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Descent with Modification
19 Descent with Modification
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Overview: Endless Forms Most Beautiful
Lepidopteran insects (moths and butterflies) have many features in common including a juvenile feeding stage called a caterpillar Lepidopteran species also have many features that are distinct from each other in both the caterpillar and adult forms Lepidopterans illustrate three key observations about life The fit between organisms and their environment The shared characteristics (unity) of life The diversity of life A new era of biology began in 1859 when Charles Darwin published The Origin of Species The Origin of Species focused biologists’ attention on the great diversity of organisms Darwin noted that current species are descendants of ancestral species Evolution can be defined by Darwin’s phrase descent with modification Evolution can be viewed as both a pattern and a process 2
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Concept 19.1: The Darwinian revolution challenged traditional views of a young Earth inhabited by unchanging species Darwin’s revolutionary ideas had deep historical roots Scala Naturae and Classification of Species The Greek philosopher Aristotle viewed species as fixed and arranged them on a scala naturae The Old Testament holds that species were individually designed by God and therefore perfect Carolus Linnaeus interpreted organismal adaptations as evidence that the Creator had designed each species for a particular purpose Linnaeus was the founder of taxonomy, the branch of biology concerned with classifying organisms He developed the binomial format for naming species (for example, Homo sapiens) 3
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Ideas About Change over Time
The study of fossils helped to lay the groundwork for Darwin’s ideas Fossils are remains or traces of organisms from the past, usually found in sedimentary rock, which appears in layers or strata Paleontology, the study of fossils, was largely developed by French scientist Georges Cuvier who speculated that each boundary between strata represents a catastrophe that destroyed many species Geologists James Hutton and Charles Lyell perceived that changes in Earth’s surface can result from slow, continuous actions still operating today. Lyell further proposed that the mechanisms of change are constant over time. This view strongly influenced Darwin’s thinking. Lamarck’s Hypothesis of Evolution Lamarck hypothesized that species evolve through use and disuse of body parts and the inheritance of acquired characteristics. The mechanisms he proposed are unsupported by evidence. Younger stratum with more recent fossils Older stratum with older fossils 4
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Concept 19.2: Descent with modification by natural selection explains the adaptations of organisms and the unity and diversity of life Some doubt about the permanence of species preceded Darwin’s ideas Darwin’s Research As a boy and into adulthood, Charles Darwin had a consuming interest in nature Darwin first studied medicine (unsuccessfully) and then theology at Cambridge University After graduating, he took an unpaid position as naturalist and companion to Captain Robert FitzRoy for a five-year around-the-world voyage on the Beagle The Voyage of the Beagle During his travels on the Beagle, Darwin collected specimens of South American plants and animals He observed that fossils resembled living species from the same region, and living species resembled other species from nearby regions He experienced an earthquake in Chile and observed the uplift of rocks Darwin was influenced by Lyell’s Principles of Geology and thought that Earth was more than 6,000 years old His interest in geographic distribution of species was kindled by a stop at the Galápagos Islands west of South America He hypothesized that species from South America had colonized the Galápagos and speciated on the islands 5
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Darwin in 1840, after his return HMS Beagle at sea from the voyage
Figure 19.5 Darwin in 1840, after his return from the voyage HMS Beagle at sea Great Britain EUROPE NORTH AMERICA ATLANTIC OCEAN The Galápagos Islands PACIFIC OCEAN AFRICA Pinta Genovesa Equator Marchena SOUTH AMERICA Malay Archipelago Equator PACIFIC OCEAN Santiago Daphne Islands Chile Brazil AUSTRALIA Fernandina Pinzón PACIFIC OCEAN Andes Mtns. Figure 19.5 The voyage of HMS Beagle Isabela Santa Cruz Santa Fe San Cristobal Argentina Cape of Good Hope Tasmania 20 40 Florenza Española Cape Horn New Zealand Kilometers 6
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The Galápagos Islands PACIFIC OCEAN Pinta Genovesa Marchena Equator
Figure 19.5c The Galápagos Islands PACIFIC OCEAN Pinta Genovesa Marchena Equator Santiago Daphne Islands Pinzón Fernandina Isabela Santa Cruz Santa Fe Figure 19.5c The voyage of HMS Beagle (part 3: Galápagos map) San Cristobal 20 40 Florenza Española Kilometers 7
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Darwin’s Focus on Adaptation
In reassessing his observations, Darwin perceived adaptation to the environment and the origin of new species as closely related processes From studies made years after Darwin’s voyage, biologists have concluded that this is what happened to the Galápagos finches In 1844, Darwin wrote an essay on natural selection as the mechanism of descent with modification but did not introduce his theory publicly Natural selection is a process in which individuals with favorable inherited traits are more likely to survive and reproduce In June 1858, Darwin received a manuscript from Alfred Russell Wallace (pictured) who had developed a theory of natural selection similar to Darwin’s Darwin quickly finished The Origin of Species and published it the next year 8
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(a) Cactus-eater (c) Insect-eater (b) Seed-eater Figure 19.6
Figure 19.6 Three examples of beak variation in Galápagos finches (b) Seed-eater 9
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Ideas from The Origin of Species
Darwin explained three broad observations about life The unity of life The diversity of life The match between organisms and their environment Descent with Modification Darwin never used the word evolution in the first edition of The Origin of Species The phrase descent with modification summarized Darwin’s perception of the unity of life The phrase refers to the view that all organisms are related through descent from an ancestor that lived in the remote past In the Darwinian view, the history of life is like a tree with branches representing life’s diversity Fossils of extinct species help to “fill in” the morphological gaps between present-day groups 10
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Hyracoidea (Hyraxes) Sirenia (Manatees and relatives) Moeritherium † †
Figure 19.9 Hyracoidea (Hyraxes) Sirenia (Manatees and relatives) Moeritherium † † Barytherium † Deinotherium † Mammut † Platybelodon † Stegodon Mammuthus † Figure 19.9 Descent with modification Elephas maximus (Asia) Loxodonta africana (Africa) Loxodonta cyclotis (Africa) 60 34 24 5.5 2 104 Millions of years ago Years ago 11
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Artificial Selection, Natural Selection, and Adaptation
Darwin noted that humans have modified other species by selecting and breeding individuals with desired traits, a process called artificial selection Darwin argued that a similar process occurs in nature Darwin drew two inferences from two observations Observation #1: Members of a population often vary in their inherited traits Observation #2: All species can produce more offspring than the environment can support, and many of these offspring fail to survive and reproduce Inference #1: Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals Inference #2: This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations Darwin was influenced by Thomas Malthus, who noted the potential for human population to increase faster than food supplies and other resources If some heritable traits are advantageous, these will accumulate in a population over time, and this will increase the frequency of individuals with these traits This process explains the match between organisms and their environment 12
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Cabbage Selection for apical (tip) bud Brussels sprouts Selection for
Figure 19.10 Cabbage Selection for apical (tip) bud Brussels sprouts Selection for axillary (side) buds Broccoli Selection for flowers and stems Figure Artificial selection Selection for stems Selection for leaves Kale Wild mustard Kohlrabi 13
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Natural Selection: A Summary
Individuals with certain heritable traits survive and reproduce at a higher rate than other individuals Over time, natural selection increases the match between organisms and their environment If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species Note that individuals do not evolve; populations evolve over time Natural selection can only increase or decrease heritable traits that vary in a population Adaptations vary with different environments A flower mantid in Malaysia A leaf mantid in Borneo 14
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Concept 19.3: Evolution is supported by an overwhelming amount of scientific evidence
New discoveries continue to fill the gaps identified by Darwin in The Origin of Species There are four types of data that document the pattern of evolution Direct observations Homology The fossil record Biogeography Direct Observations of Evolutionary Change Two examples provide evidence for natural selection: natural selection in response to introduced plant species and the evolution of drug-resistant bacteria 15
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Natural Selection in Response to Introduced Plant Species
Soapberry bugs use their “beak” to feed on seeds within fruits In southern Florida soapberry bugs feed on balloon vine with larger fruit; they have longer beaks In central Florida they feed on goldenrain tree with smaller fruit; they have shorter beaks Correlation between fruit size and beak size has also been observed in Louisiana, Oklahoma, and Australia In all cases, beak size has evolved in populations that feed on introduced plants with fruits that are smaller or larger than the native fruits These cases are examples of evolution by natural selection In Florida this evolution in beak size occurred in less than 35 years 16
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Museum-specimen average
Figure 19.14 Field Study Results 10 On native species, balloon vine (southern Florida) Beak 8 6 4 2 Number of individuals Museum-specimen average 10 On introduced species, goldenrain tree (central Florida) 8 Soapberry bug with beak inserted in balloon vine fruit Figure Inquiry: Can a change in a population’s food source result in evolution by natural selection? 6 4 2 6 7 8 9 10 11 Beak length (mm) 17
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The Evolution of Drug-Resistant Bacteria
The bacterium Staphylococcus aureus is commonly found on people’s skin or in their nasal passages Methicillin-resistant S. aureus (MRSA) strains are dangerous pathogens S. aureus became resistant to penicillin in 1945, two years after it was first widely used S. aureus became resistant to methicillin in 1961, two years after it was first widely used Methicillin works by inhibiting a protein used by bacteria in their cell walls MRSA bacteria use a different protein in their cell walls When exposed to methicillin, MRSA strains are more likely to survive and reproduce than nonresistant S. aureus strains MRSA strains are now resistant to many antibiotics Natural selection does not create new traits, but edits or selects for traits already present in the population The local environment determines which traits will be selected for or selected against in any specific population 18
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Annual hospital admissions
Figure 19.15 2,750,000 1 250,000 base pairs 400 2,500,000 350 Chromosome map of S. aureus clone USA300 500,000 300 Key to adaptations 250 2,250,000 Annual hospital admissions with MRSA (thousands) Methicillin resistance 200 Ability to colonize hosts 750,000 150 Increased disease severity 100 Increased gene exchange (within species) and toxin production 2,000,000 50 1,000,000 ’93 ’94 ’95 ’96 ’97 ’98 ’99 ’00 ’01 ’02 ’03 ’04 ’05 Figure The rise of methicillin-resistant Staphylococcus aureus (MRSA) Year 1,750,000 1,250,000 1,500,000 19
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Evolution is a process of descent with modification
Homology Evolution is a process of descent with modification Related species can have characteristics with underlying similarity that function differently Homology is similarity resulting from common ancestry Anatomical and Molecular Homologies Homologous structures are anatomical resemblances that represent variations on a structural theme present in a common ancestor Comparative embryology reveals anatomical homologies not visible in adult organisms Vestigial structures are remnants of features that served important functions in the organism’s ancestors Examples of homologies at the molecular level are genes shared among organisms inherited from a common ancestor Homologous genes can be found in organisms as dissimilar as humans and bacteria 20
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Pharyngeal arches Post-anal tail Chick embryo (LM) Human embryo
Figure 19.17 Pharyngeal arches Post-anal tail Figure Anatomical similarities in vertebrate embryos Chick embryo (LM) Human embryo 21
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A Different Cause of Resemblance: Convergent Evolution
Convergent evolution is the evolution of similar, or analogous, features in distantly related groups Analogous traits arise when groups independently adapt to similar environments in similar ways Convergent evolution does not provide information about ancestry The Fossil Record The fossil record provides evidence of The extinction of species The origin of new groups Changes within groups over time Fossils can document important transitions For example, the transition from land to sea in the ancestors of cetaceans Sugar glider Flying squirrel 22
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Cetaceans and even-toes ungulates
Figure 19.19 Most mammals Cetaceans and even-toes ungulates Figure Ankle bones: one piece of the puzzle (a) Canis (dog) (b) Pakicetus (c) Sus (pig) (d) Odocoileus (deer) 23
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Other even-toed ungulates Hippopotamuses Pakicetus † Rodhocetus †
Figure 19.20 Other even-toed ungulates Hippopotamuses Pakicetus † Rodhocetus † Common ancestor of cetaceans Dorudon † Figure The transition to life in the sea Living cetaceans 70 60 50 40 30 20 10 Key Pelvis Tibia Millions of years ago Femur Foot 24
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Pakicetus † Common ancestor of cetaceans Rodhocetus † Dorudon † Living
Figure 19.20a Pakicetus † Common ancestor of cetaceans Rodhocetus † Dorudon † Living cetaceans Figure 19.20a The transition to life in the sea (detail) 50 40 30 20 10 Key Pelvis Tibia Millions of years ago Femur Foot 25
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20 cm Diacodexis, an early even-toed ungulate Figure 19.UN01
Figure 19.UN01 In-text figure, Diacodexis, p. 377 Diacodexis, an early even-toed ungulate 26
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What Is Theoretical About Darwin’s View of Life?
Biogeography Biogeography, the geographic distribution of species, provides evidence of evolution Earth’s continents were formerly united in a single large continent called Pangaea but have since separated by continental drift An understanding of continent movement and modern distribution of species allows us to predict when and where different groups evolved Endemic species are species that are not found anywhere else in the world Islands have many endemic species that are often closely related to species on the nearest mainland or island Darwin explained that species on islands gave rise to new species as they adapted to new environments What Is Theoretical About Darwin’s View of Life? In science, a theory accounts for many observations and explains and integrates a great variety of phenomena The predictions of a scientific theory must stand up to continual testing by experimentation and observation Darwin’s theory of evolution by natural selection integrates diverse areas of biological study and stimulates many new research questions Ongoing research adds to our understanding of evolution 27
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