Mexico has jaguars in the jungle, axolotls in the canals of Xochimilco, and vaquita porpoises in the Gulf of California. None of these species existed millions of years ago: all of them formed little by little from common ancestors. How does one population become two distinct species? That is the question of this page.
The Concept of Species
A Chihuahua and a Great Dane look very different, but they are the same species. A horse and a donkey, on the other hand, look quite similar, but their offspring, the mule, is sterile. How does science decide when two living things are the same species?
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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…
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.
Distinguish between geographical isolation and reproductive isolation, and explain why reproductive isolation marks the point at which two populations have become…
Distinguish between geographical isolation and reproductive isolation, and explain why reproductive isolation marks the point at which two populations have become different species.
Speciation is the process by which a population gives rise to one or more new species. There are two main paths.
Stage by stage: (1) a population lives together and breeds freely; (2) a barrier divides it and the two groups stop exchanging genes; (3) each group accumulates different mutations and lives under different pressures, until they can no longer interbreed with fertile offspring. To find out whether two groups are already different species, you test whether, when brought together, they interbreed and have fertile young.
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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…
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…
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.
Mexico is a megadiverse country: it has only a small part of the planet's territory, but an enormous share of its species. That variety of life is the result of millions of years of speciation.
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Define biodiversity and name its three levels, providing an example of each.
Define biodiversity and name its three levels, providing an example of each.
Evolution is not only a matter of millions of years: sometimes it can be seen in just a few generations. Natural selection occurs when, within a population, some individuals have heritable traits that give them a better chance of surviving and reproducing, and so those traits become more common.
Its steps are always the same: there is variation among individuals; that variation is inherited; the environment makes some survive and reproduce more than others; and the next generation has more individuals with the favored variant.
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In a beetle population, there are individuals with lighter and darker shells, and predators spot the lighter ones more easily against dark tree bark. Explain step by…
In a beetle population, there are individuals with lighter and darker shells, and predators spot the lighter ones more easily against dark tree bark. Explain step by step how natural selection will act on the frequency of each color in subsequent generations.
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…
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 the Galápagos Islands live more than thirteen species of finches with very different beaks, even though they all descend from a single population that arrived from the mainland. How did one ancestor diversify into so many species?
Genetic Drift
Natural selection is not the only mechanism of change. Sometimes frequencies change by chance alone.
Two typical cases: the founder effect (a small group colonizes a place and carries only a sample of the variants) and the bottleneck (a catastrophe leaves very few survivors).
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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…
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.
About 66 million years ago, the non-avian dinosaurs and most marine life disappeared. Today, the vaquita porpoise of the Gulf of California and the axolotl are in danger. What is the difference between a natural extinction and the current one?
The natural ("background") rate is 1 species per million species per year. The current rate, caused by human activity, is 100 to 1,000 times higher: habitat destruction, invasive species, overexploitation (fishing, hunting, trafficking), and climate change.
After the mass extinctions of the past (Permian, Cretaceous), the biosphere took between 5 and 10 million years to recover its biodiversity through adaptive radiation. New species appear on scales of thousands to millions of years, and today species are being lost in decades. That is why new species cannot make up for the current rate of loss. In the "tree of life," entire branches are being pruned that will take millions of years to replace, and many will never return.
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The natural fossil extinction rate ("background extinction") is estimated at 1 species per million species per year, whereas the current rate due to human activity is…
The natural fossil extinction rate ("background extinction") is estimated at 1 species per million species per year, whereas the current rate due to human activity is 100 to 1 000 times higher. Knowing that after the great mass extinctions of the past (such as the Permian or Cretaceous extinctions) the biosphere took between 5 and 10 million years to recover its biodiversity through adaptive radiation, analyze and justify why the current pace of habitat destruction cannot be buffered by the emergence of new species and what impact this will have on a geological scale on the tree of life.
Back to the beginning: the jaguar, the axolotl, and the vaquita are the result of millions of years of speciation, and their loss cannot be made up on the human time scale.
Summary
Let's practice! 3 medium exercises in this section