8. Evolution is the result of genetic changes that occur in constantly changing environments. As a basis for understanding this concept: a. Students know how natural selection determines the differential survival of groups of organisms. Genetic changes can result from gene recombination during gamete formation and from mutations. These events are responsible for variety and diversity within each species. Natural selection favors the organisms that are better suited to survive in a given environment. Those not well suited to the environment may die before they can pass on their traits to the next generation. As the environment changes, selection for adaptive traits is realigned with the change. Traits that were once adaptive may become disadvantageous because of change. Students can explore the process of natural selection further with an activity based on predator-prey relationships. The main purpose of these activities is to simulate survival in predator or prey species as they struggle to find food or to escape being consumed themselves. The traits of predator and prey individuals can be varied to test their selective fitness in different environmental settings. An example of natural selection is the effect of industrial “melanism,” or darkness of pigmentation, on the peppered moths of Manchester, England. These moths come in two varieties, one darker than the other. Before the industrial revolution, the dark moth was rare; however, during the industrial revolution the light moth seldom appeared. Throughout the industrial revolution, much coal was burned in the region, emitting soot and sulfur dioxide. For reasons not completely understood, the light-colored moth had successfully adapted to the cleaner air conditions that existed in preindustrial times and that exist in the region today. However, the light-colored moth appears to have lost its survival advantage during times of heavy industrial air pollution. One early explanation is that when soot covered tree bark, light moths became highly visible to predatory birds. Once this change happened, the dark-peppered moth had an inherited survival advantage because it was harder to see against the sooty background. This explanation may not have been the cause, and an alternative one is that the white-peppered moth was more susceptible to the sulfur dioxide emissions of the industrial revolution. In any case, in the evolution of the moth, mutations of the genes produced light and dark moths. Through natural selection the light moth had an adaptive advantage until environmental conditions changed, increasing the population of the dark moths and depleting that of the light moths. 8. b. Students know a great diversity of species increases the chance that at least some organisms survive major changes in the environment. This standard is similar to the previous standard set on diversity within a species but takes student understanding one step further by addressing diversity among and between species. For the same reasons pertinent to those for intraspecies diversity, increased diversity among species increases the chances that some species will adapt to survive future environmental changes. 8. c. Students know the effects of genetic drift on the diversity of organisms in a population. If a small random sample of individuals is separated from a larger population, the gene frequencies in the sample may differ significantly from those in the population as a whole. The shifts in frequency depend only on which individuals fall in the sample (and so are themselves random). Because a random shift in gene frequency is not guaranteed to make the next generation better adapted, the shift\or genetic drift\with respect to the original gene pool is not necessarily an adaptive change. The bottleneck effect (i.e., nonselective population reductions due to disasters) and the founder effect (i.e., the colonization of a new habitat by a few individuals) describe situations that can lead to genetic drift of small populations. 8. d. Students know reproductive or geographic isolation affects speciation. Events that lead to reproductive isolation of populations of the same species cause new species to appear. Barriers to reproduction that prevent mating between populations are called prezygotic (before fertilization) if they involve such factors as the isolation of habitats, a difference in breeding season or mating behavior, or an incompatibility of genitalia or gametes. Postzygotic (after fertilization) barriers that prevent the development of

viable, fertile hybrids exist because of genetic incompatibility between the populations, hybrid sterility, and hybrid breakdown. These isolation events can occur within the geographic range of a parent population (sympatric speciation) or through the geographic isolation of a small population from its parent population (allopatric speciation). Sympatric speciation is much more common in plants than in animals. Extra sets of chromosomes, or polypoidy, that result from mistakes in cell division produce plants still capable of long-term reproduction but animals that are incapable of that process because polypoidy interferes with sex determination and because animals, unlike most plants, are usually of one sex or the other. Allopatric speciation occurs in animal evolution when geographically isolated populations adapt to different environmental conditions. In addition, the rate of allopatric speciation is faster in small populations than in large ones because of greater genetic drift. 8. e. Students know how to analyze fossil evidence with regard to biological diversity, episodic speciation, and mass extinction. Analysis of the fossil record reveals the story of major events in the history of life on earth, sometimes called macroevolution, as opposed to the small changes in genes and chromosomes that occur within a single population, or microevolution. Explosive radiations of life following mass extinctions are marked by the four eras in the geologic time scale: the Precambrian, Paleozoic, Mesozoic, and Cenozoic. The study of biological diversity from the fossil record is generally limited to the study of the differences among species instead of the differences within a species. Biological diversity within a species is difficult to study because preserved organic material is rare as a source of DNA in fossils. Episodes of speciation are the most dramatic after the appearance of novel characteristics, such as feathers and wings, or in the aftermath of a mass extinction that has cleared the way for new species to inhabit recently vacated adaptive zones. Extinction is inevitable in a changing world, but examples of mass extinction from the fossil record coincide with rapid global environmental changes. During the formation of the supercontinent Pangaea during the Permian period, most marine invertebrate species disappeared with the loss of their coastal habitats. During the Cretaceous period a climatic shift to cooler temperatures because of diminished solar energy coincided with the extinction of dinosaurs. 8. f.* Students know how to use comparative embryology, DNA or protein sequence comparisons, and other independent sources of data to create a branching diagram (cladogram) that shows probable evolutionary relationships. The area of study that connects biological diversity to phylogeny, or the evolutionary history of a species, is called systematics. Systematic classification is based on the degree of similarity between species. Thus, comparisons of embryology, anatomy, proteins, and DNA are used to establish the extent of similarities. Embryological studies reveal that ontogeny, development of the embryo, provides clues to phylogeny. In contrast to the old assertion that “ontogeny recapitulates phylogeny” (i.e., that it replays the entire evolutionary history of a species), new findings indicate that structures, such as gill pouches, that appear during embryonic development but are less obvious in many adult life forms may establish homologies between species (similarities attributable to a common origin). These homologies are evidence of common ancestry. Likewise, homologous anatomical structures, such as the forelimbs of humans, cats, whales, and bats, are also evidence of a common ancestor. Similarity between species can be evaluated at the molecular level by comparing the amino acid sequences of proteins or the nucleotide sequences of DNA strands. DNA-DNA hybridization, restriction mapping, and DNA sequencing are powerful new tools in systematics. Approaches for using comparison information to classify organisms on the basis of evolutionary history differ greatly. Cladistics uses a branching pattern, or cladogram, based on shared derived characteristics to map the sequence of evolutionary change. The cladogram is a dichotomous tree that branches to separate those species that share a derived characteristic, such as hair or fur, from those species that lack the characteristic. Each new branch of the cladogram helps to establish a sequence of evolutionary history; however, the extent of divergence between species is unclear from the sequence alone. Phenetics classifies species entirely on the basis of measurable similarities and differences with no attempt to sort homology from analogy. In recent years phenetic studies have been helped by the use of computer programs to compare species automatically across large numbers of traits. Striking a balance between these two approaches to classification has often involved subjective judgments in the final decision of

taxonomic placement. Students can study examples of cladograms and create new ones to understand how a sequence of evolutionary change based on shared derived characteristics is developed. 8. g.* Students know how several independent molecular clocks, calibrated against each other and combined with evidence from the fossil record, can help to estimate how long ago various groups of organisms diverged evolutionarily from one other. Molecular clocks are another tool to establish phylogenetic sequences and the relative dates of phylogenetic branching. Homologous proteins, such as cytochrome c, of different taxa (plants and animals classified according to their presumed natural relationships) and the genes that produce those proteins are assumed to evolve at relatively constant rates. On the basis of that assumption, the number of amino acid or nucleotide substitutions provides a record of change proportional to the time between evolutionary branches. The estimates of rate of change derived from these molecular clocks generally agree with parallel data from the fossil record; however, the branching orders and times between branches are more reliably determined by measuring the degree of molecular change than by comparing qualitative features of morphology. When gaps in the fossil record exist, phylogenetic branching dates can be estimated by calibrating molecular change against the timeline determined from the fossil record. Adaptations and Natural Selection

Natural Selection coupled with a changing environment acts like a type of pressure which directs long term adaptation in organisms. Successful (or fit) organisms are well adapted (but not perfectly adapted) to their habitat at any given time and place. But the local environment (in most cases) is constantly changing. The greater the variation in the gene pool the better able a population can adapt to any changes.

One well documented example of animal adaptation through natural selection is the English peppered moth. As tree trunks began to darken with smoke from the industrial revolution the wings of the peppered moth also darkened. This offered the moths’ better camouflage, protection against capture by hungry birds.

Theory predicted that the original light colored moth would be more visible and caught more often by the predatory birds than the darker moths. This is exactly what happened -- natural selection in action.

The genes for light color were less successful than those for darker wings. The once rare and valuable dark form became commonplace as long as smoke belched from the coal burning furnaces of English industry.

Adaptations come in three forms

1. structural (changes in shape or color of body parts) 2. physiological (changes in metabolism) 3. behavioral (change in activity or interaction with other organisms)

Let's take a quick look at each of these adaptations. You should find additional examples of each adaptation.

Structural Adaptations Most organisms; plants and animals, predators and prey, tend to evolve structural adaptations in one of the following categories.

1. Camouflage - allows an organism to blend into its surroundings. 2. Mimicry - gives an organism the appearance of another more dangerous one. 3. Warning coloration - clearly and dramatically identifies an organism which is poisonous to its

potential predators.

Physiological Adaptations

This form of adaptation provides an organism with a metabolic means of neutralizing poisons or toxins. It may also give an organism better use of its chemical resources and reduce or improve its energy needs.

Behavioral adaptations

A change in behavior can provide an organism with a rapid means to adapt to its environment. Many organisms rely on innate (genetically programmed and unalterable) behaviors. Examples include the courtship dance of some insects and birds, or the web building behavior of spiders.

Behavior can also be learned. Organisms, like ourselves, with the ability to quickly learn new behaviors have a much better chance to survive changing conditions.

Speciation (Macroevolution)

Natural Selection can eventually lead to speciation. No matter how closely individuals in two populations look alike if they cannot, under normal circumstances, produce fertile offspring then these populations are different species.

Some permanent barrier must exist between the populations to allow the degree of genetic changes necessary for complete speciation. The barriers can be physical or behavioral.

Physical barriers include geographical barriers such as rivers, mountain ranges, bodies of water, deserts etc. which can prevent two populations from interacting. The finches on the Galapagos Islands represent this type of speciation.

Behavioral isolation can involve:

• special courtship behaviors (bird songs differ) • preferring different habitats (lions live in grassy areas while tigers prefer jungles) • mating during different times of the day, or year

Barriers must be able to provide the time needed for speciation. But speciation can occur almost overnight in plants. Plants are much more tolerant of radical changes in their genomes than animals are. Occasionally chromosomes do not separate during meiosis and one gamete receives an extra chromosome or an entire added set of chromosomes.

This condition called polyploidy is nearly always fatal to an animal but in many plant species it can be tolerated. The new polyploid plant will be able to self fertilize so a new species results immediately. The new offspring cannot mate with the original species because their chromosome number is different and the pairing of homologous chromosomes will be impossible.

Reproductive Barriers Separate Species:

A reproductive barrier is any factor that will not allow the production of a fertile hybrid. These factors can be classified as prezygotic (barriers that impede mating) and postzygotic (barriers that prevent a hybrid zygote from developing).

Prezygotic Barriers:

1. Habitat Isolation. Two species may live in the same area but in different habitats. Living in these different habitats ( in water, living on land, or living in tree tops) effectively segregate these organisms from each other. Since there is little if any contact the possibility of successfully mating is drastically reduced.

2. Temporal Isolation. Two species that breed at different times of the day, season, or year cannot mix their gametes. Since the breeding times are different there is no chance of reproductive contact.

3. Behavioral Isolation. Species-specific signals and elaborate behavioral patterns are used by closely related species to insure contact with the proper mate. Birds, mammals, and insects have pre-mating rituals that attract the proper mate. These signals can be chemical or physical in nature. Other organisms pay little or no attention to these behaviors or scents.

4. Mechanical Isolation. Anatomical incompatibility may prevent sperm transfer between two closely related species. The absence of certain appendages or their modification may inhibit a male from grasping and successfully fertilizing the female. Difference in floral structure may prevent pollen from reaching the stigma of the intended flower.

5. Gametic Isolation. If for some reason foreign sperm is introduced into a female there are several preventative measures to insure that there is no union between the sperm and egg. Internal environmental conditions may cause the sperm to die. Gamete recognition sites on the sperm do not fit with the intended egg. If the two species differ in the type of fertilization (external and internal) there is no chance of the sperm ever contacting the egg.

Postzygotic Barriers: If prezygotic barriers are crossed and a hybrid zygote forms, one of several barriers will prevent development of a viable, fertile hybrid.

1. Reduced Hybrid Viability. Genetic incompatibilities between the species may abort the development of the hybrid during some stage of development. Difference in chromosome number may cause abnormal cell division. Since the chromosomes align to insure equal distribution upon cytokinesis, abnormal chromosome counts could occur based on this numerical difference.

2. Reduced Hybrid Fertility. If two species mate and produce a viable offspring, these offspring will be sterile due to the misalignment of the chromosome number. During gametogenesis the odd number of chromosomes makes it impossible for viable gametes to be produced by meiosis.

3. Hybrid Breakdown. In some cases a fertile hybrid is produced. When these hybrids mate with each other, their offspring of the next generation are feeble or sterile.

Introgression:

Introgression is the transplantation of genes between species. This occurs when alleles slip through all zygotic barriers. This occurs when two species hybridize and a small number of the offspring manage to mate with the general population. This occurs in plants more so than in animals.

Geographical Isolation:

Geographic isolation plays an important role in species development and maintenance. There are two categories of this type of isolation: allopatric speciation and sympatric speciation. These relationships deal with the contact of the new species with that of its ancestral species.

1. Allopatric Speciation. This type of speciation is produced when a physical barrier separated a species into two separate areas and does not allow any further contact. Mountain building, glacial movement, river boundary movement, etc. are examples of geographical situations that can divide up a single species into two distance areas.

When the species is split, microevolution will cause changes in the species to make them different in phenotype. Small populations have a much better chance to develop into a new species than larger populations. These small populations usually occur on the edges of a larger population. These fringe populations are good candidates for speciation. Their gene pool differs from the parent population, genetic drift will continue to cause chance changes in the gene pool of small populations until a larger population is formed. Different selection pressures are at work on there peripheral populations. Adaptive radiation is the evolution of many diversely adapted species from a common ancestor. Geographical isolation lends itself to this type of new species development.

2. Sympatric Speciation. This is a type of speciation that develops within the range of the parent population. This type of speciation does not include geographical isolation. It can occur rapidly if a genetic change results in a barrier between the mutants and the parent population.

a). Autopolyploid. An organism has more than two chromosome sets. This can occur due to nondisjunction in either mitosis or meiosis or self-fertilization.

b). Allopolyploid. A polyploid hybrid resulting from contributions by two different species. This is more common than autopolyploidy. These are usually sterile hybrids, but can reproduce asexually.

Patterns of Evolution Divergent (adaptive radiation) Convergent Fossils and Evolution A fossil is formed when an organism or part of one is preserved before it can decay. Fossils generally are found in sedimentary rock (sandstone, limestone, shale, etc.)

Fossils occur in five varieties

• imprint - a shallow mold such as a leaf or feather • mold - a depression made by the original organism • cast - a mold which is filled in and only the fill remains • petrified - a soft mineral replaced atom by atom by another harder mineral • mummified - the original material is dried out such as amber encased insects

Fossils are important in determining the relative age of sediments which is essential for locating sites of new oil deposits. Such indicator fossils, the tiny foraminifera and others, occur in great numbers. Absolute dating requires radioactive elements, their decay products, and knowledge of half-lives. Uranium-lead, potassium-argon, and rubidium-strontium three different but reliable radioactive systems have provided an agreed upon age of the earth of 4.6 billion years.

Fossils provide one of the most convincing pieces of evidence for evolution. The fossil record, although mostly incomplete, shows that throughout earth's history there have been many periods of great species diversity, but it also shows catastrophic extinction events where more that half of all life forms were wiped out.

The fossil record does show "speciation" at various taxons. Horse and camel evolution are extremely well documented at the genus level.

So called missing links found in the fossil record provide real clues as to how mammals evolved from their reptilian ancestors. Today biochemical studies especially the use of DNA provide stunning corroboration of the earlier taxonomic work and phylogenies based on the fossil record. Studies comparing DNA sequences have revealed "fossilized genes" from viruses and ancestral organisms stuck in the present day human genome.

Structures and Evolution Homologous structures in different species are structures that have a similar underlying pattern and are derived from the same embryonic body parts.

Homologous structures do not necessarily perform the same function. For instance the upper limbs of all mammals are homologous (see diagram) but the wing of a bat, foreleg of a horse, arm of a man and front flipper of a seal have different functions yet are all homologous.

Body parts that are similar in function but do not have a common embryonic origin are analogous structures. For instance Thorns are modified stems whereas spines are modified leaves. See if you can find other analogous structures.

Analogous structures frequently do not provide clues to evolutionary relationships and can even fool the unwary.

Vestigial structures are body parts that are reduced in size and appear to serve little or no function in the organism. Fish that have lived in caves for generations are often blind. Their eyes are reduced in size or are missing altogether.

Development and Evolution

Studying the stages of development from egg to embryo to adult can provide powerful leads to an organism's phylogeny (evolutionary history). For instance all mammal embryos have Brachial arches at some point during their development. The brachial arches and surrounding tissue become gills in fish. Gills and the associated pouches are essential for fish, the presumed ancestry of all vertebrates including the mammals. Since the adult mammal has no use for gills or gill pouches these tissues were lost sometime during their evolutionary past.

In the human embryo, a tail is present up to the sixth week of development but is usually reabsorbed as development continues.

DNA and Evolution All living things contain RNA and DNA. They all use the same genetic code and the associated processes of transcription and translation. By examining the actual DNA sequences for the same gene researchers have discovered that the more closely related species are the closer the corresponding DNA sequences are.

When 2 populations diverge and become separate species their gene pools begin to accumulate different mutations. Over time the number of changes in the corresponding DNA sequences increase. By counting and analyzing these mutations, researchers can, in some cases, determine how long the two species had a common ancestor.

We know that the DNA of humans and chimpanzees correspond almost 99% of the time. Common errors are also found in both genomes. For these reasons and others it is presumed that the common ancestor for humans and chimpanzees may have roamed the African veldt no more than 10 million years ago.

Schools of Taxonomy: 1. Cladistic: clades are evolutionary branches. They classify organisms according to the order in time that branches arise along a phylogenic tree. These clades all descended from one ancestral species. This gives the diagrams the appearance of a tree. 2. Phenetics: no phylogenetic assumptions are used only measurements in similarities and differences in structure. Analogy and homology are not separated. Geological Time Scale:

Relative dating: Superimposition of sedimentary rocks tells the relative age of fossils. The fossils in each layer are a local sampling of the organisms that existed when that sediment was deposited. Younger strata are superimposed on the top of older ones , and the succession of fossil species is a story of macroevolution that can be read by paleobiologists. Absolute dating: This does not mean errorless dating. The age is given in years and not in relative terms such as early, before, and after. Radioactive dating is the method often used to determine the age of the fossils and rocks. Fossils contain isotopes of elements accumulated in the organism when they were alive. Because each radioactive isotope has a fixed rate of decay, half-life, it can be used to date the species. Half-life is unaffected by pressure, temperature, or other environmental variables.

Examples of half-life:

• Potassium 40 -- 1.3 x 109 years • Rubidium 87 -- 4.9 x 1010 years • Uranium 238 -- 4.5 x 109 years • Carbon 14 -- 5.7 x 103 years

Another form of dating is called the "clock". Amino acids have right and left handed symmetry (optical isomers). The L- form is synthesized by living organisms. When an organism dies the L form is transformed into the D form . This type of dating is temperature sensitive. Climates that did not change too much over a period of time have fossils that relate well with radioactive dating. Geologic time: There are 4 eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic. These eras are divides into periods, and the periods into epochs.