Chapter 14: Big Ideas
•
Microevolution is the change in the gene pool of a
population from one generation to the next.
•
Speciation
is the process by which one species
splits into two or more species.
•
Each time speciation occurs, the diversity of life
14.1 The origin of species is the source of
biological diversity
•
Over the course of 3.5 billion years,
• an ancestral species first gave rise to two or more different species,
• which then branched to new lineages,
• which branched again,
• until we arrive at the millions of species that live, or
•
How similar are members of the same species?
• Whereas the individuals of many species exhibit•
The
biological species concept
defines a
species as a group of populations whose members
have the potential to interbreed in nature and
produce fertile offspring (offspring that themselves
can reproduce).
•
Thus, members of a biological species are united
14.2 There are several ways to define a
species
•
Reproductive isolation
• prevents genetic exchange (gene flow) and
• maintains a boundary between species.
•
But there are some pairs of clearly distinct species
that do occasionally interbreed.
• The resulting offspring are called hybrids.
• An example is the grizzly bear (Ursus arctos) and
Grizzly bear Polar bear
14.3 VISUALIZING THE CONCEPT:
Reproductive barriers keep species separate
•
Reproductive barriers
• serve to isolate the gene pools of species and
• prevent interbreeding.
•
Depending on whether they function
before
or
after
zygotes form, reproductive barriers are categorized
as
• prezygotic or
•
Five types of
prezygotic barriers
prevent mating
or fertilization between species.
1. In habitat isolation, there is a lack of opportunity for mates to encounter each other.
14.3 VISUALIZING THE CONCEPT:
Reproductive barriers keep species separate
3. In behavioral isolation, there is failure to send or receive appropriate signals.
4. In mechanical isolation, there is physical incompatibility of reproductive parts.
5. In gametic isolation, there is molecular
Mechanical isolation
(incompatible reproductive parts)
Gametic isolation (incompatible gametes)
Reduced hybrid vitality (short-lived hybrids)
Reduced hybrid fertility (sterile hybrids)
Hybrid breakdown (fertile hybrids with
sterile offspring)
Figure 14.3-1
Habitat isolation
(lack of opportunities to encounter each other)
The garter snake Thamnophis atratus lives mainly in water.
The garter snake
Thamnophis sirtalis
Temporal isolation
(breeding at different times or seasons)
The eastern spotted skunk
(Spilogale putorius) breeds in
Figure 14.3-4
Mechanical isolation
(physical incompatibility of reproductive parts)
Heliconia pogonantha is
pollinated by hummingbirds with long, curved bills.
Gametic isolation
(molecular incompatibility of eggs and sperm or pollen and stigma)
Purple sea urchin
(Strongylocentrotus
purpuratus)
Red sea urchin
(Strongylocentrotus
14.3 VISUALIZING THE CONCEPT:
Reproductive barriers keep species separate
•
Three types of
postzygotic barriers
operate after
hybrid zygotes have formed.
1. In reduced hybrid viability, interaction of parental genes impairs the hybrid’s development or
survival.
2. In reduced hybrid fertility, hybrids are vigorous but cannot produce viable offspring.
Reduced hybrid viability
(hybrid development or survival impaired by interaction of parental genes)
Figure 14.3-7
Reduced hybrid fertility (vigorous hybrids that cannot
produce viable offspring)
A mule is the sterile hybrid
•
A key event in the origin of a new species is the
separation of a population from other populations
of the same species.
• With its gene pool isolated, the splinter population can follow its own evolutionary course.
14.4 In allopatric speciation, geographic
isolation leads to speciation
•
In
allopatric speciation
, the initial block to gene
•
Several geologic processes can isolate
populations.
• A mountain range may emerge and gradually split a population of organisms that can inhabit only
lowlands.
• A large lake may subside until there are several smaller lakes, isolating certain fish populations.
• Continents themselves can split and move apart.
• Allopatric speciation can also occur when
14.4 In allopatric speciation, geographic
isolation leads to speciation
•
How large must a geographic barrier be to keep
allopatric populations apart?
• The answer depends on the ability of the organisms to move.
• Birds, mountain lions, and coyotes can easily cross mountain ranges.
• In contrast, small rodents may find a canyon or a wide river a formidable barrier. The Grand Canyon and Colorado River separate two species of
South rim North rim
14.4 In allopatric speciation, geographic
isolation leads to speciation
• Thirty species of snapping shrimp in the genus
Alpheus live off the Isthmus of Panama, the land bridge that connects South and North America.
• Morphological and genetic data group these shrimp into 15 pairs of species, with the members of each pair
being each other’s closest relative.
• In each case, one member of the pair lives on the
Atlantic side of the isthmus, while the other lives on the Pacific side.
Isthmus of Panama ATLANTIC OCEAN
14.5 Reproductive barriers can evolve as
populations diverge
•
How do reproductive barriers arise?
•
The environment of an isolated population may
include
• different food sources,
• different types of pollinators, and
• different predators.
•
As a result of natural selection acting on
•
Sympatric speciation
occurs when a new species
arises within the same geographic area as its
parent species.
•
How can reproductive isolation develop when
members of sympatric populations remain in
contact with each other?
•
Gene flow between populations may be reduced by
• polyploidy,
14.6 Sympatric speciation takes place
without geographic isolation
•
Many plant species have originated from sympatric
speciation that occurs when accidents during cell
division result in extra sets of chromosomes.
•
New species formed in this way are
polyploid
, in
•
Sympatric speciation can result from polyploidy
• within a species (by self-fertilization) orFigure 14.6a-3
Self-fertilization
Parent species 2n = 6
Tetraploid cells 4n = 12
Diploid gametes
2n = 6
Viable, fertile tetraploid
species 4n = 12
1
Species A
2n = 4 1
2
3
Species B 2n = 6
Gamete
n = 2
Chromosomes cannot pair
Gamete
n = 3
Sterile hybrid
n = 5
Can reproduce asexually
Viable, fertile hybrid species
14.7 EVOLUTION CONNECTION: The origin of
most plant species can be traced to polyploid
speciation
•
Plant biologists estimate that 80% of all living plant
species are descendants of ancestors that formed
by polyploid speciation.
•
Hybridization between two species accounts for
speciation
•
Polyploid plants include
• cotton, • oats, • potatoes, • bananas, • peanuts, • barley, • plums, • apples, • sugarcane,
• coffee, and
14.7 EVOLUTION CONNECTION: The origin of
most plant species can be traced to polyploid
speciation
•
Wheat
• has been domesticated for at least 10,000 years and
• is the most widely cultivated plant in the world.
•
Bread wheat,
Triticum aestivum
, is
• a polyploid with 42 chromosomes and
Wild Triticum (14 chromosomes) Domesticated Triticum monococcum (14 chromosomes) Sterile hybrid (14 chromosomes) T. turgidum Emmer wheat (28 chromosomes) Wild T. tauschii (14 chromosomes) Sterile hybrid (21 chromosomes) 1 2 3 4 Hybridization
Cell division error and self-fertilization
Hybridization
Cell division error and self-fertilization
AABB
AB
DD
14.8 Isolated islands are often showcases of
speciation
•
Isolated island chains are often inhabited by
unique collections of species.
•
Islands that have physically diverse habitats and
that are far enough apart to permit populations to
evolve in isolation but close enough to allow
occasional dispersions to occur are often the sites
of multiple speciation events.
•
The evolution of many diverse species from a
•
The Galápagos Archipelago
• is located about 900 km (560 miles) west of Ecuador,
• is one of the world’s great showcases of adaptive radiation,
• was formed naked from underwater volcanoes from 5 million to 1 million years ago,
• was colonized gradually from other islands and the South America mainland, and
14.8 Isolated islands are often showcases of
speciation
•
The Galápagos Islands currently have 14 species
of closely related finches, called Darwin’s finches,
because Darwin collected them during his
around-the-world voyage on the
Beagle
.
•
These birds
• share many finchlike traits,
• differ in their feeding habits and their beaks, specialized for what they eat, and
Cactus-seed-eater (cactus finch)
14.11 Speciation can occur rapidly or slowly
•
There are two models for the tempo of speciation.
1. The punctuated equilibria model draws on the fossil record, where species change most as they arise from an ancestral species and then change relatively little for the rest of their existence.
2. Other species appear to have evolved more gradually.
•
The time interval between speciation events varies
•
Convergent evolution
Patterns in Evolution
•
Divergent evolution
• Closely related species living in different environments and facing different environmental challenges
•
Adaptive radiation
• Sometimes, several species will evolve
Patterns in Evolution
•
Coevolution
• Evolution of one species affects the evolution of another species.
• As predators evolve, prey evolves. As prey evolves, predators evolve.
• Example: Cheetahs feed on Thompson’s gazelles
•
Gradualism
• Evolution may occur as a slow, gradual
process of change.
•
Theory favored by
Patterns in Evolution
• Punctuated equilibrium
• Evolution may proceed with long periods of relatively little change (stasis)
punctuated with short
periods of intense change.
• Examples: House flies,
Evolution does not always occur
•
Natural selection can
only work on existing
variation.
•
Variation cannot be
created on demand.
•
Some species lack
•
Everywhere we look on the Earth, you can find
organisms.
• Scientists have described 1.5 millions species
• Still many species yet to be described.
Darwin recognizes three special
cases of natural selection
• Sexual selection explained flamboyant appearance amongst birds.
• Sexual selection: female birds choose mates and are attracted to males with extreme features.
•
Altruism
• Individual behavior
benefits a social group often at the expense of the individual.
• Example: worker bees in a hive
•
