Phylogeny and Systematics

PHYLOGENY AND SYSTEMATICS Introduction Evolutionary biology is about both processes (e.g., natural selection and speciation) and history. A major goal of evolutionary biology is to reconstruct the history of life on earth. Systematics is the study of biological diversity in an evolutionary context. Part of the scope of systematics is the development of phylogeny, the evolutionary history of a species or group of related species. A. The Fossil Record and Geological Time Fossils are the preserved remnants or impressions left by organisms that lived in the past. In essence, they are the historical documents of biology. The fossil record is the ordered array in which fossils appear within sedimentary rocks. These rocks record the passing of geological time. 1. Sedimentary rocks are the richest source of fossils Sedimentary rocks form from layers of sand and silt that settle to the bottom of seas and swamps. As deposits pile up, they compress older sediments below them into rock. The bodies of dead organisms settle along with the sediments, but only a tiny fraction are preserved as fossils. Rates of sedimentation vary depending on a variety of processes, leading to the formation of sedimentary rock in strata. The organic material in a dead organism usually decays rapidly, but hard parts that are rich in minerals (such as bones, teeth, shells) may remain as fossils. Under the right conditions minerals dissolved in groundwater seep into the tissues of dead organisms, replace its organic material, and create a cast in the shape of the organism. Rarer than mineralized fossils are those that retain organic material. These are sometimes discovered as thin films between layers of sandstone or shale. As an example, plant leaves millions of years old have been discovered that are still green with chlorophyll. The most common fossilized material is pollen, which has a hard organic case that resists degradation. Trace fossils consist of footprints, burrows, or other impressions left in sediments by the activities of animals. These rocks are in essence fossilized behavior. These dinosaur tracks provide information about its gait. If an organism dies in a place where decomposition cannot occur, then the entire body, including soft parts may be preserved as a fossil. These organisms have been trapped in resin, frozen in ice, or preserved in acid bogs. 2. Paleontologists use a variety of methods to date fossils When a dead organism is trapped in sediment, this fossil is frozen in time relative to other strata in a local sample. Younger sediments are superimposed upon older ones. The strata at one location can be correlated in time to those at another through index fossils. These are typically well-preserved and widely-distributed species. By comparing different sites, geologists have established a geologic time scale with a consistent sequence of historical periods. These periods are grouped into four eras: the Precambrian, Paleozoic, Mesozoic, and Cenozoic eras. Boundaries between geologic eras and periods correspond to times of great change, especially mass extinctions, not to periods of similar length. The serial record of fossils in rocks provides relative ages, but not absolute ages, the actual time when the organism died. Radiometric dating is the method used most often to determine absolute ages for fossils. This technique takes advantage of the fact that organisms accumulate radioactive isotopes when they are alive, but concentrations of these isotopes decline after they die. These isotopes undergo radioactive decay in which an isotope of one element is transformed to another element. For example, the radioactive isotope, carbon-14, is present in living organisms in the same proportion as it occurs in the atmosphere. However, after an organism dies, the proportion of carbon-14 to the total carbon declines as carbon-14 decays to nitrogen-14. An isotope’s half-life, the time it takes for 50% of the original sample to decay, is unaffected by temperature, pressure, or other variables. The half-life of carbon-14 is 5,730 years. Losses of carbon-14 can be translated into estimates of absolute time. Over time, radioactive “parent” isotopes are converted at a steady decay rate to “daughter” isotopes. The rate of conversion is indicated as the half-life, the time it takes for 50% of the isotope to decay. While carbon-14 is useful for dating relatively young fossils, radioactive isotopes of other elements with longer half-lives are used to date older fossils. While uranium-238 (half life of 4.5 billion years) is not present in living organisms to any significant level, it is present in volcanic rock. If a fossil is found sandwiched between two layers of volcanic rock, we can deduce that the organism lived in the period between the dates in which each layer of volcanic rock formed. Paleontologists can also use the ratio of two isomers of amino acids, the left-handed (L) and right-handed (D) forms, in proteins. While organisms only synthesize L-amino acids, which are incorporated into proteins, over time the population of L-amino acids is slowly converted, resulting in a mixture of L- and D-amino acids. If we know the rate at which this chemical conversion, called racemization, occurs, we can date materials that contain proteins. Because racemization is temperature dependent, it provides more accurate dates in environments that have not changed significantly since the fossils formed. 3. The fossil record is a substantial, but incomplete, chronicle of evolutionary history The discovery of a fossil depends on a sequence of improbable events. First, the organism must die at the right place and time to be buried in sediments favoring fossilization. The rock layer with the fossil must escape processes that destroy or distort rock (e.g., heat, erosion). The fossil then has only a slight chance that it will be exposed by erosion of overlying rock. Finally, there is only a slim chance that someone will find the fossil on or near the surface before it too is destroyed by erosion. A substantial fraction of species that have lived probably left no fossils, most fossils that formed have been destroyed, and only a fraction of existing fossils have been discovered. The fossil record is slanted toward species that existed for a long time, were abundant and widespread, and had hard shells or skeletons. Still, the study of fossil strata does record the sequence of biological and environmental changes. 4. Phylogeny has a biogeographic basis in continental drift The history of Earth helps explain the current geographic distribution of species. For example, the emergence of volcanic islands such as the Galapagos opens new environments for founders that reach the outposts, and adaptive radiation fills many of the available niches with new species. On a global scale, continental drift is the major geographic factor correlated with the spatial distribution of life and evolutionary episodes such as mass extinctions and adaptive radiations. The continents drift about Earth’s surface on plates of crust floating on the hot mantle. About 250 million years ago, all the land masses were joined into one supercontinent, Pangaea, with dramatic impacts on life on land and the sea. Species that had evolved in isolation now competed. The total amount of shoreline was reduced and shallow seas were drained. The interior of the continent was drier and the weather more severe. The formation of Pangaea surely had tremendous environmental impacts that reshaped biological diversity by causing extinctions and providing new opportunities for taxonomic groups that survived the crisis. A second major shock to life on Earth was initiated about 180 million years ago, as Pangaea began to break up into separate continents. Each became a separate evolutionary arena and organisms in different biogeographic realms diverged. For example, paleontologists have discovered matching fossils of Triassic reptiles in West Africa and Brazil, which were contiguous during the Mesozoic era. The great diversity of marsupial mammals in Australia that fill so many of the ecological roles that eutherian (placental) mammals do on other continents is a product of 50 million years of the isolation of Australia from other continents. 5. The history of life is punctuated by mass extinctions The fossil record reveals long quiescent periods punctuated by brief intervals when the turnover of species was much more extensive. These brief periods of mass extinction were followed by extensive diversification of some of the groups that escaped extinction. A species may become extinct because: Its habitat has been destroyed. Its environment has changed in an unfavorable direction. Evolutionary changes by some other species in its community impact it for the worse. As an example, the evolution by some Cambrian animals of hard body parts, such as jaws and shells, may have made some organisms lacking hard parts more vulnerable to predation and thereby more prone to extinction. Extinction is inevitable in a changing world. During crises in the history of life, global conditions have changed so rapidly and disruptively that a majority of species have been swept away. The fossil record records five to seven severe mass extinctions. The Permian mass extinction (250 million years ago) claimed about 90% of all marine species. This event defines the boundary between the Paleozoic and Mesozoic eras. Impacting land organisms as well, 8 out of 27 orders of Permian insects did not survive into the next geological period. This mass extinction occurred in less than five million years, an instant in geological time. Factors that may have caused the Permian mass extinction include: Disturbance to marine and terrestrial habitats due to the formation of Pangaea, The massive volcanic eruptions in Siberia that may have released enough carbon dioxide to warm the global climate. The changes in ocean circulation that reduced the amount of oxygen available to marine organisms. The Cretaceous mass extinction (65 million years ago) doomed half of the marine species and many families of terrestrial plants and animals, including nearly all the dinosaur lineages. This event defines the boundary between the Mesozoic and Cenozoic eras. There are several hypotheses for the mechanism of this event. The climate became cooler, and shallow seas receded from continental lowlands. Large volcanic eruptions in India may have contributed to global cooling by releasing material into the atmosphere. Walter and Luis Alvarez proposed that the impact of an asteroid would have produced a great cloud that would have blocked sunlight and severely disturbed the climate for several months. Part of the evidence for the collision is the widespread presence of a thin layer of clay enriched with iridium, an element rare on Earth but common in meteorites and other extraterrestrial debris. Recent research has focused on the Chicxulub crater, a 65-million-year-old scar located beneath sediments on the Yucatan coast of Mexico. Critical evaluation of the impact hypothesis as the cause of the Cretaceous extinctions is ongoing. For example, advocates of this hypothesis have argued that the impact was large enough to darken the Earth for years, reducing photosynthesis long enough for food chains to collapse. The shape of the impact crater implies that debris initially inundated North America, consistent with more severe and temporally compacted extinctions in North America. Less severe global effects would have developed more slowly after the initial catastrophe, consistent with variable rates of extinction around the globe. Although the debate over the impact hypothesis has muted somewhat, researchers maintain a healthy skepticism about the link between the Chicxulub impact event and the Cretaceous extinctions. Opponents of the impact hypothesis argue that changes in climate due to continental drift, increased volcanism, and other processes could have caused mass extinctions 65 million years ago. It is possible that an asteroid impact was the sudden final blow in an environmental assault on late Cretaceous life that included more gradual processes. While the emphasis of mass extinctions is on the loss of species, there are tremendous opportunities for those that survive. Survival may be due to adaptive qualities or sheer luck. After a mass extinction, the survivors become the stock for new radiations to fill the many biological roles vacated or created by the extinctions. B. Systematics: Connecting Classification to Phylogeny To trace phylogeny or the evolutionary history of life, biologists use evidence from paleontology, molecular data, comparative anatomy, and other approaches. Tracing phylogeny is one of the main goals of systematics, the study of biological diversity in an evolutionary context. Systematics includes taxonomy, which is the naming and classification of species and groups of species. As Darwin correctly predicted, “our classifications will come to be, as far as they can be so made, genealogies.” 1. Taxonomy employs a hierarchical system of classification The Linnean system, first formally proposed by Linneaus in Systema naturae in the 18th century, has two main characteristics. Each species has a two-part name. Species are organized hierarchically into broader and broader groups of organisms. Under the binomial system, each species is assigned a two-part latinized name, a binomial. The first part, the genus, is the closest group to which a species belongs. The second part, the specific epithet, refers to one species within each genus. The first letter of the genus is capitalized and both names are italicized and latinized. For example, Linnaeus assigned to humans the scientific name Homo sapiens, which means “wise man,” perhaps in a show of optimism. A hierachical classification groups species into broader taxonomic categories. Species that appear to be closely related are grouped into the same genus. For example, the leopard, Panthera pardus, belongs to a genus that includes the African lion (Panthera leo) and the tiger (Panthera tigris). Biology’s taxonomic scheme formalizes our tendency to group related objects. Genera are grouped into progressively broader categories: family, order, class, phylum, kingdom and domain. Each taxonomic level is more comprehensive than the previous one. As an example, all species of cats are mammals, but not all mammals are cats. The named taxonomic unit at any level is called a taxon. Example: Pinus is a taxon at the genus level, the generic name for various species of pine trees. Mammalia, a taxon at the class level, includes all the many orders of mammals. Phylogenetic trees reflect the hierarchical classification of taxonomic groups nested within more inclusive groups. 2. Modern phylogenetic systematics is based on cladistic analysis A phylogeny is determined by a variety of evidence including fossils, molecular data, anatomy, and other features. Most systematists use cladistic analysis, developed by a German entomologist Willi Hennig to analyze the data A phylogenetic diagram or cladogram is constructed from a series of dichotomies. These dichotomous branching diagrams can include more taxa. The sequence of branching symbolizes historical chronology. The last ancestor common to both the cat and dog families lived longer ago than the last common ancestor shared by leopards and domestic cats. Each branch or clade can be nested within larger clades. A clade consists of an ancestral species and all its descendents, a monophyletic group. Groups that do not fit this definition are unacceptable in cladistics. Determining which similarities between species are relevant to grouping the species in a clade is a challenge. It is especially important to distinguish similarities that are based on shared ancestry or homology from those that are based on convergent evolution or analogy. These two desert plants are not closely related but owe their resemblance to analogous adaptations. As a general rule, the more homologous parts that two species share, the more closely related they are. Adaptation can obscure homology and convergence can create misleading analogies. Also, the more complex two structures are, the less likely that they evolved independently. For example, the skulls of a human and chimpanzee are composed not of a single bone, but a fusion of multiple bones that match almost perfectly. It is highly improbable that such complex structures matching in so many details could have separate origins. For example, the forelimbs of bats and birds are analogous adaptations for flight because the fossil record shows that both evolved independently from the walking forelimbs of different ancestors. Their common specializations for flight are convergent, not indications of recent common ancestry. The presence of forelimbs in both birds and bats is homologous, though at a higher level of the cladogram, at the level of tetrapods. The question of homology versus analogy often depends on the level of the clade that is being examined. Systematists must sort through homologous features or characters to separate shared derived characters from shared primitive characters. A shared derived character is unique to a particular clade. A shared primitive character is found not only in the clade being analyzed, but older clades too. Shared derived characters are useful in establishing a phylogeny, but shared primitive characters are not. For example, the presence of hair is a good character to distinguish the clade of mammals from other tetrapods. It is a shared derived character that uniquely identifies mammals. However, the presence of a backbone can qualify as a shared derived character, but at a deeper branch point that distinguishes all vertebrates from other mammals. Among vertebrates, the backbone is a shared primitive character because it evolved in the ancestor common to all vertebrates. Shared derived characters are useful in establishing a phylogeny, but shared primitive characters are not. The status of a character as analogous versus homologous or shared versus primitive may depend on the level at which the analysis is being performed. A key step in cladistic analysis is outgroup comparison which is used to differentiate shared primitive characters from shared derived ones. To do this we need to identify an outgroup: a species or group of species that is closely related to the species that we are studying, but known to be less closely related than any study-group members are to each other. To study the relationships among five vertebrates (the ingroup)—a leopard, a turtle, a salamander, a tuna, and a lamprey—on a cladogram, then an animal called the lancet would be a good choice. The lancet is closely related to the most primitive vertebrates based on other evidence and other lines of analysis. These other analyses also show that the lancet is not more closely related to any of the ingroup taxa. In an outgroup analysis, the assumption is that any homologies shared by the ingroup and outgroup must be primitive characters already present in the ancestor common to both groups. Homologies present in some or all of the ingroup taxa must have evolved after the divergence of the ingroup and outgroup taxa. In our example, a notochord, present in lancets and in the embryos of the ingroup, would be a shared primitive character and not useful. The presence of a vertebral column, shared by all members of the ingroup but not the outgroup, is a useful character for the whole ingroup. Similarly, the presence of jaws, absent in lampreys and present in the other ingroup taxa, helps to identify the earliest branch in the vertebrate cladogram. Analyzing the taxonomic distribution of homologies enables us to identify the sequence in which derived characters evolved during vertebrate phylogeny. A cladogram presents the chronological sequence of branching during the evolutionary history of a set of organisms. However, this chronology does not indicate the time of origin of the species that we are comparing, only the groups to which they belong. For example, a particular species in an old group may have evolved more recently than a second species that belongs to a newer group. Systematists can use cladograms to place species in the taxonomic hierarchy. For example, using turtles as the outgroup, we can assign increasingly exclusive clades to finer levels of the hierarchy of taxa. However, some systematists argue that the hierarchical system is antiquated because such a classification must be rearranged when a cladogram is revised based on new evidence. These systematists propose replacing the Linneaen system with a strictly cladistic classification called phylocode that drops the hierarchical tags, such as class, order, and family. So far, biologists still prefer a hierachical system of taxonomic levels as a more useful way of organizing the diversity of life. 3. Systematists can infer phylogeny from molecular data The application of molecular methods and data for comparing species and tracing phylogenies has accelerated revision of taxonomic trees. If homology reflects common ancestry, then comparing genes and proteins among organisms should provide insights into their evolutionary relationships. The more recently two species have branched from a common ancestor, the more similar their DNA and amino acid sequences should be. These data for many species are available via the internet. Molecular systematics makes it possible to assess phylogenetic relationships that cannot be measured by comparative anatomy and other non-molecular methods. This includes groups that are too closely related to have accumulated much morphological divergence. At the other extreme, some groups (e.g., fungi, animals, and plants) have diverged so much that little morphological homology remains. Most molecular systematics is based on a comparison of nucleotide sequences in DNA or RNA. Each nucleotide position along a stretch of DNA represents an inherited character as one of the four DNA bases: A (adenine), G (guanine), C (cytosine), and T (thymine). Systematists may compare hundreds or thousands of adjacent nucleotide positions from several DNA regions to assess the relationship between two species. This DNA sequence analysis provides a quantitative tool for constructing cladograms with branch points defined by mutations in DNA sequence. The rates of change in DNA sequences vary from one part of the genome to another. Some regions (e.g., rRNA) that change relatively slowly are useful in investigating relationships between taxa that diverged hundreds of millions of years ago. Other regions (e.g., mtDNA) evolve relatively rapidly and can be employed to assess the phylogeny of species that are closely related or even populations of the same species. The first step in DNA comparisons is to align homologous DNA sequences for the species we are comparing. Two closely related species may differ only in which base is present at a few sites. Less closely related species may not only differ in bases at many sites, but there may be insertions and deletions that alter the length of genes This creates problems for establishing homology. 4. The principle of parsimony helps systematists reconstruct phylogeny The process of converting data into phylogenetic trees can be a daunting problem. If we wish to determine the relationships among four species or taxa, we would need to choose among several potential trees. As we consider more and more taxa, the number of possible trees increases dramatically. There are about 3 x 1076 possible phylogenetic trees for a group of 50 species. Even computer analyses of these data sets can take too long to search for the tree that best fits the DNA data. Systematists use the principle of parsimony to choose among the many possible trees to find the tree that best fits the data. The principle of parsimony (“Occam’s Razor”) states that a theory about nature should be the simplest explanation that is consistent with the facts. This minimalist approach to problem solving has been attributed to William of Occam, a 14th century English philosopher. In phylogenetic analysis, parsimony is used to justify the choice of a tree that represents the smallest number of evolutionary changes. As an example, if we wanted to use the DNA sequences from seven sites to determine the most parsimonious arrangement of four species, we would begin by tabulating the sequence data. Then, we would draw all possible phylogenies for the four species, including the three shown here. We would trace the number of events (mutations) necessary on each tree to produce the data in our DNA table. After all the DNA sites have been added to each tree we add up the total events for each tree and determine which tree required the fewest changes, the most parsimonious tree. 5. Phylogenetic trees are hypotheses The rationale for using parsimony as a guide to our choice among many possible trees is that for any species’ characters, hereditary fidelity is more common than change. At the molecular level, point mutations do occasionally change a base within a DNA sequence, but exact transmission from generation to generation is thousands of times more common than change. Similarly, one could construct a primitive phylogeny that places humans and apes as distant clades but this would assume an unnecessarily complicated scenario. A cladogram that is not the most parsimonious would assume an unnecessarily complicated scenario, rather than the simplest explanation. Given a choice of possible trees we can draw for a set of species or higher taxa, the best hypothesis is the one that is the best fit for all the available data. In the absence of conflicting information, the most parsimonious tree is the logical choice among alternative hypotheses. A limited character set may lead to acceptance of a tree that is most parsimonious, but that is also wrong. Therefore, it is always important to remember that any phylogenetic diagram is a hypothesis, subject to rejection or revision as more character data are available. For example, based on the number of heart chambers alone, birds and mammals, both with four chambers, appear to be more closely related to each other than lizards with three chambers. But abundant evidence indicated that birds and mammals evolved from different reptilian ancestors. The four-chambered hearts are analogous, not homologous, leading to a misleading cladogram. Regardless of the source of data (DNA sequence, morphology, etc.), the most reliable trees are based on the largest data base. Occasionally misjudging an analogous similarity in morphology or gene sequence as a shared derived homology is less likely to distort a phylogenetic tree if each clade in the tree is defined by several derived characters. The strongest phylogenetic hypotheses are supported by both the morphological and molecular evidence. 6. Molecular clocks may keep track of evolutionary time The timing of evolutionary events has rested primarily on the fossil record. Recently, molecular clocks have been applied to place the origin of taxonomic groups in time. Molecular clocks are based on the observation that some regions of genomes evolve at constant rates. For these regions, the number of nucleotide and amino acid substitutions between two lineages is proportional to the time that has elapsed since they branched. For example, the homologous proteins of bats and dolphins are much more alike than are those of sharks and tuna. This is consistent with the fossil evidence that sharks and tuna have been on separate evolutionary paths much longer than bats and dolphins. In this case, molecular divergence has kept better track of time than have changes in morphology. Proportional differences in DNA sequences can be applied to access the relative chronology of branching in phylogeny, but adjustments for absolute time must be viewed with some caution. No genes mark time with a precise tick-tock accuracy in the rate of base changes. Genes that make good molecular clocks have fairly smooth average rates of change. Over time there may be chance deviations above and below the average rate. Each molecular clock must be calibrated in actual time. Typically, one graphs the number of amino acid or nucleotide differences against the times for a series of evolutionary events known from the fossil record. The slope of the best line through these points represents the evolution rate of that molecular clock. This rate can be used to estimate the absolute date of evolutionary events that have no fossil record. The molecular clock approach assumes that much of the change in DNA sequences is due to genetic drift and is selectively neutral. If certain DNA changes were favored by natural selection, then the rate would probably be too irregular to mark time accurately. Also, some biologists are skeptical of conclusions derived from molecular clocks that have been extrapolated to time spans beyond the calibration in the fossil record. The molecular clock approach has been used to date the jump of the HIV virus from related SIV viruses that infect chimpanzees and other primates to humans. Investigators calibrated their molecular clock by comparing DNA sequences in a specific HIV gene from patients sampled at different times. From their analysis, they project that the HIV-1M strain invaded humans in the 1930s. 7. Modern systematics is flourishing with lively debate Systematics is thriving at the interface of modern evolutionary biology and taxonomic theory. The development of cladistics provides a more objective method for comparing morphology and developing phylogenetic hypotheses. Cladistic analysis of morphological and molecular characters, complemented by a revival in paleontology and comparative biology, has brought us closer to an understanding of the history of life on Earth. For example, the fossil record, comparative anatomy, and molecular comparisons all concur that crocodiles are more closely related to birds than to lizards and snakes. In other cases, molecular data present a different picture than other approaches. For example, fossil evidence dates the origin of the orders of mammals at about 60 million years ago, but molecular clock analyses place their origin to 100 million years ago. In one camp are those who place more weight in the fossil evidence and express doubts about the reliability of the molecular clocks. In the other camp are those who argue that paleontologists have not yet documented an earlier origin for most mammalian orders because the fossil record is incomplete. Between these two extremes is a phylogenetic fuse hypothesis. This hypothesis proposes that the modern mammalian orders originated about 100 million years ago. But they did not proliferate extensively enough to be noticeable in the fossil record until after the extinction of dinosaurs almost 40 million years later.