Chapter 7: Evolution and Diversity
On your SAT II exam, you'll probably be faced with quite a few questions dealing with the evolution and classification of the millions of species on Earth. This chapter will give you the background you need to ace these questions. We'll be covering topics ranging from types of evidence for evolution to the taxonomic classification of various common species.
Evidence of Evolution
Evolution provides a sweeping framework for the understanding of the diversity of life on earth. Living systems, from the cell to the organism to the ecosystem, arose through a long process through geologic time, selecting solutions out of diverse possibilities. What is the evidence that supports the evolutionary view of life? The evidence takes several forms.
The Fossil Record
Fossils are preserved impressions or remains in rocks of living organisms from the past. Fossils provide some of the most direct and compelling evidence of evolutionary change and are generally found in sedimentary rock. When animals settle in sediments after death, their remains can be embedded in the sediment. These sediments then might be covered over with additional layers of sediment that turn to rock through heat and pressure over many millions of years. The embedded remains turn to stone, replaced with minerals that preserve an impression of the form of the organism, often in a quite detailed state. Most fossils are of the hard bony parts of animals, since these are preserved the most easily. Fossils of soft body parts or of invertebrates are much more unusual, probably since these parts usually decay before fossil formation can occur. In some cases, however it appears that animals died in anaerobic sediments that resisted decay to provide soft-body fossils.
One of the questions about fossils when they are discovered is their age, to place the fossil in correctly in the timeline of life on earth. One way to place the date is to compare the location of the fossil sediment to other sedimentary rock formations in which the age is already known. Dating using radioactive decay is also very useful. Carbon dating is frequently used for material that is only a few thousand years old, but cannot be used for older material since the decay rate of carbon is too rapid.
The conditions for fossil formation are relatively particular, especially for the preservation of invertebrates or soft body parts. Scientists locate fossils by luck, and overall can only look at a tiny percentage of possible fossil locations. They have over time located a great variety of fossils, including fossils that provide a clear story for the evolution of modern species. Archaeopteryx is an example of a feathered dinosaur that was probably an intermediate species in the evolution of birds. Changes in fossils over time have revealed a great deal about evolution and insight into the evolutionary paths that resulted in modern species including horses, whales, and humans. Any of the so-called "gaps" in the fossil record are probably the result of scarcity of fossils and difficulty in finding them, and is not evidence that evolution did not occur.
Comparative Anatomy
One way to find the evolutionary relationship between organisms is by examining their external and internal anatomy. Animals that evolved from a common ancestor might be expected to have anatomical features in common that they share with their common ancestor. Alternatively, two organisms might share features that look the same but evolved from different ancestors and resulted in similar structures as a result of similar functions. When we compare the anatomies of two or more living organisms, we can not only form hypotheses about their common ancestors, but we can also glean clues that shed light upon the selective pressures that led to the development of certain adaptations, such as the ability to fly. Comparative anatomists study homologous and analogous structures in organisms.
Homologous structures. Homologous structures have the same basic anatomical features and evolutionary origins. They demonstrate similar evolutionary patterns with late divergence of form due to differences in exposure to evolutionary forces. Examples of homologous structures include the wings of a bat, the flippers of a whale, the forelegs of a horse, and the arms of a human. These structures were all derived from a common ancestor but diverged to perform different functions in what is termed divergent evolution.
Analogous Structures. Analogous structures have similar functions but may have different evolutionary origins and entirely different patterns of development. The wings of a fly (membranous) and the wings of a bird (bony and covered in feathers) are analogous structures that have evolved to perform a similar function -- to fly. The wings of flies and birds might look the same but this does not indicate that they share a winged ancestor. The evolution of structures that look the same for a common function but are not derived from a common ancestor is called convergent evolution. Analogous organs demonstrate superficial resemblances that cannot be used as a basis for classification.
Comparative Embryology
Comparing the anatomy of adult organisms is one method used to derive evolutionary relationships, and comparison of embryonic structures and routes of embryo development is another way to derive these relationships. The development of the human embryo is very similar to the development of other vertebrate embryos. Adult tunicates (sea squirts) and amphibians lack a notochord, one of the key traits of the chordate phylum, but their embryos both possess notochords during development, indicating these animals are in fact vertebrates with a common evolutionary ancestor even though the adults do not resemble each other. The earlier that embryonic development diverges, the more dissimilar the mature organisms are. Thus, it is difficult to differentiate between the embryo of a human and that of an ape until relatively late in the development of each embryo, while human and flatworm embryos diverge much earlier.
Other embryonic evidence of evolution includes such characteristics as teeth in an avian embryo (recalling the reptile stage); the resemblance of the larvae of some mollusks (shellfish) to annelids (segmented worms), and the tail of the human embryo (indicating relationships to other mammals).
Molecular Evolution
If organisms are derived from a common ancestor, this should be evident not just at the anatomical level but also at the molecular level. The traits that distinguish one organism from another are ultimately derived from differences in genes. With the advent of molecular biology, the genes and proteins of organisms can be compared to determine their evolutionary relationship. The closer the genetic sequences of organisms are to each other, the more closely related they are in evolution and the more recently they diverged from a common ancestor.
Some genes change rapidly in evolution while others have changed extremely slowly. The rate of change in a gene over time is called the molecular clock. The rate of change in a gene's sequence is probably a function of the tolerance of the gene to changes without disrupting its function. Genes that change very slowly over extremely long periods of time probably do not tolerate change very well and play key roles in the life of cells and organisms. The large ribosomal RNA has changed slowly enough that it can be used to compare organisms all the way back to the divergence of eukaryotes, bacteria, and archaebacteria. The enzymes of glycolysis play an essential role in energy production for all life, and also evolve very slowly, allowing comparison of their gene sequences to illuminate evolutionary relationships over billions of years. Fossil genes are not known, but using computers to compare the gene sequences of many organisms allows researchers to determine how long ago organisms evolved from a common ancestor. More recently evolved genes and genes that evolve more rapidly can be used to compare more recent evolutionary events.
Vestigial Structures
Vestigial structures are structures that appear to be useless in the context of a particular modern-day organism's behavior and environment. It is apparent, however, that these structures used to have some function in an earlier stage of a particular organism's evolution. They serve as evidence of an organism's evolution over time, and can help scientists to trace its evolutionary path.
There are many examples of vestigial structures in humans, other animals, and plants. The appendix -- small and useless in humans -- assists digestion of cellulose in herbivores, indicating human's vegetarian ancestry, while the animal-like tail in humans is reduced to a few useless bones (coccyx) at the base of the spine. The splints on the legs of a horse are vestigial remains of the two side toes of the eohippus. Finally, adult pythons have legs that are reduced to useless bones embedded in their sides, as do whales.
Mechanisms of Evolution
The Population as the Basic Unit of Evolution
Evolution is the change in a species over time. These changes are the result of changes in the gene pool of a population of organisms. Evolution does not happen in one individual, but in a population of a organisms. What is a population? A population is a group of individuals of a species that interbreed. In classical genetics, it is observed that the genotype of organisms produces their phenotype, the physical expression of inherited traits. A population of organisms includes individuals with a range of phenotypes and genotypes. It is possible, however, to describe a population not by their individual characteristics, but by certain traits of the group as a whole, including the abundance of alleles within the whole population. The sum total of all alleles in a population is called the gene pool and the frequency of a specific allele in the gene pool is called the allele frequency. Each individual receives their specific set of alleles from the gene pool, and not every individual receives the same alleles, leading to individual variation in genotypes and phenotypes.
How is the allele frequency calculated and used? For a diploid organism, the total number of alleles for a specific gene in a population is the number of individuals multiplied by 2. If there are 1,000 rabbits in a population, and they are diploid, with two copies of every gene, then the gene pool of the rabbit population will include 2,000 alleles for genes. If the genotype of every rabbit is known, then the allele frequency in the population can be determined by adding up how many copies of each allele are found in the population. If 100 rabbits are homozygous for an allele (both gene copies are the same) and another 200 rabbits in the population are heterozygous for the same allele, then the allele frequency = (2(100) + 1(200))/2,000. Allele frequency is the decimal fraction representing the presence of an allele for all members of a population that have this particular gene locus. The letter p is used for the frequency of the dominant allele of a particular gene locus. The letter q represents the frequency of the recessive allele. For a given gene locus, p + q = 1. The total allele frequency for a gene must always equal one.
Sexual reproduction constantly shifts alleles around in a population, mixing and remixing them in new combinations through meiotic recombination, independent segregation of chromosomes during meiosis, and random matching of alleles from parents during mating and fertilization. All of these allow for mixing of alleles in a population to create variation in individual genotypes and phenotypes. Mutation in a population can create new alleles. Evolution is caused by changes in the gene pool of a population over time, as a result of changes that occur to individuals in the population caused by their phenotype and the alleles they carry.
Hardy-Weinberg and Population Changes
The allele frequencies in the gene pool of a population determine how many individuals in a population get each allele and this in turn determines the phenotypes of individuals. If nothing changes the allele frequencies, then every generation will get the same alleles in the same proportions, and the population will not change over time. This idea is the foundation of population genetics and the central idea of Hardy-Weinberg equilibrium in population genetics.
According to the Hardy-Weinberg principle, allele frequencies in a population remain constant from generation to generation and a population is maintained in equilibrium as long as certain assumptions are met. If the assumptions are met, and the allele frequencies in the gene pool of a population are constant over time, the population does not change and evolution does not occur. If the assumptions are not true, then the allele frequencies of the population will change and the population will evolve. The assumptions for Hardy-Weinberg equilibrium to be maintained are:
- Random mating (no isolation) must occur, so that no particular trait is favored. There can be no assortative mating (in other words, no organisms may select mating partners that resemble themselves).
- Migration -- that is, immigration or emigration -- cannot take place.
- There must be no total mutations.
- Large populations are required. As in all cases of probability, large samplings are needed to provide an accurate approximation of the expected occurence. In addition, it must be highly unlikely that chance alone could significantly alter gene frequencies.
- Natural selection does not occur.
Under the above conditions, there is a free flow of alleles between members of the same species, while the total content of the gene pool is continually being shuffled. A constant gene pool is nevertheless maintained for the entire species. The constancy of the gene pool is always threatened by changes in the environment (which would favor certain genes), mutations, migrations (new genes introduced), or reproductive isolation (lack of
random mating favors certain genes).
Disruption of Hardy-Weinberg Equilibrium in Evolution
The Hardy-Weinberg principle describes the stability of the gene pool. However, no population stays in Hardy-Weinberg equilibrium for very long, because the stable, ideal conditions needed to maintain it do not exist. The assumptions required for equilibrium cannot be met in the real world.
As conditions change, the gene pool changes and the population changes. Changes in the gene pool caused by breaking the assumptions are the basis of evolution.
Mutation
If the gene pool is not going to change, then there can be no new alleles that appear in the population. Mutations may be infrequent in a population as a result of the great accuracy of DNA replication, but DNA replication is never perfect and some mutations will occur at least infrequently. Radiation from the environment and environmental mutagens also contribute a low by inescapable level of mutation in any population. The mutations will not form a large part of allele frequency, but they do form an important component, as a source of variation in a population. Most mutations are harmful, but a small minority may confer a selective advantage in some way. Phenotypes are the material that evolution acts on in a population and mutations are the only source of truly new alleles that will result in truly new phenotypes.
Migration
If two populations are separated from each other and do not interbreed, then the allele frequencies in their gene pools may be different from each other. If individuals move between the populations however, carrying their alleles with them, this creates gene flow, and will alter the frequency of alleles in both of the populations involved.
Population Size
One of the assumptions for the maintenance of Hardy-Weinberg equilibrium is that a population is large. Small populations are subject to random events that can statistically alter the gene pool. Changes in the gene pool caused by random events in a small population are called genetic drift. One example is a population bottleneck. If an event like a flood suddenly and dramatically reduces the size of a population, the allele frequencies of the survivors are not necessarily the same as the allele frequencies in the original population. When the population grows in size again, the allele frequencies in the new gene pool will represent the frequencies in the small bottleneck population, not the population before the reduction in size. A similar phenomena called the founder effect occurs in the colonization of a new habitat. When a new island forms, it might be colonized by a very small number of individuals from another population. Since the new population is founded by a small number of individuals, it is unlikely statistically that the island population will represent the same allele frequencies as the population they were derived from.
Nonrandom Mating
If a population is going to maintain constant allele frequencies, then alleles must be matched randomly in each new generation. This requires that individuals mate with each other without any preference for specific traits or individuals. If the phenotype of individuals influences mating, this will change allele frequencies and disrupt Hardy-Weinberg equilibrium. Most species are quite discerning in mate selection, however, blocking maintenance of Hardy-Weinberg equilibrium.
Natural Selection
Within a population of organisms, individuals are non-identical. Mutation is a source of new alleles, and sexual reproduction leads to constant shuffling of alleles in new genotypes. The variety of genotypes created in a population in this way creates a variety of phenotypes. If individuals have different phenotypes, then these individuals probably interact with their environment with differing degrees of success in escaping predators, finding food, avoiding disease, and reproducing. The differential survival and reproduction of individuals based on inherited traits is natural selection as described first by Charles Darwin.
Fitness is a quantitative measure of the ability to contribute alleles and traits to offspring and future generations. The key to fitness is reproduction and survival of offspring. Avoiding predators, finding food, resistance to disease and other factors that improve survival are likely to improve fitness but only to the extent that they lead to more offspring and more of the alleles involved in the future gene pool. Finding a mate, mating, successful fertilization, and caring for offspring are factors that can improve fitness as well. There are different strategies for improving fitness. For example, some animals have lots of offspring but provide little parental care, while other animals have few offspring but provide lots of care for each of them.
None of the other factors that alter Hardy-Weinberg equilibrium alter it in a directed fashion. Genetic drift, mutation, and migration are all random in their effects on the gene pool. Natural selection, however, increases the prevalence of alleles in a population that increase survival and reproduction. Alleles that increase fitness will over time increase in their allele frequency in the gene pool, and increase the abundance of the associated phenotype as well. This effect will change the population in a directed manner over many generations, creating a population that is better adapted to its environment.
Different types of natural selection can occur, including stabilizing selection, disruptive selection and directional selection. Traits in a population such as the height of humans are often distributed according to a bell-shaped curve. The type of selection that occurs can affect the average value for the trait or it can alter the shape of the curve around the average. Stabilizing selection does not change the average, but makes the curve around the average sharper, so that values in the population lie closer to the average. For example if both very small fish and very large fish tend to get eaten, then stabilizing selection may not alter the average fish size, but is likely to cause future generations to be closer to average.
Disruptive selection is the opposite, in which the peak value is selected against, selecting for either extreme in a trait, so that a single peak for a trait in a population tends to be split into two peaks. Directional selection alters the average value for a trait, such as selecting for dark wings in a population of moths in an industrial area.
Natural selection acts on an individual and its direct descendants. In some cases natural selection can also act on closely related organisms that share many of the same alleles. This type of natural selection, called kin selection, occurs in organisms that display social behavior. The key to fitness is that an organism's alleles are contributed to the next generation. Contribution of alleles can happen by an individual or by close relatives like siblings, aunts, uncles, etc, who share many of the same alleles. The evolution of social organisms is the result of the increased fitness that social behavior provides. Described cases of altruistic behavior in animals is probably the result of kin selection at work, in which an animal might sacrifice its own safety to allow relatives to survive, thereby increasing the fitness of itself and the whole social group it shares alleles with.
Speciation
A species is a group of organisms that is able to successfully interbreed with each other and not with other organisms. The key to defining a species is not external appearance. Within a species, there can be great phenotypic variation, as in the domestic dog. What defines a species is reproductive isolation, an inability to interbreed and create fertile offspring. Actual interbreeding is not necessary to make organisms the same species. Two groups of animals may live in different locations and never contact each other to interbreed, but if a researcher transports some of the animals and they create fertile offspring, they are part of the same species. Horses and donkey can interbreed and create offspring, the mule. The mule, however, is sterile, meaning the horses and donkeys are two different species.
Speciation, the creation of a new species, occurs when the gene pool for a group of organisms becomes reproductively isolated. At this point, evolution can act on that group that shares a gene pool separately from all other life on earth. Two species can be derived from a single common ancestor species. In many cases this occurs when two populations of a species are separated geographically through a process known as allopatric speciation.
Separation of a widely distributed population by emerging geographic barriers causes each population to evolve specific adaptations for the environment in which it lives, in addition to the accumulation of neutral (random, nonadaptive) changes. These adaptations will remain unique to the population in which they evolve, provided that interbreeding is prevented by the barrier. In time, genetic differences will reach the point where interbreeding becomes impossible and reproductive isolation would be maintained if the barrier were removed. In this manner, geographic barriers promote evolution.
Adaptive radiation is the production of a number of different species from a single ancestral species. Radiation refers to a branching out; adaptive refers to the hereditary change that allows a species to be more successful in its environment or to be successful in a new environment. Whenever two or more closely related populations occur together, natural selection favors evolution of different living habits. This results in the occupation of different niches by each population (this process is discussed in detail in our chapter on ecology). This divergent evolution through adaptive radiation has been an extremely frequent occurrence, as demonstrated by the famous example of Darwin's finches.
Lamarckian Evolution
Until it was supplanted by Darwin's ideas, the scientist Lamarck's theory was one of the more widely accepted explanations of the mechanisms of evolution. The cornerstone of Lamarck's hypothesis was the principle of use and disuse. He asserted that organisms developed new organs, or changed their existing ones, in order to meet their changing needs. The amount of change that occurred was thought to be based on how much or little the organ in question was actually used.
Unfortunately for Lamarck, this theory of use and disuse was based upon a fallacious understanding of genetics. Any useful characteristic acquired in one generation was thought to be transmitted to the next. An oft-cited example was that of early giraffes, which stretched their necks to reach for leaves on higher branches. The offspring were believed to inherit the valuable trait of longer necks as a result of their parents' excessive use of their necks. Modern genetics has disproved this concept of acquired characteristics.
It has now been established that changes in the DNA of sex cells are the only types of changes that can be inherited; because acquired changes are changes in the characteristics and organization of somatic cells, they cannot be inherited.
Classification and Diversity
Evolution has created a great diversity of organisms on earth, but these organisms are related to each other through common ancestors they shared in the history of life. By examining organisms for common features and common ancestors, it should be possible to make sense of the diversity of life by grouping organisms into categories together. The science of classifying living things and using a system of nomenclature to name them is called taxonomy. Carolus Linnaeus invented modern taxonomy in the 1700s, grouping organisms and naming them according to a hierarchical system.
A modern classification system seeks to group organisms on the basis of evolutionary relationships. The bat, whale, horse, and human are placed in the same class of animals (mammals) because they are believed to have descended from a common ancestor. The taxonomist classifies all species known to have descended from the same common ancestor within the same taxonomic group.
Since much about early evolutionary history is not understood, there is some disagreement among biologists as to the best classification to employ, particularly with regard to groups of unicellular organisms. Taxonomic organization proceeds from the largest, broadest group to the smaller, more specific subgroups. The largest group, or kingdom, is broken down into smaller and smaller subdivisions. Each smaller group has more specific characteristics in common. Furthermore, each subgroup is distinguishable from the next. The naming system is subject to discussion and revision as research yields new insights over time into the relationship between organisms. Some classifications are clearer than others.
Viruses are obligate intracellular parasites that cannot conduct metabolic activities or replicate on their own. As such, they are not generally considered living, although they are certainly important to living systems. They are not classified within this taxonomy however.
Classification and Subdivisions
Each kingdom has several major phyla. A phylum or division has several subphyla or subdivisions, which are further divided into classes. Each class consists of many orders, and these orders are subdivided into families. Each family is made up of many genera. Finally, the species is the smallest subdivision.
Hence the order of classificatory divisions is as follows:
KINGDOM -- > PHYLUM -- > SUBPHYLUM -- > CLASS -- > ORDER -- > FAMILY -- > GENUS -- > SPECIES
The complete classification of humans is:
Kingdom: Animalia Phylum: Chordata
Subphylum: Vertebrata
Class: Mammalia
Order: Primates
Family: Hominidae
Genus: Homo
Species: Sapiens
Assignment of Scientific Names
All organisms are assigned a scientific name consisting of the genus and species names of that organism. Thus, humans are Homo sapiens, and the common housecat is Felis domestica.
One of the primary groupings of all living organisms separates prokaryotes from eukaryotic organisms. The prokaryotes include bacteria, and another type of organisms called archaebacteria. Like the bacteria, archae have no organelles and have a simple circular DNA genome. Archae were relatively unknown until recently, and tend to inhabit harsh environments like hot springs that might resemble the early earth. Archae are distinct from bacteria in many ways such as the composition of their membrane lipids, and in some ways appear to be related more closely to eukaryotes than prokaryotes. For this reason, more recent classification schemes break all living things into three domains, groups at a higher level than kingdom: bacteria, archaebacteria, and eukaryotes.
Bacteria
The ubiquitous bacteria are single-celled, lack true nuclei, lack a cytoskeleton and contain double-stranded circular chromosomal DNA that is not enclosed by a nuclear membrane. These creatures nourish themselves heterotrophically -- either saprophytically or parasitically -- or autotrophically, depending upon the species. Bacteria are classified by their morphological appearance: cocci (round), bacilli (rods), and spirilla (spiral). Some forms are duplexes (diplococci), clusters (staphylococci), or chains (streptococci).
Bacteria have cell walls made of peptidoglycan, a specialized matrix of carbohydrates and peptides. A method of staining bacteria called Gram staining separates them into two groups according to the strength of the staining of their cell wall: Gram positive and Gram negative. Gram positive cells have a thick peptidoglycan cell wall that stains strongly while Gram negative cells have a thin cell wall and an outer membrane that stains poorly.
The Protist Kingdom
The simplest eukaryotic organisms are the protists. Protists probably represent the evolution between prokaryotes and the rest of the eukaryotic kingdoms, including fungi, plants and animals. Most, but not all, protists are unicellular eukaryotes. One way to define the protists is that this group includes organisms that are eukaryotes but are not plants, animals, or fungi. The protists include heterotrophs like amoeba and paramecium, photosynthetic autotrophs like euglena and algae, and fungilike organisms like slime molds. Some protists are mobile through the use of flagella, cilia, or amoeboid motion. Protists use sexual reproduction in some cases and asexual reproduction in others.
One of the best known types of protists are the amoebas. Amoebas are large single-celled organisms that do not have a specific body shape. They move and change their shape through changes in their cytoskeleton and streaming of cytoplasm within the cell into extensions called pseudopods. Amoebas are heterotrophs that feed by engulfing a food source through phagocytosis, internalizing the food to digest it in vacuoles in the cytoplasm.
Ciliates are another well-known group of protists and are complex single-cell organisms, including paramecium. The surface of ciliates is covered with cilia that beat in a coordinated fashion to move the cell through water. Ciliates have a defined shape and contractile vacuoles that visibly beat under the microscope to remove excess water from the cell that enters through osmosis. Food (yeast cells in the case of paramecium) is internalized through an oral groove where it is internalized in digestive vacuoles. Paramecia reproduce mitotically, but also have a mechanism called conjugation for exchange of genetic material between cells.
Slime molds are an interesting group of organisms that in some cases are grouped with the fungi kingdom. They are heterotrophs, with some slime molds spending some of their time as independent cells, but at other times gathering together to form multicellular forms that produce spores.
Algae are an important group of photosynthetic protists, mostly unicellular. Algae include diatoms, single celled organisms with intricate silica shells; dinoflagellates, with flagella; and brown algae. Algae can reproduce sexually and with alternation of generations between diploid sporophytes and haploid gametophytes, as occurs in plants. The algae include large multicellular forms like giant kelp that might be grouped with the protists since they are an algae, but are also grouped with plants by others. It is likely that the plants evolved from one group of algae, the green algae.
The Fungi Kingdom
Fungi are heterotrophs that absorb nutrients from the environment. Fungi are often saprophytic, feeding off of dead material as their nutrition source, and are important along with bacteria in the decay of material in ecosystems. Absorptive nutrition involves the secretion of enzymes that digest material in the extracellular environment, followed by absorption of the digested material back into the cell. One of the distinguishing features of fungi is their cell wall made of chitin, unlike the cellulose found in plants. Fungi often form long slender filaments called hyphae. Mushrooms, molds and yeasts are all examples of fungi.
Most fungi reproduce both sexually and asexually. Asexual reproduction can occur through the production of haploid spores or through splitting of a piece of fungus that grows mitotically into a new organism. Sexual reproduction in fungi does not involve distinct male or female sexes, but multiple mating types that are not distinct in their morphology. Fungi spend most of their life cycle in a haploid form. Fertilization of haploid gametes forms a diploid zygote that usually quickly enters meiosis to produce haploid spores that can grow mitotically into mature haploid fungi.
The Plant Kingdom
Plants are multicellular eukaryotes that produce energy through photosynthesis in chloroplasts, using the energy of the sun to drive the production of glucose. A cell wall of cellulose is a common feature of all plants, along with a life cycle featuring alternation of generations between haploid gametophytes and diploid sporophytes. The sporophyte is a diploid form that makes haploid spores that grow into a complete haploid form, the gametophyte. The gametophyte in turn is a haploid form that produces haploid gametes that unite through fertilization during sexual reproduction. The resulting diploid zygote grows into the mature sporophyte once again.
Plants are distinct from animals in that plants are usually nonmotile while animals are heterotrophic and move. Plant structure is adapted for maximum exposure to light, air, and soil by extensive branching; animals, on the other hand, are adapted usually in compact structures for minimum surface exposure and maximum motility. Animals have much more centralization in their physiology while plants often exhibit delocalized control of processes and growth.
The evolution of plants has included the ongoing increase in the ability to conquer land through resistance to gravity and ability to tolerate drier conditions. The first plants probably evolved from green algae in or near shallow water. These first plants were nontracheophytes, without water transport systems called vascular systems. Nontracheophytes include mosses. They also lack woody stems. Their lack of a vascular system restricts their size and generally restricts their range to very moist environments. In non-tracheophytes, the sporophyte is larger than the gametophyte.
The evolution of vascular systems was a major adaptation in plants. The first vascular plants, tracheophytes that did not produce seeds, included ferns and horsetails, plants with cells called tracheids that form tubes for the movement of fluid in the plant tissue called xylem. This vascular system also helps to provide rigid stems that plants need to live on land. These plants colonized land about 400 million years ago, making it possible for animals like arthropods to colonize land soon after. The non-seed tracheophytes dominated the land for 200 million years before plants with seeds appeared. Ferns form large sporophytes, which release haploid spores. These spores grow into gametophytes that produce haploid gametes. Fertilization in ferns requires sperm to swim through water, restricting most ferns to moist environments.
The evolution of the seed was the next major event in plant evolution, found first in the gymnosperms and later in the flowering plants, the angiosperms. The seed is a young sporophyte that becomes dormant early in development. The embryo is usually well-protected in the seed and able to survive unfavorable conditions by remaining dormant until conditions become more favorable again and the embryo begins to grow again, sprouting. In some cases seeds can remain viable for many years, waiting for the right conditions for the sporophyte to grow. This increases the ability of plants to deal with the variable conditions found on land. Seeds are produced as the result of fertilization of male and female gametes produced by male and female gametophytes. In the seed plants, the male and female gametophytes are small.
The conifers are the most abundant gymnosperms today. About 200 million years ago gymnosperms replaced the nonseed vascular plants like ferns as the predominant plant forms on land. Pines and other conifers are large diploid sporophytes. Gymnosperms like conifers produce male and female spores in separate cones. The male cones make the male gametophytes, which are pollen grains. In conifers, pollen grains are usually dispersed by the wind to find female cones. Unlike ferns, conifers do not require male gametes to swim in water to find the female gametes to fertilize, an adaptation that allows these plants to live in drier environments. When the pollen grain finds the female cone, they grow pollen tubes from the pollen grain to the female gametophyte that contains the eggs of the plant. Male gametes swim through the pollen tube to the eggs to fertilize them and create the diploid zygote that will grow into a seed and eventually another mature plant sporophyte.
Following the evolution of the seed, the next big innovation in plant evolution was the flower. The angiosperms represent the flowering plants and are today the predominant plant group in many ecosystems. Like the gymnosperms, angiosperms produce seeds. The seeds of gymnosperms are "naked," growing without a large amount of nutritional material or protective tissues. Angiosperms produce flowers for fertilization and produce seeds that are surrounded by nutritional tissues. Angiosperm development involves a double fertilization. One of the fertilizations involves the fertilization of an egg by one sperm that grows into the embryo. The other fertilization involves the fertilization of two female nuclei by one sperm to form a triploid tissue that grows to form the nutritive component of the seed, the endosperm. When the seed embryo germinates, it first gains nutrition from the endosperm.
The Animal Kingdom
Animals are fairly easy to recognize: animals are all multicellular heterotrophs. The evolution of animals has included the evolution of a variety of body plans to solve problems like getting food, avoiding predators, and reproducing. Over time, animals have tended to become larger in size, and more complex, with greater specialization of tissues. Another trend in the animal kingdom has been the evolution of increasingly complex nervous systems to enable complex behaviors in response to the environment.
Different groups of animals have evolved different body shapes, reflecting their different life styles. Animals with radial symmetry are organized with their body in a circular shape radiating outward. The echinoderms like sea stars and the cnidarians like jellyfish are examples of animals with radial symmetry. Another common body plan is bilateral symmetry, in which the body has a left side and a right side that are mirror images of each other. Humans are a good example of bilateral symmetry, in which a plane drawn vertically through the body splits the body into left and right sides that look the same. The front of the body, where the head is located, is the anterior, and the rear of the animal is the posterior. The back of the animal, where the backbone is located in vertebrates, is the dorsal side (like the dorsal fin) and the front of the animal is the ventral side.
The method used to capture food is intimately tied to their body shape. Some animals that do not move are called sessile. These animals gather food by filtering it from the environment. Examples of sessile filter feeders include sponges and cnidarians. This is a highly successful life style that requires little energy to gather food, waiting instead for the food to come to you, but animals with this lifestyle are at the mercy of their environment and must compete for space and resources. Animals with more active life styles have evolved increasingly complex nervous systems and motor systems to enable them to navigate their environment.
During the early stages of animal development, immediately after fertilization, the embryo enters into several rapid cycles of cell division that split the zygote into increasing smaller cells. In some animals called protostomes the cells in the early embryo divide in a spiral pattern while in deuterostome animals, cells are cleaved in a radial pattern. The protostomes include annelids, arthropods, mollusks, and roundworms, while the deuterostomes include the echinoderms and chordates. These divisions reflect one of the major evolutionary divisions in the animal kingdom.
The body cavity in animals has evolved over the history of animals. The body cavity is the area between the gut and the exterior of the animal. Early animals like flatworms have only solid tissue between the gut wall and the exterior surface. Other animals like annelids and chordates have evolved a cavity called the coelom between the gut and the exterior wall. The coelom is lined with muscle both around the gut and the interior wall. The organs of the chest and abdomen in chordates reside with the coelom. The coelom in annelids makes coordinated motion with a hydrostatic skeleton possible.
The phyla that are described here do not include all of the animal phyla, but most of the more abundant and important phyla.
Phylum Porifera (sponges). Animals probably evolved from simple colonial heterotrophic protists with groups of cells living together and starting to specialize for different functions. These simple animals, probably representing the first evolutionary step between protists and animals, are the sponges, phylum porifera. Sponges resemble a colonial organism, with only a small amount of specialization of cells within the animal, no organs, and distributed function. Sponges usually only have a few different types of cells, no nervous system, and if broken apart can reassemble into new sponges. With a saclike structure, sponges have flagellated cells that move water into the animal through pores into a central cavity and back out again. Cells lining the cavity capture food from water as it moves past.
Phylum Cnidaria (hydra, sea anemone, jelly fishes). Cnidarians, also called coelenterates, have radial symmetry, with tentacles arranged around a simple gut opening. Their gut has only one opening to the environment through which food passes in and wastes pass out. They are aquatic animals and represent one of the earliest phyla of animals in evolution, with only two cell layers, the endoderm and ectoderm. With only two cell layers, cnidarians do not need circulatory or respiratory systems. These animals have a simple nerve net to respond to the environment, a decentralized system for simple responses to the environment such as retraction of tentacles or swimming motions with the body. The tentacles contain one of the trademarks of cnidarians, stinging cells called nematocysts that have a harpoon-like structure toxins to capture prey. The life cycle of cnidarians can include a polyp stage and a medusa stage. The polyp is settled on a solid surface, with the mouth opening pointed upward while the medusa is a swimming form, with the mouth opening pointed downward. Polyps are asexual. Sea anemones on a rock in a cluster are often clones of each other that have reproduced by budding, competing for space with other clones. Sexual reproduction occurs in medusa, where sperm and eggs are produced and released into the environment for fertilization.
Phylum Platylhelminthes (flatworms). Flatworms are ribbonlike with bilateral symmetry. They possess three layers of cells, including a solid mesoderm but lack a circulatory system. Their nervous system consists of simple light detection organs, an anterior brain ganglion, and a pair of longitudinal nerve cords. Their digestive system is a cavity with a single opening, and they lack a coelom. These animals are not swift moving, using cilia to move over surfaces. A common flatworm is the planaria, famous for its regeneration. The shape of the worm, elongated and without appendages, has evolved in many phyla, as a compact structure that is well designed for movement. Planaria are free-living but many flatworms are internal parasites, including flukes and tapeworms, deriving their nutrition by direct absorption into their cells from the host.
Phylum Nematoda (roundworms). Nematodes are roundworms, with three cell layers, including mesoderm, a complete digestive tract with two openings, a mouth and an anus, and a body cavity called a pseudocoelom around the gut. The pseudocoelom has muscle lining the interior body wall but not around the gut. This allows for very active movement but more of a wiggling motion than movement in a specific direction. Nematodes do not have respiratory or circulatory systems, exchanging gases directly with the environment. Roundworms are one of the most abundant animal groups, including huge numbers of free-living, scavenging species as well as parasites. The species C. elegans is a simple organism with only 950 cells that has made it popular in modern biology for studies of cell differentiation and genetics.
Phylum Annelida (segmented worms). The earthworm and leaches are examples of annelid worms, commonly called segmented worms. The annelids are worms with segmented bodies and a coelom body cavity. The division of the body into segments and the presence of the coelom body cavity filled with water creates a hydrostatic skeleton that allows annelids complex, sophisticated movement, coordinated by a nervous system. Each segment has local control by a ganglion of nerves but these nerves are coordinated by a ventral nerve cord and a larger nerve collection that might be called a brain in the front of the worm. Annelids exchange gases directly with their environment through their skin, an important reason why they have moist skin and live in moist environments. Each segment has a twin set of excretory organs called nephridia. Annelids have a complete digestive tract with some specialization into organs along the tract and they also have a closed circulatory system with five pairs of hearts.
Phylum Arthropoda. Arthropods have jointed appendages, exoskeletons of chitin, and open circulatory systems. With an exoskeleton, the coelom of arthropods is reduced and less important in movement. The three most important classes of arthropods are insects, arachnids, and crustaceans. The exoskeleton of arthropods has muscles attached to their interior for movement. The exoskeleton provides protection, and has a variety of specialized appendages. The exoskeleton prevents gas exchange between the skin and exterior, however, as occurs in annelids, requiring the evolution of a respiratory system. Insects possess three pair of legs, spiracles, and tracheal tubes designed for breathing outside of an aquatic environment. Arachnids have four pair of legs and "book lungs"; examples include the scorpion and the spider. Most arthropods have complex sensory organs, including compound eyes, that provide information about the environment to their increasingly complex nervous systems. Crustaceans have segmented bodies with a variable number of appendages. Crustaceans like the lobster, crayfish, and shrimp possess gills for gas exchange. The exoskeleton of arthropods allowed them to colonize land and become the first winged organisms as well. Arthropods, particularly insects, remain one of the most abundant and varied groups of organisms on earth.
Phylum Mollusca. The mollusks include animals like clams, squid, and snails. In their body shape these animals do not resemble each other very much but they do share a few basic traits that lead biologists to classify them together as mollusks. These shared mollusk traits include a muscular foot, a mantle that secretes a shell, and a rasping tongue called the radula. Most mollusks are covered by a hard protective shell secreted by the mantle. Mollusks are mostly aquatic and use gills for respiration that are enclosed in a space created by the mantle, the mantle cavity. The gills are also involved in feeding, and move water over their surface with the beating of cilia.
Phylum Echinodermata. The echinoderms, which include sea stars and sea urchins, are spiny and have radial symmetry, contain a water-vascular system, and possess the capacity for regeneration. The echinoderms may not resemble the vertebrates but they share in common with chordates that they are deuterostomes. The water vascular system is a unique adaptation of the echinoderms, with a network of vessels that carry water to extensions called tube feet. The tube feet are the small suckerlike extensions in sea stars, sea urchins and sand dollars that allow the animals to adhere and to move. Echinoderms also have a hard internal skeleton formed from calcium deposits that assists in protection and locomotion.
Phylum Chordata. The chordates have a stiff, solid dorsal rod called the notochord at some stage of their embryologic development, as well as paired gill slits. Chordata have dorsal hollow nerve cords, tails extending beyond the anus at some point in their development, and a ventral heart. These adaptations may not sound impressive but they paved the way for the evolution of the vertebrates, a major subphylum of chordates.
The chordates probably originated from animals like tunicates, commonly called sea squirts. Adult tunicates are sessile filter feeders that do not resemble vertebrates at all. Tunicate larvae however are free-swimming, with a notochord and a dorsal nerve cord, and resemble tadpoles.
The vertebrates are a subphylum of the chordates that includes fish, amphibians, reptiles, birds and mammals. In vertebrates the notochord is present during embryogenesis but is replaced during development by a bony, segmented vertebral column that protects the dorsal spinal cord and provides anchorage for muscles. Vertebrates have bony or cartilaginous endoskeletons, chambered hearts for circulation, and increasingly complex nervous systems. The vertebrate internal organs are contained in a coelom body cavity.
The first vertebrates were probably filter-feeding organisms that evolved into swimming jawless fishes that were still filter feeders. Jawless fish such as lampreys still exist today. The evolution of fish with jaws led to the development of the cartilaginous and bony fishes that are dominant today. These fish use gills for respiration, and move water over the gills through paired gill slits. The jaw allows fish to adopt new life styles other than filter feeding, grabbing food with their jaws. Cartilaginous fish (class Chondrichthyes) like sharks and rays have an endoskeleton that is made entirely of cartilage rather than hard, calcified bone. Bony fishes (class Osteichthyes) have swim bladders for the regulation of buoyancy in water.
Two adaptations were important to set the stage for vertebrates to colonize the land. One was the presence of air-sacs that allowed some fish in shallow water to absorb oxygen from air for breath periods. The other adaptation was a change in the structure of fins to have lobes that allowed some degree of movement on land. Fish with these features evolved into the amphibians about 350 million years ago. Most amphibians like frogs and salamanders still live in close association with water and have only simple lungs or gills supplemented by oxygen absorbed through the skin. Another reason that amphibians are mostly associated with water is that amphibian eggs lack hard shells and will dry out on land. Amphibian larvae often live in water and then metamorphose into the adult form.
Reptiles became independent of water for reproduction through the evolution of hard-shelled eggs that do not dry out on land. The egg shell protects the developing embryo but still allows gas exchange with the environment. Reptiles also evolved more effective lungs and heart and thicker dry skins to allow them a greater metabolic activity than amphibians and the ability to survive on land.
Birds evolved from reptilian relatives of dinosaurs with the development of wings, feathers, and light bones for flight. Birds also have four-chambered hearts and uniquely adapted lungs to supply the intense metabolic needs of flight. Birds have hard-shelled eggs and usually provide a great deal of parental care during embryonic development and maturation after hatching. A famous evolutionary intermediate from the fossil record is Archaeopteryx, which is dinosaurlike in some respects, but had feathers and wings.
Mammals are the remaining major class of vertebrates. Mammals have hair, sweat glands, mammary glands, and four-chambered hearts. The fossil record indicates that mammals evolved 200 million years ago and coexisted with the dinosaurs up until the major extinction 65 million years ago. At this time, mammals diversified to occupy many environmental niches and become the dominant terrestrial vertebrates in many ecosystems. Mammals are highly effective in regulation of their body temperature, and most mammals provide extensive care for their young. One small group of mammals, the Monotremes (for example, the duck-billed platypus), lay eggs. Other mammals gestate their embryos internally and give birth to young. Marsupial mammals give birth after a short time and complete development of young in an external pouch. Placental mammals gestate their young to a more mature state, providing nutritition to the embryo with the exchange of material in the placenta. Marsupial mammals were once widespread across the globe, but were replaced in most cases by placental mammals. Australia being isolated was a haven for marsupial mammals until the present day.
Among the mammals, the primates have opposable thumbs and stereoscopic vision for depth perception, adaptations for life in the trees and traits that have been important factors in the evolution of humans. Many primates have complex social structures. The ancestors of humans included australopithecines. Fossils indicate these ancestors were able to walk upright on two legs on the ground. Fossil remains of hominids such as Homo habilus from 2-3 million years ago display an increasing size of the cortex. Homo habilus probably used tools, setting the stage for modern humans, Homo sapiens.
Congratulations! You've made it through the chapter, and you should now be well versed in topics evolution, speciation, the origin of early life, and classification and diversity. See just how much you've learned as you tackle the quiz on the following pages.
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