Date post: | 18-Jan-2018 |
Category: |
Documents |
Upload: | bernard-may |
View: | 254 times |
Download: | 0 times |
If you can't read please download the document
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.