Explain how populations become distinct species, compare historical and modern explanations of evolution, evaluate molecular, anatomical and fossil evidence including convergent evolution, and read cladograms as hypotheses of relatedness

Learning goal: Explain speciation and the variation of organisms within and between populations; identify geographical barriers that account for distinct but similar populations and propose analyses to decide whether neighboring populations are distinct species; compare the work of Lamarck, Darwin and Wallace; explain how natural selection has affected real species; use molecular, anatomical and paleontological evidence to support the theory of evolution through natural selection and to explain convergent evolution; and use cladograms, expanded to molecular evidence, to describe degrees of relatedness.

What a species is. The biological species concept defines a species as a group of populations whose members can interbreed and produce fertile offspring under natural conditions, and that do not do so with members of other groups. The concept is useful and has limits: it cannot be applied to organisms that reproduce asexually or to fossils, and some clearly different groups occasionally hybridize. Geneticists add a molecular criterion, comparing DNA sequences, and field biologists add ecological and behavioral ones. Deciding whether two neighboring populations are distinct species is therefore an analysis with several possible tests: do they interbreed where they meet, are hybrids fertile, do they differ consistently in measured traits, do their mating signals differ, and how different are their DNA sequences.

Speciation. Speciation is the process by which one species becomes two. The commonest route begins with a geographical barrier. A population is divided by a river, a lake, a mountain range, a canyon or the sea; gene flow between the halves stops; and the halves diverge because they experience different selection and different drift. Given enough generations, differences accumulate until the two no longer interbreed when they meet, whether because they cannot mate, because they mate at different times or with different signals, or because hybrids are sterile or unfit. Sibling species, pairs that look nearly alike but do not interbreed, are what this process looks like near its end. Two closely related forms of tassel-eared squirrel live on opposite rims of the Grand Canyon, separated by the canyon itself (the north-rim Kaibab squirrel is usually classed as a subspecies of the south-rim Abert's squirrel, a divergence not yet complete); the yellow-rumped warbler has an eastern and a western form that differ in throat color and meet in a narrow zone where some hybridize. In Ohio, the water snakes of the Lake Erie islands are paler and less banded than the mainland snakes of the same species; open water limits gene flow, and on the islands' pale limestone shores, banded snakes are more visible to predators, so selection on the islands and immigration from the mainland pull in opposite directions. The island snakes are not a separate species, but they show the first stage of the process and how a barrier accounts for distinct yet similar forms.

Variation within and between populations. The raw material of speciation is variation, and a good analysis measures it. Take three pine species that grow in Ohio: eastern white pine has five needles per bundle, red pine two and pitch pine three, and their needle lengths differ as well. Measure 30 needles from several trees of each species and plot the distributions. Within a species, needle length varies around a mean; between species, the means differ and the distributions may not overlap at all. Traits whose distributions do not overlap separate species reliably; traits whose distributions overlap heavily do not. Interpreting the pattern through natural selection means asking what each species' needle length does for it: long needles may be favored where water is plentiful and short ones where it is scarce or winds are drying. An engineering question follows: if two neighboring populations should be kept from interbreeding, a physical barrier or a difference in timing must block gene flow; if they should be reconnected, a corridor restores it.

Historical and modern theory. Jean-Baptiste Lamarck, in the early 1800s, proposed that species change over time, and that they do so because organisms strengthen features by use and lose them by disuse and then pass the acquired changes to their offspring. His first idea was right and important; his mechanism was wrong, because acquired changes to the body do not alter the DNA in gametes. Charles Darwin and Alfred Russel Wallace independently arrived at natural selection, presented jointly in 1858, and Darwin's 1859 book set out the evidence at length: populations produce more offspring than survive, individuals vary, some variants survive and reproduce more, and the variants are inherited. Neither knew of genes. The modern synthesis of the twentieth century joined natural selection to Mendelian genetics and population genetics, giving the mechanism the previous lesson describes. Darwin's finches on the Galapagos are the classic case of natural selection observed: during a drought that left mainly large, hard seeds, finches with deeper beaks survived better, and the next generation's average beak depth was measurably larger. Peppered moths in industrial England became predominantly dark when soot darkened tree trunks and lighter again when pollution controls cleaned them, because the better-camouflaged form was eaten less. Predicting a species' response to environmental change follows the same reasoning: given its range and population size, ask which existing variants the new conditions favor, whether the population is large enough to contain them, and whether it can shift its range, as cold-water fish in a warming Gulf of Maine have shifted northward.

Evidence from several disciplines. The theory of evolution through natural selection is supported because independent lines of evidence agree. Paleontology: the fossil record shows organisms appearing, changing and disappearing in a consistent order, with transitional forms such as fishes with limb-like fins and whales with hind limbs. Anatomy: homologous structures, such as the one-bone, two-bone, many-bone pattern of a human arm, a bat wing and a whale flipper, share construction because they were inherited from a common ancestor, even though they now serve different uses; vestigial structures, such as hip bones in whales, are remnants of ancestral function. Molecular biology: all known cellular life stores its genetic information in DNA and reads it with a nearly universal genetic code (a few small variations exist, for example in mitochondria), and the more closely two species are related by other evidence, the more similar their DNA and protein sequences are. Recent molecular sequence data have generally confirmed the lineages earlier proposed from anatomy, while correcting some, such as placing fungi closer to animals than to plants. Because the disciplines use different methods and reach the same tree, the agreement is far stronger evidence than any one line alone.

Convergent evolution. Similar environments can favor similar solutions in unrelated lineages. Cacti of the American deserts and euphorbias of African deserts are both spiny, leafless, water-storing succulents, yet their flowers and DNA show they belong to different plant families; sharks, dolphins and the extinct ichthyosaurs all have streamlined bodies and fins, though one is a fish, one a mammal and one a reptile. These are analogous structures: similar in function, different in origin. Distinguishing analogy from homology needs the details of construction and the molecular evidence; convergence is why appearance alone can mislead a classifier, and why a public exhibit of look-alike desert plants from different continents makes a fine demonstration of selection shaping form.

Classification and cladograms. Classification systems are frameworks scientists build to describe diversity and indicate relatedness. Every species has a two-part scientific name, and species are grouped into genera, families, orders, classes, phyla and kingdoms. A cladogram is a branching diagram presenting a hypothesis of descent from common ancestors with modification: each branch point is a shared ancestor, and groups that share a more recent branch point are more closely related. Cladograms are built from shared derived characters, which may be anatomical (hair, amniotic egg, jaws) or molecular (shared sequences). Worked in the prose: for lamprey, trout, frog, lizard and mouse, jaws are shared by all but the lamprey; four limbs by frog, lizard and mouse; an amniotic egg by lizard and mouse; hair by mouse alone. The cladogram branches in that order, and reading it, the mouse is more closely related to the lizard than to the frog, because the mouse and lizard share a more recent common ancestor. Two cladograms that differ are two hypotheses, to be judged by which requires fewer changes and which agrees with molecular data.

Misconceptions. "Two populations that look different are different species" is not settled by looks; the tests are interbreeding, fertility, signals and DNA. "Lamarck was simply wrong" undervalues him; he was right that species change and wrong about how. "Similar-looking organisms are closely related" fails for convergent evolution. "A cladogram shows which species is more advanced" is wrong; it shows relatedness, and every living species has been evolving for the same length of time.

Where the rest of the standard is taught. The protocols for slowing antibiotic, herbicide and pesticide resistance, which Ohio lists under this standard, are worked in the mechanisms-of-evolution lesson.

Investigation you can do. Collect 30 leaves or needles from at least three trees of each of two local species, measure the length of each to the nearest millimeter, and record the tree each came from. Draw a histogram for each species on the same axes and mark the means. Then compute, for each species, how many measurements fall within the other species' range. A result in which the within-species spread is narrower than the between-species difference supports the claim that the trait distinguishes the species; a heavily overlapping result means this trait alone cannot. Interpret the difference through natural selection by proposing what each leaf size might do for its species and what conditions would favor it. What the model cannot show: measurement of one trait cannot show whether the trees interbreed, cannot show relatedness without a comparison of many characters or DNA, and cannot show whether the difference is inherited or caused by the trees' growing conditions.

Worked cladogram from a character table: Characters for five animals: lamprey has none of jaws, four limbs, amniotic egg, hair; trout has jaws; frog has jaws and four limbs; lizard has jaws, four limbs and an amniotic egg; mouse has all four including hair. Build the tree: the first branch splits the lamprey from all the rest (jaws arose after that split); the next splits the trout from the four-limbed animals; the next splits the frog from the amniotes; the last splits lizard from mouse. Reading relatedness: lizard and mouse are each other's closest relatives on this tree; the frog is next; the trout is more distantly related; the lamprey most distant. Adding molecular data, if lizard and mouse DNA sequences were more alike than either is to frog DNA, the hypothesis would be supported; if not, the tree would need revision.

Worked hypothesis for sibling species: Two closely related ground squirrels occupy opposite sides of a large canyon, with the northern form living in cooler, higher forest and the southern in warmer, lower forest. Hypothesis: an ancestral population spanned the region before the canyon deepened; the canyon then blocked gene flow, and each side adapted to its own climate while drift added further differences. Predictions that test it: the two forms should differ in DNA in proportion to the time since separation; their fur and body size should match their climates; and if brought together they should interbreed rarely or produce less fit hybrids. An engineering proposal to keep them distinct would preserve the barrier; a proposal to reconnect them would need a corridor with suitable habitat on both sides.

Worked prediction for an environmental change: A cold-water shellfish occupies a Northern Hemisphere coast from a southern limit to a northern one, with the largest populations near the middle of its range, and water temperatures rise rapidly over two decades. Prediction: individuals near the southern limit experience temperatures beyond those they tolerate, survival and reproduction there fall, and the range's southern edge shifts north; at the northern limit, formerly too cold, populations establish and grow. If the species cannot disperse fast enough, or a barrier such as a bay lies to the north, the range contracts and the population shrinks. The prediction assumes temperature is the limiting factor; it must be checked against food, predators and competitors that also respond to warming.

Worked analysis of convergence: Desert plant A (Americas) and desert plant B (Africa) are both leafless, ribbed and spiny succulents. Anatomical evidence: A's spines grow from cushion-like structures found only in cacti, while B's spines grow from different structures (paired outgrowths at the leaf bases or old flower stalks, depending on the species) and B has milky sap, as euphorbias do; their flowers are built on different plans. Molecular evidence: DNA sequences place A with other cacti and B with other euphorbias, far apart. Conclusion: their similar forms are analogous, produced by natural selection in similar desert conditions favoring water storage, reduced surface area and defense against herbivores. The exhibit would show the two side by side, then the flowers and DNA that separate them.

Supported record: Using the five-animal character table, draw the cladogram, mark on it where each of the four characters arose, and write two sentences stating which pairs are most and least closely related and why. Then make a two-column comparison of Lamarck's and Darwin-Wallace's explanations of how a giraffe population came to have long necks, ending with the modern statement in terms of alleles.

Independent performance: Carry out the leaf or needle measurement investigation on two local species, and write a report. Success criteria: (a) at least 30 measurements per species from three or more trees are tabulated with the tree of origin; (b) histograms for both species share one set of axes and the means are marked; (c) the report states how much the distributions overlap and whether this trait alone can distinguish the species; (d) it interprets the difference through natural selection by proposing conditions that would favor each form; (e) it proposes two further analyses that would test whether the populations are distinct species; (f) it names a geographical or ecological barrier that could reduce gene flow between them and states what the measurement cannot show about relatedness or inheritance.