The evolution of species

Explain what a species is according to the biological species concept (interbreeding to produce fertile offspring) and give an example of two closely related animals belonging to different species.

A horse and a donkey can interbreed and produce a mule, but they belong to different species. Explain, applying the biological species concept, why the mule does not "unite" the two species.

Distinguish between geographical isolation and reproductive isolation, and explain why reproductive isolation marks the point at which two populations have become different species.

Look at the diagram, which shows three stages in the evolution of a population divided by a geographical barrier. What is speciation? Explain stage by stage how geographical isolation can lead to the formation of a new species, and state what you would test in the two groups labelled ? to know if they are already distinct species.

On opposite sides of the Grand Canyon, there are squirrel species that are very similar yet distinct. Explain what type of speciation this example illustrates and reconstruct the process that caused it.

Some flies lay their eggs on apple trees and others, closely related to them, on hawthorns, and each group almost always breeds within its own plant. Explain what sympatric speciation is and how it differs from the classic case of the geographical barrier.

Explain what an adaptive radiation is and describe it with the example of the Galápagos finches: from an ancestral species that arrived from the mainland, several species arose with beaks adapted to different foods.

Explain why islands are considered "natural laboratories of evolution": what role isolation, new environments with few competitors and adaptive radiations play. Give an example (the Galápagos, the Canary Islands or Madagascar).

In Lake Victoria, hundreds of cichlid species coexist, having originated in barely 15 000 years thanks to the exploitation of micro-niches and females visually choosing males based on very bright coloration patterns in clear waters. In recent decades, discharges and deforestation have severely muddied the lake's water. Predict and justify the immediate evolutionary consequences that the loss of visibility has on the reproductive barriers between cichlid species, and explain what will happen to genetic diversity and the number of distinct species.

A patient diagnosed with a bacterial infection stops an antibiotic treatment prescribed for 10 days after only 4 days because they no longer have symptoms. Two weeks later, the infection recurs more aggressively, and the same antibiotic proves ineffective. Their relative claims: "The bacteria got used to the medication in order to survive and have now become immune." Refute their relative's claim based on the theory of natural selection and reasonably predict how the frequencies of sensitive and resistant bacteria in the bacterial population will have changed between the start of the treatment, day 4, and the relapse.

On an oceanic island newly formed after an eruption, a storm washes up a small raft of vegetation carrying 8 beetles of a mainland species (6 green and 2 brown). On the mainland, millions of these beetles live, where 95 % are camouflaged brown and only 5 % are green, as birds easily prey on the green ones. After several generations on the island (where there are no predatory birds), 75 % of the beetles are green. Justify which evolutionary mechanism accounts for the initial difference and predict how genetic drift will influence the population compared to natural selection if the island population remains small.

An agricultural research center produces a cloned wheat variety (100 % genetically identical) that yields 40 % more than any traditional variety under moderate drought. The agronomist claims: "We have achieved the optimal phenotype; eliminating genetic variability from our fields is the safest long-term evolutionary strategy." Reasonably predict what will happen to this crop if an unforeseen new fungal pest emerges compared to a diverse traditional crop, and justify why, in changing environments, retaining genetic variability is advantageous for natural populations instead of homogenizing into a single supposedly optimal type.

Explain what an extinction is and when we speak of a mass extinction, naming two examples from Earth's history (for example, the one at the end of the Permian or the one that wiped out the dinosaurs at the end of the Cretaceous).

Name the main causes why human activities are accelerating the loss of biodiversity today (habitat destruction, invasive species, overexploitation, pollution and climate change) and briefly explain how each one acts.

The polar bear (Ursus maritimus) and the brown bear (Ursus arctos) diverged approximately 500 000 years ago and exhibit very different morphological and metabolic adaptations. Due to the retreat of Arctic ice, their ranges increasingly overlap and fertile hybrids have been documented in the wild ("grolar bears"). With these data: a) Question the limitations of the biological species concept applied to this case. b) Predict and justify what long-term genetic consequences this gene flow could have for the conservation of the specific adaptations of the polar bear if the ice continues to disappear.