A Chihuahua and a Great Dane look nothing alike: one fits in a bag and the other looks you straight in the eye. Yet they are the same species. A horse and a donkey, on the other hand, look quite similar, but they are different species. What criterion decides when two populations are a single species or already two? And, above all, how does one become two? That is the question of this page.
The concept of species
A horse and a donkey can mate: their offspring is the mule. But the mule is sterile; it cannot have offspring. That is the key.
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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.
The stages of allopatric speciation are:
A single population occupies a territory.
A geographic barrier divides it in two and gene flow is interrupted.
On each side, different mutations appear and natural selection acts in slightly different environments.
After many generations, the populations have diverged so much that, if they meet again, they no longer produce fertile offspring: they are two species.
To find out whether two separated groups are already different species, you check whether, when brought together, they breed and have fertile offspring.
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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.
Speciation requires populations to change. The main driver of that change is natural selection, which acts in three steps:
Variability: individuals in a population differ from one another, and part of those differences is hereditary (it comes from mutations and sexual reproduction).
Differential survival and reproduction: in each environment, some traits give a higher probability of surviving and leaving offspring.
Inheritance: the offspring inherit those traits, so in the next generation the advantageous trait is more common.
Let's see it with numbers. In a population of 100 beetles there are 50 light ones and 50 dark ones. Birds see the light ones better against the dark bark. Suppose 40% of the light ones survive and 80% of the dark ones, and that the survivors reproduce until they are back to 100 individuals with the same color as their parents.
Generation
Light
Dark
% light
0
50
50
50 %
1
33
67
33 %
2
20
80
20 %
Each generation the light beetles lose ground: the dark color becomes more common. Nobody "decided" to change color: the environment simply filtered what already existed.
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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 a dozen species of finches with very different beaks (thick ones for cracking seeds, thin ones for catching insects, long ones for sipping nectar). All of them descend from a founding population that arrived from the mainland. How does a single species become so many?
Biodiversity
The result of billions of years of speciation (and extinctions) is biodiversity.
Mexico is a megadiverse country, and CONABIO is responsible for studying and recording that richness.
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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.
Natural selection is not the only evolutionary mechanism. Genetic drift is the random change in allele frequencies from one generation to the next, unrelated to whether the allele is advantageous or not. It matters much more in small populations: just as 10 coin flips give very uneven results, while 10,000 flips come close to 50%.
A typical case is the founder effect: a few individuals colonize a new place and carry only a random sample of the alleles of the original population.
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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.
Everything above rests on genetic information. Before each cell division the DNA is copied (replication) and, to produce a trait, it is expressed: it is transcribed into RNA and translated into proteins. If an error occurs in the copying, a mutation appears; if it alters a protein, it may change a trait, and natural selection can then act on it. Thus, replication, gene expression, and their errors are the molecular basis of heredity and of the variability that fuels evolution. More details in the pages "Mutations" and "Gene expression."
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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.
An extinction is the complete disappearance of a species. A mass extinction occurs when, in a short geological time, a large part of the planet's species disappear. About 66 million years ago, the impact of the Chicxulub asteroid, in Yucatán, marked the end of the non-avian dinosaurs and of many other species. Today the loss of biodiversity is due mainly to habitat destruction, invasive species, overexploitation, and climate change. The vaquita porpoise and the monarch butterfly are Mexican examples of threatened species.
In addition, the boundaries between species are not always sharp. Polar bears and brown bears can hybridize and have fertile offspring, and they are still considered different species because of their ecological, anatomical, and genetic differences. The biological criterion is a useful tool, but nature does not always draw perfect lines.
Go back to the beginning: a Chihuahua and a Great Dane are a single species because they can breed and have fertile offspring, while the horse and the donkey, whose mule is sterile, are two. And new species arise when isolation lets each population accumulate changes, until they can no longer mix.
Summary
Let's practice! 3 medium exercises in this section