CK-12 Biology I - Honors
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Figure 12.19
Vestigial structures show evolutionary reduction or loss of unneeded structures which were useful to ancestors. A: Whales retain remnants of their mammalian ancestors leg bones (c). B: Cavefish lack the eyes and pigments important to their relatives who live in lighted habitats. C: True flies have reduced insects second pairs of wings to balancing knobs. D: We still show the reflex which raises hairs for insulation in cold air in our furry relatives, but all we have to show for our follicles efforts are goosebumps.
True flies have reduced the second pair of wings found in most insects to halteres for balance shown in Figure B above. Cavefish lose both eyes and pigment, because both would require energy to build and are useless in the lightless habitat they have adopted shown in Figure C above. You are probably very familiar with a fine example of a vestigial behavior: goosebumps raise the sparse hairs on your arms even though they are no longer sufficiently dense to insulate you from the cold by trapping warm air next to your skin; in most mammals, this reflex is still quite functional shown in Figure D above. Most vestigial structures are homologous to similar, functioning structures in closely related species, and as such, support both common ancestry and (incomplete!) natural selection.
Molecular Biology
Did you know that your genes may be 50% the same as those of a banana?
Unknown in Darwin’s time, the “comparative anatomy” of the molecules which make up life has added an even more convincing set of homologies to the evidence for evolution. All living organisms have genes made of DNA. The order of nucleotides – As, Ts, Cs, and Gs - in each gene codes for a protein, which does the work or builds the structures of life. Proteins govern the traits chosen (or not) in natural selection. For all organisms, a single Genetic Code translates the sequence of nucleotides in a gene into a corresponding chain of 20 amino acids. By itself, the universality of DNA genes and their code for proteins is strong evidence for common ancestry. Yet there is more.
If we compare the sequence of nucleotides in the DNA of one organism to the sequence in another, we see remarkable similarities. For example, human DNA sequences are 98-99% the same as those of chimpanzees, and 50% the same as a banana’s! These similarities reflect similar metabolism. All organisms have genes for DNA replication, protein synthesis, and processes such as cellular respiration. Although metabolic processes do not leave fossils, similar DNA sequences among existing organisms provide excellent evidence for common ancestry.
The differences in DNA sequences are even more intriguing. Many are single base substitutions resulting from mutations accumulated through time. Assuming mutations occur randomly, the number of differences in bases between any two species measures the time elapsed since two organisms shared a common ancestor. This type of "molecular clock" has confirmed traditional classification based on anatomy. Most scientists consider it sufficiently powerful to clarify or correct our understanding of evolutionary history. For example, human DNA differs 1.2% from chimpanzees, 1.6% from gorillas, and 6.6% from baboons; we can infer from this data that humans and chimpanzees share a relatively recent common ancestor, and that the common ancestor we share with gorillas lived much longer ago. Figure below shows a cladogram depicting hypothetical evolutionary relationships constructed with this data. Similarities and differences in the sequences of amino acids in proteins support common ancestry in the same way, because they are determined by DNA.
Figure 12.20
Cladograms use comparison data to construct diagrams showing evolutionary relationships. This cladogram uses comparisons of DNA nucleotide sequences to reveal patterns of descent from common ancestors. Molecular biology has supported and extended our understanding of evolutionary relationships based on traditional anatomy.
Heritability and variation in traits are essential parts of Darwin’s theory of evolution by natural selection. Since he published The Origin of the Species, rediscovery of Mendel’s identification of genes and how they are inherited has confirmed Darwin’s ideas. Molecular biology has clarified the nature of genes and the sources of variation. Comparative analysis of DNA and proteins continues to give us an exquisitely detailed view of patterns of variation, common ancestry, and how evolution works.
Biogeography
Australia, Africa, and South America occupy the same latitude, at least in part, and therefore have roughly the same climate. If plants and animals were distributed only according to their adaptations to habitat, we would expect the same species to occupy similar regions of these continents. However, the short-tailed monkeys, elephants, and lions in Africa differ significantly from the long-tailed monkeys, llamas, and jaguars of South America, and even more from the koalas, kangaroos, and Tasmanian devils of Australia. Biogeography studies the distribution of plants and animals and the processes that influence their distribution – including evolution and natural selection. Only geologic change and evolution can explain the distributions of many species, so biogeography is another kind of evidence for the theory of evolution.
Alfred Russel Wallace, who developed his own ideas of evolution and natural selection at the same time as Darwin, explained the distributions of many species in terms of changes in geography (such as formation of land bridges) and environment (for example, glaciations) and corresponding evolution of species. Figure below shows the six biogeographical regions he identified: Nearctic, Neotropical, Palaearctic, Ethiopian, Oriental, and Australian.
Figure 12.21
Alfred Russel Wallace identified six major biogeographic regions: Nearctic, Neotropical, Palaearctic, Ethiopian, Oriental, and Australian Regions. Wallace explained the distributions of many animals and plants as a result of changes in geography and evolution.
Let’s consider just the camel family as an example, shown in Figure below of how biogeography explains the distribution of species. Fossils suggest that camel ancestors originated in North America. Distant fossils show structural similarities which suggest that their descendants migrated across the Bering land bridge to Asia and across the Isthmus of Panama into South America. These two isolated populations evolved in different directions due to differences in chance variations and habitat. Today’s descendants are llamas and guanacos in South America, and camels in Asia. Asian camels continued to migrate west into Africa, giving rise to two species – the dromedary in Africa, and the Bactrian in eastern Asia.
Figure 12.22
Biogeography explains the distribution of camel-like animals as a result of geographical changes and independent evolution. Today, the descendants of early camel ancestors are the dromedary in Africa, the Bactrian camel in Asia (center), and the guanaco (right) and llamas of South America.
The distribution of some older fossils shows an opposite pattern; for example, fossils of a single species of fern, Glossopteris, have been found in South America, Africa, India, Antarctica, and Australia (Figure below). Putting together many such distributions and a great deal of geologic data, Alfred Wegener showed that the continents were long ago united as Gondwanaland, and have since drifted apart. His theory of continental drift and its modern form, plate tectonics, help to further explain patterns of evolutionary descent in space and time.
Figure 12.23
The locations of fossils such as Glossopteris on widely separated continents form contiguous patterns if the continents are joined. These patterns led to the theory of plate tectonics. Gondwanaland, a supercontinent of long ago, played an important part in evolution, natural selection and the history of life.
Island Biogeography
Island biogeography studies archipelagos (oceanic island chains) as isolated sites for evolution. Both Darwin and Wallace used examples from isolated oceanic islands, such as the Galapagos and Hawaii, in their arguments for evolution and natural selection. Until humans arrived, terrestrial mammals and amphibians were completely absent on these islands. Darwin and Wallace showed that the animals and plants which were present had blown or drifted from one of the continents, or had descended – with mo
difications which suited the new habitats – from one of the original colonists. Terrestrial mammals and amphibians, having no powers of dispersal across oceans (until humans came along), were understandably absent.
Darwin's Finches
Only long after returning from his voyage did Darwin, with help from ornithologist John Gould, realize that the Galapagos birds he had collected but dismissed as uninteresting blackbirds, grosbeaks, finches, and a wren, were actually all closely related descendants of a single ancestral finch which had relatives on the South American mainland. Careful analysis showed that each of the 12 new species was confined and adapted to a specific habitat on a specific island. The finches, now known as “Darwin’s finches” (Figure A below), clearly support both descent with modification and natural selection. Hawaiian honeycreepers (Figure B below) are a more colorful but also more endangered example of the same evolutionary process of adaptive radiation. Bills ranging from thick and heavy (finch-like) for seed-eaters to long and curved for probing flowers illustrate the variations by which descendants of a single, original finch-like colonizer adapted to multiple ecological niches on the islands. Unfortunately, human destruction of habitat and introductions of rodents, the mongoose, and the mosquito which carries avian malaria have caused the extinction of 15 honeycreeper species, and still threaten the species which remain.
Figure 12.24
Darwins finches (above) on the Galapagos and honeycreepers (right) on Hawaii show the adaptive radiation of single finch ancestors which first colonized the islands. Each species show descent with modification, and the variety of bill shapes show adaptation to a specific niche. Many similar examples from island biogeography support evolution and natural selection. Honeycreepers are the finch-like palila (top right), the flower-probing Iiwi (center), and another nectar feeder, the amakihi (bottom).
Scientific Evidence
Altogether, the fossil record, homologies, analogies, vestigial structures, molecular uniformity and diversity, and biogeography provide powerful scientific evidence for the descent of today’s species from common ancestors. Some details of natural selection have been and are still being modified. However, the remarkable biological discoveries of the 150 years since Darwin published The Origin of the Species have dramatically strengthened support for his theory. Moreover, Darwin’s theory continues to enlighten new discoveries. Perhaps we could paraphrase Dobzhansky: Everything in biology makes sense in the light of evolution. The only piece still missing from the evidence puzzle is direct observation of the process itself. Darwin thought that humans could never witness evolution in action because of the vast time periods required. For once, however, he was mistaken; evolution in action is the subject of the next lesson.
Lesson Summary
Evolution is not “just a theory” as a scientific theory, it explains and unifies the entire field of biology and has a great deal of evidence supporting it.
The evidence includes the comparisons and observations Darwin included in his Origin, and new knowledge from genetics and molecular biology, added since the Origin was published.
Darwin used the fossils known in his time as evidence for his ideas, and today’s record is even more convincing.
Often, fossil species first appear in older rocks, and disappear in younger rocks, providing evidence that species change.
Changes in climate indicated by geology correlate with changes in fossil species and their adaptations, supporting the idea of natural selection.
The fossil record for horses shows gradual changes which correspond to changes in the environment.
Many basic similarities in comparative anatomy support recent common ancestry.
Similarities in structure for closely related species are homologous.
Similarities in structure among distantly related species are analogous if they evolved independently in similar environments. They provide good evidence for natural selection.
Examples of evidence from embryology which supports common ancestry include the tail and gill slits present in all early vertebrate embryos.
Vestigial structures are reduced and perhaps even nonfunctional, but homologous to fully developed and functional similar structures are in a closely related species; these support the idea of natural selection.
Cavefish without sight or pigment and humans with goose bumps illustrate the concept of vestigiality.
The universality of DNA for genes, amino acids to build protein enzymes and the Genetic Code is strong evidence for common ancestry.
Similarities in metabolic pathways such as DNA replication and transcription and cellular respiration are further evidence for common ancestry.
Within these similarities are differences in the sequence of As, Ts, Cs, and Gs due to the accumulation of mutations.
Comparison of DNA sequences supports descent with modification and can be used to clarify evolutionary relationships.
A Cladogram is a tree-like diagram showing evolutionary relationships which can be construction from one or a number of kinds of comparison data; DNA sequence comparisons are often used.
Darwin’s Theory of Evolution is strongly supported and also helps to explain many more recent discoveries, such Mendel’s work in genetics and the molecular biology of DNA and protein.
Changes in geographic features such as land bridges explain puzzling fossil species distributions.
Older fossil distributions suggest that the continents have joined and separated during Earth’s history.
Plate tectonics explain the distant locations of closely related species as the result of continental drifting.
Both Darwin and Wallace proposed that oceanic island chain species often descended from a single colonizing mainland species and adapted to open niches through natural selection.
Galapagos finches (Darwin’s finches) and Hawaiian honeycreepers each fill many different ecological niches as the result of adaptive radiation from a single colonizing finch-like ancestor.
Review Questions
Why is it wrong to say that the Theory of Evolution is “just a theory”?
How did Darwin use the fossil record to support descent from common ancestors and natural selection?
Summarize how the fossil record for ancestors and relatives of the horse supports the relationship between evolution and changing environments.
Compare and contrast homologous and analogous structures as evidence for evolution.
Give two examples of evidence from embryology which support common ancestry.
Use an example to show how vestigial structures support evolution by natural selection.
List the molecular similarities found in all species of organisms, which support common ancestry.
Interpret the following cladogram in terms of evolutionary relationships and the DNA data which could have been used to construct it.
Relate the distribution of plants and animals to changes in geography and climate, using at least one specific example.
Use a specific example to illustrate the explanation given by both Darwin and Wallace for the distribution of few, closely related species across island chains.
Further Reading / Supplemental Links
David Quammen. 1997. The Song of the Dodo: Island Biogeography in an Age of Extinctions. Scribner.
Jonathan Weiner, The Beak of the Finch: A Story of Evolution in Our Time (Alfred A. Knopf, 1994).
http://darwin-online.org.uk/
http://www.ucmp.berkeley.edu/history/evolution.html
http://www.pbs.org/wgbh/evolution/
http://people.delphiforums.com/lordorman/light.htm
http://ibc.hbw.com/ibc/phtml/familia.phtml?idFamilia=196
http://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html
Vocabulary
absolute (radioisotope) dating
A technique for dating fossils based on exponential decay of a radioactive isotope incorporated into the rock at the time of its formation or the fossil at the time of the organism’s death.
adaptive radiation
A pattern of speciation which involves the relatively rapid evolution from a single species to several species to fill a diversity of available ecological niches.
analogous traits
Similar structures with identical functions shared by distantly related species; analogous traits result from natural selection in similar environments, but they evolve independently.
biogeography
The study of patterns of distribution of species on continents and islands.
cladogram
A tree-like diagram showing evolutionary relationships according to a given set of data, such as molecular data.
comparative anatomy
The study of the similarities and differences in organisms’ structures.
comparative embryology
The study of the similarities during the embryological development of vertebrate animals; reveals homologies which form during development but may later disappear.
embryology