Chapter 17 The Origin of Species.

chapter 17 The Origin of Species

Chapter 17 At a Glance 17.1 What Is a Species?
17.2 How Is Reproductive Isolation Between Species Maintained? 17.3 How Do New Species Form? 17.4 What Causes Extinction? © 2017 Pearson Education, Ltd.

17.1 What Is a Species? Darwinian ideas did not fully explain the diversity of life The process of natural selection alone cannot explain how living things came to be divided into groups © 2017 Pearson Education, Ltd.

17.1 What Is a Species? Each species evolves independently
Today, a species is defined as a group of populations that evolves independently The most widely used standard defines species as “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups” © 2017 Pearson Education, Ltd.

17.1 What Is a Species? Each species evolves independently (continued)
The biological species concept is based on the observation that reproductive isolation (inability to successfully breed outside the group) ensures evolutionary independence © 2017 Pearson Education, Ltd.

17.1 What Is a Species? Each species evolves independently (continued)
There are two limitations to the biological species concept Because the definition is based on patterns of sexual reproduction, it does not help us determine species boundaries among sexually reproducing organisms It is not always practical or even possible to directly observe whether members of two different groups interbreed © 2017 Pearson Education, Ltd.

17.1 What Is a Species? Appearance can be misleading
Organisms of different appearance sometimes belong to the same species The Northwestern garter snake may be brown, black, gray, green, or some shade in between and may or may not have stripes These snakes are still members of the same species © 2017 Pearson Education, Ltd.

Figure 17-1 Members of a species may differ in appearance
(a) A green-striped Northwestern garter snake (b) A red-striped Northwestern garter snake © 2017 Pearson Education, Ltd.

17.1 What Is a Species? Appearance can be misleading (continued)
Organisms of similar appearance sometimes belong to different species The cordilleran flycatcher and Pacific slope flycatcher are so similar that birdwatchers can’t tell them apart These birds were considered to be a single species, but research revealed that they do not interbreed and are in fact two different species © 2017 Pearson Education, Ltd.

Figure 17-2 Members of different species may be similar in appearance
(a) Cordilleran flycatcher (b) Pacific-slope flycatcher © 2017 Pearson Education, Ltd.

17.1 What Is a Species? Appearance can be misleading (continued)
There is no visible difference between Anopheles dirus and Anopheles harrisoni The appearance disguises a crucial difference important to people A. dirus spreads the often deadly disease malaria from person to person © 2017 Pearson Education, Ltd.

17.2 How Is Reproductive Isolation Between Species Maintained?
Traits that prevent interbreeding and maintain reproductive isolation are called isolating mechanisms The benefit of isolating mechanisms is the production of no offspring that are unfit or sterile Isolating mechanisms prevent the wasting of reproductive effort and failing to contribute to future generations Natural selection favors traits that prevent reproduction across species boundaries © 2017 Pearson Education, Ltd.

Premating isolating mechanisms prevent mating between species Reproductive isolation can be maintained by a variety of mechanisms Those mechanisms that prevent mating attempts are especially effective © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following Members of different species may not meet Geographical isolation prevents interbreeding between populations that do not come into contact because they live in different, physically separated places © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Members of different species may be prevented from meeting This type of isolation is usually considered to be a mechanism that allows new species to form rather than one that maintains reproductive isolation between species © 2017 Pearson Education, Ltd.

(a) Kaibab squirrel (b) Abert’s squirrel Figure 17-3
Figure Geographic isolation (a) Kaibab squirrel (b) Abert’s squirrel © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Different species may occupy different habitats Ecological isolation occurs when species do not mate because they occupy different habitats White-crowned sparrows inhabit fields and meadows, while white-throated sparrows inhabit dense thickets © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Ecological isolation occurs when species do not mate because they occupy different habitats (continued) In another example, each species of fig wasp breeds in the fruit of a different species of fig, and the wasps thus do not come into contact with one another © 2017 Pearson Education, Ltd.

Figure 17-4 Ecological isolation
© 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Different species may breed at different times Temporal isolation (time-based isolation) prevents breeding between two species occupying the same habitat because of different breeding seasons The spring field cricket and the fall field cricket both occur in many areas of North America They are unable to interbreed because of their varying breeding seasons © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Different species may breed at different times (continued) In nature, Bishop pines and Monterey pines do not interbreed Bishop pine pollination occurs in summer Monterey pine pollination occurs in early spring © 2017 Pearson Education, Ltd.

(a) Bishop pine (b) Monterey pine Figure 17-5
Figure Temporal isolation (a) Bishop pine (b) Monterey pine © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Different species may have different courtship signals Among animals, elaborate courtship colors and behaviors can prevent mating between species Behavioral isolation is created by signals and behaviors that differ from species to species © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Different species may have different courtship signals (continued) The plumes and arresting pose of a courting male Raggiana bird of paradise are conspicuous indicators of this species There is little chance females of another species will mate with him by mistake © 2017 Pearson Education, Ltd.

Figure 17-6 Behavioral isolation

Premating isolating mechanisms include the following (continued) Different species may have different courtship signals (continued) Male frogs embrace any female regardless of species Female frogs encountering males of a different species utter the “release call,” which causes the males to let go As a result, few hybrids—offspring of parents of different species—are produced © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Differing sexual organs may foil mating attempts If a male and female of different species attempt to mate, the attempt may be disrupted by physical differences between the species collectively known as mechanical incompatibilities © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Differing sexual organs may foil mating attempts (continued) In animals with internal fertilization, male and female sexual organs may not fit together Incompatible body shapes may make copulation between species impossible For example, snails of species whose shells have left-handed spirals may be unable to copulate successfully with snails whose shells have right-handed spirals © 2017 Pearson Education, Ltd.

Premating isolating mechanisms include the following (continued) Differing sexual organs may foil mating attempts (continued) In plants, differences in flower size or structure may attract different pollinators © 2017 Pearson Education, Ltd.

On the basis of differences in DNA and structure, West African elephants are thought to be a different species from Central, East, and South African savanna elephants and Central African forest elephants. Which isolation mechanism has occurred? Geographical isolation Ecological isolation Temporal isolation Behavioral isolation Question: 17-2 Answer: a Diff: Moderate Text Ref: Section 17.2 Skill: Application Notes: The answer should be obvious in that the different populations are from different geographic areas. © 2017 Pearson Education, Ltd.

Two populations of the columbine flower had lived at different altitudes in the same area. When the two were reunited, they could no longer breed. Which isolation mechanism had occurred? Geographical isolation Ecological isolation Temporal isolation Behavioral isolation Question: 17-3 Answer: b Diff: Hard Text Ref: Section 17.2 Skill: Application Notes: This question can be difficult for students in that they may first guess geographical isolation, but the habitats are in the same general area. © 2017 Pearson Education, Ltd.

One firefly population mates right after sundown, and another mates in the middle of the night. Which isolation mechanism is occurring? Geographical isolation Ecological isolation Temporal isolation Behavioral isolation Question: 17-4 Answer: c Diff: Hard Text Ref: Section 17.2 Skill: Application Notes: This question is difficult for students who do not know what “temporal” means. © 2017 Pearson Education, Ltd.

A female cricket does not understand the mating call of a male cricket
A female cricket does not understand the mating call of a male cricket. Which isolation mechanism is occurring? Geographical isolation Ecological isolation Temporal isolation Behavioral isolation Question: 17-5 Answer: d Diff: Moderate Text Ref: Section 17.2 Skill: Application Notes: A dance, a change in color, or a mating call can all be involved in behavioral isolation. © 2017 Pearson Education, Ltd.

Sea urchins spawn their gametes freely into the surrounding ocean waters. The sperm of one type of urchin will not fertilize the egg of another type of urchin. Which isolation mechanism is occurring? Geographical isolation Ecological isolation Temporal isolation Gametic incompatibility Question: 17-6 Answer: d Diff: Easy Text Ref: Section 17.2 Skill: Application Notes: Remind students that they can glean information from the words alone, which provide clues to these different types of isolation mechanisms. The answer should be obvious from the words “gametic incompatibility” that the gametes are unsuitable for one another. © 2017 Pearson Education, Ltd.

Postmating isolating mechanisms limit hybrid offspring Failing or unevolving premating isolation mechanisms may cause members of different species to mate If the resulting offspring die during development, the two species remain reproductively isolated from each other Mechanisms that prevent the formation of vigorous, fertile hybrids between species are called postmating isolating mechanisms © 2017 Pearson Education, Ltd.

Postmating isolating mechanisms limit hybrid offspring (continued) One species’ sperm may fail to fertilize another species’ egg Gametic incompatibility occurs when sperm from one species cannot fertilize eggs of another In animals with internal fertilization, fluids of the female reproductive tract may weaken or kill sperm of another species © 2017 Pearson Education, Ltd.

Postmating isolating mechanisms limit hybrid offspring (continued) One species’ sperm may fail to fertilize another species’ egg (continued) Gametic incompatibility may be an especially important isolating mechanism in certain species Marine invertebrates and wind-pollinated plants reproduce by scattering gametes in the water or in the air © 2017 Pearson Education, Ltd.

Postmating isolating mechanisms limit hybrid offspring (continued) One species’ sperm may fail to fertilize another species’ egg (continued) Sea urchin sperm cells contain a protein that allows them to bind only to eggs of their own species © 2017 Pearson Education, Ltd.

Postmating isolating mechanisms limit hybrid offspring (continued) Hybrid offspring may fail to survive or reproduce If cross-species fertilization occurs, the resulting hybrid may be unable to survive, a situation called hybrid inviability © 2017 Pearson Education, Ltd.

Postmating isolating mechanisms limit hybrid offspring (continued) Hybrid offspring may fail to survive or reproduce (continued) The genetic instructions directing development of the two species may be so different that hybrids abort early in development The hybrid may abort early in development or die shortly after birth © 2017 Pearson Education, Ltd.

Postmating isolating mechanisms limit hybrid offspring (continued) Hybrid offspring may fail to survive or reproduce (continued) The hybrid may be unable to reproduce because it displays behaviors that are mixtures of the two parental types Lovebird hybrids have great difficulty learning to carry nest materials during flight and probably could not reproduce in the wild © 2017 Pearson Education, Ltd.

Postmating isolating mechanisms limit hybrid offspring (continued) Hybrid offspring may be infertile Mule hybrids (a cross between a horse and a donkey) are sterile Liger hybrids (a zoo-based cross between a male lion and a female tiger) are sterile Hybrid infertility prevents hybrids from passing on their genetic material to offspring, thus blocking gene flow between the two parent populations © 2017 Pearson Education, Ltd.

Postmating isolating mechanisms limit hybrid offspring (continued) Hybrid offspring may be infertile (continued) Infertility is often caused by the failure of chromosomes to pair properly during meiosis, so eggs and sperm never develop © 2017 Pearson Education, Ltd.

Figure 17-7 Hybrid infertility

A leopard frog and a wood frog mate and produce offspring, which die within a few days. Which isolation mechanism has occurred? Geographical isolation Ecological isolation Hybrid inviability Hybrid infertility Question: 17-7 Answer: c Diff: Moderate Text Ref: Section 17.2 Skill: Application Notes: Many students confuse hybrid inviability with hybrid infertility. If students think about what the word “viable” means, they will have an easier time with this one. © 2017 Pearson Education, Ltd.

A cross between a male tiger and a female lion is called a tigon
A cross between a male tiger and a female lion is called a tigon. The tigon is sterile. Which isolation mechanism has occurred? Geographical isolation Ecological isolation Hybrid inviability Hybrid infertility Question: 17-8 Answer: d Diff: Moderate Text Ref: Section 17.2 Skill: Application Notes: Because of the movie Napoleon Dynamite, students may be familiar with ligers—but when the sexes are reversed, a tigon is produced. A liger is produced by a male lion and a female tiger. © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Darwin did not propose a complete mechanism of speciation, the process by which new species form Speciation depends on two processes Isolation Genetic divergence © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Speciation depends on two processes (continued) Isolation of populations If individuals move freely between two populations, interbreeding and the resulting gene flow will cause changes in one population to become widespread in the other Two species cannot grow increasingly different unless something happens to block interbreeding between them © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Speciation depends on two processes (continued) Genetic divergence of populations This must occur through the evolution of isolating mechanisms, driven by genetic drift or natural selection © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Speciation always requires isolation followed by divergence Evolutionary biologists group the different pathways to speciation into two broad categories In allopatric speciation, two populations are geographically separated from one another In sympatric speciation, two populations share the same geographic area © 2017 Pearson Education, Ltd.

Animation: The Process of Speciation

17.3 How Do New Species Form? Geographic separation of a population can lead to allopatric speciation New species can arise by allopatric speciation when an impassible barrier physically separates different parts of a population © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Geographic separation of a population can lead to allopatric speciation (continued) Organisms may colonize isolated habitats A small population may become isolated if it moves to a new location Some members of a land-dwelling population might colonize an island © 2017 Pearson Education, Ltd.

Figure 17-8 Allopatric isolation and divergence
Part of a mainland population reaches an isolated island. many generations pass Over many generations, the isolated populations begin to diverge due to genetic drift and natural selection. Figure Allopatric isolation and divergence many generations pass Divergence may eventually become sufficient to cause reproductive isolation. © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Geographic separation of a population can lead to allopatric speciation (continued) Geological and climate changes may divide populations Isolation can result from landscape changes that divide a population Rising sea levels might change a coastal hilltop into an island, isolating the residents © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Geographic separation of a population can lead to allopatric speciation (continued) Isolation can result from landscape changes that divide a population (continued) New rock from a volcanic eruption can divide a previously continuous sea or lake, splitting populations A river that changes course can also divide populations Continental drifts have divided many populations © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Geographic separation of a population can lead to allopatric speciation (continued) Natural selection and genetic drift may cause isolated populations to diverge If two populations become geographically isolated for any reason, there will be no gene flow between them If the environments of the locations differ, then natural selection may favor different traits in the different locations © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Geographic separation of a population can lead to allopatric speciation (continued) Natural selection and genetic drift may cause isolated populations to diverge (continued) If one or more of the separated populations is small enough that genetic drifts occur, genetic differences may arise Geographic isolation followed by allopatric speciation has been the most common source of new species © 2017 Pearson Education, Ltd.

Animation: Allopatric Speciation

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation Ecological isolation can reduce gene flow Two populations of a species living in the geographical area may occupy the two habitats Through the action of natural selection, different members of the species begin to specialize in one habitat or the other © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation (continued) Ecological isolation can reduce gene flow (continued) Two populations of a species living in the geographical area may occupy the two habitats (continued) The two isolated populations may diverge sufficiently genetically so that they can’t interbreed, and split into two species © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation (continued) Ecological isolation can reduce gene flow (continued) Two sympatric populations of fruit flies (Rhagoletis pomonella) may be evolving into two separate species Rhagoletis is a parasite of the American hawthorn tree © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation (continued) Ecological isolation can reduce gene flow (continued) One population lays its eggs in hawthorn fruit The fruit is eaten after the maggots hatch Today, Rhagoletis is splitting into two species: one that breeds on apples and one that breeds on hawthorns © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation (continued) Ecological isolation can reduce gene flow (continued) The two kinds of flies will become two species only if they maintain reproductive separation Apple flies and hawthorn flies do not interbreed Female flies usually lay their eggs in the same type of fruit in which they developed Males tend to prefer the same type of fruit in which they developed © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation (continued) Ecological isolation can reduce gene flow (continued) Apple flies and hawthorn flies do not interbreed (continued) Apple-liking males will encounter and mate with apple-liking females Apples mature 2–3 weeks later than do hawthorn fruits © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation (continued) Ecological isolation can reduce gene flow (continued) Apple flies and hawthorn flies do not interbreed (continued) The two types of flies emerge with timing appropriate for their chosen host fruit The two types of flies have very little chance of meeting © 2017 Pearson Education, Ltd.

Figure 17-10 Sympatric isolation and divergence
Part of a fly population that lives only on hawthorn trees moves to an apple tree. many generations pass Figure Sympatric isolation and divergence If flies living on the apple tree rarely encounter flies living on the hawthorn tree, the populations may diverge over many generations. © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation (continued) Mutations can lead to genetic isolation New species may arise almost instantaneously as a result of mutations that change the number of chromosomes in their cells © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation (continued) The acquisition of multiple copies of each chromosome is known as polyploidy and is a frequent cause of sympatric speciation In general, polyploid individuals cannot mate successfully with normal diploid individuals and so are genetically isolated from the parent species © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Genetic isolation without geographic separation can lead to sympatric speciation (continued) The acquisition of multiple copies of each chromosome is known as polyploidy and is a frequent cause of sympatric speciation (continued) Polyploid plants are more likely than polyploid animals to be able to reproduce © 2017 Pearson Education, Ltd.

Animation: Speciation by Polyploidy

Two populations of similar-appearing snapping shrimp live on either side of the isthmus of Panama. When the populations are brought together, neither displays courtship behaviors; they are hostile toward each other. What type of speciation is occurring? Hybrid infertility Hybrid inviability Allopatric Sympatric Question: 17-9 Answer: c Diff: Easy Text Ref: Section 17.3 Skill: Application Notes: Allopatric speciation has occurred in many marine animals on either side of the Isthmus of Panama, which closed about 3 million years ago. © 2017 Pearson Education, Ltd.

More than 300 species of cichlids live in Lake Malawi in Africa
More than 300 species of cichlids live in Lake Malawi in Africa. Although the species are similar, the fish have different diets, gill structures, jaws, and teeth. What type of speciation has occurred? Hybrid infertility Hybrid inviability Allopatric Sympatric Question: 17-10 Answer: d Diff: Easy Text Ref: Section 17.3 Skill: Application Notes: Sympatric speciation is harder for students to understand than allopatric speciation, because the species are living in the same habitat or ecological area. This is a good time to talk about niche specialization and its influence in sympatric speciation. © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Under some conditions, many new species may arise The history of life can be represented by an evolutionary tree similar to the representation of a family tree The base of the evolutionary tree of life represents Earth’s earliest organisms, ending with branches representing today’s species © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Under some conditions, many new species may arise (continued) The history of life can be represented by an evolutionary tree similar to the representation of a family tree (continued) Hypotheses and discoveries about the evolutionary relationships among species are often communicated by depictions of a portion of life’s evolutionary tree © 2017 Pearson Education, Ltd.

Forks represent speciation events. Each line represents a species.
Figure 17-11a Forks represent speciation events. Each line represents a species. Figure 17-11a Interpreting evolutionary trees time past present (a) Evolutionary tree © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Under some conditions, many new species may arise (continued) The mechanisms of speciation and reproductive isolation lead to forking branches in the evolutionary tree of life, as one species splits into two Adaptive radiation is the rise of many new species over a relatively short period of time © 2017 Pearson Education, Ltd.

In an adaptive radiation, multiple speciation events
Figure 17-11b In an adaptive radiation, multiple speciation events may occur rapidly enough that biologists cannot be certain of their order. Figure 17-11b Interpreting evolutionary trees time past present (b) Evolutionary tree representing adaptive radiation © 2017 Pearson Education, Ltd.

17.3 How Do New Species Form? Under some conditions, many new species may arise (continued) This process occurs when populations of one species invade a variety of new habitats with few competitors Silversword plant colonization of the Hawaiian Islands resulted in 30 species © 2017 Pearson Education, Ltd.

(b) Waialeale dubautia
Figure 17-12 (a) Ahinahina (b) Waialeale dubautia Figure Adaptive radiation (c) Kupaoa (d) Na’ena’e ’ula © 2017 Pearson Education, Ltd.

17.4 What Causes Extinction?
Extinction is the death of all members of a species At least 99.9% of all species that ever existed are now extinct The immediate cause of extinction is probably always environmental change Environmental changes that can lead to extinction include habitat destruction and increased competition among species © 2017 Pearson Education, Ltd.

Localized distribution makes species vulnerable Species inhabiting extremely limited ranges may become extinct if the area is disturbed The Devil’s Hole pupfish is found in only one spring-fed waterhole in the Nevada desert If the Devil’s Hole dries up due to a drought or well drilling, its pupfish will immediately vanish Wide-ranging species normally do not succumb to local environmental catastrophes © 2017 Pearson Education, Ltd.

Specialization increases the risk of extinction Species that develop adaptations that favor survival in a specific environment are at risk of becoming extinct The Karner blue butterfly feeds only on the blue lupine plant The habitat of the lupine has been significantly reduced by development Loss of the lupine will lead to extinction of the Karner blue butterfly © 2017 Pearson Education, Ltd.

Figure 17-13 Extreme specialization places species at risk

Interactions with other species may drive a species to extinction Competition and predation serve as agents of natural selection, but can lead to extinction (in some cases) rather than to adaptation Species that are unable to exploit resources more efficiently and effectively than their competitors may become extinct © 2017 Pearson Education, Ltd.

Interactions with other species may drive a species to extinction (continued) Three million years ago, the isthmus of Panama rose above sea level and formed a land bridge between North and South America North American species displaced the vast majority of South American species, many of which became extinct © 2017 Pearson Education, Ltd.

Habitat change and destruction are the leading causes of extinction Habitat change, both contemporary and prehistoric, is the single greatest cause of extinctions Human activities are the primary cause of present-day habitat destruction Clearing of tropical rain forests could lead to the loss of up to half of all current species over the next 50 years © 2017 Pearson Education, Ltd.

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