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Species and Speciation

Chapter 15

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What is a species?

• The “species” is a fundamental concept in biology – paradoxically, there is no definition of species that is universally applicable

• Freeman and Herron suggest that “most biologists agree … [that a species] is the smallest evolutionarily independent unit” (p. 584)– Evolutionary independence occurs when gene flow between

populations ceases (or becomes very low); and when drift, mutation, and selection operate on populations separately

• The “essence of speciation is lack of gene flow,” and speciation involves at least two stages:– Genetic isolation

– Differentiation

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Species concepts

• Biological species concept

• Phylogenetic species concept

• Morphological species concept

• Ecological species concept

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Biological species concept:(Ernst Mayr 1942)

• A biological species is a group of interbreeding (or potentially interbreeding) individuals, that is reproductively isolated from other groups of interbreeding individuals– A species is a gene pool that is not exchanging genes

other gene pools

– Reproductive isolation means that individuals of different species will not mate and produce offspring, or offspring will be sterile

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Biological species concept – 2• The BSC is the “textbook” standard definition of

species• It is applicable to sexually reproducing organisms• It can be difficult to apply in practice because we

generally do not know if populations that have separate ranges can interbreed, unless we do the experiment

• Doesn’t apply to asexual organisms• Can’t be applied to fossils• How to apply to many plant and some animal

species that hybridize freely?

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Phylogenetic species concept= genealogical species concept

• A species is the smallest monophyletic group• Suppose we sample 5 individuals from each of two

populations, A and B, and construct a tree – based perhaps on genetic information

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Phylogenetic species concept – 2

• If our tree looks like this, then populations A and B represent different species:

A1 A2 A3 A4 A5 B1 B2 B3 B4 B5

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Phylogenetic species concept – 3

• If our tree looks like this, then populations A and B are NOT different species:

A1 A2 A3 A4 A5 B1 B2 B3 B4 B5

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Phylogenetic species concept – 4

• What happens if our tree looks like this?

A1 B4 A3 A4 B2 B1 A5 B3 A2 B5

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Phylogenetic species concept – 5

• The logic of the phylogenetic species concept is that traits can only distinguish populations on a phylogeny if the populations have been isolated in terms of gene flow and have diverged genetically and/or morphologically

• To be considered separate phylogenetic species, populations must have been separated long enough to have evolved unique derived diagnostic characters

• Unlike the BSC, the PSC can be applied to asexual organisms

• Like the BSC, the PSC may be difficult to apply in practice because it requires a lot of information to construct good trees that will identify monophyletic groups

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BSC and PSC overlap

• A “good” biological species is a monophyletic group of individuals – individuals of the same species are more closely related to one another than they are to individuals of other species

• Therefore, the BSC and PSC may agree on species designations some of the time

• However, the PSC could greatly increase the number of species because separate populations of a biological species might be monophyletic but still able to interbreed – if individuals from population A can interbreed with individuals from population B, we have one biological species but two phylogenetic species

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BSC and PSC overlap – 2

• If individuals from population A can interbreed with individuals from population B, then we have one biological species but two phylogenetic species

A1 A2 A3 A4 A5 B1 B2 B3 B4 B5

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Morphological Species Concept – 1

• A species is a group of phenotypically similar individuals

• This is, in fact, how most species are actually defined in the absence of detailed information about reproductive compatibility and/or phylogenetic relationship

• Only species concept applicable to fossils

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Morphological Species Concept – 2

• Disadvantages– Not “evolutionary” (genetic or phylogenetic)– Arbitrary and idiosyncratic: two people may

disagree about where to draw species boundaries, or statistical phenetic methods may disagree about how to create groups of morphologically similar individuals

– Cryptic species cannot be distinguished

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Ecological Species Concept

• A species is a group of phenetically similar organisms that occupy a given ecological niche (or set of niches)

• Species integrity is maintained not so much by reproductive isolation, but by selection to adapt each species to its niche

• Hybridization is not a problem if hybrids are less fit than parental species or have very restricted ranges

• Works for asexual species• Problem:

– Hard to define niches independently of the species that occupy them

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Applying Species ConceptsThe marine copepod, Eurytemora affinis

• Small estuarine copepod (1 – 2 mm), world -wide distribution, important part of zooplankton and marine food chains.

• Lee (2000) sequenced 2 genes in individuals from 38 populations and performed matings between individuals from different populations in the laboratory

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Eurytemora affinishttp://life.bio.sunysb.edu/marinebio/pl_23.jpg

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A phylogeny of E. affinis populations(Lee 2000) (Fig. 15.3 b)

• This phylogeny supports “at least 8” phylogenetic species

• Breeding tests indicate that individuals from two different phylogenetic species are reproductively isolated

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How many species of elephants?(Roca et al. 2001) (Fig. 15.4 b)

• This phylogeny supports two African elephant species

• 195 elephants from 21 African populations

• 4 genes sequenced

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Mechanisms of genetic isolation

• Geographic (physical) isolation – allopatric speciation

• Isolation based on differential resource use without geographic isolation – sympatric speciation

• Isolation by selection and limited gene flow in continuously distributed populations – parapatric speciation

• Isolation based on changes in chromosome number or chromosomal rearrangements – chromosomal speciation

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Allopatric speciation(Ernst Mayr 1942, 1963)

• Two or more populations of a species become geographically isolated from one another (either by dispersal or vicariance)

• Separated populations will evolve independently of one another provided there is no (or very little) gene flow between them

• In a “pure” allopatric model, speciation is an “accidental” by-product of separate evolutionary trajectories that eventually result in genetic (reproductive) incompatibility between individuals from different populations

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Isolation by dispersal and vicariance (Fig. 15.5)

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Geographic isolation by dispersal:Hawaiian drosophilids

• Over 500 endemic species in 2 genera• Occupy a wide variety of ecological niches• Many species have ranges restricted to single islands• One of the most famous adaptive radiations• “Founder hypothesis” – one gravid female (or a few

individuals) disperse to another island and start a new isolated population

• Predictions:– Closely related species should tend to be on neighboring islands– At least some phylogenetic branching sequences should

correspond to the sequence in which islands were formed

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Evidence for speciation by dispersal and colonization

events – Hawaiian

Drosophila (DeSalle and

Giddings 1986) (Fig.

15.7)

Based on mtDNA

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Geographic isolation by vicarianceSnapping shrimp and the Isthmus of Panama

(Knowlton et al. 1993)• Before the formation of the Panamanian land bridge about

3 million years ago, the Pacific Ocean and Carribean Sea were connected. The formation of the land bridge was a vicariant event that divided populations of marine organisms

• Seven species pairs identified on the basis of morphology – one member of each pair in the Carribean and the other member in the Pacific. This is what would be expected if there were seven species originally, and each was subsequently split by the formation of the land bridge.

• Data on mtDNA sequences confirms this hypothesis

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mtDNA phylogeny of 7 morphological species pairs of snapping shrimp (Knowlton et al. 1993)(Fig. 15.8 b)

(P = Pacific Ocean; C = Carribean Sea; numbers designate morphospecies)

http://www.dnr.sc.gov/marine/sertc/images/photo%20gallery/Alpheus%20armillatus.jpg

Alpheus armillatus

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Mechanisms of divergence

• Given that populations become geographically isolated, what then causes them to diverge genetically?– Founder effect speciation – maybe, if number

of founders is very small and new population stays small for a long time

– Genetic drift – likely to require a long time unless populations are very small

– Natural selection, sexual selection, mutation operating differently in isolated populations

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Sympatric Speciation

• Genetic isolation without geographic isolation• A single population becomes subdivided by

alternative habitat preferences or alternative resource use

• Sympatric speciation commonly invoked for herbivorous insects – alternative host plants give rise to host races that ultimately become separate species

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Apple and hawthorn maggot flies (Rhagoletis pomonella) – 1

• Fly is native to northeastern and north-central US• Native host plant is hawthorn (Crategus)• Flies first recorded as a pest of apples in the mid

1800’s• Female flies lay eggs on the fruit, larvae hatch and

burrow into the fruit: after about a month the fruit falls to the ground, the larvae leave and pupate in the soil, and emerge as adults the following summer

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Rhagoletis pomonella(Photo: Guy Bush) (http://www.ento.psu.edu/mcpheronlab/schwarz/Copy

%20of%20Rhagoletis1.jpg

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Apple and hawthorn maggot flies (Rhagoletis pomonella) – 2

• Are flies that parasitize apple fruits and hawthorn fruits distinct populations?– host race hypothesis

– implies selection for exploiting different hosts

• Or, do flies that parasitize apple fruits and hawthorn fruits interbreed freely and are they a single population?– this hypothesis seems more likely because the two host trees occur

together (sympatrically) throughout their ranges, and flies are known to travel distances of more than a mile

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Apple and hawthorn maggot flies (Rhagoletis pomonella) – 3

• Flies that parasitize apple and hawthorn fruits are distinct host races– genetically differentiated– show preference for their own fruit type in choice tests

• Field observations indicate that only 6% of matings take place between apple and hawthorn flies

• This is still a fair amount of gene flow– it would prevent genetic divergence of host races unless selection

was acting differently on the two hosts• Natural selection for divergence appears to be based on

different times of fruit ripening– apple fruits ripen 3 – 4 wks before hawthorn– natural selection for apple flies to complete development well

before hawthorn flies

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Allele frequency changes caused by differences in temperature experienced by hawthorn maggot flies

(Feder et al. 1997) (Fig. 15.10)

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How common is sympatric speciation?

• Historically, a controversial idea• Simple population models suggest that genetic

isolation in sympatry is likely to be difficult• However, host race formation is much more likely

to occur if individuals tend to mate and lay eggs on the same kind of fruit that they grew up on – assortative mating reduces gene flow

• There is more and more evidence that sympatric speciation may be relatively common – most examples involve herbivorous insects