copyright © 2010 pearson education, inc. biol102

Copyright © 2010 Pearson Education, Inc.

BIOL102


Origin of Species

Part 1 – A few reminders from lecture 2

• Modern Synthesis of Genetics and Evolution

• Hardy-Weinberg Principle

• Factors Changing Allele Frequencies

BIOL102

Source of cover picture: Reece et al. (2010) , Campbell Biology, 9th edition, Pearson Benjamin Cummings, San Francisco (CA), Figure 24.4c


Part 2 – Species Concepts

• Species

• Biological Species Concept

• Morphological Species Concept

• Ecological Species Concept

• Phylogenetic Species Concept

BIOL102

Origin of Species


Part 3 – Speciation

• Allopatric Speciation

• Sympatric Speciation

• Rates of Speciation

• Dynamics

BIOL102

Origin of Species


• A population is the smallest biological unit that can

evolve and is defined as a group of individuals of the

same species that live, interbreed and produce fertile

offspring in a particular geographic area

• A gene pool consists of all alleles (forms of genes)

for all loci in a population and is the source of

genetic variation that produces the phenotypes

and their traits on which natural selection acts

• A population evolves when individuals with different

genotypes survive or reproduce at different rates

Modern Synthesis of Genetics and Evolution



• states that frequencies of alleles and genotypes in a

population remain constant from generation to generation

if certain conditions are met (Hardy-Weinberg equilibrium)

no mutations

random mating

no natural selection

extremely large population size (no effect of

genetic drift)

no gene flow (migration into or out of a population)


Hardy-Weinberg Principle


• Hardy-Weinberg equilibrium is a null hypothesis, which

assumes that allele frequencies are not changed

• However, there are at least four mechanisms of evolution,

which cause changes in allele frequencies of populations:

mutations

gene flow

genetic drift

natural selection

Factors Changing Allele Frequencies



• is defined as an evolutionarily independent

population or group of populations

• Biologists commonly use the following four

approaches to identify species:

the biological species concept

the morphological species concept

the ecological species concept

the phylogenetic species concept

A. Species



Species


• defines a species as a population or group of populations

whose members have the potential to interbreed and

produce fertile offspring

• considers populations to be evolutionarily independent

if they are reproductively isolated from each other and

no gene flow occurs between them

B. Biological Species Concept



• Biologists categorize the mechanisms that stop gene

flow between populations into prezygotic barriers

(before fertilization) and postzygotic barriers (after

fertilization)

prezygotic barriers: individuals of different

species are prevented from mating

postzygotic barriers: individuals from different

populations do mate, but the hybrid offspring

produced have low fitness and do not survive or

produce offspring

Prezygotic and Postzygotic Barriers

Biological Species Concept


Prezygotic and Postzygotic Barriers

Individuals ofdifferent species

Mating attempt

Fertilization(zygote forms)

Viable, fertileoffspring

Prezygoticbarriers

Postzygoticbarriers


• block fertilization from occurring by:

impeding different species from attempting to mate

preventing the successful completion of mating

hindering fertilization if mating is successful

Prezygotic Barriers



• Habitat isolation: two species encounter each other

rarely, or not at all, because they occupy different

habitats, even though not isolated by physical barriers

• Temporal isolation: Species that breed at different

times of the day, different seasons, or different years

cannot mix their gametes

• Behavioral isolation: courtship rituals and other

behaviors unique to a species are effective barriers

Prezygotic Barriers


• Floral traits of plants can influence the behavior of

pollinators, and thus whether plants can hybridize

two species of columbines (Aquilegia) in California

can produce fertile hybrids, but flower structure

determines that one species is pollinated by

hummingbirds, the other by hawkmoths, so

hybridization is rare

Behavioral Isolation


Behavioral Isolation


• Mechanical isolation: morphological differences (e. g.,

size and shape of reproductive organs) can prevent

successful mating

• Gametic isolation: sperm of one species may not be

able to fertilize eggs of another species

Prezygotic Barriers


• In plants, mechanical isolation may involve pollinators

many orchid flowers look and smell like the females of

particular pollinator species

male insects attempt

to mate, thereby

transferring

pollen

Mechanical Isolation


Prezygotic barriers

HabitatIsolation

TemporalIsolation

BehavioralIsolation

MechanicalIsolation

GameticIsolation

Individuals of

differentspecies

MATINGATTEMPT FERTILIZATION

(a) (c) (e) (f)

(b)

(g)

(d)

Prezygotic Barriers


• prevent the hybrid zygote from developing into a

viable, fertile adult due to:

reduced hybrid viability

reduced hybrid fertility

hybrid breakdown

• Hybrids are the offspring of crosses between

different species

Postzygotic Barriers



• Reduced hybrid viability: genes of the different parent

species may interact and impair the hybrid’s

development

• Reduced hybrid fertility: even if hybrids are vigorous,

they may be sterile

• Hybrid breakdown: some first-generation hybrids

are fertile, but when they mate with another species

or with either parent species, offspring of the next

generation are feeble or sterile

Postzygotic Barriers


Reduced Hybrid Fertility


Reduced HybridViability

Reduced HybridFertility

HybridBreakdown

FERTILIZATIONVIABLE,FERTILE

OFFSPRING

Postzygotic barriers

(k)

(h) (i)

(j)

(l)

Postzygotic

Barriers


• If two formerly isolated populations are reunited

before complete reproductive isolation has developed,

interbreeding can occur with three possible outcomes:

if hybrid offspring are as fit as those resulting from

matings within each population, hybrids will mate

with individuals of both parental species. The gene

pools will gradually become completely mixed

(no speciation)

Hybrid Zones



if hybrid offspring are less fit, reinforcement may result

in more prezygotic barriers and complete reproductive

isolation may evolve (speciation)

a hybrid zone may develop in the absence of

reinforcement, or before reinforcement is complete, and

may contain recombinant individuals resulting from

many generations of hybridization

Hybrid Zones


• Example: two species of European toads have a long

narrow hybrid zone

the toad hybrids have many defects, some of which

are lethal

on average, a hybrid toad is significantly less fit as

a purebred individual

the hybrid zone is narrow, because there is strong

selection against hybrids. But it persists because

individuals of both species continue to move into it

and mate

Hybrid Zones


Hybrid Zones


Limitations


• The criterion of reproductive isolation cannot be evaluated

in fossils or in species that reproduce asexually

for example, prokaryotic and viral species must be

defined differently

• this concept can only be applied to populations that

overlap geographically

• it also emphasizes absence of gene flow, which can occur

between distinct species

for example, grizzly bears and polar bears can mate

to produce “grolar bears”


Grizzly bear (U. arctos)

Polar bear (U. maritimus)

Hybrid “grolar bear”

Limitations of

the Biological

Species

Concept


• defines a species by differences in morphological or

structural features

is based on the idea that distinguishing features are

most likely to arise if populations are independent

and isolated from gene flow

applies to sexual and asexual species but relies on

subjective criteria

also cannot identify cryptic species that differ in

non-morphological traits

C. Morphological Species Concept



• views a species in terms of its ecological niche

applies to sexual and asexual species and

emphasizes the role of disruptive selection

is widely used for viral species (in addition

to genetic homologies)

D. Ecological Species Concept



• defines a species as the smallest group of individuals

on a phylogenetic tree (monophyletic group)

applies to sexual and asexual species, but it can be

difficult to determine the degree of difference

required for separate species

on phylogenetic trees, an ancestral population plus

all of its descendants is called a monophyletic group

or clade, which is identified by synapomorphies,

homologous traits inherited from a common ancestor

that are unique to certain populations or lineages

E. Phylogenetic Species Concept



Phylogenetic Species Concept


• This concept can be applied to any population, but

there are disadvantages:

phylogenies are currently available for only a

tiny (though growing) subset of populations on

the tree of life

would probably lead to recognition of many

more species than either of the other species

concepts

Phylogenetic Species Concept


• A key event in the potential origin of a species occurs

when a population is somehow severed from other

populations of the parent species. With its gene pool

isolated, the splinter population can follow its own

evolutionary course and become reproductively

incompatible

• Two modes leading to reproductive barriers can be

distinguished

allopatric speciation

sympatric speciation



• Allopatric speciation

occurs when geographic

isolation creates a

reproductive barrier

(extrinsic mechanisms)

• Sympatric speciation

occurs when a reproductive

barrier is created by

something other than

geographic isolation

(intrinsic mechanisms)



• Genetic isolation happens routinely when populations

become physically separated. Physical isolation, in turn,

occurs in one of two ways: dispersal or vicariance.

dispersal occurs when a population moves to a new

habitat, colonizes it, and forms a new population

vicariance occurs when a physical barrier splits a

widespread population into subgroups that are

physically isolated from each other

• Speciation that begins with physical isolation via either

dispersal or vicariance is known as allopatric speciation

A. Allopatric Speciation



Allopatric Speciation by Dispersal or Vicariance


• Geographic separation prevents species from mating

• Speciation occurs only with the evolution of reproductive

barriers between the

isolated population

and its parent

population

Allopatric Speciation


• The definition of barrier depends on the ability of a

population to disperse

for example, a canyon may create a barrier for

small rodents, but not birds, coyotes, or pollen

• Separate populations may evolve independently

through mutation, natural selection, and genetic drift

for example, speciation of snapping shrimp (Alpheus)

populations due to separation by the Isthmus of

Panama



A. harrisii A. leucurus

Physical Isolation and Reproductive Barriers


Physical Isolation and Reproductive Barriers


• Regions with many geographic barriers typically have

more species than do regions with fewer barriers

• Reproductive isolation between populations generally

increases as the distance between them increases

however, barriers to reproduction are intrinsic;

separation itself is not a biological barrier



EXPERIMENT

Initial populationof fruit flies(Drosophila

pseudoobscura)

Some flies raisedon starch medium

Mating experimentsafter 40 generations

Some flies raised onmaltose medium

Allopatric Populations and Reproductive Isolation


RESULTS

Female

Starch Maltose

Ma

leM

alt

ose

Sta

rch

Number of matingsin experimental group

22 9

8 20

FemaleStarch

population 1

Ma

leS

tarc

hp

op

ula

tio

n 2

Number of matingsin control group

18 15

12 15

Starchpopulation 2

Sta

rch

po

pu

lati

on

1

Allopatric Populations and Reproductive Isolation


B. Sympatric Speciation


• In sympatric speciation, speciation takes place in

geographically overlapping populations

• can occur if a genetic change produces a reproductive

barrier between mutants and the parent population

• may be the result of:

polyploidy

extreme habitat differentiation

sexual selection


Sympatric Speciation


Polyploidy


• is the presence of extra sets of chromosomes due to

accidents during cell division

an autopolyploid is an individual with more than

two chromosome sets, derived from one species

an allopolyploid is a species with multiple sets of

chromosomes derived from different species

• is much more common in plants than in animals

many important crops (oats, cotton, potatoes,

tobacco, and wheat) are polyploids


Autopolyploidy


Allopolyploidy


Extreme Habitat Differentiation


• Sympatric speciation can result from the appearance

of new ecological niches

for example, populations of the North American

maggot fly prefer to live either on native hawthorn

trees or on more recently introduced apple trees

although they are not yet separate species on the

basis of any species concept, apple flies and

hawthorn flies are diverging


Extreme Habitat Differentiation


Sexual Selection


• Sexual selection can drive sympatric speciation

such selection for mates of different colors

has likely

contributed

to speciation

in cichlid fish

in Lake

Victoria

Normal lightMonochromatic

orange light

P. pundamilia

P. nyererei


Comparison with Allopatric Speciation


• In allopatric speciation, geographic isolation restricts gene

flow between populations

reproductive isolation may then arise by e. g., natural

selection or genetic drift, in the isolated populations

• In sympatric speciation, a reproductive barrier isolates a

subset of a population without geographic separation

from the parent species

sympatric speciation can result from polyploidy,

natural selection, or sexual selection


• differ among organisms and can occur in time scales

a slow rate of speciation evidenced by a living horseshoe

crab (13 species) and a 300 million year-old fossil

a rapid rate of speciation evidenced by Galapagos finches

which have diversified into 13 species within the last

100,000 years


C. Rates of Speciation


• Gradual model

traditional evolutionary

trees diagram the

descent of species as

gradual divergence


D. Dynamics


• Punctuated equilibrium

is a contrasting model

of evolution

states that species most

often diverge in spurts

of relatively sudden change

accounts for the relative

rarity of transitional fossils

and hence appears to be a

more accurate view of

speciation dynamics

Dynamics


Learning Objectives and Check of Understanding

BIOL101 Introduction to Biology B

• Compare and contrast the different species concepts?

• Distinguish prezygotic and postzygotic barriers.

• Differentiate allopatric and sympatric speciation.

• Compare and contrast the models describing the

dynamics of speciation.


Reading Assignments

• Campbell: Chapter 24

• Sadava: Chapter 23

BIOL101 Introduction to Biology B



Part 2 – Classification and Taxonomy

Part 3 – Phylogeny or Systematics

Lecture 4 – 07/08/2011

Classification and Phylogeny

Brief Outline of the Upcoming Lecture

copyright © 2010 pearson education, inc. biol102

Documents

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biological species conceptpart

independent population

large population

pearson benjamin cummings

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