Origin of Life: The Heterotroph Hypothesis
Life on Earth began about 3.5 billion years ago. At that point in the development of the Earth, the atmosphere was very different from what it is today. As opposed to the current atmosphere, which is mostly nitrogen and oxygen, the early Earth atmosphere contained mostly hydrogen, water, ammonia, and methane.
In experiments, scientists have showed that the electrical discharges of lightning, radioactivity, and ultraviolet light caused the elements in the early Earth atmosphere to form the basic molecules of biological chemistry, such as nucleotides, simple proteins, and ATP. It seems likely, then, that the Earth was covered in a hot, thin soup of water and organic materials. Over time, the molecules became more complex and began to collaborate to run metabolic processes. Eventually, the first cells came into being. These cells were heterotrophs, which could not produce their own food and instead fed on the organic material from the primordial soup. (These heterotrophs give this theory its name.)
The anaerobic metabolic processes of the heterotrophs released carbon dioxide into the atmosphere, which allowed for the evolution of photosynthetic autotrophs, which could use light and CO2 to produce their own food. The autotrophs released oxygen into the atmosphere. For most of the original anaerobic heterotrophs, oxygen proved poisonous. The few heterotrophs that survived the change in environment generally evolved the capacity to carry out aerobic respiration. Over the subsequent billions of years, the aerobic autotrophs and heterotrophs became the dominant life-forms on the planet and evolved into all of the diversity of life now visible on Earth.
Evidence of Evolution
Humankind has always wondered about its origins and the origins of the life around it. Many cultures have ancient creation myths that explain the origin of the Earth and its life. In Western cultures, ideas about evolution were originally based on the Bible. The book of Genesis relates how God created all life on Earth about 6,000 years ago in a mass creation event. Proponents of creationism support the Genesis account and state that species were created exactly as they are currently found in nature. This oldest formal conception of the origin of life still has proponents today.
However, about 200 years ago, scientific evidence began to cast doubt on creationism. This evidence comes in a variety of forms.
Rock and Fossil Formation
Fossils provide the only direct evidence of the history of evolution. Fossil formation occurs when sediment covers some material or fills an impression. Very gradually, heat and pressure harden the sediment and surrounding minerals replace it, creating fossils. Fossils of prehistoric life can be bones, shells, or teeth that are buried in rock, and they can also be traces of leaves or footprints left behind by organisms.
Together, fossils can be used to construct a fossil record that offers a timeline of fossils reaching back through history. To puzzle together the fossil record, scientists have to be able to date the fossils to a certain time period. The strata of rock in which fossils are found give clues about their relative ages. If two fossils are found in the same geographic location, but one is found in a layer of sediment that is beneath the other layer, it is likely that the fossil in the lower layer is from an earlier era. After all, the first layer of sediment had to already be on the ground in order for the second layer to begin to build up on top of it. In addition to sediment layers, new techniques such as radioactive decay or carbon dating can also help determine a fossil’s age.
There are, however, limitations to the information fossils can supply. First of all, fossilization is an improbable event. Most often, remains and other traces of organisms are crushed or consumed before they can be fossilized. Additionally, fossils can only form in areas with sedimentary rock, such as ocean floors. Organisms that live in these environments are therefore more likely to become fossils. Finally, erosion of exposed surfaces or geological movements such as earthquakes can destroy already formed fossils. All of these conditions lead to large and numerous gaps in the fossil record.
Comparative Anatomy
Scientists often try to determine the relatedness of two organisms by comparing external and internal structures. The study of comparative anatomy is an extension of the logical reasoning that organisms with similar structures must have acquired these traits from a common ancestor. For example, the flipper of a whale and a human arm seem to be quite different when looked at on the outside. But the bone structure of each is surprisingly similar, suggesting that whales and humans have a common ancestor way back in prehistory. Anatomical features in different species that point to a common ancestor are called homologous structures.
However, comparative anatomists cannot just assume that every similar structure points to a common evolutionary origin. A hasty and reckless comparative anatomist might assume that bats and insects share a common ancestor, since both have wings. But a closer look at the structure of the wings shows that there is very little in common between them besides their function. In fact, the bat wing is much closer in structure to the arm of a man and the fin of a whale than it is to the wings of an insect. In other words, bats and insects evolved their ability to fly along two very separate evolutionary paths. These sorts of structures, which have superficial similarities because of similarity of function but do not result from a common ancestor, are called analogous structures.
In addition to homologous and analogous structures, vestigial structures, which serve no apparent modern function, can help determine how an organism may have evolved over time. In humans the appendix is useless, but in cows and other mammalian herbivores a similar structure is used to digest cellulose. The existence of the appendix suggests that humans share a common evolutionary ancestry with other mammalian herbivores. The fact that the appendix now serves no purpose in humans demonstrates that humans and mammalian herbivores long ago diverged in their evolutionary paths.
Comparative Embryology
Homologous structures not present in adult organisms often do appear in some form during embryonic development. Species that bear little resemblance to each other in their adult forms may have strikingly similar embryonic stages. In some ways, it is almost as if the embryo passes through many evolutionary stages to produce the mature organism. For example, for a large portion of its development, the human embryo possesses a tail, much like those of our close primate relatives. This tail is usually reabsorbed before birth, but occasionally children are born with the ancestral structure intact. Even though they are not generally present in the adult organism, tails could be considered homologous traits between humans and primates.
In general, the more closely related two species are, the more their embryological processes of development resemble each other.
Molecular Evolution
Just as comparative anatomy is used to determine the anatomical relatedness of species, molecular biology can be used to determine evolutionary relationships at the molecular level. Two species that are closely related will have fewer genetic or protein differences between them than two species that are distantly related and split in evolutionary development long in the past.
Certain genes or proteins in organisms change at a constant rate over time. These genes and proteins, called molecular clocks because they are so constant in their rate of change, are especially useful in comparing the molecular evolution of different species. Scientists can use the rate of change in the gene or protein to calculate the point at which two species last shared a common ancestor. For example, ribosomal RNA has a very slow rate of change, so it is commonly used as a molecular clock to determine relationships between extremely ancient species. Cytochrome c, a protein that plays an important role in aerobic respiration, is an example of a protein commonly used as a molecular clock.
Theories of Evolution
In the nineteenth century, as increasing evidence suggested that species changed over time, scientists began to develop theories to explain how these changes arise. During this time, there were two notable theories of evolution. The first, proposed by Lamarck, turned out to be incorrect. The second, developed by Darwin, is the basis of all evolutionary theory.
Lamarck: Use and Disuse
The first notable theory of evolution was proposed by Jean-Baptiste Lamarck (1744–1829). He described a two-part mechanism by which evolutionary change was gradually introduced into the species and passed down through generations. His theory is referred to as the theory of transformation or Lamarckism.
The classic example used to explain Lamarckism is the elongated neck of the giraffe. According to Lamarck’s theory, a given giraffe could, over a lifetime of straining to reach high branches, develop an elongated neck. This vividly illustrates Lamarck’s belief that use could amplify or enhance a trait. Similarly, he believed that disuse would cause a trait to become reduced. According to Lamarck’s theory, the wings of penguins, for example, were understandably smaller than the wings of other birds because penguins did not use their wings to fly.
The second part of Lamarck’s mechanism for evolution involved the inheritance of acquired traits. He believed that if an organism’s traits changed over the course of its lifetime, the organism would pass these traits along to its offspring.
Lamarck’s theory has been proven wrong in both of its basic premises. First, an organism cannot fundamentally change its structure through use or disuse. A giraffe’s neck will not become longer or shorter by stretching for leaves. Second, modern genetics shows that it is impossible to pass on acquired traits; the traits that an organism can pass on are determined by the genotype of its sex cells, which does not change according to changes in phenotype.
Darwin: Natural Selection
While sailing aboard the HMS Beagle, the Englishman Charles Darwin had the opportunity to study the wildlife of the Galápagos Islands. On the islands, he was amazed by the great diversity of life. Most particularly, he took interest in the islands’ various finches, whose beaks were all highly adapted to their particular lifestyles. He hypothesized that there must be some process that created such diversity and adaptation, and he spent much of his time trying to puzzle out just what the process might be. In 1859, he published his theory of natural selection and the evolution it produced. Darwin explained his theory through four basic points:
- Each species produces more offspring than can survive.
- The individual organisms that make up a larger population are born with certain variations.
- The overabundance of offspring creates a competition for survival among individual organisms. The individuals that have the most favorable variations will survive and reproduce, while those with less favorable variations are less likely to survive and reproduce.
- Variations are passed down from parent to offspring.
Natural selection creates change within a species through competition, or the struggle for life. Members of a species compete with each other and with other species for resources. In this competition, the individuals that are the most fit—the individuals that have certain variations that make them better adapted to their environments—are the most able to survive, reproduce, and pass their traits on to their offspring. The competition that Darwin’s theory describes is sometimes called the survival of the fittest.
Natural Selection in Action
One of the best examples of natural selection is a true story that took place in England around the turn of the century. Near an agricultural town lived a species of moth. The moth spent much of its time perched on the lichen-covered bark of trees of the area. Most of the moths were of a pepper color, though a few were black. When the pepper-color moths were attached to the lichen-covered bark of the trees in the region, it was quite difficult for predators to see them. The black moths were easy to spot against the black-and-white speckled trunks.
The nearby city, however, slowly became industrialized. Smokestacks and foundries in the town puffed out soot and smoke into the air. In a fairly short time, the soot settled on everything, including the trees, and killed much of the lichen. As a result, the appearance of the trees became nearly black in color. Suddenly the pepper-color moths were obvious against the dark tree trunks, while the black moths that had been easy to spot now blended in against the trees. Over the course of years, residents of the town noticed that the population of the moths changed. Whereas about 90 percent of the moths used to be light, after the trees became black, the moth population became increasingly black.
When the trees were lighter in color, natural selection favored the pepper-color moths because those moths were more difficult for predators to spot. As a result, the pepper-color moths lived to reproduce and had pepper-color offspring, while far fewer of the black moths lived to produce black offspring. When the industry in the town killed off the lichen and covered the trees in soot, however, the selection pressure switched. Suddenly the black moths were more likely to survive and have offspring. In each generation, more black moths survived and had offspring, while fewer lighter moths survived to have offspring. Over time, the population as a whole evolved from mostly white in color to mostly black in color.
Types of Natural Selection
In a normal population without selection pressure, individual traits, such as height, vary in the population. Most individuals are of an average height, while fewer are extremely short or extremely tall. The distribution of height falls into a bell curve.
Natural selection can operate on this population in three basic ways. Stabilizing selection eliminates extreme individuals. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures act to favor plants of medium height.
Directional selection selects against one extreme. In the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks.
Disruptive selection eliminates intermediate individuals. For example, imagine a plant of extremely variable height that is pollinated by three different pollinator insects: one that was attracted to short plants, another that preferred plants of medium height, and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against, and the population would tend toward both short and tall plants, but not plants of medium height.
The Genetic Basis for Evolution
Darwin’s theory of natural selection and evolution rests on two crucial ideas:
- Variations exist in the individuals within a population.
- Those variations are passed down from one generation to the next.
But Darwin had no idea how those variations came to be or how they were passed down from one generation to the next. Mendel’s experiments and the development of the science of genetics provided answers. Genetics explains that the phenotype—the physical attributes of an organism—is produced by an organism’s genotype. Through the mechanism of mutations, genetics explains how variations arose among individuals in the form of different alleles of genes. Meiosis, sexual reproduction, and the inheritance of alleles explain how the variations between organisms are passed down from parent to offspring.
With the modern understanding of genes and inheritance, it is possible to redefine natural selection and evolution in genetic terms. The particular alleles that an organism inherits from its parents determine that organism’s physical attributes and therefore its fitness for survival. When the forces of natural selection result in the survival of the fittest, what those forces are really doing is selecting which alleles will be passed on from one generation to the next.
Once you see that natural selection is actually a selection of the passage of alleles from generation to generation, you can further see that the forces of natural selection can change the frequency of each particular allele within a population’s gene pool, which is the sum total of all the alleles within a particular population. Using genetics, one can create a new definition of evolution as the change in the allele frequencies in the gene pool of a population over time. For example, in the population of moths we discussed earlier, after the trees darkened, the frequency of the alleles for black coloration increased in the gene pool, while the frequency of alleles for light coloration decreased.
Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that a sexually reproducing population will have stable allelic frequencies and therefore will not undergo evolution, given the following five conditions:
- large population size
- no immigration or emigration
- random mating
- random reproductive success
- no mutation
The Hardy-Weinberg principle proves that variability and inheritance alone are not enough to cause evolution; natural selection must drive evolution. A population that meets all of these conditions is said to be in Hardy-Weinberg equilibrium. Few natural populations ever experience Hardy-Weinberg equilibrium, though, since large populations are rarely found in isolation, all populations experience some level of mutation, and natural selection simply cannot be avoided.
Development of New Species
The scientific definition of a species is a discrete group of organisms that can only breed within its own confines. In other words, the members of one species cannot interbreed with the members of another species. Each species is said to experience reproductive isolation. If you think about evolution in terms of genetics, this definition of species makes a great deal of sense: if species could interbreed, they could share gene flow, and their evolution would not be separate. But since species cannot interbreed, each species exists on its own individual path.
As populations change, new species evolve. This process is known as speciation. Through speciation, the earliest simple organisms were able to branch out and populate the world with millions of different species. Speciation is also called divergent evolution, since when a new species develops, it diverges from a previous form. All homologous traits are produced by divergent evolution. Whales and humans share a distant common ancestor. Through speciation, that ancestor underwent divergent evolution and gave rise to new species, which in turn gave rise to new species, which over the course of millions of years resulted in whales and humans. The original ancestor had a limb structure that, over millions of years and successive occurrences of divergent evolution, evolved into the fin of the whale and the arm of the human.
Speciation occurs when two populations become reproductively isolated. Once reproductive isolation occurs for a new species, it will begin to evolve independently. There are two main ways in which speciation might occur. Allopatric speciation occurs when populations of a species become geographically isolated so that they cannot interbreed. Over time, the populations may become genetically different in response to the unique selection pressures operating in their different environments. Eventually the genetic differences between the two populations will become so extreme that the two populations would be unable to interbreed even if the geographic barrier disappeared.
A second, more common form of speciation is adaptive radiation, which is the creation of several new species from a single parent species. Think of a population of a given species, which we’ll imaginatively name population 1. The population moves into a new habitat and establishes itself in a niche, or role, in the habitat (we discuss niches in more detail in the chapter on Ecology). In so doing, it adapts to its new environment and becomes different from the parent species. If a new population of the parent species, population 2, moves into the area, it too will try to occupy the same niche as population 1. Competition between population 1 and population 2 ensues, placing pressure on both groups to adapt to separate niches, further distinguishing them from each other and the parent species. As this happens many times in a given habitat, several new species may be formed from a single parent species in a relatively short time. The immense diversity of finches that Darwin observed on the Galápagos Islands is an excellent example of the products of adaptive radiation.
Convergent Evolution
When different species inhabit similar environments, they face similar selection pressures, or use parts of their bodies to perform similar functions. These similarities can cause the species to evolve similar traits, in a process called convergent evolution. From living in the cold, watery, arctic regions, where most of the food exists underwater, penguins and killer whales have evolved some similar characteristics: both are streamlined to help them swim more quickly underwater, both have layers of fat to keep them warm, both have similar white-and-black coloration that helps them to avoid detection, and both have developed fins (or flippers) to propel them through the water. All of these similar traits are examples of analogous traits, which are the product of convergent evolution.
Convergent evolution sounds as if it is the opposite of divergent evolution, but that isn’t actually true. Convergent evolution is only superficial. From the outside, the fin of a whale may look like the flipper of a penguin, but the bone structure of a whale fin is still more similar to the limbs of other mammals than it is to the structure of penguin flippers. More importantly, convergent evolution never results in two species gaining the ability to interbreed; convergent evolution can’t take two species and turn them into one.
Classifying Life
The diversity of life on Earth is staggering. The science of identifying, describing, naming, and classifying all of these organisms is called taxonomy. Carolus Linnaeus, an eighteenth-century Swedish botanist, is considered the father of modern taxonomy. He carefully observed and compared different species, grouping them according to the similarities and differences he found. Taxonomists today still use his system of organization, though they classify organisms based on their evolutionary relationships, or phylogeny, rather than on simple physical characteristics. The classification system used in taxonomy is hierarchical and contains seven levels. The seven levels of taxonomic classification, from broadest to most specific, are:
KingdomPhylumClassOrderFamilyGenusSpecies
A good way to remember the sequence of taxonomic categories is to use a mnemonic:
King Philip Came Over From German Shores
Each kingdom contains numerous phyla; each phylum contains numerous classes; each class contains numerous orders; etc. It is more accurate to draw the diagram of the taxonomic categories in a tree structure, with each level of the hierarchy branching into the next:
As one moves through the hierarchy from species to kingdom, the common ancestor of all the species at a certain level dates further back in evolutionary history than the common ancestor of organisms in more specific levels. For example, the common ancestor of humans and chimpanzees (which are both in the order Primates) was alive more recently than the common ancestor of humans and dogs (which are both in the class Mammalia). Much in the same way, members of the same genus are more closely related than members of the same family; members of the same family are more closely related than members of the same order.
Each species is placed into the classification system with a two-part name. The first half of the name is the species’ genus, while the second is the species’ own specific name. The genus name is capitalized, and the species name is lowercase. Humans belong to the genus Homo and the species sapiens, so the name for humans is Homo sapiens.
The Five Kingdoms
Taxonomy splits all living things into five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. For the SAT II Biology, you should know the basic characteristics of the organisms that belong in each of these kingdoms, and you should also be familiar with the names and features of the major phyla within each kingdom.
Kingdom Monera
Monerans are prokaryotic: they are single-celled organisms that lack a nucleus and membrane-bound organelles. Of the four kingdoms, monerans are the simplest, and they generally evolved the earliest. Of all the kingdoms, only monerans are prokaryotic.
Monerans are characterized by a single circular chromosome of DNA, a single cell membrane that controls the transport of substances into and out of the cell, and a process of asexual reproduction called binary fission that involves dividing into two identical clones. Some monerans have a cell wall made of a sugar-protein complex called peptidoglycan, which can be determined by Gram staining. A Gram-positive moneran has a thick peptidoglycan cell wall, while a Gram-negative moneran has a much thinner one. Monerans are broken down into phyla according to their means of procuring food.
We cover the structure and function of monerans in more detail in the section on microorganisms in the Organismal Biology chapter.
PHYLUM BACTERIA
Bacteria are heterotrophic and can act as symbionts, parasites, or decomposers.
PHYLUM CYANOBACTERIA (BLUE-GREEN ALGAE)
Cyanobacteria are autotrophs that can perform photosynthesis.
Kingdom Protista
Protists are eukaryotic. In general, protists are less complex than the other eukaryotes and originated earlier in evolutionary history. Most are unicellular, though some are organized in colonies and some others are multicellular. The kingdom Protista can be separated into three primary divisions: animal-like, plantlike, and funguslike.
The animal-like protists are heterotrophic and motile. The most important protozoa for the SAT II Biology are the amoebas, sporozoa, and ciliates:
PHYLUM RHIZOPODA
The members of phylum Rhizopoda are amoebas, known for their constantly changing body structure. Amoebas use membrane extensions called pseudopods (“false feet”) to move and to surround food particles, which they then engulf into their cytoplasm via phagocytosis. Amoebas generally live in fresh water, but some are found in soil or salt water. If an amoeba finds its way inside a human through contaminated drinking water, it can cause severe dysentery.
PHYLUM APICOMPLEXA
The phylum Apicomplexa consists of spore-forming parasitic organisms, also known as sporozoa. The adult form lives inside the cells of animals. The spores are transmitted to other host animals, usually by a carrier animal. For example, a mosquito bite transmits plasmodium, an apicomplexan that lives in red blood cells and causes malaria.
PHYLUM CILIOPHORA
All members of the phylum Ciliophora propel themselves by waving many short, hairlike structures called cilia in a coordinated fashion; cilia also help draw food particles into the oral groove. Unlike other protozoa, ciliates have two nuclei: the smaller micronucleus is involved in reproduction, while the macronucleus controls the organism’s metabolic processes. A paramecium is the classic example of a ciliate protozoan.
The plantlike protists include euglenoids and various kinds of algae. They are all photo-synthetic autotrophs, transforming light energy into food. Some are unicellular, but many are multicellular, forming fibrous seaweed structures.
PHYLUM EUGLENOPHYTA
Euglenoids are classified with the plantlike protists because many of them photosynthesize. But these unicellular organisms have flagella that allow them to move.
PHYLUM PHAEOPHYTA
Brown algae of phylum Phaeophyta are all multicellular seaweeds, ranging in size from an inch to almost the length of a football field (the large varieties are called kelp). Brown algae provide both food and shelter to many animals in the coastal marine ecosystem.
PHYLUM CHLOROPHYTA
Green algae of phylum Chlorophyta have the same photo-synthetic pigments and the same cell wall structure as plants. In fact, they are believed to be the ancestors of modern plants. Some are unicellular, and some are multicellular; however, none have specialized tissues like plants, and therefore they remain classified with the simpler organisms in kingdom Protista.
The funguslike protists are called slime molds, which belong to the phyla Myxomycota and Acrasiomycota. All slime molds are heterotrophs.
PHYLUM MYXOMYCOTA
This phylum includes the plasmodial (acellular) slime molds. A plasmodium consists of a single cell with multiple nuclei. Plasmodial slime molds creep slowly along the decaying vegetation they digest; when food or water is scarce, they produce small tough spores that germinate when environmental conditions improve.
PHYLUM ACRASIOMYCOTA
The cellular slime molds belong to. The mold is really a large collection of individual amoebalike protists which congregate into a “pseudo-plasmodium” or “slug” only when food is scarce. In this cooperative form, they produce a single stalk that releases spores.
Kingdom Fungi
Fungi are typically nonmotile and, like plants, have cell walls. Unlike plants, fungi are heterotrophic and have cell walls made of chitin rather than cellulose. Fungi secrete enzymes to digest their food externally and then absorb the nutrients. They usually live as decomposers, living off dead and decaying organisms, or as parasites, growing on or in other living organisms. With the exception of yeast, most fungi are multicellular. Structurally, multicellular fungi are composed of filaments called hyphae; some have hyphae that are segmented by divisions called septa, while others have a continuous cytoplasm with many nuclei in each hyphae. Many fungi exist as a tangle of hyphae, called a mycelium. Examples of fungi are yeast and mushrooms.
Most fungi can also exist in the form of a spore, a microscopic reproductive structure that is much more resistant to lack of food or water. Unlike most plants and animals, which exist predominantly in a diploid state, fungi spend most of their time in a haploid state, with only a brief diploid phase during the reproductive cycle.
Some fungi grow in a mutually beneficial relationship with a photosynthetic algae or plant. Lichen is an example of such a partnership between a fungus and an algae. The benefits of the merger are apparent: lichen can grow in a wider range of temperatures than any individual plant or fungus, and lichen can often colonize rocks that will not support any other multicellular life forms.
Kingdom Plantae
Plants are complex multicellular photosynthetic autotrophs, with cellulose in their cell walls and a waxy cuticle covering their aboveground parts. They are easily distinguishable from members of all other kingdoms, with the possible exception of their simpler ancestors in the Protista kingdom, the green algae. Over evolutionary time, plants improved their ability to live on land by developing a variety of important features. Plants can be divided into four major groups, displaying a progressively greater degree of adaptation to the terrestrial environment.
NONVASCULAR PLANTS—BRYOPHYTES
Bryophyta is the only phylum in the group of nonvascular seedless plants. These mosses and worts are the most primitive true plants. Because they lack a vascular system (vascular systems are discussed in much more detail in the section on Structure and Function of Plants, which is part of the Organismal Biology chapter), bryophytes do not have a stem, leaves, or roots; they must distribute water and nutrients throughout their bodies by absorption and diffusion. As a result, they cannot grow beyond a small size and must keep their bodies close to moist earth. Bryophytes reproduce by spores and need water in order to bring about fertilization. Because the male gamete is a flagellated sperm, reproduction requires water in which the sperm can swim. Unlike all other plants, which have a diploid adult stage, adult bryophytes are haploid, passing only briefly through a diploid phase during the reproductive cycle.
SEEDLESS VASCULAR PLANTS
There are three phyla of seedless vascular plants: Lycophyta (club mosses), Sphenophyta (horsetails), and, most likely to appear on the SAT II Biology, Pterophyta (ferns). Vascular plants have a dual fluid transport system: xylem transports water and inorganic minerals from the roots upward, and phloem transports sugars and other organic nutrients up and down. This vascular system represents a major evolutionary step in the adaptation to life on land. The ability to transport water and nutrients across long distances allows plants to grow much larger, sending specialized photosynthetic structures (leaves) upward toward sunlight and specialized root structures downward toward the water and minerals in the ground. Like bryophytes, seedless vascular phyla reproduce by spores and have flagellated sperm that require water in which to swim, limiting these plants to relatively moist environments.
FLOWERLESS SEED PLANTS—GYMNOSPERMS
The evolution of seeds provided plants with another advantage in their prolonged pilgrimage onto land. Unlike the spores of more primitive plants, seeds are multicellular, containing both a complete diploid embryo and a food supply. Having a food supply inside the seed provides the newborn plant with a period of growth that is independent of food resources in the environment. This independence allows seed plants to grow in a greater variety of environments. Further freeing seed plants, the male gametes of the seed plants take the form of pollen, making reproduction independent of water.
The seed plants that evolved first, called gymnosperms (“naked seeds”), do not produce flowers. Their seeds are exposed directly to the air, without any capsule or fruit enclosing them. The most important group of gymnosperms is phylum Coniferophyta; these plants, commonly called conifers, produce cones that carry seeds on their scales. Examples of gymnosperms are pines, firs, cedars, and sequoias.
FLOWERING SEED PLANTS—ANGIOSPERMS
Flowering plants, called angiosperms (“covered seeds”), are vascular seed plants with specialized reproductive structures, which include both flowers and fruit. Instead of depending on currents of wind or water for the dispersal of their gametes and seeds, plants with flowers and fruit provide protection and attract animals that then serve as the means of fertilization.
Flowering plants are divided into two classes, monocots and dicots. Monocot seeds have a single cotyledon, while dicots have two cotyledons in each seed. Monocots and dicots are covered in more detail in the section on the Structure and Function of Plants.
Kingdom Animalia
Animals are eukaryotic, multicellular, and heterotrophic. Animals also have specialized tissues to perform various functions. Most animals are motile, at least during part of their life cycle, reproduce sexually, and have nervous systems that allow them to respond rapidly to changes in their environment.
Taxonomists use several observable features to classify animals into groups according to their evolutionary relationships. One of the most important of these features is body symmetry. In bilateral symmetry, the left half of the organism is the mirror image of the right half, but the top does not resemble the bottom, and the front is dissimilar to the back. In radial symmetry, the organism has a circular body plan, with similar structures arranged like spokes on a wheel, such as a starfish. Most animals have three layers of cells: the ectoderm, mesoderm, and endoderm. Almost all animals have a hollow tube inside, which acts as a digestive tract; the opening where food enters is called the mouth, and the opening where digested material exists is called the anus.
Animals are the most diverse of the kingdoms. Any of their various phyla may come up on the SAT II Biology, though the vertebrates come up most often.
PHYLUM PORIFERA (SPONGES)
Sponges are sessile (nonmoving), complex colonies of flagellated unicellular protozoalike organisms. They do not exhibit any clear symmetry, and they are the only animal phylum that does not possess at least two distinct embryonic tissue layers. Their unique lack of tissue organization has prompted taxonomists to classify sponges as parazoa (“next to animals”). Nonetheless, some sponge cells are specialized for reproductive or nutritional purposes, and this slight organizational complexity gives them a toehold on the edge of the animal kingdom. Although sponges do have a hollow space inside, they do not have a digestive gut like other animals. Water flows into the central space through the many pores in the sponge’s outer surface and flows out through the large opening at the top of the sponge. The flow of water brings food and oxygen and carries away waste and carbon dioxide. All sponges secrete a skeleton that maintains their shape (you might use these skeletal remains as “natural sponges” in bathing).
PHYLUM CNIDARIA
Phylum Cnidaria includes all stinging marine organisms that exhibit radial symmetry, such as jellyfish, hydras, sea anemones, and coral. Cnidarians have a true digestive gut like other animals, but one opening serves as both the mouth and anus. Additionally, their body walls are made up of only two layers of cells: endoderm and ectoderm.
PHYLUM PLATYHELMINTHES (FLATWORMS)
Flatworms are bilaterally symmetric and are the most primitive animals to possess all three embryonic tissue layers. Like cnidarians, most flatworms have a digestive gut with only a single opening. Flatworms are also the most primitive animals to exhibit discernable organs, internal structures with at least two tissue layers and a specialized function. There are three main kinds of flatworms: free-living carnivorous planarians, parasitic flukes that feed off the blood of other animals, and parasitic tapeworms that live inside the digestive tracts of other animals.
PHYLUM NEMATODA (ROUNDWORMS)
Most nematodes, also called roundworms, are free-living; however, some live as parasites in the digestive tracts of humans and other animals. Soil-dwelling roundworms play an important ecological role by helping to decompose and recycle organic debris. Roundworms are bilaterally symmetric, have a complete gut tube with two openings, and possess all three embryonic tissue layers with a cavity in between the mesodermal and endodermal tissues. The roundworm species Caenorhabditis elegans was the first animal to have its entire genome sequence determined.
PHYLUM MOLLUSCA
Phylum Mollusca includes many familiar animals such as snails, slugs, squid, octopuses, and shellfish such as clams and oysters. Mollusks are bilaterally symmetric and have a complete digestive tract and a circulatory system with a simple heart. They move by means of a muscular structure called a foot, and they have a rasping tongue called a radula and a mantle that secretes a hard shell. Mollusks generally live in aquatic regions.
PHYLUM ANNELIDA (SEGMENTED WORMS)
Annelida means “ringed” and refers to the repeated ringlike segments that make up the bodies of annelids such as earthworms and leeches. Annelids exhibit bilateral symmetry have a complete digestive tract with two excretory organs called nephridia in each segment and a closed circulatory system. Their nervous system consists of a simple brain in front and a ventral (near the belly) nerve cord connecting smaller clusters of nerve cells, or ganglia, within each segment. Earthworms live freely within the soil, while most leeches, on the other hand, are bloodsucking parasites. All annelids must live in moist environments. Having not yet developed more sophisticated respiratory systems, they exchange gases directly with their surroundings.
PHYLUM ARTHROPODA
Arthropoda is the most diverse and numerous animal phylum. Insects, spiders, and crustaceans—which include lobsters, shrimp, and crabs—constitute the major arthropod groups. The name Arthropoda means “jointed feet”; arthropods have jointed appendages and, like annelids, exhibit segmentation. Insects and crustaceans have three body segments consisting of the head, thorax, and abdomen, while arachnids only have two body segments. Arthropods are unique among animals in having a hard exoskeleton made of chitin. The arthropod nervous system resembles the annelid nervous system, with a simple brain, a ventral nerve cord, and smaller ganglia within the various body segments. However, many arthropods have very highly developed sensory perception, including hearing organs, antennae, and compound eyes. Arthropods have an open circulatory system, a full digestive tract, and structures called Malphigian tubules to eliminate waste.
PHYLUM ECHINODERMATA
The name Echinodermata means “spiny skin,” and this phylum includes spiny marine animals such as starfish, sea urchins, and sand dollars, all of which exhibit radial symmetry. Echinoderms have several characteristic features, including an endoskeleton that secretes a spiny skin and an unusual vascular system of water-filled vessels that regulates the movement of their many tube feet and also permits the exchange of carbon dioxide for oxygen. Echinoderms have a very simple nervous system, with a ring of nerves around their mouth and no brain. Some echinoderms filter food out of the water, while others, like starfish, are carnivorous predators or scavengers. Despite their primitive appearance, patterns in early embryonic development strongly suggest that echinoderms are most closely related to the chordates, the animal phylum that developed most recently in evolutionary time.
PHYLUM CHORDATA
Human beings belong to Chordata, the phylum that evolved most recently in the animal kingdom. Chordates have three embryonic tissues, a complete digestive tract, and well-developed circulatory, respiratory, and nervous systems. Several features distinguish chordates from all other animal phyla. The primary feature, for which chordates are named, is the notochord, a tubular rod of tissue that runs longitudinally down the back. Just above the notochord runs a single, hollow nerve cord, the center of the nervous system. Other animals, such as earthworms, also have nerve cords; however, these run in ventral pairs along the belly and are not hollow. Two other features, gill slits and tails, are present in all chordates during embryonic development but disappear by adulthood in many members of the phylum.
There are two groups of chordates, subphylum Urochordata and subphylum Vertebrata. The former subphylum includes invertebrate marine animals such as tunicates and lancelets, and almost never appears on the SAT II Biology. Much more important for the test are the vertebrates.
Subphylum Vertebrata contains those chordates that have replaced the simple notochord with a segmented skeletal rod that wraps around and protects the brain and nerve cord. The skeletal segments, called vertebrae, are made of bone or cartilage, and the entire series of segments is called the vertebral column. The portion encasing the brain is called the skull. There are seven main classes of vertebrates.
JAWLESS FISH:
These fish are bottom-dwelling filter feeders without jaws. They breathe through gills and lay eggs. Examples are lampreys and hagfish.
CARTILAGINOUS FISH:
With a flexible endoskeleton made of cartilage, these fish have well-developed jaws and fins, and they breathe through gills. Their young hatch from eggs. Examples are sharks, eels, and rays.
BONY FISH:
Bony fish mark an advance since they have much stronger skeletons made of bone rather than cartilage. Bony fish are found in both salt water and fresh water. They breathe through gills and lay soft eggs. Almost every fish you can think of is a bony fish, from goldfish to trout.
AMPHIBIANS:
Amphibians such as frogs and salamanders embody the transition from aquatic to terrestrial living. Born initially as fishlike tadpoles living in the water, they undergo a metamorphosis and develop legs and move onto land as adults. Most adult amphibians breathe through lungs that develop during their metamorphosis, though some can breathe through their skin. Their eggs lack shells, must be laid in water, and receive little parental care.
REPTILES:
With the development of the fluid-filled amniotic sac, reptiles, including dinosaurs, were the first animals to be able to hatch their eggs on land and make the full transition to terrestrial life. Reptiles lay few eggs and provide some parental care. Reptiles also have thick, scaly skin that resists water loss and efficient lungs.
All classes of vertebrates that evolved before birds are cold-blooded (ectothermic). The metabolism of these earlier classes is dependent on the environment. When the temperature drops, their metabolism slows and speeds up as the temperature rises. Birds and mammals, in contrast, are warm-blooded (endothermic). They have developed structures such as feathers, hair, and fur to help them maintain body temperature. The metabolism of birds and mammals stays constant through far larger extremes of temperature, making these two classes much more versatile.
BIRDS:
Birds have specially evolved structures such as wings, feathers, and light bones that allow for flight. In addition, birds have four-chambered hearts and powerful lungs that can withstand the extreme metabolic demands of flight. Birds lay hard eggs but provide a great deal of care for their eggs and developing young.
MAMMALS:
Mammals have a number of unique features that have allowed them to adapt successfully to many different environments. They have the most highly developed nervous systems in the animal kingdom, providing them with complex and adaptable behaviors. With the exception of a few species such as the platypus, mammals do not lay eggs like other vertebrates; instead, mammalian embryos develop inside the mother and are not released until nearly or fully developed and equipped for survival. Mammals are also unique in having milk glands that provide nourishment for their infants. In this way, the protection and feeding of their young is built directly into mammalian bodies, dramatically increasing the ability of these animals to raise surviving offspring in diverse environments. Examples of mammals are whales, cows, mice, monkeys, and humans.
