a. Three types of evolution are typically distinguished:

b. The overall trends in most phylogenies are toward divergence, where one type of organism evolves into a variety of diverse forms.

c. Divergence is expected, as evolution depends on accidental, unpredictable events such as mutation, recombination, and selection.

d. Some organisms have appeared to "degenerate" into simpler forms, but this is not considered a reversal of evolution.

e. Evolution should not be seen as progressive, on the whole, as it does not produce progress in and of itself.

f. Convergent evolution involves species evolving similar forms, due to similar ways of life, generally.

g. The streamlining of several swimming vertebrates that had diverse ancestors (Figure 22.1) illustrates convergent evolution.

h. The internal structures of these animals are not necessarily similar, however.

i. The wings of three flying vertebrates (Figure 22.2) shows independent responses to the same opportunity.

j. These wings are only partly homologous, as they are constructed very differently internally.

k. Vertebrate wings, compared to insect wings, are an example of analogy.

a. Every science has a system for categorizing the objects it tries to explain.

b. In discussing evolution, the concept of a species must be defined.

c. Classification begins with putting similar individuals (Figure 22.3) into groups that can be called morphological species, which have similar features that are distinguishable from those of other groups.

d. Examples of animals that are in the same species, though they have different morphs, include:

e. Flycatchers of the genus Empidonax (Figure 22.6), however, are members of different species, in spite of their extremely similar morphologies.

f. The biological species concept states that a species is a group of organisms that can breed with one another but not with the members of other similar groups.

g. A phylogenetic tree like the one in Figure 22.7 helps amplify the concept of a biological species.

h. All individuals in a species share a gene pool, which theoretically includes all the gametes of all the individuals in the species, and is thus the source of new members of the species.

i. A group of asexual organisms can be called a species only on the basis of their common features; an asexual "species" is merely a convenient category of similar but independent individuals.

j. Figure 22.8 shows one species diverging into two.

k. Populations often do not jump from one species to the next, but can have intermediate stages between two species, at which point they can exchange some gametes.

a. Natural populations contain a great deal of genetic variation, with different alleles of many genes and many different chromosomal arrangements.

b. A population that contains many different forms for a given trait (e.g. fur color) is polymorphic for that trait.

c. Polymorphism is common, as evidenced most obviously by the human population.

d. Richard Lewontin analyzed the alleles of Drosophila melanogaster (Figure 22.9) and found that over 30% of the genes have two or more alleles.

e. Geographic differences among populations are common; local populations that have differences are called demes.

f. If a characteristic of a species changes gradually across the species' range, it is said to form a cline.

g. The cyanide content of white clover forms a cline in Europe (Figure 22.10).

h. Subspecies or geographic races are groups within species that have distinct traits, as in the American song sparrow (Figure 22.11).

a. Karl von Linné, also known as Carolus Linnaeus, devised the binomial system for naming organisms.

b. A group of organisms is a taxon (plural: taxa).

c. The standard taxa, in order from least inclusive to most inclusive, are: species, genus, family, order, class, phylum, and kingdom.

d. The most closely related species are grouped into a single genus (plural: genera).

e. In writing the technical name of an organism, which is derived from Latin or Greek, the genus is written first, and capitalized, and the species is written second in lower case; the entire name is italicized in print, or underlined if written longhand.

f. The Linnaean system provides for keeping names straight, and a standard type specimen for each species is placed in a museum, and its name revised as agreed upon by taxonomists.

g. Over a million species of animals and 400,000 species of plants have been named; biologists estimate that somewhere between 3 and 30 million species currently exist.

a. As Figure 22.12 shows, related genera are placed in a family, related families are placed in an order, related orders are placed in a class, and related classes are placed in a phylum.

b. Similar phyla are placed in a single kingdom.

c. Members of the same taxon share similar features.

d. Section 2.14 presents current kingdom concepts.

e. The modern method of classifying organisms looks for groups that share a common ancestor, and thus takes into account the evolutionary history of the organisms.

a. Before Darwin, biologists merely catalogued organisms based on similar morphologies.

b. Modern systematists attempt to show that taxa share common ancestors.

c. Figure 22.13 shows a contemporary classification.

d. Often, there is no direct evidence about the evolutionary history of an organism or group.

e. The phylogenetic tree is an inference about that evolutionary history.

f. The goal in making a tree is to have each taxon be monophyletic, consisting of organisms derived from just one branch of the tree.

g. Polyphyletic taxa represent groups that have evolved convergently from separate ancestors.

h. A taxon is paraphyletic if it includes some, but not all of the descendants of a single ancestor.

i. No matter the taxon being analyzed in a phylogenetic tree, each branch point represents a new speciation.

a. Analysis of DNA structure, including that of maternally inherited mitochondrial DNA, has provided evolutionary biologists with useful phylogenetic information.

b. Restriction-fragment length polymorphisms (Section 9.10) have been used to compare the DNA of different organisms and to show similarities and differences in inheritance.

c. Relatively small changes in DNA can produce large changes in morphologies, which might fool taxonomists into grouping organisms incorrectly; DNA analysis is considered to give more conservative conclusions about phylogenies.

d. The polymerase chain reaction (Section 18.11) is also used to amplify segments of DNA from organisms of interest (Figure 22.14).

e. Charles Sibley hybridized DNA samples from many bird species, and some revisions to some traditional classifications, previously based only on external morphologies, were the result.

f. Use of DNA hybridization data to produce a "molecular clock" has been demonstrated, but remains a controversial mode of analysis.

a. Taxonomists attempt to find the classification that "makes sense" for a given group.

b. Evolutionary ("orthodox") taxonomy is the traditional approach.

c. Figure 22.15 illustrates the use of two other methods: phenetic and cladistic grouping methods.

d. A phenetic classification emphasizes appearance alone.

e. A cladistic classification emphasizes evolutionary history.

f. One phenetic approach involves using a computer to analyze equally weighted morphological traits, to create a numerical taxonomy.

g. This results in a matrix (Figure 22.16) showing common features, and a branching tree known as a phenogram.

h. The strict objectiveness of this approach creates problems, such as polyphyletic taxa, that are obvious misinterpretations of the phylogeny.

i. The cladistic approach weighs traits according to their supposed evolutionary significance.

j. Cladistics thus depends on distinguishing primitive from derived traits.

k. Shared primitive characteristics, such as a notochord in chordates, define ancestral groups.

l. Shared derived characteristics, such as hairy skin in mammals, define distinctive groups derived from a common ancestor.

m. Cladists also compare members of a group to another group known to be primitive.

n. Embryological evidence and fossil evidence are also both considered in cladistic analyses.

o. A cladogram is a partial phylogenetic tree drawn for a limited group of organisms.

p. A clade is a monophyletic branch on a cladogram; a sister clade is a related branch.

q. Though a cladogram could be transformed directly into a phylogeny (Figure 22.17), a combination of phenetic and cladistic methods is more often used to consider the complete phylogenetic picture for any given group.

r. Figure 22.18 illustrates the differences between strictly phenetic and strictly cladistic approaches to classification.

s. Taxonomists often look for a complex feature that is thought to have evolved only once, in order to guide the direction of the phylogeny in the tree.

a. Intermediate stages in speciation cause the biological species concept to have difficulties.

b. James Patton, for instance, showed that American pocket gophers (Figure 22.19) have polyphyletic populations that have evolved independently three times, though they are still classified in the same species.

c. Stephen Freeman applied DNA methods to North American orioles (Figure 22.20), and found sister species, which would be expected to interbreed; however, nonsister species are also known to interbreed where their territories overlap, supporting the classification of these more distantly related groups into the same species.

d. The phylogenetic species concept is one adopted by some taxonomists; it regards a species as the smallest monophyletic group of organisms identifiable as a distinct group on the basis of morphology, without consideration of interbreeding.

e. This approach also has its problems, as it elevates some groups to the level of species when they are known to interbreed with each other.

a. Aristotle's "ladder of Nature" concept and the Medieval Great Chain of Being (Figure 22.21) are ideas to be avoided by biologists attempting to construct an accurate tree of life.

b. A phylogenetic tree of organisms on earth is multibranched, and no species is at the top.

c. An alternative distinction is between ancient and derived lineages, or between more plastic and more restricted, rather than anything implying "higher" and "lower."