chapter 14 the origin of species © 2012 pearson education, inc. bowerbirds, native to new guinea...

Chapter 14 The Origin of Species

© 2012 Pearson Education, Inc.

Bowerbirds, native to New Guinea and Australia, are named for the structure, called a bower, that the male weaves from twigs and grasses to attract females.

After building his bower, the male collects objects such as fruits, seeds, insect parts, rocks, flowers, and leaves and arranges them artfully by color and type.

Figure 14.01

Females are dull colored (as are males) and tour the bowersof local males, inspecting eachwhile its owner courts her with a song and dance.

Vogelkop bowerbird photograph by Barrie Britton

DEFINING SPECIES


14.1 The origin of species is the source of biological diversity

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.

– Every time speciation occurs, the diversity of life increases.

– The many millions of species on Earth have all arisen from an ancestral life form that lived around 3.5 billion years ago.


14.2 There are several ways to define a species

The word species is from the Latin for “kind” or “appearance.”

Although the basic idea of species as distinct life-forms seems intuitive, devising a more formal definition is not easy and raises questions.

– How similar are members of the same species?

– What keeps one species distinct from others?


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.

– Therefore, members of a species are similar because they reproduce with each other.



Reproductive isolation

– prevents members of different species from mating with each other,

– prevents gene flow between species, and

– maintains separate species.

– Therefore, species are distinct from each other because they do not share the same gene pool.



Figure 14.2A Similarity between two species: the eastern meadowlark (left) and western meadowlark (right). Similar looking but different songs and mating behavior

Figure 14.2BDiversity within one species

The biological species concept can be problematic.

– Some pairs of clearly distinct species occasionally interbreed and produce hybrids.

– For example, grizzly bears and polar bears may interbreed and produce hybrids called grolar bears.

– Melting sea ice may bring these two bear species together more frequently and produce more hybrids in the wild.

– Reproductive isolation cannot usually be determined for extinct organisms known only from fossils.

– Reproductive isolation does not apply to prokaryotes or other organisms that reproduce only asexually.

– Therefore, alternate species concepts can be useful.



Figure 14.2C Hybridization between two species of bears

Grizzly bear Polar bear

Hybrid “grolar” bear

The morphological species concept

– classifies organisms based on observable physical traits and

– can be applied to

– asexual organisms and

– fossils.

– However, there is some subjectivity in deciding which traits to use.



The ecological species concept

– defines a species by its ecological role or niche and

– focuses on unique adaptations to particular roles in a biological community.

– For example, two species may be similar in appearance but distinguishable based on

– what they eat or

– where they live.



The phylogenetic species concept

– defines a species as the smallest group of individuals that shares a common ancestor and thus

– forms one branch of the tree of life.

– Biologists trace the phylogenetic history of a species by comparing its

– morphology or

– DNA.

– However, defining the amount of difference required to distinguish separate species is a problem.



14.3 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

– postzygotic.


Five types of prezygotic barriers prevent mating or fertilization between species.

1. In habitat isolation, two species live in the same general area but not in the same kind of place.

2. In temporal isolation, two species breed at different times (seasons, times of day, years).



Figure 14.3

Habitat isolation(lack of opportunities to encounter each other)

The garter snake Thamnophisatratus lives mainly in water.

The garter snake Thamnophis sirtalis lives on land.

Figure 14.3

Temporal isolation(breeding at different times or seasons)

The eastern spotted skunk(Spilogale putorius) breeds inlate winter. The western spotted skunk

(Spilogale gracilis) breeds inthe fall.

Prezygotic Barriers, continued

3. In behavioral isolation, there is little or no mate recognition between females and males of different species.

4. In mechanical isolation, female and male sex organs are not compatible.

5. In gametic isolation, female and male gametes are not compatible.



Figure 14.3

Behavioral isolation(different courtship rituals)

The blue-footed booby(Sula nebouxii) performs anelaborate courtship dance.

The masked booby(Sula dactylatra) performsa different courtship ritual.

Figure 14.3

Mechanical isolation(physical incompatibility of reproductive parts)

Heliconia pogonantha ispollinated by hummingbirdswith long, curved bills.

Heliconia latispatha is pollinated by hummingbirds with short, straight bills.

Figure 14.3

Gametic isolation(molecular incompatibility of eggs and sperm

or pollen and stigma)

Purple sea urchin(Strongylocentrotuspurpuratus)

Red sea urchin(Strongylocentrotusfranciscanus)

Three types of postzygotic barriers operate after hybrid zygotes have formed.

1. In reduced hybrid viability, most hybrid offspring do not survive.

2. In reduced hybrid fertility, hybrid offspring are vigorous but sterile.

3. In hybrid breakdown,

– the first-generation hybrids are viable and fertile but

– the offspring of the hybrids are feeble or sterile.



Figure 14.3

Reduced hybrid viability(hybrid development or survival impaired

by interaction of parental genes)

Some salamander species can hybridize,but their offspring do not develop fully orare frail and will not survive long enoughto reproduce.

Figure 14.3

Reduced hybrid fertility(vigorous hybrids that cannot

produce viable offspring)

A mule is the sterile hybridoffspring of a horse and a donkey.

Figure 14.3

Hybrid breakdown(viable and fertile hybrids with feeble

or sterile offspring)

The rice hybrids on the left and rightare fertile, but plants of the nextgeneration (middle) are sterile.

MECHANISMS OF SPECIATION


14.4 In allopatric speciation, geographic isolation leads to speciation

In allopatric speciation, populations of the same species are geographically separated, isolating their gene pools.

Isolated populations will no longer share changes in allele frequencies caused by

– natural selection,

– genetic drift, and/or

– mutation.


Gene flow between populations is initially prevented by a geographic barrier. For example

– the Grand Canyon and Colorado River separate two species of antelope squirrels, and

– the Isthmus of Panama separates 15 pairs of snapping shrimp.

14.4 In allopatric speciation, geographic isolation leads to speciation


Figure 14.4A Allopatric speciation of geographically isolated antelope squirrels(Ammospermophilus)

South rim

A. harrisii

North rim

A. leucurus

Figure 14.4B Allopatric speciation in snapping shrimp (Alpheus)

Isthmus of Panama

A. millsae

A. nuttingiA. formosus

A. panamensis

ATLANTIC OCEAN

PACIFIC OCEAN

14.5 Reproductive barriers can evolve as populations diverge

How do reproductive barriers arise?

Experiments have demonstrated that reproductive barriers can evolve as a by-product of changes in populations as they adapt to different environments.

These studies have included

– laboratory studies of fruit flies and

– field studies of monkey flowers and their pollinators.


Figure 14.5A Evolution of reproductive barriers in laboratory populations of fruit flies adapted to different food sources

Starch medium Maltose medium

Initial sampleof fruit flies

Mating experiments

FemaleFemale

Results Population#1

Population#2Starch Maltose

Ma

le

Ma

lto

se

Sta

rch

22

8 20

9 18 15

1512Ma

le

Po

p#

2P

op

#1

Number of matingsin experimental groups

Number of matingsin starch control groups

Figure 14.5B Transferring an allele between monkey flowers changes flower color and influences pollinator choice in Mimulus.

Pollinator choice intypical monkey flowers

Typical M. lewisii(pink)

M. lewisii withred-color allele

Typical M. cardinalis(red)

M. cardinalis withpink-color allele

Pollinator choice aftercolor allele transfer

14.6 Sympatric speciation takes place without geographic isolation

Sympatric speciation occurs when a new species arises within the same geographic area as a 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,

– habitat differentiation, or

– sexual selection.


Many plant species have evolved by polyploidy in which cells have more than two complete sets of chromosomes.

Sympatric speciation can result from polyploidy

– within a species (by self-fertilization) or

– between two species (by hybridization).

14.6 Sympatric speciation takes place without geographic isolation


Figure 14.6A Sympatric speciation by polyploidy within a single species

Parentspecies2n = 6

Tetraploidcells

4n = 12

Diploidgametes2n = 6

Viable, fertiletetraploidspecies4n = 12

Self-fertilization

31

2

Figure 14.6B Sympatric speciation producing a hybrid polyploid from two different species

Species A2n = 4

Gameten = 2

Gameten = 3

Species B2n = 6

Chromosomescannot pair

Can reproduceasexually

Sterile hybridn = 5

1

2

Viable, fertilehybrid species

2n = 10

3

14.7 EVOLUTION CONNECTION: Most plant species trace their origin 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 most of these species.



Polyploid plants include

– cotton,

– oats,

– potatoes,

– bananas,

– peanuts,

– barley,


– plums,

– apples,

– sugarcane,

– coffee, and

– bread wheat.

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

– the result of hybridization and polyploidy.



Figure 14.7 The evolution of bread wheat, Triticum aestivum

DomesticatedTriticum monococcum(14 chromosomes)

AA

DDAABB

Wild Triticum(14 chromo-somes)

Hybridization

AB

Sterile hybrid(14 chromosomes)

1

2

3

4

Cell division errorand self-fertilization

Hybridization

WildT. tauschii(14 chromosomes)

T. turgidumEmmer wheat(28 chromosomes)

ABD

Sterile hybrid(21 chromosomes)

Cell division errorand self-fertilization

AABBDD

T. aestivumBread wheat(42 chromosomes)

BB

14.8 Isolated islands are often showcases of speciation

Most of the species on Earth are thought to have originated by allopatric speciation.

Isolated island chains offer some of the best evidence of this type of speciation.

Multiple speciation events are more likely to occur in island chains that have

– physically diverse habitats,

– islands far enough apart to permit populations to evolve in isolation, and

– islands close enough to each other to allow occasional dispersions between them.



The evolution of many diverse species from a common ancestor is adaptive radiation.

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,

– was colonized gradually from other islands and the South America mainland, and

– has many species of plants and animals found nowhere else in the world.



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 finches

– share many finchlike traits,

– differ in their feeding habits and their beaks, specialized for what they eat, and

– arose through adaptive radiation.


Figure 14.8

Cactus-seed-eater (cactus finch)

Tool-using insect-eater (woodpecker finch)

Seed-eater (medium ground finch)

14.10 Hybrid zones provide opportunities to study reproductive isolation

What happens when separated populations of closely related species come back into contact with each other?

Biologists try to answer such questions by studying hybrid zones, regions in which members of different species meet and mate to produce at least some hybrid offspring.


14.10 Hybrid zones provide opportunities to study reproductive isolation

Over time in hybrid zones (Fig 14.10A)

– reinforcement may strengthen barriers to reproduction, such as occurs in flycatchers (Fig. 14.10B), or

– fusion may reverse the speciation process as gene flow between species increases, as may be occurring among the cichlid species in Lake Victoria (Fig. 14.10C).

In stable hybrid zones, a limited number of hybrid offspring continue to be produced.


Threepopulationsof a species

Figure 14.10A Formation of a hybrid zone

Newly formedspecies

PopulationBarrier togene flow

Geneflow

Hybrid individual

Hybrid zone1 2

4

3

Gene flow

Figure 14.10B Reinforcement of reproductive barriers

Malecollaredflycatcher

Malepiedflycatcher

Allopatricpopulations

Sympatricpopulations

Pied flycatcherfrom allopatricpopulation

Pied flycatcherfrom sympatricpopulation

Figure 14.10C Fusion: males of Pundamilia nyererei and Pundamilia pundamilia contrasted with a hybrid from an area with turbid water

Pundamilia nyererei Pundamilia pundamilia

Hybrid: Pundamilia “turbid water”

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

– experience relatively little change for the rest of their existence.

2. Other species appear to have evolved more gradually = gradualism.


Figure 14.11 Two models for the tempo of speciation

Punctuated pattern

Gradual pattern

Time

14.11 Speciation can occur rapidly or slowly

What is the total length of time between speciation events (between formation of a species and subsequent divergence of that species)?

– In a survey of 84 groups of plants and animals, the time ranged from 4,000 to 40 million years.

– Overall, the time between speciation events averaged 6.5 million years.


chapter 14 the origin of species © 2012 pearson education, inc. bowerbirds, native to new guinea...

Documents

species distinct

distinct species

origin of species

millions of species

bear species

biological species concept

andmaintains separate

basic idea of species