Charles Lyell wrote that present rock formations have developed through gradual changes over long periods of time. Darwin applied his ideas to present life forms.
Observations of animal breeding helped Darwin appreciate the importance of heritable variations.
Malthus’ work showed that populations produce more offspring than the environment can support.
Charles Babbage and John Herschel believed that natural laws governed the origin of species.
Alfred Russel Wallace formulated a theory very similar to Darwin’s. Although they collaborated on a joint paper, Darwin’s clear and forceful Origin of Species earned him greater credit.
The two general ideas of Darwin’s Theory are evolution and natural selection.
The concept of natural selection includes these observations and conclusions:
By chance, heritable variations exist within a species.
Species produce more offspring than can survive.
Offspring with favorable variations are more likely to survive to reproduce.
Gradually, individuals with favorable variations make up more of the population.
Variation among individuals within species ensures that some will survive environmental change.
Because some variations help survival in a specific habitat more than others, individuals having those variations are more likely to survive and reproduce.
This differential survival and reproduction results in a population which is adapted to its environment.
The result of natural selection is gradual change in species, and when enough changes have accumulated, new species form. This is “descent with modification.”
The idea that natural selection has led to the origin of all species, together with evidence from the fossil record, means that all existing species are related by “common ancestry.”
Evolution by natural selection explains the history of life as recorded in the fossil record.
Common ancestry explains the similarities, and natural selection in the face of environmental change explains the differences among present-day species.
Like Lyell’s Principles of Geology, Darwin’s Theory of Evolution supports the general principle that the present arises from the materials and forms of the past.
Review Questions
State 3 of the 5 ideas Darwin developed during the Voyage of the Beagle. For each idea, give and example of a specific observation he made which supports the idea.
Compare and contrast Darwin’s position as a “gentleman scientist” with today’s professional scientists.
What does the expression “standing on the shoulders of giants” say about Darwin and his Theory of Evolution? Support your interpretation with at least three specific examples.
Explain the importance of Lyell’s Principles of Geology to Darwin’s work.
Discuss the influence of animal breeding on Darwin’s thinking.
Clarify the relationship between Darwin and Alfred Russel Wallace.
Summarize in your own words the two basic ideas which make up Darwin’s Theory of Evolution.
Compare and contrast Lamarck’s and Darwin’s ideas using the evolution of the human brain as an example.
Why is it incorrect to say that evolution means organisms adapt to environmental change?
Why is it not correct to say that evolution means “we came from monkeys?”
Further Reading / Supplemental Links
http://www.aboutdarwin.com/voyage/voyage03.html
http://darwin-online.org.uk/
http://www.ucmp.berkeley.edu/history/evolution.html
http://www.pbs.org/wgbh/evolution/
http://www.literature.org/authors/darwin-charles/the-origin-of-species/
http://www.life.umd.edu/emeritus/reveal/pbio/darwin/darwindex.html
Vocabulary
adaptation
A characteristic which helps an organism survive in a specific habitat.
artificial selection
Animal or plant breeding; artificially choosing which individuals will reproduce according to desirable traits.
inheritance of acquired characteristics
The idea that organisms can increase the size or improve the function of a characteristic through use, and then pass the improved trait on to offspring.
law
A statement which reliably describes a certain set of observations in nature; usually testable.
natural selection
The process by which a certain trait becomes more common within a population, including heritable variation, overproduction of offspring, and differential survival and reproduction.
theory
An explanation which ties together or unifies a large group of observations.
Points to Consider
How might the Theory of Evolution help us to understand and fight disease?
What other aspects of medicine could benefit from an understanding of evolution?
How can evolution and natural selection improve conservation of species and their environments?
How would you put into words the ways in which evolution has changed the way we look at ourselves?
How do you think it has altered the way we relate to other species? To the Earth?
Consider the human brain. If Lamarck’s hypothesis about inheritance of acquired characteristics were true, how would your knowledge compare to your parents?
Lesson 12.2: Evidence for Evolution
Lesson Objectives
Clarify the significance of a scientific theory.
Recognize that Darwin supported his theory with a great deal of evidence, and that many kinds of evidence since his time have further strengthened the theory of evolution.
Describe how Darwin used the fossil record to support descent from common ancestors.
Compare and contrast homologous structures and analogous structures as evidence for evolution.
Give examples of evidence from embryology which supports common ancestry.
Explain how vestigial structures support evolution by natural selection.
Discuss the molecular similarities found in all species of organisms.
Describe how evolution explains the remarkable molecular similarities among diverse species.
Analyze the relationship between Darwin’s Theory of Evolution and more recent discoveries such as Mendel’s work in genetics and the molecular biology of DNA and protein.
Relate the distribution of plants and animals to changes in geography and climate.
Explain how biogeography supports the theory of evolution by natural selection.
Summarize the explanation given by both Darwin and Wallace for the distribution of few, closely related species across island chains.
Introduction
You are probably aware that the concept of evolution still generates controversy today, despite its wide acceptance. In The Origin of the Species, Darwin mentioned humans only once, predicting, "Light will be thrown on the origin of man and his history." Nevertheless, some people immediately distorted its far-reaching message about the unity of life into near-sighted shorthand: humans “came from” monkeys (Figure below).
Figure 12.13
In Darwins time and today, many people incorrectly believe that evolution means humans come from monkeys. This interpretation does not do justice to Darwins theory, which holds that all species share common ancestry.
In the last lesson, you learned that evolution relates all of life – not just humans and monkeys. In this lesson, you will learn that biological evolution, like all scientific theories, is much more than just an opinion or hypothesis, it is based on evidence.
In science, a theory is an explanation which ties together or unifies a large group of observations. Scientists accept theories if they have a great deal of supporting evidence. In The Origin of the Species, Darwin took the time to compile massive amounts of fossil and biological evidence to support his ideas of natural selection and descent from common ancestors. He clearly a
nd effectively compared animal breeding (artificial selection), which was familiar to most people, and natural selection. Because Darwin provided so much evidence and used careful logic, most scientists readily accepted natural selection as a mechanism for change in species. Since Darwin’s time, additional fossil and biological data and new fields of biology such as genetics, molecular biology, and biogeography have dramatically confirmed evolution as a unifying theory – so much so that eminent biologist Theodosius Dobzhansky wrote that “Nothing in biology makes sense except in the light of evolution.”
In this lesson, you can explore and evaluate for yourself the many kinds of evidence which support the theory of evolution by natural selection. You will also have the opportunity to appreciate the power of evolution to explain observations in every branch of biology.
The Fossil Record: Structural Changes Through Time
Few would argue that dinosaurs roamed Earth in the past, but no longer exist. The fossil record is a revealing window into species that lived long ago. Paleontologists have carefully analyzed the preserved remains and traces of animals, plants, and even microorganisms to reconstruct the history of life on Earth (see the History of Life chapter for more detail). Relative (rock layer position) and absolute (radioisotope) dating techniques allow geologists to sequence the fossils chronologically and provide a time scale. Geology also reveals the environmental conditions of past species.
For many reasons, the fossil record is not complete. Most organisms decomposed or were eaten by scavengers after death. Many species lacked hard parts, which are much more likely to fossilize. Some rocks and the fossils they contained have eroded and disappeared. Moreover, much of evolution happens in the small populations that survive changes in environmental conditions, so the chance that intermediates will fossilize is low. Nevertheless, the current record includes billions of fossils – over 300 million from Los Angeles’ LaBrea Tar Pits alone, and an estimated 800 billion in South Africa’s Beaufort Formation. Analysts have identified 250,000 species among these remains.
Although the fossil record is far more detailed today than in Darwin’s time, Darwin was able to use it as powerful evidence for natural selection and common descent. Throughout geological history, species that appear in an early rock layer disappear in a more recent layer. Darwin argued that a species’ appearance recorded its origin, and that its disappearance showed extinction. Moreover, he noted remarkable similarities among structures in differing species, supporting common ancestry. Finally, he could often correlate environmental conditions with structures, supporting his idea that natural selection led to adaptations which improved survival within certain habitats.
Figure 12.14
The fossil record for relatives of the modern horse is unusually complete, allowing us to select a few which show major change over time. These changes can be correlated with environmental changes, supporting the ideas of evolution and natural selection. However, the linear arrangement is misleading; addition of all known fossils would show a branching, bushy path of descent and common ancestry.
As an example, let’s analyze a relatively complete set of fossils which record the evolution of the modern horse. Figure above sequences five species which show major evolutionary changes. The oldest fossil shows a fox-sized animal with slender legs and nearly vertical digits: Hyracotherium bit and chewed soft leaves in wooded marshlands. Geology and paleontology suggest that the climate gradually dried, and grasslands slowly replaced the marshes. Mesohippus was taller, with fewer, stronger digits – better able to spot and run from predators, and thus more likely to survive and reproduce in the new grasslands. Merychippus was taller still, and kept only one, enlarged digit – a hoof to run fast on the hard ground. By Pliohippus time, molar teeth had widened and elongated to grind the tough grasses. These fossils show gradual structural changes which correspond to changes in the environment. They appear to show a smooth, linear path directed toward the “goal” of the modern horse, but this is deceiving. These five fossils are merely “snapshots” of a bushy family tree containing as many as 12 genera and several hundred species. Some transitions are smooth progressions; others are abrupt. Together, they support natural selection and descent with modification from common ancestors.
Comparative Anatomy and Embryology
The evidence Darwin presented in The Origin of Species included not only fossils but also detailed comparisons of living species at all life stages. Naturalists in Darwin’s time were experts in comparative anatomy – the study of the similarities and differences in organisms’ structures (body parts). At different times during his life, Darwin studied the comparative anatomy of closely related species of marine mammals, barnacles, orchids, insectivorous plants, and earthworms.
Species which share many similarities are closely related by a relatively recent common ancestor. For example, all orchids share parallel-veined leaves, two-sided flowers with a “lip,” and small seeds (Figures A and B below). Species which share fewer similarities, sharing only basic features, are related by relatively distant ancestor. The sundew, one of the insectivorous plants Darwin studied, shares leaves and petals with orchids, but the leaves are wide with branching veins and the flowers are radially symmetrical rather than two-sided (Figure C below). The many species of orchids, then, share a recent common ancestor, but they also share a more distant ancestor with the sundew.
Figure 12.15
Darwins Theory of Evolution explains both the similarities and the differences among living things. All flowering plants share leaves, petals, stamens, and pistil, but orchids have parallel-veined leaves and flowers with lips and fused stamens and pistil, while sundews have leaves with branching veins and flowers with equal petals and separate stamens and pistil. The two species of orchid (A and B) share a recent common ancestor, whereas all three species share a more distant common ancestor.
Homologous and Analogous Structures
Figure 12.16
are similarities throughout a group of closely related species. The similar bone patterns in bats wings, dolphins flippers, and horses legs support their descent from a common mammalian ancestor.
Similarities can show two different kinds of relationships, both of which support evolution and natural selection.
(1) Similarities shared by closely related species (species who share many characteristics) are homologous, because the species have descended from a common ancestor which had that trait. Homologous structures may or may not serve the same function. Figure above shows the forelimbs of mammals, considered homologous because all mammals show the same basic pattern: a single proximal bone joins a pair of more distal bones, which connect to bones of the wrist, “hand,” and digits. With this basic pattern, bats build wings for their lives in the air, whales form fins for their lives in the sea, and horses, as we have seen, construct long, hoofed legs for speed on land. Therefore, homologous structures support common ancestry.
Figure 12.17
The wings of pterosaurs, bats, and birds illustrate both homologous and analogous structures. Similarities in the patterns of bones are due to descent from a common vertebrate (reptilian) ancestor, so they are homologous. However, the wings of each evolved independently, in response to similar environments, so they are analogous, and provide evidence for natural selection.
(2) Similarities shared by distantly related species may have evolved separately because they live in similar habitats. These structures are analogous because they serve similar functions, but evolved independently. Figure above compares the wings of bats, bird, and pterosaurs. Bats evolved wings as mammals, pterosaurs as dinosaurs, and birds from a separate line of reptiles. Their wings are analogous structures, each of which evolved independently, but all of which suit a lifestyle in the air. Note that although the wings are analogous, their bones are homologous: all three share a common but more distant vertebrate ancestor, in which the basic forelimb pattern evolved. Because analogous structures are independent adaptations to a common environment, they
support natural selection.
Embryology
Embryology is a branch of comparative anatomy which studies the development of vertebrate animals before birth or hatching. Like adults, embryos show similarities which can support common ancestry. For example, all vertebrate embryos have gill slits and tails, shown in Figure below. The “gill slits” are not gills, however. They connect the throat to the outside early in development, but in many species, later close; only in fish and larval amphibians do they contribute to the development of gills. In mammals, the tissue between the first gill slits forms part of the lower jaw and the bones of the inner ear. The embryonic tail does not develop into a tail in all species; in humans, it is reduced during development to the coccyx, or tailbone. Similar structures during development support common ancestry.
Figure 12.18
reveals homologies which form during development but may later disappear. All vertebrate embryos develop tails, though adult humans retain only the coccyx. All vertebrate embryos show gill slits, though these develop into gill openings only in fish and larval amphibians. In humans, gills slits form the lower jaw and Eustachian tube. Many scientists consider developmental homologies evidence for ancestry, although some embryologists believe that these particular drawings exaggerate the similarities.
Vestigial Structures
Structures which are reduced and perhaps even nonfunctional, such as the human tail and the human appendix, are considered vestigial structures. The tail, of course, functions for balance in many mammals, and the human appendix may have served digestive functions in herbivorous ancestors. Whales, which evolved from land mammals, do not have legs or hair as adults; both begin to develop in embryos, but then recede. Vestigial leg bones remain, buried deep in their bodies, shown in Figure A below.
CK-12 Biology I - Honors Page 53
