In 2019, while a new airport was being built in the State of Mexico, workers uncovered hundreds of mammoth bones. Today no mammoths live on the planet, but elephants do. Have species always been the way we see them now? How do they change, and why do some disappear? Evolutionary theories try to answer exactly that.
What Is Evolution? From Fixism to the First Ideas
For a long time, people thought each species had always been the same: a dog was the same as its ancestors and corn was the same as it was thousands of years ago. This idea is called fixism. But fossils brought a surprise: there were remains of living things that no longer exist, such as mammoths or trilobites.
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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.
In the early 1800s, the French naturalist Jean-Baptiste Lamarck was one of the first to propose that species change. His explanation was based on two ideas:
According to Lamarck, giraffes had short-necked ancestors. By stretching to eat leaves from tall trees, their necks grew a little longer, and their young inherited that longer neck. Repeated over generations, the neck ended up long.
Today we know that the second idea is false. Your body has two types of cells: somatic cells (muscle, skin, bone…) and germ cells (eggs and sperm). Only germ cells pass information on to children. If you work out and your muscles grow, somatic cells change, not the DNA in your gametes.
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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.
Charles Darwin traveled around the world and observed many species. Alfred Russel Wallace, another naturalist, independently arrived at the same explanation. In 1858 they both presented their ideas, and in 1859 Darwin published On the Origin of Species. It is one of the best-supported explanations for the origin of biodiversity, and science has continued to refine it.
The key ideas:
For Darwin, giraffes were already born with necks of different lengths. When low leaves became scarce, the longer-necked ones ate more, survived, and left more offspring with long necks. The variation existed before any effort.
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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.
How do we know that species descend from one another? There are several lines of evidence that agree.
The same bone pattern with different functions is a sign of a common ancestor. If two organs do the same job but have a different origin (a bird's wing and a butterfly's wing), they are analogous and do not prove kinship.
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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 where variation came from or how it was inherited. In the 20th century, genetics explained it. A gene is a segment of DNA with the information for a trait; an allele is one of its versions. The modern synthesis (neo-Darwinism) joins both ideas.
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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.
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.
Back to the beginning: mammoths disappeared because species are not fixed; they change through natural selection and, when the environment changes too much, some go extinct.
Summary
Let's practice! 3 medium exercises in this section