Figure 4.3 (b)

24 The Origin of species

Species and Speciation

• Fundamental unit of classification is the species.

• Species = a group of populations in which genes are actually, or potentially, exchanged through interbreeding.

• Problems– Reproductive criterion must be assumed based on

phenotype and ecological information.– Asexual reproduction– Fossil – Geographical isolation

• The origin of new species, or speciation– Is at the focal point of evolutionary theory,

because the appearance of new species is the source of biological diversity

• Evolutionary theory– Must explain how new species originate in

addition to how populations evolve

Microevolution, Macroevolution, and Evidence of Macroevolutionary change

~ Bacteria gain resistance to antibiotics over time• A change in frequency of alleles in populations over time is

called Microevolution.

• Over longer timescales, microevolutionary processes result in large scale changes that result in formation of new species called Macroevolution (species level)

• Evidence of Macroevolution- patterns of plant and animal distribution, fossils, anatomical structures, and developmental processes

• Concept 24.1: The biological species concept emphasizes reproductive isolation

• Species– Is a Latin word meaning “kind” or

“appearance”

Reproductive isolation leads to Speciation

- the formation of new species• Requirement

– Subpopulations are prevented from interbreeding

– Gene flow does not occur (Reproductive isolation)

• Reproductive isolation can result in evolution• Natural selection and genetic drift can result in

evolution

Speciation of Darwin’s Finches

Warbler

Large ground finch

Similarity between different species.

The eastern and western meadowlark (Sturnella magna, left) (Sturnella neglecta, right)

songs and other behaviors are different enough

to prevent interbreeding

(a)

Diversity within a species. As diverse as we

may be in appearance, all humans belong to

a single biological species (Homo sapiens),

defined by our capacity to interbreed.

(b)

Figure 24.3 A, B

Reproductive Isolation

• Reproductive isolation– Is the existence of biological factors that

impede members of two species from producing viable, fertile hybrids

– Is a combination of various reproductive barriers

• Prezygotic barriers– Impede mating between species or hinder the

fertilization of ova if members of different species attempt to mate

• Postzygotic barriers– Often prevent the hybrid zygote from developing

into a viable, fertile adult

• Prezygotic and postzygotic barriers

Figure 24.4

Prezygotic barriers impede mating or hinder fertilization if mating does occur

Individualsof differentspecies

Matingattempt

Habitat isolation

Temporal isolation

Behavioral isolation

Mechanical isolation

HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION

(b)

(a)

(c)

(d)

(e)

(f)

(g)

Viablefertile

offspring

Reducehybrid

viability

Reducehybridfertility

Hybridbreakdown

Fertilization

Gameticisolation

GAMETIC ISOLATION REDUCED HYBRID VIABILITY

REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

(h)(i)

(j)

(k)

(l)

(m)

Limitations of the Biological Species Concept

• The biological species concept cannot be applied to– Asexual organisms– Fossils– Organisms about which little is known

regarding their reproduction

Other Definitions of Species

• The morphological species concept– Characterizes a species in terms of its body shape, size,

and other structural features

• The paleontological species concept– Focuses on morphologically discrete species known only

from the fossil record

• The ecological species concept– Views a species in terms of its ecological niche

• The phylogenetic species concept– Defines a species as a set of organisms with a unique

genetic history

(a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population.

(b) Sympatric speciation. A smallpopulation becomes a new specieswithout geographic separation.

Figure 24.5 A, B

• Concept 24.2: Speciation can take place with or without geographic separation

• Speciation can occur in two ways– Allopatric speciation– Sympatric speciation

Allopatric (“Other Country”) Speciation

• In allopatric speciation– Gene flow is interrupted or reduced when a

population is divided into two or more geographically isolated subpopulations

Figure 24.6

A. harrisi A. leucurus

• Once geographic separation has occurred– One or both populations may undergo

evolutionary change during the period of separation

Sympatric (“Same Country”) Speciation

• In sympatric speciation– Speciation takes place in geographically

overlapping populations

Habitat Differentiation and Sexual Selection

• Sympatric speciation– Can also result from the appearance of new

ecological niches

• In cichlid fish– Sympatric speciation has resulted from

nonrandom mating due to sexual selection

Figure 24.10

Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orangelight, the two species appear identical in color. The researchers then observed the mating choices of the fish in each tank.

EXPERIMENT

P. nyererei

Normal lightMonochromatic

orange light

P. pundamilia

Under normal light, females of each species mated only with males of their own species. But under orange light, females of each species mated indiscriminately with males of both species. The resulting hybrids were viable and fertile.

RESULTS

The researchers concluded that mate choice by females based on coloration is the main reproductive barrier that normally keeps the gene pools of these two species separate. Since the species can still interbreed when this prezygotic behavioral barrier is breached in the laboratory, the genetic divergence between the species is likely to be small. This suggests that speciation in nature has occurred relatively recently.

CONCLUSION

Allopatric and Sympatric Speciation: A Summary

• In allopatric speciation– A new species forms while geographically

isolated from its parent population

• In sympatric speciation– The emergence of a reproductive barrier

isolates a subset of a population without geographic separation from the parent species

Adaptive Radiation• Adaptive radiation

– Is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities (typical for long-distance dispersal)

Black noddy ternAustralian coast

• The Hawaiian archipelago– Is one of the world’s great showcases of

adaptive radiation

Dubautia laxa

Dubautia waialealae

KAUA'I

5.1millionyears O'AHU

3.7millionyears

LANAI

MOLOKA'I

1.3 million years

MAUI

HAWAI'I0.4

millionyears

Argyroxiphium sandwicense

N

• The punctuated equilibrium model – Contrasts with a model of gradual change

throughout a species’ existence

Figure 24.13

Gradualism model. Species descended from a common ancestor gradually diverge more and more in their morphology as they acquire unique adaptations.

Time

(a)Punctuated equilibrium model. A new species changes most as it buds from a parent species and then changes little for the rest of its existence.

(b)

25 Phylogeny and Systematics

• Overview: Investigating the Tree of Life

• This chapter describes how biologists trace phylogeny– The evolutionary history of a species or group

of related species

• Biologists draw on the fossil record– Which provides information about ancient

organisms

Figure 25.1

• Biologists also use systematics– As an analytical approach to understanding

the diversity and relationships of organisms, both present-day and extinct

• Currently, systematists use– Morphological, biochemical, and molecular

comparisons to infer evolutionary relationships

Figure 25.2

• Concept 25.1: Phylogenies are based on common ancestries inferred from fossil, morphological, and molecular evidence

The Fossil Record

• Sedimentary rocks– Are the richest source of fossils– Are deposited into layers called strata

Figure 25.3

1 Rivers carry sediment to the ocean. Sedimentary rock layers containing fossils form on the ocean floor.

2 Over time, new strata are

deposited, containing fossils from each time period.

3 As sea levels change and the seafloor is pushed upward, sedimentary rocks are exposed. Erosion reveals strata and fossils.

Younger stratum with more recent fossils

Older stratum with older fossils

• The fossil record– Is based on the sequence in which fossils have

accumulated in such strata

• Fossils reveal– Ancestral characteristics that may have been

lost over time

• Though sedimentary fossils are the most common– Paleontologists study a wide variety of fossils

(a) Dinosaur bones being excavated from sandstone (d) Casts of ammonites,

about 375 million years old

(f) Insects preserved whole in amber

(b) Petrified tree in Arizona, about 190 million years old

(c) Leaf fossil, about 40 million years old

Morphological and Molecular Homologies

• In addition to fossil organisms– Phylogenetic history can be inferred from

certain morphological and molecular similarities among living organisms

• In general, organisms that share very similar morphologies or similar DNA sequences– Are likely to be more closely related than

organisms with vastly different structures or sequences

Sorting Homology from Analogy

• A potential misconception in constructing a phylogeny– Is similarity due to convergent evolution,

called analogy, rather than shared ancestry

• Convergent evolution occurs when similar environmental pressures and natural selection– Produce similar (analogous) adaptations in

organisms from different evolutionary lineages

Figure 25.5

Marsupial Australianmole

Eutherian North Am. mole

• Analogous structures or molecular sequences that evolved independently– Are also called homoplasies

Figure 25.6

C C A T C A G A G T C C

C C A T C A G A G T C C

C C A T C A G A G T C C

C C A T C A G A G T C C

G T A

Deletion

Insertion

C C A T C A A G T C C

C C A T G T A C A G A G T C C

C C A T C A A G T C C

C C A T G T A C A G A G T C C

1 Ancestral homologous DNA segments are identical as species 1 and species 2 begin to diverge from their common ancestor.

2 Deletion and insertion mutations shift what had been matching sequences in the two species.

3 Homologous regions (yellow) do not all align because of these mutations.

4 Homologous regions realign after a computer program adds gaps in sequence 1.

1

2

1

2

1

2

1

2

• Concept 25.2: Phylogenetic systematics connects classification with evolutionary history

• Taxonomy– Is the ordered division of organisms into

categories based on a set of characteristics used to assess similarities and differences

Binomial Nomenclature

• Binomial nomenclature– Is the two-part format of the scientific name of

an organism– Was developed by Carolus Linnaeus 1707-

1778 (Father of Taxonomy or Systematics)

• The binomial name of an organism or scientific epithet– Is latinized– Is the genus and species

Hierarchical Classification

• Linnaeus also introduced a system– For grouping species in increasingly broad

categories

Figure 25.8

Pantherapardus

Panthera

Felidae

Carnivora

Mammalia

Chordata

Animalia

EukaryaDomain

Kingdom

Phylum

Class

Order

Family

Genus

Species

Linking Classification and Phylogeny

• Systematists depict evolutionary relationships– In branching

phylogenetic trees

Figure 25.9

Panthera pardus

(leopard)

Mephitis mephitis

(striped skunk)

Lutra lutra (European

otter)

Canis familiaris

(domestic dog)

Canislupus (wolf)

Panthera Mephitis Lutra Canis

Felidae Mustelidae Canidae

Carnivora

Ord

er

Fa

mil

yG

enu

sS

pe

cie

s

• Each branch point– Represents the divergence of two species

Leopard Domestic cat

Common ancestor

• Concept 25.3: Phylogenetic systematics informs the construction of phylogenetic trees based on shared characteristics

• A cladogram– Is a depiction of patterns of shared characteristics

among taxa

• A clade within a cladogram– Is defined as a group of species that includes an

ancestral species and all its descendants

• Cladistics– Is the study of resemblances among clades

Cladistics

• Clades– Can be nested within larger clades, but not all

groupings or organisms qualify as clades

• A valid clade is monophyletic– Signifying that it consists of the ancestor

species and all its descendants

Figure 25.10a

(a) Monophyletic. In this tree, grouping 1, consisting of the seven species B–H, is a monophyletic group, or clade. A mono-phyletic group is made up of an ancestral species (species B in this case) and all of its descendant species. Only monophyletic groups qualify as legitimate taxa derived from cladistics.

Grouping 1

D

C

E G

F

B

A

J

I

KH

• A paraphyletic clade– Is a grouping that consists of an ancestral

species and some, but not all, of the descendants

Figure 25.10b

(b) Paraphyletic. Grouping 2 does not meet the cladistic criterion: It is paraphyletic, which means that it consists of an ancestor (A in this case) and some, but not all, of that ancestor’s descendants. (Grouping 2 includes the descendants I, J, and K, but excludes B–H, which also descended from A.)

D

C

E

B

G H

F

J

I

K

A

Grouping 2

• A polyphyletic grouping– Includes numerous types of organisms that

lack a common ancestor

Figure 25.10c

(c) Polyphyletic. Grouping 3 also fails the cladistic test. It is polyphyletic, which means that it lacks the common ancestor of (A) the species in the group. Further-more, a valid taxon that includes the extant species G, H, J, and K would necessarily also contain D and E, which are also descended from A.

D

C

B

E G

F

H

A

J

I

K

Grouping 3

Shared Primitive and Shared Derived Characteristics

• In cladistic analysis– Clades are defined by their evolutionary

novelties (new chars)

Outgroups

• Systematists use a method called outgroup comparison– To differentiate between shared derived

(unique to a clade but not found in beyond that taxon) and shared primitive (ancestral) characteristics

• As a basis of comparison we need to designate an outgroup– which is a species or group of species that is

closely related to the ingroup, the various species we are studying

• Outgroup comparison– Is based on the assumption that homologies

present in both the outgroup and ingroup must be primitive characters that predate the divergence of both groups from a common ancestor

• The outgroup comparison– Enables us to focus on just those characters

that were derived at the various branch points in the evolution of a clade

Figure 25.11a, bSal

aman

der

TAXA

Tur

tle

Leop

ard

Tun

a

Lam

prey

Lanc

elet

(out

grou

p)0 0 0 0 0 1

0 0 0 0 1 1

0 0 0 1 1 1

0 0 1 1 1 1

0 1 1 1 1 1

Hair

Amniotic (shelled) egg

Four walking legs

Hinged jaws

Vertebral column (backbone)

Leopard

Hair

Amniotic egg

Four walking legs

Hinged jaws

Vertebral column

Turtle

Salamander

Tuna

Lamprey

Lancelet (outgroup)

(a) Character table. A 0 indicates that a character is absent; a 1 indicates that a character is present.

(b) Cladogram. Analyzing the distribution of these derived characters can provide insight into vertebrate phylogeny.

CH

AR

AC

TE

RS

The Universal Tree of Life • The tree of life– Is divided into three great clades called domains: Bacteria,

Archaea, and Eukarya

• The early history of these domains is not yet clear

Figure 25.18

Bacteria Eukarya Archaea4 Symbiosis of

chloroplast ancestor with ancestor of green plants

3 Symbiosis of mitochondrial ancestor with ancestor of eukaryotes

2 Possible fusion of bacterium and archaean, yielding ancestor of eukaryotic cells

1 Last common ancestor of all living things

4

3

2

1

1

2

3

4

0B

illio

n ye

ars

ago

Origin of life

26 The Tree of LifeAn Introduction to Biological Diversity

• Overview: Changing Life on a Changing Earth

• Life is a continuum– Extending from the earliest organisms to the

great variety of species that exist today

• Geological events that alter environments– Change the course of biological evolution

• Conversely, life changes the planet that it inhabits

Figure 26.1

• The analogy of a clock– Can be used to place major events in the

Earth’s history in the context of the geological record

Figure 26.10

Land plants

Animals

Multicellulareukaryotes

Single-celledeukaryotes

Atmosphericoxygen

Prokaryotes

Origin of solarsystem andEarth

Humans

Ceno-zoicMeso-

zoic

Paleozoic

ArchaeanEon

Billions of years ago

ProterozoicEon

1

2 3

4

• Concept 26.3: As prokaryotes evolved, they exploited and changed young Earth

• The oldest known fossils are stromatolites– Rocklike structures

composed of many layers of bacteria and sediment

– Which date back 3.5 billion years ago

The First Prokaryotes

• Prokaryotes were Earth’s sole inhabitants– From 3.5 to about 2 billion years ago

Electron Transport Systems• Electron transport systems of a variety of

types– Were essential to early life– Have: some aspects that possibly precede life itself

Photosynthesis and the Oxygen Revolution

• The earliest types of photosynthesis– Did not produce oxygen

• Oxygenic photosynthesis– Probably evolved about 3.5 billion years ago

in cyanobacteria

Figure 26.12

• When oxygen began to accumulate in the atmosphere about 2.7 billion years ago– It posed a challenge for life– It provided an opportunity to gain abundant

energy from light– It provided organisms an opportunity to exploit

new ecosystems

• Concept 26.4: Eukaryotic cells arose from symbioses and genetic exchanges between prokaryotes

• Among the most fundamental questions in biology– Is how complex eukaryotic cells evolved from

much simpler prokaryotic cells

The First Eukaryotes

• The oldest fossils of a simple eukaryotic cell– Date back 2.1 billion years

Endosymbiotic Origin of Mitochondria and Plastids

• The theory of endosymbiosis– Proposes that mitochondria and plastids were

formerly small prokaryotes living within larger host cells

• The prokaryotic ancestors of mitochondria and plastids– Probably gained entry to the host cell as

undigested prey or internal parasites

Figure 26.13

CytoplasmDNA

Plasmamembrane

Ancestralprokaryote

Infolding ofplasma membrane

Endoplasmicreticulum

Nuclear envelope

Nucleus

Engulfingof aerobic

heterotrophicprokaryote Cell with nucleus

and endomembranesystem

Mitochondrion

Ancestralheterotrophiceukaryote Plastid

Mitochondrion

Engulfing ofphotosyntheticprokaryote insome cells

Ancestral Photosyntheticeukaryote

• In the process of becoming more interdependent– The host and endosymbionts would have

become a single organism

• The evidence supporting an endosymbiotic origin of mitochondria and plastids includes– Similarities in inner membrane structures and

functions– Both have their own circular DNA

• Concept 26.5: Multicellularity evolved several times in eukaryotes

• After the first eukaryotes evolved– A great range of unicellular forms evolved– Multicellular forms evolved also

The Earliest Multicellular Eukaryotes

• Molecular clocks– Date the common ancestor of multicellular

eukaryotes to 1.5 billion years

• The oldest known fossils of eukaryotes– Are of relatively small algae that lived about 1.2

billion years ago

• Larger organisms do not appear in the fossil record– Until several hundred million years later

• Chinese paleontologists recently described 570-million-year-old fossils– That are probably animal embryos

Figure 26.15a, b

150 m 200 m(a) Two-cell stage (b) Later stage

The Colonial Connection

• The first multicellular organisms were colonies– Collections of autonomously replicating cells

Figure 26.16

• Some cells in the colonies– Became specialized for different functions

• The first cellular specializations– Had already appeared in the prokaryotic

world

Colonization of Land by Plants, Fungi, and Animals

• Plants, fungi, and animals– Colonized land about 500 million years ago

• Robert Whittaker proposed a system with five kingdoms– Monera, Protista, Plantae, Fungi, and

Animalia

Figure 26.21

Plantae Fungi Animalia

Protista

Monera

Eukaryo

tes

Prokaryotes

Reconstructing the Tree of Life: A Work in Progress

• A three domain system– Has replaced the five kingdom system– Includes the domains Archaea, Bacteria, and

Eukarya

• Each domain– Has been split by taxonomists into many

kingdoms