EVOLUTION MAKING SENSE OF LIFE PDF
PDF | A comprehensive, engaging textbook about evolution for biology majors now in its second edition. PDF | On Jan 11, , Carl Zimmer and others published Ebook Evolution: Making Sense of Life By. These separate lines of evidence all support the same scenario for the evolution of marsupials (Springer et al. ). Marsupial-like mammals were living in.
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by カール・ジンマー. Carl Zimmer; Douglas J Emlen; Isao Sarashina; Makiko Ishikawa; Yoshiki Kunitomo. Print book. Japanese. 講談社, Tōkyō: Kōdansha. (Download) Evolution: Making Sense of Life By Carl Zimmer PDF #Audiobook ppti.info?book= #readOnline. Evolution: Making Sense of Life Amazon Barnes & Noble IndieBound Co-author Doug Emlen is interviewed by Nature about creating an ebook version.
But by using powerful examples, beautiful images, and finely wrought prose, Zimmer and Emlen have produced a book that not only conveys the explanatory power of evolution, but is also permeated with the joy of doing science. Their text can only be described as an exciting moment for our field: Sean B.
Carroll, University of Wisconsin, Madison A richly illustrated and very clearly written text, Evolution: Making Sense of Life brings forth the excitement, power, and importance of modern evolutionary biology in an accessible, yet sophisticated overview of the field. Peter R. Grant, Princeton University Two master craftsmen in the art of scientific communication have combined to produce an excellent basic text on Evolution: The illustrations are outstanding.
Thompson, University of California, Santa Cruz Carl Zimmer and Douglas Emlen have captured in this stunning new book the excitement and richness of twenty-first-century evolutionary biology. They describe clearly and elegantly not only what, but also how, we are learning about evolutionary processes and the patterns they produce.
The writing is compelling, the illustrations beautiful and truly informative, and the balance between breadth and depth of discussion on each topic just right. This is a book that would make anyone think about becoming an evolutionary biologist, today.
Breathtakingly illustrated, this book covers not only the usual topics in evolution—adaptation, drift, phylogenetic analysis—but also a host of new and exciting areas where groundbreaking research is occurring.
It also shows how evolutionary biology is done, with glimpses of the real people behind the discoveries.
Richard E. This book is bound to attract many more students into the field of evolutionary biology. Bronwyn H. Bleakley, Stonehill College I think my students will be genuinely more at ease with their reading assignments and more able to assimilate and retain information from this text. We will begin with the geographic patterns of macroevolution—why certain clades are found only in some parts of the world, for example, or why some regions have more species than others. We will consider the fac- tors that drive some clades to diversify into many new species while other clades remain species-poor Figure These extinction pulses offer a sobering warning for our own future.
Mapping Macroevolution When we look at the diversity of life across the planet, certain patterns jump out.
Some regions of the world, such as the tropics, are more species- Figure Certain groups of species are common in some places , species of beetles.
Their and rare in others. Marsupial mammals, for example, are mostly found in Australia, high diversity is the result of an origi- whereas other regions of the world have much lower levels of marsupial diversity. Before the nineteenth extinction rate. Environmental conditions such as climate alone could not completely geography and time. Darwin also observed that environmental conditions often failed to explain the similarities among groups of species.
The rodents that live on South American moun- tains showed many signs of being closely related to the rodents that live on the South American plains—as opposed to the rodents of the environmentally similar moun- tains of North America. Darwin argued that an adaptive divergence had occurred Figure This map shows levels of verte- brate diversity.
Some scientists have proposed that the tropics are rich in species because of the high speciation rate there. Others have argued that extinction rates are low. Lowest Highest vertebrate vertebrate diversity diversity When Darwin traveled to islands, he could see that those ecosystems were domi- nated by species that were good dispersers.
They were home to many species of bats and birds, for example, but few amphibians or elephants. This distribution of island species made sense as the result of species colonizing the islands from the mainland and then adapting to those habitats. Alfred Russel Wallace, co-discoverer of evolution by natural selection, also made important observations about biogeography. He separated the world into six major biogeographical provinces, each one having its own distinctive balance of species Figure The boundary between two of these provinces—southeast Asia on one side and Australia and New Zealand on the other—is particularly striking.
It actually runs through the middle of the Indonesian Archipelago. Islands on both sides of the boundary have very similar environments but very different species assemblages.
The science of biogeography seeks to explain such intriguing patterns of species dis- tribution. Birds of populations from one geographic flew to islands; elephants walked over mountain ranges; seeds were carried by water region to another with very limited to distant shores.
But dispersal fell short in explaining some patterns of biogeogra- return exchange, or none at all. This process is known as vicariance.
Rising sea levels can turn pen- gene flow, resulting in the separa- insulas into islands. Mountain ranges can rise up, splitting a population. Rivers can tion of once continuously distributed suddenly change course. Continents can break up and drift apart. Figure A continent breaks in two, and those two land- masses split in turn. If a clade does not disperse during this breakup, its phylogeny should reflect the history of the landmass.
The most closely related clades should be found on the most recently separated landmasses.
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The history of marsupials is a spectacular example of how both vicariance and dispersal produce complex patterns of biodiversity.
Although most living species of marsupials are found today in Australia, the oldest marsupial fossils come from chapter fourteen macroevolution: Here, a single continent drifts apart into four fragments. The phylogeny of the clade reflects the geological history. China and North America. Yet there is only one species of marsupial in all of North America today the Virginia opossum , and none at all in China.
How did we get to this puzzling situation? We can understand this pattern by integrating several separate lines of evidence: In , for example, Maria Nilsson of the University of Munster and her colleagues published a detailed molecular phy- logeny of all the major marsupial groups in the world Nilsson et al.
After analyzing 53 retroposons, Nilsson and her colleagues concluded that all Australian marsupials form a clade, nested within a clade of South American species, that represents a single dispersal event from South America to Australia Figure A molecular phylogeny can show only the relationships of species from which scientists can obtain DNA.
The addition of fossil evidence complements the molecular evidence for the evolu- tion of marsupials. A phylogeny of the major marsupial lineages known from fos- sils, based on morphological characters Figure It also reveals that marsupials lived in Antarctica when it was warmer, and that these extinct Antarctic marsupials were more closely related to liv- ing Australian marsupials than to South American ones.
Finally, it shows that extinct North American marsupials belong to the deepest of the branches. This branching pattern parallels the order in which these continents separated from each other. North America split off first; next, South America separated from Australia and Antarctica; and lastly, Australia and Antarctica became separate landmasses.
These separate lines of evidence all support the same scenario for the evolution of marsupials Springer et al. Marsupial-like mammals were living in China by million years ago, the age of the oldest fossils yet found. By million years ago, they had dispersed into North America, which at the time was linked to Asia. Many new lineages of marsupials evolved in North America over the next 55 million years. Another group of North American marsupials dispersed to South America around 70 million years ago.
A cladogram showing the ranges of marsupial lineages dispersal. Molecular phylogeny shows that Australian marsupials known as an area cladogram shows that marsupial phylogeny tracks form a clade nested with South American marsupials.
The only living the major separations of continents top and tracks a phylogenetic North American marsupial, the Virginia opossum Didelphis , is also cladogram of known fossil lineages based on morphological char- nested within the South American clade.
Dots denote retroposons acters bottom. By combining this evidence, we can construct a uniquely shared by marsupial clades. Adapted from Nilsson et al. Marsupials arrived in Australia no later than 55 million years ago, the age of the oldest marsupial fossils found there. Later, South America, Antarctica, and Australia began to drift apart, each carrying with it a population of marsupials vicariance.
The fossil record shows that marsupials were still in Antarctica 40 million years ago. But as the conti- nent moved nearer to the South Pole and became cold, these animals became extinct. Meanwhile, marsupials in South America diversified into a wide range of species, including cat-like marsupial sabertooths. These large, carnivorous species became extinct, along with many other unique South American marsupials, when the con- tinent reconnected to North America a few million years ago.
Competition with pla- cental mammals dispersing from North America may have been an important factor driving this extinction. However, there are still many different species of small and medium-sized marsupials living in South America today. One South American mar- supial, the Virginia opossum, even expanded back into North America, where marsu- pials had disappeared millions of years before.
Australia, meanwhile, drifted in isolation for over 40 million years. The fossil record of Australia is currently too patchy for paleontologists to say whether there were any placental mammals in Australia during that time.
Abundant Australian fos- sils date back to about 25 million years ago, when all of the therian mammals in Australia were marsupials.
They evolved into a spectacular range of forms, includ- ing kangaroos and koalas.
Table of Contents for: Evolution : making sense of life
It was not until 15 million years ago that Australia moved close enough to Asia to allow placental mammals—rats and bats—to begin coloniz- ing the continent. Dispersal occurs when a taxon crosses a preexisting barrier, like an ocean. Vicariance occurs when a barrier interrupts the preex- isting range of the taxon, preventing gene flow between the now separated populations.
Speciation and Extinction To measure species diversity through time, scientists have adapted a method from population ecology. Population ecologists chart the growth of populations through time by a formula. In the formula shown here, N1 stands for the current size of a population, and N2 stands for the size of the population in the next time step.
N1 1 births 1 immigrations 2 deaths 2 emigrations 5 N2 When scientists study macroevolution, they can adapt the formula, making spe- cies or higher clades their units of analysis instead of individual organisms. Likewise, they can substitute the originations and extinctions of species for the birth and death of individuals.
The number of species in a region can also be altered through immi- gration and emigration. Here, D stands for the diversity—that is, the total number of species in a particular clade. D1 1 originations 1 immigrations 2 extinctions 2 emigrations 5 D2 In these cases, we can eliminate the immigration and emigra- tion terms from the equation: D1 diversity 1 originations 2 extinctions 5 D2 new diversity This simple equation brings into focus a fundamental idea about macroevolu- tion: We can look at these processes in entire faunas many different kinds of organisms in single ecosystems, regions, or at a worldwide scale.
We can also look at them within a single clade, tracking its fluctuations in diversity through time. We can illustrate this method with a hypothetical example, shown in Figure A vertical bar represents the temporal range of fossils belonging to each species. The fossils were deposited in four consecutive geological stages marked here as A, B, C, and D. If we add up the fossils present at any point during stage A, we find a total of 24 species. During stage B, 10 new ones evolved, bringing the total to But 6 species did not survive beyond B; the total in C is thus 36—only 2 species more than in B, despite the emergence of 8 new species.
During D, another combination of origina- tions and extinctions dropped the total to 30 species. In the example shown in Figure Turnover rates can tell us many impor- replacement by others origination tant things about extinct clades. Paleontologists have found that some clades have in studies of macroevolution. The especially high turnover rates, while others have low ones. Trilobites, for example, turnover rate is the number of spe- had very high turnover rates. Each trilobite species had only a brief duration in the cies eliminated and replaced per unit fossil record, but the trilobite clade persisted because new species continued to arise of time.
Other groups, such as clams and snails, appear to have had relatively low turnover rates. Their species last much longer, and new species appear less frequently. Paleontologists can calculate rates of processes such as origination and Older Younger extinction from these kinds of data. Time chapter fourteen macroevolution: We can represent the rate at which new species evolve, or originate, in a clade by the Greek letter a alpha , and the rate at which species go extinct by V omega.
In Figure Although our sample is not very large, we can see that 12 extinctions in stage C is more than usual, and that on average the extinc- tion rate is slightly greater than the origination rate.
Radiometric dating of stages makes it pos- sible to measure these rates by using years instead. If the stages shown in Figure The origination and extinction rates in a clade determine the total diversity at any given time—a value known as standing diversity.
If V becomes of species or other taxonomic unit higher than a, the diversity of a clade will decline. But diversity can also decline if a present in a particular area at a given drops. Likewise, either a drop in V or a rise in a will lead to an increase in diversity.
Changes in both of these rates can lead to superficially similar patterns, but paleon- tologists can discriminate between these alternatives by carefully inspecting the fossil record. Scientists first developed methods for estimating a and V by using fossils, but more recently, they have also developed methods for analyzing molecular phylog- enies Harvey et al.
In these analyses, scientists use a molecular clock see Chapter 9 to date the nodes. They then test models of a and V that produce patterns most closely resembling the actual phylogeny. These molecu- lar methods are especially useful for measuring a and V in clades with a poor fossil record but a large number of living species. Scientists are also developing new meth- ods to combine molecular and fossil data into a single macroevolutionary model.
Measuring origination and extinction rates can shed light on many striking pat- terns of biological diversity. As we noted earlier, for example, the tropics have higher levels of diversity than temperate regions. Scientists have proposed several hypoth- eses to explain this pattern Mannion et al. Some scientists argue that high levels of tropical diversity are the result of high a.
In other words, new species origi- nate faster in the tropics than elsewhere. On the other hand, some scientists argue that a low V is the cause of tropical diversity. In other words, species are less likely to become extinct in the tropics than elsewhere. And still other researchers argue that both factors are involved—they view the tropics as both a nursery and a museum.
The clade may be a single species or a higher group. Trilobites, for example, were a clade containing many species; the entire trilobite clade became extinct million years ago. One challenge is the task of distinguishing species based on their fossils alone see Box Another challenge is that the fossil record is far from a complete picture of past biodiversity.
There must be many, many other fossil species we have yet to find International Institute for Species Exploration A pioneering paleobiologist, David Raup of the University of Chicago, took on these challenges by amassing a tremendous database of marine invertebrate fossils Raup These animals had several advantages over other taxa: In the s, Raup started to build a database that by included , species with information about each species such as the geological age in which it lived.
He then calculated changes in diversity from the Cambrian to the present, finding intriguing rises and falls. Ever since, other researchers have been building on that database and starting others, and applying new statistical methods to get a more accurate understanding of macroevolutionary patterns Alroy ; Jablonski Sepkoski found that the diversity of groups through time was not random.
In fact, certain groups rose and fell together with other groups with remarkable consistency. These three faunas are called the Cambrian, Paleozoic, and Modern faunas. The names refer to when the faunas reached their peak diversity. The Cambrian fauna arose at the beginning of the Paleozoic. It was dominated by trilobites, inarticulate brachiopods, and coil-shelled mollusks known as monoplacophorans. Following a quick start, this fauna went into decline. Today, only one group each of inarticulate brachiopods and monoplacophorans survive.
The Cam- brian fauna arose at the beginning of the Paleozoic and quickly declined. The Paleozoic fauna arose in the Ordovician. The so-called Modern fauna has its roots in the Cambrian but came to dominate the planet after the end of the Permian, million years ago. From Sepkoski Carboniferous Perm. To do so, we must be lineage able to count separate species in a reliable and accurate way.
This is with different no easy task. As we saw in Chapter 13, biologists who study living morphology species use several criteria to delineate their boundaries, such as breeding ability, morphological or genetic differences, geographi- Gap in fossil record cal separation, and ecological differentiation. Paleontologists, on the due to erosion other hand, can look only at the morphology of fossils to determine whether they belong to a previously described species or represent Older fossil lineage a new one.
Using morphology to identify paleontological species creates some special challenges when we try to study the process of spe- ciation in the fossil record. Box Figure The vertical axis represents the position of the fossils in the rock matrix. The fossils at the bot- tom are older, and the ones at the top are younger. In Box Figure As we move up in the section, we encounter some variability in this species, but nothing remarkable. Then we see a gap where there are no preserved fossils. Above the gap, there are more fossils.
They are similar to the lower ones, but measurably different in the shape of their shell. Do these younger fossils belong to the same species as the C older ones? Now two species existed where there had once been Gradual only one. Otherwise, we can choose between two possibilities.
On morphological the one hand, the old species might have split in two, but this split change without speciation occurred when no fossils were being deposited. The original species went extinct during this time, while the new species survived.
The other possibility is that during the undocumented time, the shape of the shell in the original species evolved into the new form.
In the latter case, no speciation would Box Figure Above a discontinuity, they find would be dealing with a case transformation of a lineage from a new species with morphological differences, as shown in panel A. In macro- There are two possible explanations for this pattern: This between species, as illustrated in Box Figure If the fossil record were complete, it would show a gentle ria Box Figure Equilibria referred to the stasis in lineages transformation from one form to the next. They argued that most of the lineages documented most species undergo relatively little change for most of their in the fossil record experienced stasis; in other words, they exhib- geologic history.
These periods of stasis are punctuated by brief ited little or no directional change for millions of years. The stasis periods of rapid morphological change, often associated with was punctuated by relatively rapid change—enough to produce the speciation.
Eldredge and Gould argued that these bursts were consis- fossil and living bryozoans separate into species in a similar man- tent with a principal model of speciation recognized by population ner?
The researchers compared morphological differences in fossil biologists—the peripheral isolate model. Instead of the gradual and living bryozoans against molecular and genetic assessments divergence of a big population into two new lineages, the periph- of difference in living forms. They found that morphology was a eral isolate model proposed that geographic isolation can cause very good guide to the taxonomy of living bryozoans, as it was for fairly rapid evolution if a relatively small portion of the species fossil ones.
Jackson and Cheetham later extended their study to a vari- and Gould argued, then we should expect abrupt morphological ety of other marine invertebrates and microorganisms and found breaks in the fossil record.
Paleobiologist cies in the act of splitting. Gene Hunt of the Smithsonian Institution found a similar pattern, In Chapter 8 we saw how evolutionary biologists have docu- in which stasis dominated the fossil record Hunt Conse- mented that natural selection can change allele frequencies in a quently, punctuated equilibria endures as an influential model of matter of years or less.
Such examples of rapid natural evolution are macroevolution.
Hatched areas marked A—D are preserved tern in the fossil record with gradual anagenesis, as in this diagram sediment; white spaces are inferred gaps in the stratigraphic record. Adapted from Eldredge and Gould Species that best fit the punctuated equilibria model. This diagram shows how experienced long periods of stasis, punctuated by rapid morphological a lineage of bryozoans Metrarabdotos evolved rapidly into new spe- change during speciation.
This is in contrast to the traditional model cies, but changed little once those species were established. Adapted of slow, steady directional change in fossil lineages, which turns out from Benton Most of these groups never recovered, and some major groups became extinct. The Modern fauna has its roots in the Cambrian era, although its members were of relatively low diversity at the time.
After the Cambrian, it climbed gradually in numbers. The Modern fauna, which has been dominant since the end-Permian extinc- tion, consists mainly of gastropods snails and bivalves clams. Scientists have found evidence for two types of factors involved in macroevolutionary change. Intrinsic factors, such as the physi- ology of clades, can play a role. Extrinsic factors in the environment can as well. Sepkoski, for example, proposed that the transition between faunas may have been driven by their differences in origination a and extinction V rates.
The inver- tebrates that dominated the Cambrian fauna especially trilobites had very high turnover rates, those of the Paleozoic fauna had moderate ones, and those of the Modern fauna particularly clams and snails had low ones. On the other hand, Shanan Peters, a paleontologist at the University of Wiscon- sin, has found possible physical factors involved in these changes: He observed that most fossils of the Paleozoic fauna Figure Most of the Modern fauna fos- tion from the sun and the chemical sils are found in rocks known as silicoclastics, which formed from the sediments composition of the atmosphere.
The ecosystems built on these rocks may favor preser- Large changes in the climate—both vation of certain clades over others. The numbers by each clastic rocks have become more common, possibly because of sediment delivered to arrow shows the watts per square the oceans by rivers. Peters proposed that as the seafloor changed, the Modern fauna meter being absorbed or released by could expand across a greater area while the Paleozoic fauna retreated to a shrinking the atmosphere and Earth.
Solar radiation Thermal radiation Another physical factor drives long-term changes in biodiversity: Earth is warmed by incoming radiation from the sun Fig- Earth: Once this energy reaches Earth, it Space surface: The chemical composition of the atmosphere can change Atmosphere captured by the amount of heat it traps.
Warm water has a higher concentra- tion of the isotope oxygen than cool water does, for example, and Earth so rocks that form in warm water will lock in those isotopes as well. Over a span of million years, the global mean temperature red circles fluctuated dramatically.
Global levels of biodiversity also fluctuated blue circles; both shown as deviations from their mya average. Warmer periods had higher global levels of standing diversity than cooler periods. Adapted from Mayhew et al. One major source of this variation is the amount of carbon dioxide in the atmosphere. When certain types of volcanoes erupt more, they deliver more of these heat-trapping greenhouse gases to the atmosphere.
Peter Mayhew of the University of York examined how these changes in climate might affect the diversity of life. As we saw earlier, differences in climate have been proposed to explain the different levels of diversity found today in the tropics and in temperate zones.
By carefully comparing the fossil record with the climate record, Mayhew and his colleagues did indeed find a correla- tion. After correcting for sampling biases in the fossil record, they found that periods with warmer ocean temperatures also had increased standing diversity of marine invertebrates Figure Statistical analyses can control for known biases and help scientists make and test predictions about the processes that shaped the observed patterns.
Sep- koski and Raup, for example, studied marine fossils collected across the whole world. But even on a local scale, macroevolution can produce striking patterns that intrigue scientists. Take, for example, the islands of Hawaii. As we discussed in Chapter 13, Hawaii is home to 37 species of swordtail crickets found nowhere else.
Evolution: Making Sense of Life
Hawaii is also home to other remarkable clades, such as more than 50 species of honeycreeper birds. Silversword plants also diversified into an equally impressive range of forms Figure Clades like More than half of these species have since gone extinct. Adapted from Losos and 8 7 6 5 4 3 2 1 0 Ricklefs ; Lerner et al. Dubautia arborea D. Hawaiian silversword alliance D. After the ancestors of modern D.
From A. Argyroxiphium grayanum these, which have rapidly diversified by adapting to a wide range of resource zones, are known as adaptive radiations Losos The Great ary lineages that have undergone Lakes of East Africa are geologically very young, in many cases having formed in just exceptionally rapid diversification the past few hundred thousand years Sturmbauer et al. Once the lakes formed, into a variety of lifestyles or ecologi- cichlid fishes moved into them from nearby rivers. The fishes then diversified explo- cal niches.
Along the way, the cichlids adapted to making a living in an astounding range of ways—from crushing mollusks to scraping algae to eating the scales off other cichlids Kocher ; Salzburger et al. During adaptive radiations, new lineages expand to occupy new ecological roles. As a result, adaptive radiations produce some of the most striking examples of evo- lutionary convergence. Cichlid fish diversified independently within adjacent African Great Lakes, and these simultaneous radiations resulted in striking parallels in feeding ecology and morphology.
The African Great Lakes cichlids also experienced convergent evolution as lineages adapted to the same lifestyles and habitats in different lakes Figure Adaptive radiations may occur when clades evolve to occupy ecological niches in the absence of competition. These opportunities can arise with the emergence of a new island or lake. But they can arise in other ways as well.
When extinctions remove certain species from an ecological resource zone, other lineages can evolve that take their place. Such appears to be the case for mammals. When large dinosaurs became extinct at the end of the Cretaceous, large mammals rapidly evolved and diversified Smith et al.
In other cases, clades may radiate because new adaptations, known as key inno- vations, evolve that allow them to occupy habitats or adaptive zones that were simply off limits to earlier clades Table That seems to be what happened in the most diverse clade of animals on Earth, the insects. Insects first evolved about million years ago, and today a million species of insects have been named.
Probably millions more have yet to be described. Their closest relatives are a group called the entogna- thans, which includes springtails. The entognathans comprise only 10, species. And although entognathans generally look very similar to one another, insects have diversified impressively, from carnivorous dragonflies to ants that tend mushroom gardens to wasps that inject their eggs into living hosts.
Unlike the entognathans, the insects evolved wings that allowed them to occupy ecological roles unavailable to flightless invertebrates. Wings permitted insects to occupy new adaptive zones, and they also permitted them to colonize new habitats Grimaldi and Engel ; Mayhew ; Nicholson et al.
Other potential fac- tors in the success of insects may include the evolution of herbivory. Cambrian radiation of Environmental change; Increased O2 availability; increased animals key innovations genetic developmental capacity to diversify toolkit, body segments, in form; colonization of new lifestyles skeletal structures e.
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Key innovations can transform how organisms interact with their environments in ways that take them into new and undercontested habitats or permit them to exploit novel ways of life. These opportunities can trigger explosive subsequent diversification and adaptive radiation.
Macroevolution at the Dawn of the Animal Kingdom As spectacular as the adaptive radiation of insects may have been, it was, in some respects, a modest event. Every species of insect retains the same body plan. It gave rise to many of the major groups of animals found on Earth today, each with its own distinctive body plan.
Known as the Cambrian Explo- sion, it stands as one of the most important macroevolutionary events in the history of life Erwin and Valentine The first signs of the Cambrian Explosion emerged in the nineteenth century, as paleontologists began to organize the fossil record.Alfred Russel Wallace, co-discoverer of evolution by natural selection, also made important observations about biogeography.
Meredith, J. Biological Reviews 82 3: Later, South America, Antarctica, and Australia began to drift apart, each carrying with it a population of marsupials vicariance. Impacts of Climate Change on the Future of Biodiversity. Australia, meanwhile, drifted in isolation for over 40 million years.