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BIOL102 . 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.

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Origin of Species

Part 1 – A few reminders from lecture 2

• Modern Synthesis of Genetics and Evolution

• Hardy-Weinberg Principle

• Factors Changing Allele Frequencies

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


Origin of Species

Part 2 – Species Concepts

• Species

• Biological Species Concept

• Morphological Species Concept

• Ecological Species Concept

• Phylogenetic Species Concept


Origin of Species

Part 3 – Speciation

• Allopatric Speciation

• Sympatric Speciation

• Rates of Speciation

• Dynamics

• 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

Part 1 – A few reminders from lecture 2

Modern Synthesis of Genetics and Evolution

Part 1 – A few reminders from lecture 2

Hardy-Weinberg Principle

  • • 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 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

Part 1 – A few reminders from lecture 2

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

Part 2 – Species Concepts

A. 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

Part 2 – Species Concepts

B. Biological Species Concept

• Biologists categorize the mechanisms that stop gene

flow between populations into prezygotic barriers

(before fertilization) and postzygotic barriers (after


 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

Biological Species Concept

Prezygotic and Postzygotic Barriers

Prezygotic and Postzygotic Barriers

Individuals of

different species



Mating attempt


(zygote forms)



Viable, fertile


block fertilization from occurring by:

 impeding different species from attempting to mate

 preventing the successful completion of mating

 hindering fertilization if mating is successful

Biological Species Concept

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

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



Mechanical Isolation

Prezygotic Barriers

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

Biological Species Concept

Postzygotic Barriers

Reduced hybrid viability: genes of the different parent

species may interact and impair the hybrid’s


• 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

Postzygotic barriers

Postzygotic Barriers



Reduced Hybrid


Reduced Hybrid











• 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)

Biological Species Concept

Hybrid Zones

 if hybrid offspring are less fit, reinforcement may result

in more prezygotic barriers and complete reproductive

isolation may evolve (speciation)

 ahybrid 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

Biological Species Concept


  • • 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)

Limitations of the Biological Species Concept

Polar bear (U. maritimus)

Hybrid “grolar bear”

• 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

Part 2 – Species Concepts

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)

Part 2 – Species Concepts

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

Part 2 – Species Concepts

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


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


• Two modes leading to reproductive barriers can be


 allopatric speciation

sympatric speciation

Part 3 – Speciation

Part 3 – Speciation

• Allopatric speciation

occurs when geographic

isolationcreates 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

Part 3 – Speciation

A. Allopatric Speciation

Allopatric Speciation by Dispersal or Vicariance

Allopatric Speciation

• Geographic separation prevents species from mating

• Speciation occurs only with the evolution of reproductive

barriers between the

isolated population

and its parent


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

Allopatric Speciation

• 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

Allopatric Populations and Reproductive Isolation


Initial population

of fruit flies



Some flies raised on

maltose medium

Some flies raised

on starch medium

Mating experiments

after 40 generations

Allopatric Populations and Reproductive Isolation





population 2


population 1




population 1









population 2






Number of matings

in experimental group

Number of matings

in control group

Part 3 – Speciation

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


• 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

Sympatric Speciation

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

Sympatric Speciation

Sexual Selection


orange light

Normal light

• Sexual selection can drive sympatric speciation

 such selection for mates of different colors

has likely


to speciation

in cichlid fish

in Lake


P. pundamilia

P. nyererei

Sympatric Speciation

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

Part 3 – Speciation

C. Rates of Speciation

Part 3 – Speciation

D. Dynamics

• Gradual model

 traditional evolutionary

trees diagram the

descent of species as

gradual divergence


• 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

Biol101 introduction to biology b
BIOL101 Introduction to Biology B

Learning Objectives and Check of Understanding

• 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.

Biol101 introduction to biology b1
BIOL101 Introduction to Biology B

Reading Assignments

• Campbell: Chapter 24

• Sadava: Chapter 23

Brief Outline of the Upcoming Lecture

Lecture 4– 07/08/2011

Classification and Phylogeny

Part 1 – A few reminders from lecture 3

Part 2 – Classification and Taxonomy

Part 3 – Phylogeny or Systematics