Speciation

most important problem in evolutionary biology- change over time- diversification

Species concepts

Mechanisms of speciation

Species Concepts

Mayden (1997) 22 different species concepts

species as categories -- taxa

species as evolutionary groups -- “smallestindependent evolutionary unit”

importance -- conservation of biological diversity understanding the process of diversification

Biological Species Concept -- Ernst Mayr (1947)

“Species are actual or potentially interbreeding natural populations which are reproductively isolated from other populations”

emphasizes: genetic exchange within species reproductive isolation between species

can be difficult to apply

and, doesn’t apply to many species:

asexual species -- bacteria, fungi, many invertebrates and plants

hybridizing species -- wolves and coyotes; cottonwood and balsam poplar

fossil species

Phylogenetic Species Concept -- Joel Cracraft (1989)

“An irreducible (basal) cluster of organisms diagnosably differentfrom other such clusters, and within which there is a parentalpattern of ancestry and descent”

species are monphyletic groups

can be applied to any type of organism

can be difficult to implement

difficulties with matching gene trees to speciation events

Morphospecies Concept

fossil species

problems of cryptic species because of limited characters

Geographical classification (pass)

allopatricvicariancefounder event

parapatricsympatric

Classical model of allopatric speciation

isolation of populations allopatry

genetic divergence of populations allopatry

development of reproductive isolation when populations come back into contact sympatry (secondary contact and reinforcement)

geographic isolation due to range splitting -- vicariance

populations may diverge due to:natural selectionsexual selection(genetic drift)

type of divergence may be pre-mating or post-mating or both

Vicariant speciation in snapping shrimp

Isthmus of Panama closes ~3 mya

Caribbean, Pacific populations

both pre-mating and post-mating divergence

geographic isolation due to a founder event

founder population is genetically different from source population;genetic drift is important at the start of the process

divergence via: natural selection in a novel environmentsexual selection

type of divergence may be pre-mating or post-mating or both

Patterns of speciation in Hawaiian Drosophila are consistent with founder events

outcomes of divergence in allopatry

insufficent divergence ---> fusion

reproductive isolation due to assortative mating withinpopulations ---> speciation

mating between populations leading to a hybrid zone

hybrid fitness hybrid zone eventual outcome

lower than narrow, short- reinforcementparental forms lived

equal to wide, parental populationsparental forms persistent coalesce

greater than ecotone stable hybrid zone,parental forms new habitat new species

Classical model of allopatric speciation

isolation of populations allopatry

genetic divergence of populations allopatry

development of reproductive isolation when populations come back into contact sympatry (secondary contact and reinforcement)

Not unique to allopatric speciation

*first two stages frequently happen in ‘sympatry’differentiation despite gene flow due to selection

reproductive isolation is a consequence of selection onecological attributes

Ecological speciation in three-spined sticklebacks

Rundle et al. 2000 Science 287:306

pairs of species in freshwater lakes; independently derived from a marine stickleback species

benthic form -- large, heavy-bodied; feeds on invertebrates in the near-shore habitats

limnetic form – slender-bodied; feeds on plankton in open water

Predict: limnetic forms from different lakes should not be reproductivelyisolated from each other

limnetic forms from one lake should be isolated from benthicforms from a different lake

species pairs from three different lakes

different lakessame lake

Natural selection, sexual selection selection and speciation in the cichlids of Lake Victoria

Verheyen et al. 2003 Science 300:325

Lake Victoria -- more than 500 species of haplochromine cichlids

morphological, ecological, and behavioral diversity

Lake Victoria cichlids are a monophyletic group most closely related to taxa from Lake Kivu

algal grazer insect eater piscivore algal grazer

Sexual selection drives speciation in African great lakes cichlids

Couldridge and Alexander 2002 Behavioral Ecology 13:59

Pseudotropheus zebracomplex of at least four species (Lake Malawi)differ in color pattern; similar in other morphology, ecology, behavior

Test for assortative matingfemale preference with choice (conspecifics and heterospecifics)female preference without choice (heterospecifics only)

C P. callainos pale blue, no bars

R P. zebra “red dorsal” pale blue; dark blue

bars; orangedorsal fin

G P. zebra “gold” pale orange, faint

brown bars

L P. lombardoi yellow, 5 vertical

black bars

Predict: 1) in choice tests, females will prefer conspecific males 2) in no choice tests, females will prefer the heterospecific

male whose color pattern is most similar

C P. callainos pale blue, no bars

R P. zebra “red dorsal” pale blue; dark blue

bars; orangedorsal fin

G P. zebra “gold” pale orange, faint

brown bars

L P. lombardoi yellow, 5 vertical

black bars

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Disruptive selection and speciation in red crossbills, Loxia curvirostra

highly specialized -- seeds of coniferous trees (seeds retained in cones)use the crossed bill to extract seeds from within the cone extraction efficiency depends on bill shape and palate structurebill structure and cone structure affect feeding performance

---> no generalist crossbill

nine different call types; seven are specialized on different species of conifers

Benkman 2003 Evolution 57:1176

detailed study of one population in southern Idahofeeds on Rocky Mountain lodgepole pine (Pinus contorta latifolia)

estimated form of selection on bill shape (bill depth)

relationship between bill depth and fitness (survival)

fitness surface based on feeding performance (bill depthand palate structure) for five species

Selection on bill depth is stabilizing

Selection and speciation

Natural Selection for different feeding methods or habitat preferences

Sexual Selection for mate preferences

Hybrids do poorly because they do not work well inthe environment (physical or social).

They may function properly as organismsi.e. viable and fertile

What are the requirements for this to work?

Strong selection

Little genetic exchange/assortative mating

Predictions

Prerequisite of close linkage/pleiotropy between the traits of ‘mating’ and ‘performance’

or,

Recombination will erode weaker associations leading to no speciation

Genetics of speciation -- performance and habitat preference in pea aphids

Via 2002

two host races/incipient species -- alfalfa and clover

known trade-off between performance on clover and alfalfa

reaction norms mean fitness of clones tested on both hosts

Genetics of speciation -- performance and habitat preference in pea aphids

Via 2002

two host races/incipient species -- alfalfa and cloverknown trade-off between performance on clover and alfalfa

what is the genetic architecture of this trade-off?

are genes for performance independent of genes forhabitat choice/assortative mating?

negative genetic correlation:pleiotropy or LDE of closely linked loci

vs. LDE of unlinked loci

QTL analysis

Performance

Acceptance andPerformance on alfalfa

Acceptance and per-formance on clover

Intrinsic postmating incompatibility

Drosophila frogs Lepidoptera birds

Lepidoptera

Post

Pre- plus post

Hybrid sterility and inviability evolve gradually; positively correlatedwith the time since divergence -- gradual accumulation ofdeleterious epistatic interactions between species

Hybrid sterility evolves faster than inviability

Haldane’s Rule: if hybrid sterility or inviability is present in only one sex of F1 offspring, it will be the heterogametic

sexFor Drosophila it is males--for Lepidoptera it is females

Haldane’s Rule: if hybrid sterility or inviability is present in only one sex of F1 offspring, it will be the heterogametic sex

inviability due to lethal incompatibilities between autosomal and X-linked loci

dominance theory: hybrid incompatibilities are partially recessive XX hybrids are heterozygous and protected X Y hybrids express all X-linked recessives

faster-male evolution: - sexual selection drives rapid divergence of male fertility

factors ---> hybrid male sterility- spermatogenesis is inherently more sensitive to perturbation

Species are evolutionarily independent groups. A species is often defined as a group of populations that are capable of interbreeding, but this definition applies best to diploid, sexually reproducing taxa. Populations which are phenotypically or ecologically distinct, yet regularly interbreed, or species that reproduce largely asexually may still be valid species.

The process of adaptive divergence that leads to speciation may occur either in allopatry or sympatry.

Speciation is a result of divergent natural (ecological) or sexual selection. Genetic drift may accelerate the process of divergence during founder events.

Reproductive isolation between species may result from assortative mating (prezygotic) or from hybrid sterility/inviability (postzygotic).

Haldane’s rule is a general description of the early stages of speciation.

Genetic correlations for performance in different environments and for habitat choice may facilitate the rate at which divergence occurs.