, using evidence from the data. (b) Explain how this could be an example of sympatric speciation, and compare it with allopatric speciation. (c) Explain why gene flow between the two forms, if it increased, would tend to reduce their divergence.