In open-air markets (tianguis) all over Mexico you can find huge ears of corn with yellow, white, blue, and red kernels. Their wild relative, teosinte, has a "cob" of just a few hard seeds. How did we get from one to the other? Do species change over time, or have they always been the way we see them today? On this page you will see how that question was gradually answered and what evidence supports the answer.
What is evolution? From fixism to the first cracks
Dogs have puppies of their own breed, and a wheat field always produces wheat. For centuries, this apparent stability led people to think that living things have never changed.
Fossils were the first crack: remains of animals that no longer existed anywhere kept turning up. In Mexico, mammoth bones have been found in places such as Tultepec, in the State of Mexico. If species never change or disappear, why are there mammoths in the rock and not in the pastures?
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Explain what biological evolution is and what it means to state that present-day species descend from other species of the past.
Explain what biological evolution is and what it means to state that present-day species descend from other species of the past.
A blacksmith develops very strong arms after years of hammering at the anvil. Will his children be born with those muscles thanks to their father's effort?
Lamarck explained the long neck of giraffes like this: short-necked giraffes stretched their necks to reach the high leaves, their necks lengthened through use, and their young were born with already longer necks.
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Summarise Lamarck's two main ideas (use and disuse of organs and inheritance of acquired characteristics) and explain how he interpreted the long neck of giraffes.
Summarise Lamarck's two main ideas (use and disuse of organs and inheritance of acquired characteristics) and explain how he interpreted the long neck of giraffes.
Analyze the following two situations: 1) If the tails of mice are surgically amputated for several successive generations, all offspring in subsequent litters continue…
Analyze the following two situations: 1) If the tails of mice are surgically amputated for several successive generations, all offspring in subsequent litters continue to be born with intact tails. 2) If the testes of male mice are irradiated with X-rays, many of their offspring are born with tail malformations. A) Justify these results based on the difference between somatic cells and germline cells. B) If a bodybuilder were to use gene therapy to modify the DNA of their arm muscle fibers to induce hypertrophy, would their descendants inherit that muscle mass? Justify your answer.
Think of a litter of field mice on a ground of black volcanic rock: some are born with dark fur and others with light fur. If owls hunt by sight, do all the mice have the same chance of surviving to reproduce?
Compare the two explanations of the giraffe. The key difference is what is inherited: Lamarck says what the individual acquired during its life is inherited; Darwin says the variation the individual was already born with is inherited.
Selection does not only happen in nature. When Mesoamerican peoples saved the seeds from the best ears of teosinte to plant them, they were the "environment" doing the selecting (artificial selection). Over thousands of years, this produced corn. Go back to the beginning: the teosinte did not need to "want" to change; it was enough that there was heritable variation and that someone chose which seeds to plant.
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Summarise the key ideas of Darwinism: variation among individuals, the struggle for existence and natural selection of the best adapted.
Summarise the key ideas of Darwinism: variation among individuals, the struggle for existence and natural selection of the best adapted.
Look at the diagram, which shows two different explanations (A and B) of how giraffes came to have long necks. State which corresponds to Lamarck and which to Darwin,…
Look at the diagram, which shows two different explanations (A and B) of how giraffes came to have long necks. State which corresponds to Lamarck and which to Darwin, and point out the fundamental difference between the two regarding what is inherited.
Evolution by natural selection does not happen because of a single cause. It is explained by the combined action of four factors.
Factor
What it means
Mexican example
Reproductive potential
Populations can have more offspring than the environment can support
A olive ridley sea turtle lays hundreds of eggs, but very few hatchlings reach adulthood
Genetic variation
Individuals differ in heritable traits (through mutation and recombination)
Different colors and sizes of kernels among corn varieties
Intraspecific and interspecific interactions
Competition, predation, parasitism, and cooperation, within the species and with other species
Males competing for territory; the jaguar preying on other animals
Natural selection
Traits that give greater survival and reproductive success become more common
The trait that helps escape the predator becomes predominant
Interactions are part of the selection pressure: a predator, a parasite, or a competitor is part of the "environment" that selects. If the predator changes, which traits are advantageous changes too.
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In an English industrial region, the proportion of dark (carbonaria*) and light (typica*) peppered moths (Biston betularia*) was measured. After anti-coal pollution laws…
In an English industrial region, the proportion of dark (carbonaria) and light (typica) peppered moths (Biston betularia) was measured. After anti-coal pollution laws were passed in 1960, records showed: in 1960, 90% dark moths; in 1980, 50%; and in 2000, only 10%. A) Interpret these data by identifying the natural selection factor and explaining why the allele frequencies changed. B) Predict with reasoned arguments what would happen to the proportion of both varieties if a new nocturnal predator were introduced that locates its prey exclusively by smell and not by sight.
If species descend from other species, there must be traces. Several lines of evidence point to the same conclusion.
Biogeography studies where species live. Mexican islands show this well: on Cozumel Island live species found nowhere else, such as the pygmy raccoon, similar to those on the mainland. This is explained if they descend from mainland ancestors that reached the island and were modified there.
And DNA provides independent evidence: human and chimpanzee DNA match in more than 98 %, and humans are less similar to more distant species. The greater the similarity, the more recent the common ancestor.
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Name and briefly describe three lines of evidence for evolution: fossils, comparative anatomy (homologous bones) and biogeography.
Name and briefly describe three lines of evidence for evolution: fossils, comparative anatomy (homologous bones) and biogeography.
Darwin did not know how traits were inherited or where variation came from. Twentieth-century genetics answered those questions.
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What does Neo-Darwinism or the synthetic theory add to Darwin's ideas? Explain how genetics (mutations and inheritance) completed the theory of natural selection.
What does Neo-Darwinism or the synthetic theory add to Darwin's ideas? Explain how genetics (mutations and inheritance) completed the theory of natural selection.
On a small island, a prolonged drought wipes out soft-seeded plants, and only those with large, hard seeds survive. A team of biologists measures beak depth in a finch…
On a small island, a prolonged drought wipes out soft-seeded plants, and only those with large, hard seeds survive. A team of biologists measures beak depth in a finch population before the drought (mean of 8.8 mm) and one year later among the survivors (mean of 9.8 mm). A) Justify using the synthetic theory what has happened in the population at the phenotypic and genotypic levels. B) A student comments: "The finches had to strive to crack the hard seeds, which caused their beaks to grow 1 mm during their lifetime, and that is why their offspring were born with larger beaks." Refute their statement by pointing out why it falls into Lamarckism and how natural selection actually operates.
Genetic drift changes allele frequencies by chance, especially in small populations. Gene flow changes them when individuals that reproduce arrive or leave (migration). Selection, in contrast, does it through differences in reproductive success.
Try drawing a few chips several times: the proportions move away from 50 % just by chance. With many chips they get closer to 50 %.
Let's practice! 3 medium exercises in this section