Figure 4.3 (b)

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

• 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.

• 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

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

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

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

• 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

Systematics and Clades

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

comparisons to infer evolutionary relationships

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

species and all its descendants

(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

(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

(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

Sal

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