Bio413 07 sympatric_speciation_species concepts_allopatric_speciation_winter2012_posted

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Heliconius butterflies

Real Example: Reproductive isolation + Sexual Selection+ Natural Selection = sympatric species

• live in various habitats in South America

• Müllerian mimics to other Heliconius species in or close to their range, all are unpalatable (taste bad to predators)

Heliconius butterflies

Real Example: Reproductive isolation + Sexual Selection+ Natural Selection = sympatric species

• live in various habitats in South America

• Müllerian mimics to other Heliconius species, all unpalatable (taste bad to predators)

• (incomplete) pre-zygotic isolation by habitat isolation

H. cydno: live in forest understory H. melpomene: live in disturbed re-growth forest

Species overlap in intermediate habitats and found flying together: rare hybrids found in wild

1) Why aren’t hybrids more common?

2) How do overlapping (sympatric) species remain distinct species?

Real Example: Reproductive isolation + Sexual Selection+ Natural Selection

Experiment

• Mating confirmed by identifying male spermatophore in female via dissection

• Mate choice (and sexual selection pressure) is by MALES (male choice) & based on female color pattern

• A male mates with a newly hatched female before her first flight

• Males and females exposed to each other in

no-choice trials (1 hr)

Results: Assortative mating in both species and their hybridMales prefer to mate with females of their “type”

Results: Sexual Selection against hybrids because they don’t have attractive traits (in this case color pattern)

Hybrids females are mated with 50% less often by males of either “parent” species

Results: Natural Selection: hybrids eaten by Jacamar birds (and other predators) more than parent species are.

Hybrids aren’t protected from predators by Müllerian mimicry, thus more eaten

by predators (N.S) =

Results: Natural Selection: hybrids eaten by Jacamar birds (and other predators) more than parent species are.

Hybrids are eaten moreby predators. The parent

species are favored by N.S.: Disruptive selection

Darwin & The Modern Synthesis (Natural Selection, Heredity & Genetics)

• Speciation was introduced by Darwin, in On the Origin of Species by Natural Selection (1859)

• Darwin didn’t discuss allopatric speciation or genetic drift

• The Modern Synthesis focused on geographical isolation and genetic drift (isolation + time )

Allopatric speciation

Darwin & The Modern Synthesis (Natural Selection, Heredity & Genetics)

•Allopatric speciation: evolution of reproductive isolation and divergence between populations that are geographically separate

Allopatric speciation: - requires geographic separation. - Most documented kind of speciation

In this model, speciation results from differences in mating preference that arise from 2 possible sources:

(1) genetic drift:

Allopatric Speciation

http://evolution.berkeley.edu/evosite/evo101/IIIDGeneticdrift.shtml

In this model, speciation results from differences in mating preference that arise from 2 possible sources:

(1) genetic drift: Change in allele frequencies due to chance.

(2) disruptive selection: Natural Selection and/or Sexual Selection that favors more extreme phenotypes over the original average phenotype

Secondary contact: occurs after evolving in isolation, two populations come back into contact with each other

•If the divergence is recent, these two species (or populations) may hybridize producing fertile offspring

•If the divergence is substantial, the two species may hybridize producing unfertile offspring or may not hybridize at all

Allopatric speciation: - requires geographic separation,

Geographic separation of one population into two occurs by:

(A) vicariance –separation of a population by geological forces

Allopatric speciation: - requires geographic separation,

(A) vicariance –separation of a population by geological forcesExamples:

- a new river, glacier, or mountain range forms- a land bridge forms or is submerged- plate tectonics and continent movements

Allopatric speciation: - requires geographic separation, which causes reproductive isolation.

(B) dispersal - colonization of a new habitat by foundersExamples:

- islands- postglacial lakes (Canada), rift lakes (Africa)

Initially, there is migration between 2 nearby populations of a forest-dwelling animal with two alleles controlling color

Allopatric speciation : Vicariance + Genetic drift

because of gene flow, the allele frequencies will be the same in the two populations

Over time, a barrier to migration arises between populations:

mountain range

The allele frequencies in each population will start to change due to genetic drift

(Step 1: gene flow has been interrupted)

The populations will slowly diverge as different alleles become fixed at many loci throughout the genome

(Step 2: populations differentiate)

mountain range

Different alleles may eventually fix in the 2 populations

(Step 3: the populations become reproductively isolated)

mountain range

Differences accumulate BY CHANCE in allelescontrolling mate preference: mating signals, genital shapes, etc.

... but because of assortative mating, the two types do not interbreed

have formed sister species that will now evolve separately

If the mountain range disappears, the two populations can mix = Secondary Contact.

(Step 4: speciation has occurred)

Consider a species found in a desert and a neighboring forest

Hot, dry desert

Cool, rainy forest

Allopatric speciation : Vicariance + Disruptive Selection

Selection will favor different alleles in the desert and forest

Hot, dry desert

Cool, rainy forest

Natural selection will favor forest-adapted individuals

Natural selection will favor desert-adapted individuals

After selection, the two populations will be genetically different

Hot, dry desert

Cool, rainy forest

forest-adapted individualshave survived here

desert-adapted individualshave survived here

However, migration will keep mixing alleles between the populations

Hot, dry desert

Cool, rainy forest

Maybe some evolution, but NO SPECIATION unless there is reproductive isolation

Now: a barrier to gene flow arises between the 2 habitats

Hot, dry desert

Cool, rainy forest

mountains

Now: a barrier to gene flow arises between the 2 habitats

Hot, dry desert

Cool, rainy forest

Natural selection will favor forest-adapted individuals

Natural selection will favor desert-adapted individuals

mountains

Each population evolves into a distinct, well-adapted species

Hot, dry desert

Cool, rainy forest

Forest-adapted population Desert-adapted population

mountains

Species #1 Species #2

No migration

Hawaiian Drosophila

Allopatric speciation : Dispersal & founders

Speciation and the Isthmus of Panama

RESULTS of mating experiments:(1) Sister-species shrimp snapped at each other instead of mating!This is pre-zygotic reproductive isolation (sexual isolation): mating signals not understood by potential mates

(2) when a male of one species was held with a female of its sister species [from the other side of Panama] for a month,

no offspring were produced except for one single pair

This is either pre-zygotic isolation (gamete isolation) or post-zygotic reproductive isolation: zygote does not develop.

For snapping shrimp: 3.5 million years is enough time for complete reproductive isolation to occur

Allopatric speciation: Vicariance

Caribbean and Pacific oceans were linked until the Isthmus of Panama formed ~3.5 million years ago Prevented any more gene flow between marine organisms on each side of the new land barrier

Did this result in the evolution of new sister species pairs separated by the Isthmus?

Speciation and the Isthmus of Panama

Google maps

Allopatric speciation : Vicariance

Speciation and the Isthmus of PanamaKnowlton et al. studied pairs of snapping shrimps that were morphologically similar, where one member of the pair was found on the Caribbean side and the other on the Pacific side

Knowlton & Weight 1998

Alphaeus cylindricus

The sister species still closely resembled each other – were they different species?

Sequenced part of a gene and also compared allele frequencies

Finally, did mating crosses to assess reproductive compatibility

would shrimp mate or fight with their sister species?

Allopatric speciation : Vicariance

Knowlton & Weigt 1998

• based on DNA sequences, the members of each pair were indeed each other’s closest

relatives

- That is, each sister-group pair were were descendants of a common ancestor

Allopatric speciation: Vicariance

What is a species?• Species are the fundamental units of

biodiversity• Note: one species, two species

http://carabidae.pro/carabidae/elaphrinae.htmlElaphrus beetles

Importance of recognizing species

• Survival depends on our ability to recognize species

What is a species?

Universality of recognizing species

What is a species?Ti

Amount of divergence (morphology)

What is a species?• Races • Varieties• “kinds”• Incipient species • Ecotypes• Morphs• Subspecies• Cultivars (Artificial Selection)

• Breeds (Artificial Selection)

Amount of divergence

What is a species?

1) speciation by cladogenesis

2) speciation by anagenesis between a & b

anagenesis

Anagenesis: no cladogenesis

cladogenesis

Cladogenesis: speciation

Anagenesis + Cladogenesis

A B C D E

Most of the time we don’t know the ancestral states for

What is a species?• Biologists do not agree on ONE way to define a species – thus, “species concepts”

At least 22 different definitions have been proposed to explain what a “species” is.

Three main ones:1. Morphological2. Biological3. Phylogenetic

Species concepts

Morphological species concept

: Two organisms that display “substantial” and consistent morphological differences are different species

• Advantages: Easy to apply, can be used with fossil species

– Disadvantages: It is not testable, the definition of “substantial” is subjective. Problems with convergence, cryptic species, hybridization (= gene flow) & intermediate phenotype

Morphological species concept

: Chronospecies: a morphology that can be identified as a stage in an evolving lineage (anagenesis)

Note increasing number of pleura (or “ribs”) on

trilobites

Biological species concept: a group of actually or potentially inter-

breeding individuals

• Advantages: testable in many cases, objective• Disadvantages: difficult to apply and test

(impractical), cannot be used with fossils, irrelevant to asexual populations

Fertile Hybrids: Violations of both the Morphological v. Biological species

Concepts

Phylogenetic species concept: Lineages with different evolutionary histories

are different species – these lineages must be the smallest units of evolution

• Advantage: it is testable, objective and can be applied to living and fossil species

– Disadvantage: it requires comprehensive phylogenetic analyses = need a cladogram!

Phylogenetic species concept: Lineages with different evolutionary histories

are different species – these lineages must be the smallest units of evolution

•Instead of depending on reproductive isolation, this concept revolves around fixed differences between populations

•Species are the smallest population that you don’t have any reason to divide into even smaller clades or populations

Phylogenetic species concept

Issues & Solutions

Issue: How genetically different do species have to be? Solutions:- from a single DNA base change that only exists in one population, to many consistent genetic differences across genes, also consistent measurable phenotypic differences

- Ideally, multiple fixed differences should be used

- Need a well-supported cladogram.

Phylogenetic Species Concept

High level of cryptic species diversity revealed by sympatric

lineages of Southeast Asian forest frogs

Bryan L Stuart, Robert F Inger, and Harold K VorisBiol Lett. 2006 September 22; 2(3): 470–474.

Odorrana livida

Cryptic species by the phylogenetic species concept

cf. = similar to

Odorrana livida

Cryptic species: organisms that appear identical or almost identical to close relatives, but are genetically distinct- they are their own lineage

Odorrana livida

cf. = similar to

Odorrana livida

Rana chalconota

cf. = similar to

Morphology + DNA

Forest habitat Savanna habitat

Cryptic species by the phylogenetic species concept + extinct organisms!

Ring species by the phylogenetic & morphological & biological species concepts

How do you describe a species?1. Compare it to all similar described species

2. Name it• Must be latinized & binominal

3. Designate a type• Designate a holotype • Designate paratypes (if possible)

4. Describe it

5. Provide a picture or illustration

6. Designate a type locality

7. Deposit it (the type or types) into a museum

8. Publish the description in a peer-reviewed journal

Bio413 07 sympatric_speciation_species concepts_allopatric_speciation_winter2012_posted

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