Geographic Barriers
Geographic Barriers
The most commonly studied type of isolation is geographic isolation. Geographic isolation involves physical barriers that divide a population into two or more groups. These barriers can include rivers, mountains, and dried lakebeds.
As shown in FIGURE 5.2, the formation of the Isthmus of Panama created a barrier for many marine species. Marine organisms could no longer easily cross between the Atlantic and Pacific Oceans. Over time, the isolated populations became genetically different. Several species of snapping shrimp have evolved through geographic isolation. These species appear almost identical to one another. However, when males and females from opposite sides of the isthmus are placed together, they snap at each other instead of courting. Because they will no longer mate, these shrimp are classified as different species.
Temporal Barriers
Barriers can also involve timing. Temporal isolation exists when timing prevents reproduction between populations. Some members of a population may show signs of courtship at different times if there is a lot of competition for mates. Reproductive periods may change to a different time of the yearora different part of the day. These differences in timing can lead to speciation.
For example, two tree species that grow on the Monterey Peninsula in California are very closely related. However, they have different pollination periods. The Monterey pine sheds its pollen in February, while the Bishop pine sheds is pollen in April. These pine species have likely evolved thousa temporal isolation.
Compare and Contrast What are the differences and similarities between behavioral isolation and temporal isolation?
CRITICAL THINKING
1. Apply Why are the flash patterns of fireflies considered to be behavioral isolation?
2. Analyze How did geographic isolation affect the diversity Darwin observed in Galápagos finches? KEY CONCEPT Evolution occurs in patterns.
MAIN IDEAS
* Evolution through natural selection is not random.
* Species can become extinct. Species can shape each other over time. Speciation often occurs in patterns.
1. ~ Connect to Your World
2. People adapt their behavior to their situation. As you go through school, you are likely to change how you dress, talk, and study. When you learn something that makes your life better, you hold on to that new skill. On a genetic level and over multiple generations, species hold onto traits that benefit them in their environment.
3. Natural selection is the process that preserves these adaptive traits in a population.
4. However, sudden changes in an environment can wipe out a species quickly. The rise and fall of species over time reveal clear evolutionary patterns.
5. • MAIN IDEA
6. Evolution through natural selection is not random.
7. FIGURE 6.1 PATTERNS IN NATURAL SELECTION
8. In this hypothetical population, green body color is favored by natural selection. With each generation, alleles associated with green body color increase in frequency. Over time, more and more individuals in the population will have the advantageous phenotype.
9. In science, the terms chance and random relate to how easily an outcome can be predicted. Because mutations and genetic drift cannot be predicted, they are called random events. These random events are sources of genetic diver-sity. However, natural selection, which acts on this diversity, is not random.
10. Individuals with traits that are better adapted for their environment have a better chance of surviving and reproducing than do individuals without these traits.
11. You have learned about directional, stabilizing, and disruptive selection. In each of these modes of selection, the effects of natural selection add up over many generations. In other words, natural selection pushes a population's traits in an advantageous direction. As you can see in FIGURE 6,1, alleles associ-
12. ated with these traits add up in the population's gene pool.
13. Remember, however, that having
14. direction is not the same as having purpose or intent. The environment controls the direction of natural selection. When the environment changes, different traits may become advantageous. The response of species to environmental challenges and opportunities is not random.
15.
16. Convergent Evolution
17. Different species often must adapt to similar environments. Evolution toward similar characteristics in unrelated species is called convergent evolution.
18. Analogous structures, such as wings on birds and insects, are common examples of convergent evolution. Another example is the tail fin of fish and marine mammals, as shown in FIGURE 6.2. Sharks, which are fish, and dolphins, which are mammals, are separated by about 300 million years of evolution.
19. Separately, they have both evolved similar tail fins to propel themselves through the water. However, the tail fins of sharks and other fish are vertical, while those of dolphins are horizontal.
20. Divergent Evolution
21. When closely related species evolve in different directions, they become increasingly different through divergent evolution. The evolution of the red fox and the kit fox is an example of this trend. Though closely related, the two species have different appearances that are the result of adapting to different environments. The red fox lives in temperate regions, usually in forests. Its dark reddish coat helps it to hide from predators. The sandy-colored coat of the kit fox allows it to blend in with its desert surroundings.
22. Kit foxes also have large ears relative to their body size. This adaptation helps them to keep cool in the desert heat.
23. Infer Are the shells of turtles and snails examples of convergent or divergent evolution? Explain.
24. FIGURE 6.2 Convergent and Divergent Evolution
25. Natural selection is not random. It can have direction, and its effects are cumulative through generations.
26. CONVERGENT EVOLUTION
27. Dolphins, which are mammals, and sharks, which are fish, have evolved similar tail fins, as each has adapted to similar environmental conditions.
28. DIVERGENT EVOLUTION
29. The kit fox and the red fox evolved from a common ancestor while adapting to different environments.
30. ne 27 5
31. Red fox
32. Kit fox
33. mid elationsh
34. Hom aacia is 2
35. mil herbivores su
36. さささい。
37. these ants live ins cheat the plant by st
38. exacin and the ants s mhip is much mo
39. .are not invelved in
40. se 1o he relationshit
41. recar producing leave
42. Ams Races
43. amary arms races?
44. occur
45. aare species evolve in Tr a readianchips form I
46. in competitio
47. many generations
48. tarots competing, pla tom growing nearby pants produce dete
49. Through better ad
50. Dolphin
51. Shark
52. Analyze How do convergent and divergent evolution illustrate the directional nature of natural selection?
53. 336
54. Unit 4: Evolution
55. Ancestor
56.
57. O MAIN IDEA
58. Species can shape each other over time.
59. Species interact with each sherin man liferent ways. For example, they may compete for the same foad ouse orbe involved in a predator- prey theyon chip. Most of these interactioa do not involve evolutionary changes thieve . sometimes the evolutionary paths of two species become connanges.
60. Beneficial Relationships Through Coevolution
61. The bull-thorn acacia is a plant species with branches covered in hollow thorns. Although the thorns rots a the plant from being eaten by large animals, smal herbivores such as caterpillars can fit between them. To the rescue comes Pseudomyrmex ferrugineus, a species of stinging ants As shown in FIGURE 6.3, these ants live inside the thorns and feed on the plants nectar.
62. The ants protect the plant by stinging animals that try to eat the leaves.
63. This relationship is much more than a simple cooperation between two species. The acacia and the ants share an evolutionary history. The holiv thorns and nectar-producing leaves of the acacia and the stinging of the ants have evolved due to the relationship between the two species. Relatives of these species that are not involved in this type of relationship do not have these traits. Such relationships form through coevolution, the process in which two or more species evolve in response to changes in each other.
64. Evolutionary Arms Races
65. Coevolution can also occur in competitive relationships. These interactions can lead to "evolutionary arms races," in which each species responds to pressure from the other through better adaptations over many generations.
66. For example, many plants produce defense chemicals in the soil to discourage other plants from growing nearby and competing for resources.
67. Natural selection then favors competing plants that can overcome the effects d the chemicals. After many generations, most competitors have some level of resistance and are again able to grow near the defensive plant. Natural reection then favors plants that have evolved even more potent chemicals. In anther case, the thick shells and spines of murex snails are an adaptive itsponse to predation by crabs. In turn, crabs have evolved powerful claws at are strong enough to crack the snails' shells.
68. * What do you think will happen in future generations of crabs and snais?
69. FIGURE 6.3 The relationship between this ant and the acacia plant has developed through coevolution. The ant lives Inside the hollow thorn and protects the acacla by stinging any potential predators.
70. Red tor
71. ni predator
72. Natural selection favors snails with thicker shells
73. and spines.
74. Through natural selection. crabs evolve more powerful claws that can pierce the snails' thick, spiny shells.
75. In response, natural selection favors snails with even thicker shells and spines.
76. Chapter It: The
77.
78. Species can become extinct.
79. Just as birth and death are natura even evolution- an individual, the rise and fall of species are natural processes of evolution. The elimination of a species from Barth is called extinction. Exittion rten eccurs when a species as a whole is unable to adapt to a change in its environment. Biologists divide extinction events into two categories - background extinctions and mass extinctions. Although they differ in degree, the effect of both is the same: the permanent loss of species from Earth.
80. Background Extinctions
81. Extinctions that occur continuously but at a very low rate are called background extinctions. They are part of the cycle of life on Barth. Background extinctions occur at roughly the same rate as speciation. Unlike catastrophic mass extinctions, background extinction events usually affect only one or a fe species in a relatively small area, such as a rain forest or a mountain range. They can be caused by local changes in the environment, such as the introductionof a new predator species or a decrease in food supply. From a human perspective, such extinctions seem to occur randomly but at a fairly constant rate.
82. Mass Extinctions
83. Mass extinctions are much more rare than background extinctions. However as illustrated in FIGURE 65, they are much more intense. These events often occural the global level. Therefore, they destroy many species— even entire orders or families. Mass extinctions are thought to occur suddenly in geologic time, usually because of a catastrophic event such as an ice age or asteroid impact.
84. The fossil record confirms that there have been at least five mass extinctions in the past 600 million years. Some scientists also think that we are in the midst of a sixth mass extinction that has been caused by human impact on the biosphere.
85. Compare and Contrast What are the differences and similarities between background extinctions and mass extinctions?
86. FIGURE 6.5 EXTINCTION RATES THROUGH TIME
87. 20
88. Approximate background extinctions
89. Extinction rate
90. N
91. SP pa ref
92. rest
93. cipl peri
94. follo punc sudde ary ch
95. Museu
96. Jay Go
97. Both m four clo sudden linked to defense :
98. The th idea that i species. It Species, D: modificati comparisol
99. FIGURE 6.6
100. The concept ti periods of tim
101. alism.
102. Extinction rate
103. (families per million years)
104. 0
105. When extinction rate is plotted against time, mass extinctions appear as periodic peaks rising above background extinction levels.
106. 5
107. 600
108. Source; University of California, Berkeley
109. 200
110. 400
111.
112. Speciation often occurs in patterns. pacontalogis have long no Am neg eating patterns in the history of life, releted in the fos radial am, is these patterns, two stand out from the rat. In eroutionary gradialsry discussed in Section 2 of the cha prem tine: dipt routin evolutionars, change are thought to occur over cong pinto con of te to parents cates couton aheate tin den.
113. In the second of the two patterns, bursts of evolutionary activity are calone by long periods orstab staye This pattern is described by the theory of punctuated equilibrium, which states that episodes of speciation hetr suddenly in geologic time and are followed by long periods of litte evolution: any change, or stasis, Paleontologist Niles Eldredge, a curator at The American Museum of Natural History in New York, and evolutionary biologist Stephen lay Gould originally proposed the theory of punctuated equilbrium in 1972.
114. Bach men were graduate students at Columbia University, studying fossils of four closely related species of trilobite. The fossils showed evidence of the sudden appearance of a new form of eye. This new trilobite eye appears to be inked to an increased ability to roll into a protective ball, allowing for better defense against predators.
115. The theory of punctuated equilibrium was written as a revision of Darwin's idea that new species arise through gradual transformations of ancestral species. It must be noted that in the sixth edition of his book On the Origin of Species, Darwin wrote that "the periods, during which species have undergone modification, though long as measured by years, have probably been short in comparison with the periods during which they retained the same form."
116. - CONNECT TO GEOLOGY
117. Refer back to Section 1 of the chapter Principles of Evolution to review James Hutton's theory that led to the concept of evolutionary gradualism.
118. HSURE 6.6 Evolutionary Gradualism and Punctuated Equilibrium
119. The concept that species evolve slowly, over long
120. Punctuated equilibrium proposes that species
121. parioas of time, is known as evolutionary gradu-
122. show little evolutionary change for millions of years, followed by periods of rapid speciation.
123.
124. Modern studies show us that in stable ecosystems, most species are well adapted and generally resistant to change, unless some outside force causes disruption. In the case of punctuated equilibrium, this is believed to occur because a portion of a population becomes isolated and undergoes a speciation event. This isolation may be due to some sort of catastrophe, after which those organisms able to evolve quickly are more likely to survive. Isolation may also occur as a result of long-term environmental changes, like the formation of mountains or deserts, or due to a mutation that gives the organism a significant survival advantage over competitors.
125. When an ecosystem is greatly damaged, such as after the 1980 volcanic eruption of Mt. Saint Helens, in Washington state, other organisms will rapidly move into the area to fill empty niches. Although rapid evolutionary bursts can be compared in some ways to what is seen when such modern ecosystems are seriously disturbed, there is a distinct difference between the two. Rather than existing species moving in to fill vacancies in a changed ecosystem, in punctuated equilibrium, new speciation occurs suddenly following a long
126. interval of stasis. Both evolutionary gradualism and punctuated equilibrium are viable scientific
127. VISUAL VOCAB
128. Adaptive radiation is the rapid evolution of many diverse species from ancestral species.
129. descendent species
130. theories, and both are supported by evidence found in the fossil record. Although scientists still debate which of these two ideas best accounts for observed patterns of evolutionary change, most accept that it is likely that evolution occurs by a combination of these two major theories.
131. time
132. The process involving the diversification of one ancestral species into many descendent species is referred to as adaptive radiation. These descendent
133. ancestral species
134. species are usually adapted to a wide range of environments. One example of adaptive radiation
135. is the variation found in Galápagos
136. FIGURE 6.7 LEPTICTIDIUM
137. finches, which were discussed in the chapter Principles of Evolution.
138. Another rather dramatic example is seen in the radiation of mammals following the mass extinction at the end of the Cretaceous period about 65 million years ago.
139. According to the fossils that have been found thus far, the earliest mammals were tiny, mostly noctur-nal, and probably insect eaters, such as the shrew-like Leptictidium, seen. in FIGURE 6.7, allowing them to coexist with the dinosaurs for about
140. This model of Leptictidium was made based on the bone structures found in fossil specimens.
141. 150 million years.
142.
143. FIGURE 6.8 The K-T Boundary
144. The K-T boundary layer is marked clearly in rock layers in many places around the world.
145. It is inked to the collision of an asteroid with Earth around 65 milion years ago. The layer contains high concentrations of the element ridium. Iridium is very rare on Earth, but is found in much greater abundance in objects from space.
146. K-T BOUNDARY
147. Iridium-tich layer
148. The extinction of the dinosaurs about 65 million years ago left environments fill of open niches for other types of animals. In the first 10 million ras of the Tertiary period following the mass extinction event, more than 400 mammal species had evolved, including the ancestors of modern whales, bai, rodents, and primates. Evidence of this mass extinction, known as the Cretaceous-Tertiary (K-T) boundary, can be seen in FIGURE 6.8.
149. The fossil record indicates that there have been at least five mass extinctions in the past 600 million years, where large percentages of global populations were decimated. Studying these extinctions reveals that following each was a period of rapid evolutionary changes and the appearance of new species. hesize The adaptive radiation of mammals followed the extinction of the finosaurs. How do these events support the theory of punctuated equilibrium?
150. Formative Assessment
151. KEVEWING O MAIN IDEAS
152. 1. Explain what it means to say that natural selection is not random.
153. 1. How does coevolution shape two species over time?
154. . ho can mass extinctions lead to the sudden appearance of new
155. Pecies?
156. 'What atten is described by the theory of punctuated equilibrium?
157. CRITICAL THINKING
158. Synthesize Defensive chemicals are usually found in unripe fruit, but not in ripe fruit. In terms of coevolution, why might this be?
159. Infer Analogous structures are often examples of convergent evolution. What types of structures would likely be examples
160.
161. The K-T boundary layer is marked clearly in rock layers in many places around the world.
162. It is inked to the collision of an asteroid with Earth around 65 milion years ago. The layer contains high concentrations of the element ridium. Iridium is very rare on Earth, but is found in much greater abundance in objects from space.
163. K-T BOUNDARY
164. Iridium-tich layer
165. The extinction of the dinosaurs about 65 million years ago left environments fill of open niches for other types of animals. In the first 10 million ras of the Tertiary period following the mass extinction event, more than 400 mammal species had evolved, including the ancestors of modern whales, bai, rodents, and primates. Evidence of this mass extinction, known as the Cretaceous-Tertiary (K-T) boundary, can be seen in FIGURE 6.8.
166. The fossil record indicates that there have been at least five mass extinctions in the past 600 million years, where large percentages of global populations were decimated. Studying these extinctions reveals that following each was a period of rapid evolutionary changes and the appearance of new species. hesize The adaptive radiation of mammals followed the extinction of the finosaurs. How do these events support the theory of punctuated equilibrium?
167. Formative Assessment
168. KEVEWING O MAIN IDEAS
169. 1. Explain what it means to say that natural selection is not random.
170. 1. How does coevolution shape two species over time?
171. . ho can mass extinctions lead to the sudden appearance of new
172. Pecies?
173. 'What atten is described by the theory of punctuated equilibrium?
174. CRITICAL THINKING
175. Synthesize Defensive chemicals are usually found in unripe fruit, but not in ripe fruit. In terms of coevolution, why might this be?
176. Infer Analogous structures are often examples of convergent evolution. What types of structures would likely be examples of divergent evolution?
177. SELF-CHECK Online
178. HMHScience.com
179. GO ONLINE
180. * CONNECT TO HUMAN BIOLOGY
181. 7. Through mutation, HIV can accumulate resistance to drugs developed for treat-ment. Describe the relationship between HIV and the humans who develop these drugs in terms of an evolutionary arms race.
182. Chapter 11:
Do you ever consider how much the wield and is chibatants have change inte past 15, 50, or 100 years? Have you studied ancient vizations that existed tha sands of years ago? These time frames are singosis, on the ale read, the fossil record. Some of the world's oldest fossils, found at the Burges Shade so.
Canada, offer a glimpse of what life was like 500 million years before Tollund Near lived. These specimens are keys to understanding the history of life on Earth.
O MAIN IDEA
Fossils can form in several ways.
Fossils are far more diverse than the giant dinosaur skeletons we see in museums. The following processes are some of the ways fossils form.
FIGURE 11 shows examples of fossils produced in these different ways.
* Permineralization occurs when minerals carried by water are deposited around a hard structure. They may also replace the hard structure itsel
* Natural casts form when flowing water removes all of the original bone tissue, leaving just an impression in sediment. Minerals fill in the mo recreating the original shape of the organism.
* Trace fossils record the activity of an organism. They include nests, burrows, imprints of leaves, and footprints.
* Amber-preserved fossils are organisms that become trapped in tree res that hardens into amber after the tree gets buried underground.
* Preserved remains form when an entire organism becomes encased in material such as ice or volcanic ash or immersed in bogs.
*
* Most fossils form in sedimentary rock, which is made by many layers of sediment or small rock particles. The best environments for any type of foilization include wetlands, bogs, and areas where sediment is continuously deposited, such as river mouths, lakebeds, and floodplains.
* The most common fossils result from permineralization. Several circumstances are critical for this process, as shown in FIGURE 1.2. The organism must beburied or encased in some type of material-such as sand, sediment, mud, or tar— very soon after death, while the organism's features are still intact.
* After burial, groundwater trickles into tiny pores and spaces in plants, bones, and shells. During this process, the excess minerals in the water are deposited on the remaining cells and tissues. Many layers of mineral deposits are left behind, creating a fossilized record by replacing organic tissues with hard aunts. The resulting fossil has the same shape as the original structure and may contain some original tissue.
* With such specific conditions needed for fossilization, it is easy to see ay olya tiny percentage of living things that ever existed became fossils. host man decompose or are destroyed before they can be preserved. Even aces fositis no guarantee that an organism's remains will beaded o ta strod. aural events such as earthquakes and the recyling of
* complete fosis discovered
Radiometric dating provides a close estimate of a fossil's age.
Recall that geologists in the 1700s had realized that rock layers at the born of an undisturbed sequence of rocks were deposited before those at the tip and therefore are older. The same logic holds true for the fossils found in rost layers. Relative dating estimates the time during which an organism lived by comparing the placement of fossils of that organism with the placement of fossils in other layers of rock. Relative dating allows scientists to infer the order in which groups of species existed, although it does not provide the
actual ages of fossils.
To estimate a fossil's actual, or absolute, age, scientists use radiometric dating a technique that uses the natural decay rate of unstable isotopes found in materials in order to calculate the age of that material.
Isotopes are atoms of an element that have the same number of protons bet a different number of neutrons. Most elements have several isotopes. For example, the element carbon (C) has three naturally occurring isotopes. All carbon isotopes have six protons. Isotopes are named, however, by their number of protons plus their number of neutrons. Thus, carbon-12 (**C) has six neutrons, carbon-13 (C) has seven neutrons, and carbon-14 (*C) has eight neutrons. More than 98 percent of the carbon in a living organism is °C
Some isotopes have unstable nuclei. As a result, their nuclei undergo radioactive decay-they break down-over time. This releases radiation in the form of particles and energy. As an isotope decays, it can transform into a different element. The decay rate of many radioactive isotopes has been measured and is expressed as the isotopes half-life, as shown in FIGURE 13.
A half-life is the amount of time it takes for half of the isotope in a sampie.to decay into a different element, or its product isotope. An element's half life t not affected by environmental conditions such as temperature or pressure Both "C and "C are stable, but *C decays into nitrogen-14 (*N), with a half-life of roughly 5700 years,
Radiocarbon Dating
The isotope "C is used commonty for radiometric dating of recent renata. sue iss those of Tollund Man shown at the beginning of this chapter. Oes isms absorb carbon through eating and breathing, so "C is constany bets, resupplied. When an organism dies, its intake of carbon stops, but the des
of "C continues. The fossil's age can be estimated by comparing the ratio of a stable isotope, such as *C IC. The longer the organism has been dead, the kite the difference between the amounts of "Cand * there will be. The half-life of carbon-4 is roughy 7, years, which means that after 570 years at te in a fossil will have dea 1,40 ,
two half lives, 75 percent of the
One-quarter of the original C remains. Radioactive decay of "C is shown in FIGURE 14. Carbon-14 dating can be used to date objects only up to about 45,000 years old. If the objects are older than that, the fraction of "C will be too small to measure accurately. Older objects can be dated by using an isotope that has a longer half-life, such as uranium.
Determining Earth's Age
Scientists have used radiometric dating to determine the age of Earth. Because Earth constantly undergoes erosion and rock recycling, rocks on Earth do not remain in their original state. Unlike Earth's rocks, meteorites-which are mostly pieces of rock and iron that have fallen to Earth's surface from space-do not get recycled or undergo erosion. Meteorites are thought to have formed at about the same time as Earth. Therefore, meteorites provide an unspoiled sample for radiometric dating. Uranium-to-lead isotope ratios in many meteorite samples consistently estimate Earth's age at about 4.5 billion years.
Summarize Why are meteorites helpful for determining the age of Earth?
Connect to Your World
Life is marked by increments of progress. From your first year of school through high school graduation, each new year is a step in your life development. Earth's life spans about 4.5 billion years. Scientists have divided the Earth's progress into manageable units based on the occurrence of major geologic changes.
* MAIN IDEA
Index fossils are another tool to determine the age of rock layers.
You have learned that both relative dating and radiometric dating can help scientists determine the age of rock layers. Scientists who are trying to determine the age of a rock layer almost always use two or more methods to confirm results. Index fossils provide an additional tool for determining the age of fossils or the strata in which they are found. Index fossils are fossils of organisms that existed only during specific spans of time over large geographic areas.
Using index fossils for age estimates of rock layers is not a new idea. In the late 1700s, English geologist William Smith discovered that certain rock layers contained fossils unlike those in other layers. Using these key fossils as markers, Smith could identify a particular layer of rock wherever it was exposed.
The shorter the life span of a species, the more precisely the different strata can be correlated. The best index fossils are common, easy to identify, found widely around the world, and existed only for a relatively brief time.
The extinct marine invertebrates known as ammonites, shown in FIGURE 2.1, are one example of an index fossil. They were at one time very common, but disappeared after a mass extinction event about 251 million years ago. The presence of ammonites indicates that a rock layer must be between 251 million and 359 million years old. Ammonite fossils are useful for dating fossils of other organisms in strata, because the presence of both organisms in one layer shows that they lived during the same time period.
Apply Could a rock layer with ammonite fossils be 100 million years old? Explain.
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Chapter 12: The History
mya-present This period continues today and includes all modern forms of life. TERTIARY PERIOD (PALEOGENE AND NEOGENE)
65-1.8 mya Mammals, flowering plants, grasslands, insects, fish, and birds diversified. Primates evolved.
MESOZOIC ERA
CRETACEOUS PERIOD
145-65 mya Dinosaur populations peaked and then went extinct.
Birds survived to radiate in the Tertiary period. Flowering plants arose.
JURASSIC PERIOD
200-145 mya Dinosaurs diversified, as did early trees that are common today. Oceans were full of fish and squid. First birds arose.
TRIASSIC PERIOD
251-200 mya Following the largest mass extinction to date, dinosaurs evolved, as did plants such as ferns and cycads. Mammals and flying reptiles (pterosaurs) arose.
PALEOZOIC ERA
PERMIAN PERIOD
299-251 mya Modern pine trees first appeared, and Pangaea supercontinent was formed as major landmasses joined together.
CARBONIFEROUS PERIOD
359-299 mya Coal-forming sediments were laid down in vast swamps. Fish continued to diversify. Life forms included amphib-ians, winged insects, early conifers, and small reptiles.
DEVONIAN PERIOD
416-359 mya Fish diversified. First sharks, amphibians, and insects appeared. First ferns, trees, and forests arose.
SILURIAN PERIOD
444-416 mya Earliest land plants arose. Melting of glaciers allowed seas to form. Jawless and freshwater fishes evolved.
ORDOVICIAN PERIOD
488-444 mya Diverse marine invertebrates evolved, as did the earliest vertebrates. Massive glaciers formed, causing sea levels to drop and a mass extinction of marine life to occur.
CAMBRIAN PERIOD
542-488 mya All existing animal phyla developed over a relatively short period of time known as the Cambrian Explosion.
The geologic time seale, shi de in FIGURE 2.2, Is a representation of the history of Barth. It organizes Bart's development by major changes or evens that have occurred, using evidence from the fossil and geologic records, Scientists worked out the entire geologic time scale during the 18005 and eaty 1900s, Although the scale is still being changed a little bit here and there, the main divisions of geologic time have stayed the same for over a hundred yeara The time scale is divided into a series of units based on the order in which
consists of three basic units of time.
diferent groups of rocks and fossils were formed. The geologic time scale
* Eras last tens to hundreds of millions of years and consist of two or more periods.
* Periods are the most commonly used units of time on the geologic time scale, lasting tens of millions of years. Each period is associated with a particular type of rock system.
* Epochs (EHP-uhks) are the smallest units of geologic time and last several million years. The names of the eras came from early ideas about life forms preserved as fossils. Paleozoic means "ancient life, Mesozoic means "middlelife" and Cenozoic means "recent life. Within the eras, the boundaries between many of the geologic periods are defined by mass extinction events. These events help to define when one period ends and another begins. The largest adaptive radiations tend to follow large mass extinctions. Recall that adaptive radiation happens when a group of organisms diversifies into several species. Those species adapt to different ecological niches because mass extinctions make many niches available. Over generations, the adaptive traits favored within these newly opened niches may become common for that population of organisms, and speciation may occur.
Summarize Why do adaptive radiations often occur after mass extinctions?
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- CONNECT TO
ADAPTIVE RADIATION
Recall from the chapter The Evolution of Populations that adaptive radiation refers to the change of a single species into several forms that are each adapted to a specific environmental niche.
12.2
Formative Assessment
REVIEWING O MAIN IDEAS
1. How are index fossils used to date rock layers?
2. What is the usefulness of categorizing Earth's history into the geologic time scale?
CRITICAL THINKING
1. Infer The most common index fossils are shells of invertebrates. Give two reasons why this is so.
2. Analyze Scientists have inferred that there have been at least five mass extinctions in Earth's history.
How would fossil evidence support