Chapter 14

The Origin of Species

RAPID EVOLUTION• One example of rapid evolution occurred among

mosquitoes who migrated into the London underground

• In less than 150 years, Culex pipiens evolved into a new mosquito species, Culex molestus

• The origin of new species is called speciation

• The isolated mosquitoes adapted to their new underground environment

– They altered their prey, mating habits, and breeding patterns

• Environmental barriers that isolate populations are just one of many mechanisms in the evolution of species

• Linnaeus used physical appearance to identify species when he developed the binomial system of naming organisms– This system established the basis for taxonomy

What is a species?CONCEPTS OF SPECIES

• But appearance alone does not always define a species

Figure 14.1A

– Example: eastern and western meadowlarks

• Similarities between some species and variation within a species can make defining species difficult– Humans exhibit extreme physical diversity

Figure 14.1B

• A ring species may illustrate the process of speciation

Figure 14.1C

OREGONPOPULATION

1

2

COASTALPOPULATIONS

Yellow-eyed

Monterey3

SierraNevada

Yellow-blotched

Gap in ring Large-

blotched

INLANDPOPULATIONS

• The biological species concept is not applicable to fossils or asexual organisms

• Most organisms are classified based on observable phenotypes– The morphological species concept

• The genealogical species concept defines a species as a cluster of organisms representing a specific evolutionary lineage

• The ecological species concept defines a species by its ecological role

• Prezygotic and postzygotic reproductive barriers prevent individuals of different species from interbreeding

Reproductive barriers keep species separate

Table 14.2

• Courtship ritual in blue-footed boobies is an example of one kind of prezygotic barrier, behavioral isolation

• Many plant species have flower structures that are adapted to specific pollinators– This is an example of

mechanical isolation, another prezygotic barrier

Figure 14.2A, B

• Hybrid sterility is one type of postzygotic barrier

– A horse and a donkey may produce a hybrid offspring, a mule

– Mules are sterile

Figure 14.2C

• When a population is cut off from its parent stock, species evolution may occur– An isolated population may become genetically

unique as its gene pool is changed by natural selection, genetic drift, or mutation

– This is called allopatric speciation

Geographic isolation can lead to speciation

MECHANISMS OF SPECIATION

Figure 14.3

• On the Galápagos Islands, repeated isolation and adaptation have resulted in adaptive radiation of 14 species of Darwin’s finches

Islands are living laboratories of speciation

Figure 14.4A

• Adaptive radiation on an island chain

Figure 14.4B

Species Afrom mainland

1

A

2B

B

3BC 4

C

C5

BC

D

C D

• In sympatric speciation, a new species may arise without geographic isolation– A failure in meiosis can produce diploid gametes– Self-fertilization can then produce a tetraploid zygote

New species can also arise within the same geographic area as the

parent species

Figure 14.5A

Parent species

2n = 6Diploid

Unreduced diploid gametes

4n = 12Tetraploid

Zygote

Meioticerror

Self-fertilization

Offspring maybe viable andself-fertile

• Sympatric speciation by polyploidy was first discovered by Dutch botanist Hugo de Vries in the early 1900s

Figure 14.5B

• Many plants are polyploid– They are the products of

hybridization– The modern bread wheat is

an example

Connection: Polyploid plants clothe and feed us

Figure 14.6A

• The evolution of wheat

Figure 14.6B

AA BB

WildTriticum(14 chromo-somes)

Triticummonococcum(14 chromosomes)

Sterile hybrid(14 chromosomes)

AB

Meiotic error andself-fertilization

AABB DD

Sterile hybrid

ABD

T. turgidumEMMER WHEAT(28 chromosomes)

T. tauschii(wild)(14 chromosomes)

AA BB DD

T. aestivumBREAD WHEAT(42 chromosomes)

Meiotic error andself-fertilization

• This has been documented by– laboratory

studies (fruit flies)

Reproductive barriers may evolve as populations diverge

Figure 14.7A

Starch medium

Initial sampleof fruit flies Maltose medium

Results ofmating experiments

FemaleStarch Maltose

Ma

le Sta

rch

Ma

lto

se

22 9

8 20

Mating frequenciesin experimental group

Femalepopulations

Same Different

Ma

lep

op

ula

tio

ns

Sa

me

Dif

fere

nt

18 15

12 15

Mating frequenciesin control group

– examples in natural populations (pupfish in Death Valley)

Figure 14.7B

• According to the gradualist model of the origin of species– new species evolve by

the gradual accumulation of changes brought about by natural selection

• However, few gradual transitions are found in the fossil record

The tempo of speciation can appear steady or jumpy

Figure 14.8A

• The punctuated equilibrium model suggests that speciation occurs in spurts– Rapid change occurs

when an isolated population diverges from the ancestral stock

– Virtually no change occurs for the rest of the species’ existence

Figure 14.8B