Discover Biology SIXTH EDITION
Anu Singh-Cundy Gary Shin Discover Biology SIXTH EDITION CHAPTER 15 The Origin of Species The Notes field in these PowerPoint slides contain figure captions from the textbook, and sometimes additional explanatory text from the textbook enclosed in parentheses (like this). Any extra contentthat is, content not found in the textbookis identified by enclosing the relevant notes within square brackets [like this]. Generally, the extra content notes explicate supplementary photographs, graphs, or line drawings that are not found in the printed or electronic version of the textbook and are therefore unique to these PowerPoint slides. 2015 W. W. Norton & Company, Inc. CHAPTER 15 The Origin of Species
Cichlid Mysteries 15.1 What Are Species? Species are often morphologically distinct Species are reproductively isolated from one another 15.2 Speciation: Generating Biodiversity Speciation can be explained by the same mechanisms that cause the evolution of populations Speciation can result from geographic isolation Speciation can occur without geographic isolation 15.3 Adaptive Radiations: Increases in the Diversity of Life 15.4 Evolution Can Explain the Unity and Diversity of Life 15.5 Rates of Speciation BIOLOGY MATTERS: ISLANDS ARE CENTERS FOR SPECIATIONAND EXTINCTION APPLYING WHAT WE LEARNED: Lake Victoria: Center of Speciation How could so many species have evolved in just 15,000 years?
Cichlid Mysteries Changes on Earth separate populations of organisms, alter the environments in which they live, and set the stage for evolution. Until the 1970s, Lake Victoria was home to more than 500 species of cichlids, descended from just two different ancestor species. Environmental influences have decreased the number of cichlid species; this process may lead to accelerated evolutionary change. Cichlid Diversity. Cichlids are an extremely diverse group of fishes. Africa alone is believed to have at least 1,600 species of cichlids. Hundreds more have evolved in other parts of the world, from Madagascar, India, and Syria to Central America, Mexico, and the southern United States. (Over time, the surface of Earth changes slowly but dramatically. Chains of islands rise from the sea; new lakes form and old ones disappear; mountains thrust upward, miles above sea level; and rivers cut massive canyons that divide continents. Such changes separate populations of organisms, alter the environments in which they live, and set the stage for evolution. Some of the most remarkable examples of rapid evolutionary change are the cichlid fishes of Lake Victoria in East Africa. Since its formation 400,000 years ago, Victoria has dried up and filled with water again and again. It last filled with water about 15,000 years ago, and until the 1970s it was home to about 500 species of cichlidsmore kinds of fishes than in all the lakes and rivers of Europe. In the 1970s, researchers reported so many fish that, in just 10 minutes, they were able to catch 1,000 fish of 100 different species. Amazingly, all of these cichlid fishes descended from just two ancestor species from nearby Lake Kivu, and all had evolved in 15,000 years. Environmental degradation and introduction of invasive Nile perch has driven 200 species of cichlids to extinction. Can this loss of biodiversity be reversed?) How could so many species have evolved in just 15,000 years? Four species of Hawaiian
What Are Species? Various species concepts have been advanced by biologists, which, taken together, help us understand what defines a species. Four species of Hawaiian honeycreepers (There are several ways to answer the question What are species? While the word is commonly applied to members of a group that can mate with each other to produce fertile offspring, not all species can be defined by their ability to interbreed. For example, many species, including all bacteria, reproduce asexually.) Morphological Concept of Species: Different Species Look Different
According to the morphological species concept a species is a distinct group of organisms with a unique set of morphological characteristics (unique external form). The morphological species concept is limited because groups that look quite different can be members of the same species (as in Helicornis erato butterflies) and groups that look similar could belong to different species (as in grizzlies and different types of brown bears around the world). Figure Members of a Single Species May Look Different from One Another Heliconius butterflies exhibit an astonishing range of morphological variation, even within a species. Shown here are different variants of H. erato. Some forms bear a stronger resemblance to Heliconius butterflies of other species than to members of their own species. (Morphology is sometimes the only way we can identify and distinguish fossil species. However, the morphological species concept does not always work well. Sometimes distinct and separate species have members that look very much alike. For example, researchers from the Smithsonian Institution were startled to discover that the three species of Starksia blennies they had been studying in the Caribbean islands were really 10 different species. Each of the 10 species of these reef fishes inhabits a quite different habitat in the western Atlantic, and the species cannot interbreed; however, even experts cannot tell all 10 species apart by appearance alone. Conversely, different populations can vary in appearance [sometimes dramatically] yet be assigned to the same species. All the brown bears of the world, for exampleincluding the grizzly bear of inland Alaska and the much larger coastal brown bearbelong to one species: Ursus arctos. Likewise, Heliconius butterflies of Central and South America exhibit extraordinary morphological variation within a single species.) Limitations of the morphological concept: these butterflies look very different but they belong to the same species, Helicornis erato. Biological Concept of Species: One Species Cannot Breed with Another Species
The biological species concept defines a species as one or more population that can interbreed to produce fertile offspring but are reproductively isolated from other groups. When two species are prevented from interbreeding, we say that the species are reproductively isolated from each other. Barriers to reproduction ensure that the members of a species share a unique genetic heritage, a particular set of genes and alleles that is typical of the species but different from that of all other species. Figure Distinct Species May Look Similar to One Another The (a) eastern meadowlark and the (b) western meadowlark are very similar in form, but they are distinct species. How do you recognize species? What makes these two birds distinct species? Why do these birds look so similar? (Note that in the biological species concept, reproductive isolation is distinct from geographic isolation. For example, the eastern and western meadowlarks are different species, although they look very similar. The two species do not interbreed, even though their ranges overlap in parts of the upper midwestern United States. These grassland species are reproductively isolated because each sings a distinctly different song, and a female will mate only with the male that sings the melody unique to her species. The biological species concept has important limitations. For example, it cannot be used to define fossil species, since no information can be obtained about whether two fossil forms were reproductively isolated from each other. Instead, fossil species are defined on the basis of morphology. Nor does the biological species concept apply to organisms that reproduce mainly by asexual meansfor example, bacteria and dandelions.) Reproductive isolation: these larks look very similar but their songs are different and they cannot breed with each other even though their geographical range overlaps in the midwestern United States. Prezygotic and Postzygotic Barriers
Table 15.1 Barriers That Can Reproductively Isolate Two Species in the Same Geographic Region Discover Biology, 6/e Table 15.1 2015 W. W. Norton & Company, Inc. TYPE OF BARRIER DESCRIPTION EFFECT Prezygotic barriers Temporal isolation The two species breed at different times. Mating is prevented. Ecological isolation The two species breed in different portions of their habitat. Behavioral isolation The two species respond poorly to each other's courtship displays or other mating behaviors. Mechanical isolation The two species are physically unable to mate. Gametic isolation The gametes of the two species cannot fuse, or they survive poorly in the reproductive tract of the other species. Fertilization is prevented. Postzygotic barriers Zygote death Zygotes fail to develop properly, and they die before birth. No offspring are produced. Poor hybrid performance Hybrids survive poorly or reproduce poorly. Hybrids are not successful. (Reproductive isolation creates separate gene pools in the isolated populations, which can then diverge, leading the two lineages to acquire unique forms that make them look different from each other. The biological species concept remains the most useful definition for most biologists. However, while reproductive isolation [the cornerstone of the biological species concept] is very useful in establishing a species definition, the concept of a species in nature can be considerably more complicated.) Prezygotic barriers prevent fertilization or the formation of a zygote. Postzygotic barriers prevent zygotes from developing into healthy and fertile offspring. Speciation: Generating Biodiversity
Speciation is the process in which one species splits to form two or more species that are reproductively isolated from each other. Reproductive isolation is caused by many factors, including geographic separation. Over time two populations that are separated from each other may accumulate so many genetic changes that they are no longer able to breed with each other. In order for speciation to occur, reproductive isolation must halt gene flow, allowing the two populations to acquire unique gene pools and develop into different species. [The illustration is from a previous edition of Discover Biology.] The diversity of life results from the splitting of one species into two or more species. Speciation Can Be Explained by the Same Mechanisms That Cause the Evolution of Populations
Populations evolve genetic differences from one another because of mutation, genetic drift, or natural selection, and these genetic differences sometimes result in reproductive isolation. Gene flow limits the genetic divergence of populations; therefore, the factors that promote speciation must have a greater effect than the amount of ongoing gene flow. [The line drawings are from a previous edition of Discover Biology.] When Gene Flow between Two Populations Decreases or Stops, Evolutionary Processes Begin to Act on Each Gene Pool Independently An experiment with fruit flies illustrates population divergence: Population of fruit flies split into two populations isolated from each other and fed different food. Over the generations, flies raised on the two different foods start to prefer mating with flies raised on the same food supply (behavioral isolation). After long periods of time, mutation, genetic drift, and natural selection can cause the two behaviorally isolated populations to evolve so many additional genetic differences that they become distinct species. Figure When What You Eat Affects Who You Love Fruit flies in an initial sample were separated into four populations and raised on two different kinds of food (starch or maltose) for several generations. In the experimental group, flies from the populations that had become adapted to feed on starch were then given the opportunity to mate with other flies that had adapted to feed on starch or with flies adapted to feed on maltose. As the mating frequencies show, scientists found that flies adapted to feed on starch preferred mating with other flies adapted to starch. Flies adapted to maltose likewise preferred other flies adapted to maltose. These preferences are the early stages of reproductive isolation that can eventually lead to speciation. Speciation Can Result from Geographic Isolation
Geographic isolation can occur when populations of a single species become separated, or geographically isolated, from one another. The distance required for geographic isolation to occur varies from species to species depending on how easily the species can travel across anygiven barrier. The formation of new species from geographically isolated populations is allopatric speciation. Figure Physical Barriers Can Produce Speciation by Blocking Gene Flow New species can form when populations are separated by a geographic barrier, such as a rising sea. (Geographic isolation can also occur when a few members of a species colonize a region that is difficult to reach, such as an island located far outside the usual geographic range of the species. For example, Darwins finches on the separate Galpagos Islands were geographically isolated from one another and from finches on the South American mainland by ocean waters.) An Example of Geographic Isolation
Figure The Grand Canyon Is a Geographic Barrier for Squirrels The Kaibab squirrel is confined to the ponderosa pine forests on the north rim of the Grand Canyon. Aberts squirrel lives on the south rim and also farther south on the Colorado Plateau and in the southern Rocky Mountains to Mexico. The Kaibab population became isolated from the Aberts squirrels when the Colorado River cut a canyonas deep as 6,000 feet in some places. With gene flow between them blocked, probably beginning about 5 million years ago, the Kaibab population diverged from the ancestral Aberts squirrel. The two squirrels used to be considered separate species, but now most experts classify them as subspecies of the Aberts squirrel, Sciurus aberti. (Populations of squirrels and other rodents that live on opposite sides of the Grand Canyona formidably deep and large barrier for a rodenthave diverged considerably. Meanwhile, populations of birdswhich can easily fly across the canyonhave not diverged. In general, geographic isolation is said to occur whenever populations are separated by a distance that is great enough to limit gene flow.) Speciation on Islands Because of geographic isolation, descendants of a mainland population can evolve into a very different population when isolated on an island. Tiny human relatives, also as hobbit people or little people of Indonesia, were discovered in 2004 on the island of Flores. Measuring 3 feet tall as adults, Homo floresiensis lived as recently as 17,000 years ago on their isolated island along with giant Komodo dragon lizards and pygmy elephants. They used stone tools and fire. Figure Island Isolation May Have Driven the Evolution of Homo Floresiensis The foot structure and other anatomical features suggest that H. floresiensis was a distinctly different species, not a scaled-down version of H. erectus or H. sapiens. Ring Species Are a Result of Geographic Isolation
Ring species have been found in salamander populations that loop around the San Joaquin Valley in California. Figure Ring Species of Salamanders from the Genus Ensatina Although each of the adjoining populations of salamanders can interbreed, E. croceater and E. eschscholtzii cannot. Theoretically, alleles from E. croceater can reach E. eschscholtzii by going around the ring through the various subspecies, but the amount of gene flow is so low that reproductive isolation has occurred. (Another line of evidence for the importance of geographic isolation in speciation comes from cases in which individuals of a population found at one end of a species geographic range reproduce poorly with individuals of a population at the other end of its range, even though individuals from both ends reproduce well with individuals from intermediate portions of the species range.) Ring species can develop when populations loop around a geographic barrier in which populations at the two ends of the loop are in contact with each other, yet individuals from these populations cannot interbreed. Speciation Can Occur without Geographic Isolation
The formation of new species in the absence of geographic isolation is called sympatric speciation. Ecological specialization has led to the diversification of cichlid fishes within the same lake, for example, bottom feeders are isolated from surface feeders. Differences in feeding preferences explain ongoing diversification in Rhagoletis populations that eat apples instead of hawthorns. Figure Food Preferences and Female Mate Preferences May Have Driven Speciation among Lake Victoria Cichlids The four species shown here illustrate some of the differences in feeding behavior and morphology among Lake Victoria cichlids. [The sketch of Rhagoletis is from a previous edition of Discover Biology.] (North American populations of the apple maggot fly, Rhagoletis pomonella, are in the process of diverging into new species, even though their geographic ranges overlap. Historically, Rhagoletis ate native hawthorn fruits, but starting in the mid-nineteenth century these flies became pests to apple farmers [apples were an introduced nonnative species]. Over time the two populations of Rhagoletis diverged. Rhagoletis populations that feed on apples are now genetically distinct from populations that feed on hawthorns. Members of the apple and hawthorn populations mate at different times of year and usually lay their eggs only on the fruit of their particular food plant. As a result, there is little gene flow between the apple- and hawthorn-eating fly populations. In addition, researchers have identified alleles that benefit flies that feed on one host plant but are detrimental to flies that feed on the other host plant. Natural selection operating on these alleles acts to limit whatever gene flow does occur. Over time, the ongoing research on Rhagoletis may well provide a dramatic case history of sympatric speciation.) Apple maggot flies (Rhagoletis pomonella) that have taken to feeding on apples (a non-native crop) dont breed with individuals that feed on hawthorns (a native plant). Chromosomal Changes Can Lead to Allopatric Speciation in a Single Generation
In plants, rapid changes in chromosome numbers can cause sympatric speciation. New plant species can form in a single generation as a result of polyploidy, a condition in which an individual has more than two sets of chromosomes. Populations that differ in chromosome number cannot interbreed, resulting in reproductive isolation within the same habitat. (The photo is from page 346, Chapter 15.) (Polyploidy can also occur when a hybrid spontaneously doubles its chromosome number. Chromosome doubling can lead to reproductive isolation because the chromosome number in the gametes of the polyploid no longer matches the number in the gametes of either of its parents. For example, a cross between a diploid organism (2n) and a tetraploid one (4n) would result in a triploid (3n) offspring. After undergoing meiosis, it would not be able to produce gametes with the proper number of chromosomes to mate with either parent. Polyploidy has had a large effect on life on Earth: more than half of all plant species alive today are descended from species that originated by polyploidy.) Throughout Earths History, Adaptive Radiations Have Contributed Greatly To The Diversity Of Life
Under certain conditions, a lineage may experience multiple speciation events in a relatively short period of time. One lineage may quickly give rise to many descendant species. This phenomenon is known adaptive radiation. The conditions that lead to adaptive radiations include: Colonization of a new location Mass extinctions that remove existing species Evolution of a novel trait that confers a significantcompetitive advantage Figure Extinction of the Dinosaurs Enabled Mammals to Radiate Early mammals (such as Morganucodon, were small and are thought to have been nocturnal. Following the extinction of the dinosaurs, mammals radiated to occupy the ecological niches vacated by the dinosaurs. Because of the huge range in size between the smallest (Morganucodon, about the size of a shrew) and the largest (the blue whale, which can reach 2530 meters long), none of these animals are drawn to scale. (Adaptive radiations occur when the process of speciation is made easier by a relaxation of natural selection. When ecological niches are more freely available, a greater variety of lifestyles can be sustained. In this way, speciation can proceed rapidly, leading to the formation of many new species. Finches underwent adaptive radiation after one species of finch from the South American mainland colonized the Galpagos Islands a few million years ago. Once on the islands many niches not available on the mainland were open to these colonizers, since there were no other bird species to compete with. While originally adapted for seed eating, the finches on the Galpagos were now able to feed on flowers and insects. The lack of competitors in the new environment made it possible for ecological isolation to rapidly lead to many descendant species.) Organisms Share Characteristics as a Result of Common Descent
Two species derived from the same ancestral lineage will share some traits because of their common ancestry. Homologous traits are features that organisms share because they have inherited them from a common ancestor. Figure Shared Characteristics The human arm, the whale flipper, and the bat wing are homologous structures, all of which have what are essentially a matching set of five digits and a matching set of arm bones that have been altered by evolution for different functions. Some organisms look and behave very differently, yet have some striking shared characteristics due to their common ancestry. Common Descent Explains Vestigial Organs
Figure A Python Has Rudimentary Hind Legs Visible Externally and Internally Vestigial organs are reduced or degenerate parts whose function is no longer needed. Similar Characteristic Can Evolve in Very Different Organisms Because They Live in Similar Environments Convergent evolution occurs when natural selection causes distantly related organisms to evolve similar structures in response to similar environmental challenges. Figure The Power of Natural Selection (ac) These three plants evolved from very different groups of leafy plants. They resemble one another because of convergent evolution: each was similarly driven by natural selection to adapt to life in a desert. Their shared structures (fleshy stems, spines, reduced leaves) are therefore analogous, not homologous. (a) Euphorbia belongs to the spurge family and can be found in Africa. (b) Echinocereus is a cactus found in North America. (c) Hoodia, a fleshy milkweed, can be found in Africa. (d, e) Convergent evolution can be a powerful force shaping animals as well. Here we see how natural selection has caused two distantly related animals(d) sharks and (e) dolphinsto look very similar. Sharks are a kind of fish; dolphins are mammals. (Cacti found in North American deserts share many features with distantly related plants found in African and Asian deserts. Similarly, both sharks and dolphins have streamlined bodies that make it easier for them to swim rapidly. The two are distantly related, and their overall similarities do not represent homology. Instead, these organisms share features as a result of convergent evolution, which occurs when natural selection causes distantly related organisms to evolve similar structures in response to similar environmental challenges.) Characteristics that result from convergent evolution are said to be analogous. Rates of Speciation Species formation generally takes thousands of years, but can also happen in a generation (as a result of polyploidy, for example). Freshwater fishes can speciate in a time span ranging from 3,000 years (in pupfishes) to over 9 million years in carp, piranhas, and many aquarium fishes. Rates of speciation and rates of extinction are affected by various factors, including doubling time (time it takes for a population to double in size), and behavior (such as processes by which mates are chosen), how fast reproductive isolation occurs, and whether changes in the physical environment are sudden or gradual. Some populations can be geographically isolated for a long time without evolving reproductive isolation (for example, polar bears and brown bears). [The sketch of salsify, Tragopogon species, is from a previous edition of Discover Biology. One of the new species, Tragopogon miscelllus, arose as result of polyploidy in a hybrid of two of the European introductions, T. dubius and T. pratensis. The other new species, T. mirus, evolved from polyploidy in a hybrid between T. dubius and the third European introduction, T. porrifolius.] [Genetic studies and field surveys revealed that between 1920 and 1950, two new species of salsify evolved in Idaho and Washington from three species introduced from Europe.] BIOLOGY MATTERS: ISLANDS ARE CENTERS FOR SPECIATIONAND EXTINCTION
Encompassing only 5 percent of Earths surface, islands contain roughly 20 percent of Earths species. Reasons for high species diversity: - Geographical isolation among different islands leading to allopatric speciation - Ecological isolation because of highly variable environments (coast versus inland), with diverse microclimates, especially with mountains - High rates of endemism (species not found elsewhere in the world) Island biodiversity is unusually vulnerable because it is susceptible to: - Effects of genetic drift - Invasive species (The figure is page 345, Biology Matters, Chapter 15.) (A major reason for the high level of biodiversity is that individual islands in a chain are geographically isolated from one another and provide many opportunities for allopatric speciation. Because migration between the islands is rare, there is little gene flow between populations on different islandscertainly not enough to maintain the populations as a single gene pool. When migration does occur, the new migrants are unable to interbreed with the resident populations, and (because they are now isolated from their source population) will eventually become new species. While islands are rich in biodiversity, the species found there are particularly vulnerable to extinction. Approximately 40 percent of critically endangered species reside on islands, and 80 percent of all known extinctions have occurred on islands. The small size of islands makes populations highly subject to genetic drift. On average, island species maintain about 29 percent less genetic diversity than their mainland counterparts. Furthermore, the relationships between species within an island community are easily upset by the introduction of invasive species that may act as predators or competitor.) Geographic isolation leading to allopatric speciation in island groups. Rare migratory events increase species diversity. Lake Victoria: Center of Speciation
Cichlids in Lake Victoria have diversified into hundreds of new species over the past 400,000 years, in a classic example of adaptive radiation. A combination of the specialized color vision and the range of light color in the water helps to reproductively isolate each cichlid species. Recent pollution has reduced visibility, and hybridization among species has increased. One previously rare, surface-feeding cichlid, Yssichromis pyrrhocephalus, appears to have adapted: it has evolved extra large gills, feeds at the bottom now, and somehow avoids predation by Nile perch. (Like other fishes, cichlids are masters of speciation. Half of all species of vertebrates are fish. Of the roughly 30,000 fish species, nearly 10 percent are cichlids. One key aspect of cichlid biology partly explains the fishes rapid diversification. At the surface of a lake, red light dominates, while deeper down, blue light dominates. Fish near the surface can see red well and prefer reddish males, whereas in deep water females prefer bluish males because they can see blue better. The combination of cichlids specialized color vision and the range of light color in the water helps to reproductively isolate each cichlid species, setting the stage for speciation. But because of pollution in the last 50 years, these fish can hardly see. Instead of separating into distinct mating groups, now the fish swim all over and hybridize with other species. With about two-thirds of Lake Victorias cichlid species extinct, the lake is awash in empty niches that an evolving fish can exploit. One species, Yssichromis pyrrhocephalus, seems to have jumped at this opportunity. Before the lakewide extinctions, this cichlid fed on plankton floating on the surface of the lake. It had nearly gone extinct when, in 2008, biologists discovered a surviving population that fed by rooting around in the mud at the bottom of the lake. The new Yssichromis pyrrhocephalus had gills for absorbing oxygen that were two-thirds larger than beforepossibly because eutrophication had reduced oxygen levels in the water. Y. pyrrhocephalus 2.0 also had a strikingly different head shape, which apparently helped it prey on tough invertebrates in the muddy lake bottom. The fish had also changed its behavior: instead of avoiding the voracious Nile perch, it somehow managed to live in close proximity to this dangerous predator; how, biologists didnt know.) List of Key Terms: Chapter 15
adaptive radiation (p. 348) allopatric speciation (p. 344) analogous (p. 350) ancestral species (p. 348) biological species concept (p. 341) convergent evolution (p. 350) genetic heritage (p. 341) geographic isolation (p. 343) homologous (p. 349) morphological species concept (p. 340) polyploid (p. 346) postzygotic barrier (p. 341) prezygotic barrier (p. 341) reproductive isolation (p. 342) ring species (p. 346) speciation (p. 342) species (p. 340) sympatric speciation (p. 346) vestigial organ (p. 349) [Phenotypic diversity in bat sea stars, Patitria miniata. The sea anemone are Anthopleura elegantissima.] Class Quiz, Part 1 One species of frog in a pond splits into two
species because males develop two different mating calls. This is an example of endemism. polyploidy. behavioral isolation. The correct answer is C. Mouse click brings up arrow pointing to the correctanswer. The frogs live in the same pond but demonstrate different behaviors by theirmating calls. Because they live in the same pond, ecological isolation isincorrect. Class Quiz, Part 2 Which of the following is not a reproductive
isolation mechanism? hybrid fertility zygote death gametic isolation The correct answer is A. Mouse click brings up arrow pointing to the correctanswer. Hybrids being fertile does not keep species separated; it allows them to mix. Class Quiz, Part 3 Gene flow between two populations
A. is likely to prevent allopatric speciation between the two populations. B. is necessary for sympatric speciation. C. is the main mechanism by which prezygotic barriers are formed. D. is the main mechanism by which postzygotic barriers are formed. The correct answer is A. Mouse click brings up arrow pointing to the correctanswer. Gene flow must be blocked before two populations will diverge via allopatricspeciation. It will not promote sympatric speciation either, and certainly isntnecessary. Gene flow is not relevant in the context of prezygotic andpostzygotic reproductive barriers. Relevant Art from Other Chapters
All art files from the book are available in JPEG and PPT formats online and on the Instructor Resource Disc An Example of Rapid Evolution
(From a previous edition of Discover Biology) Soapberry Bugs Have Rapidly Adapted to Changes in the Fruits They Eat (a) In Florida, soapberry bugs traditionally fed on seeds of a native plant species, the balloon vine. The bugs had to pierce to the center of the balloon vines fruit to reach the seeds at the center. (b) Over the past 3050 years, some populations of soapberry bugs have evolved short beaks, enabling them to feed on seeds within the narrower fruit of an introduced species, the golden rain tree. 15.1 Concept Check, Part 1 1. Distinguish between the morphological and biological species concepts. ANSWER: The morphological species concept defines species by similarities in their physical form. The biological species concept defines species as organisms that can actually or potentially interbreed. 15.1 Concept Check, Part 2 2. What is the fundamental requirement for speciation? ANSWER: Reproductive isolation 15.2 Concept Check, Part 1 1. What is allopatric speciation?
ANSWER: The formation of new species that occurs when two populations become reproductively isolated because of a geographic barrier 15.2 Concept Check, Part 2 2. Why is sympatric speciation generally thought to be more difficult than allopatric speciation? ANSWER: When organisms inhabit the same area, the likelihood of gene flow between them is higher. Gene flow reduces the probability of speciation. 15.3 Concept Check, Part 1 1. Under what conditions might an adaptive radiation occur? ANSWER: Colonization of a new habitat, mass extinction of competitor species, or the evolution of a novel trait that confers significant evolutionary advantage 15.3 Concept Check, Part 2 2. How might a small amount of gene flow affect speciation rates? ANSWER: Gene flow homogenizes gene pools, making speciation more difficult. 15.4 Concept Check, Part 1 1. What is the relationship between speciation and homologous traits? ANSWER: Speciation occurs when a single lineage diverges to become two or more. The descendant lineages share similarities due to their common ancestry. These traits are called homologous traits. 15.4 Concept Check, Part 2 2. What is the main force that drives two separate species to exhibit convergent evolution? ANSWER: Natural selection 15.5 Concept Check, Part 1 1. What factors govern the rate of speciation? ANSWER: Speciation is thought to happen most often when populations are geographically isolated. However, some species exhibit more rapid speciation because of ecological specialization, while others may be separated for long periods of time without significant genetic differentiation. 15.5 Concept Check, Part 2 2. How might a small amount of gene flow affect speciation rates? ANSWER: Since speciation requires reproductive isolation, a small amount of gene flow will slow speciation rates.