What makes a species invasive? f) Micro-evolutionary change hypothesis

3) What makes a species invasive?f) Micro-evolutionary change hypothesis

Micro-evolutionary change hypothesis• Frequent colonizing events are a central feature of invasive

plants


Micro-evolutionary change hypothesis• Frequent colonizing events

Founder effects = founders of a new population carry only a fraction of the total genetic variation of the source populations



Founder effects = founders of a new population carry only a fraction of the total genetic variation of the source populations

Climate matching = different genetic stock succeed in different parts of introduced range; often correspond to similar latitudes or climate in native range. e.g. tamarisk in US.



Founder effectsGenetic bottlenecks = loss of genetic variation when population

size drastically decreases; often associated with catastrophic events that result in mass mortality



Founder effectsGenetic bottlenecksGenetic drift = loss of genetic variation by chance when

populations are small and do not have complete, random interbreeding



Founder effectsGenetic bottlenecksGenetic drift

• Natural selection: strong selective forces often apply to successful colonizers




• Natural selectionNew abiotic environment – rapid adaptive responses over short

times and within short distances to new environment




• Natural selectionNew abiotic environment – rapid adaptive responses over short

times and within short distances to new environmente.g.. Bearded Goatgrass in CA: ‘competitive’ VS ‘seed

producing’ phenotypes in invasion front VS monoculture.




• Natural selectionNew abiotic environmentNew biotic environment – shifts in relative proportions of

competition vs. defense vs. reproductive pressures e.g. Leger 2003.




• Natural selectionNew abiotic environmentNew biotic environment

• Hybridization – a natural process that occurs in plants





• Hybridization – a natural process that occurs in plants↑ genetic diversity





• Hybridization – a natural process that occurs in plants↑ genetic diversity↓reproductive barriers





• Hybridization – a natural process that occurs in plants↑ genetic diversity↓reproductive barriersTransfers or originates adaptations





• Hybridization – a natural process that occurs in plants

2834 plant species in the British IslesOf these, 715 (25%) are hybrids

74 are native X alien21 are alien X alien95 (13% of hybrids) involve aliens





• HybridizationInterspecific – often with other species in new environment





• HybridizationInterspecificIntraspecific – often with populations from native range that would

not normally occur

Determining genetic change and hybridization

• How?

Determining genetic change and hybridization

• How?• Morphology and cytology• Genetic analyses – chloroplast, nuclear, ribosomal DNA • Experiments (common garden)


Hybridization Evidence:• Stabilized introgressants

Introgression = back cross with 1 or more parentsP1 X P2 → F1

F1 X {P1, P2} → F2 introgressant



Introgression = back cross with 1 or more parentsStabilized = viable, fertile hybrids



Hybrids form new Intraspecific taxa



Hybrids form new intraspecific taxaHybrids form new Species – Note: all examples are alien X alien


Hybridization Evidence:• Stabilized introgressants• Allopolyploids = hybrid between different species in which

chromosomes of both parents are retained


Hybridization Evidence:• Stabilized introgressants• Allopolyploids

Hybrids form new Species – Note: both Tragopogon’s are alien X alien


Specific Example: Rhododendron ponticum in British Isles*Milne & Abbott (2000) Molecular Ecology 9:541-556• Natural distribution: south of Black Sea with disjunct

populations in Lebanon, Spain, & Portugal• Extensively naturalized throughout British Isles


Specific Example: Rhododendron ponticum in British Isles*Milne & Abbott (2000) Molecular Ecology 9:541-556• Natural distribution: south of Black Sea with disjunct populations in

Lebanon, Spain, & Portugal• Extensively naturalized throughout British Isles• Origin unclear:

Earliest known introduction (1763) from SpainBut subsequent introductions likely, especially from Black Sea

areaAlso can’t tell from morphological information where it came from,

but know from morphology that had to hybridize at some timeR. ponticum cultivated along with other introduced species


Specific Example: Rhododendron ponticum in British Isles*Milne & Abbott (2000) Molecular Ecology 9:541-556• Most individuals from naturalized populations had genotypes from

Spain (88%), followed by Portugal (10%)• No genotypes from Black Sea region


Specific Example: Rhododendron ponticum in British Isles*Milne & Abbott (2000) Molecular Ecology 9:541-556• Most individuals from naturalized populations had genotypes from

Spain (88%), followed by Portugal (10%)• No genotypes from Black Sea region• Small number of individuals had hybridized with at least 3 other

speciesOccurrence of R. catawbiense genotypes most common in

Scotland (coldest area of British Isle)Introgression with catawbiense appears to have conferred cold

tolerance into ponticum

Another example: Tamarisk in US

• Gaskin and Shaal 2002. PNAS 99 (17): 11256-11259

• Used DNA sequencing to determine origins and relationships of plants (nuclear gene intron)

• Little similarity in genetic types between US and Eurasia (4 common of 58 types)

• Most common species in US invasion was hybrid between 2 Eurasian species (Tamarix chinensis and Tamarix ramosissima)

• Hybrid unknown in Asia (species geographically isolated)

Evidence for genetic change in invasive species

• Leger and Rice 2003, Eschultzia californica • Used common garden to look for genetic VS phenotypic

traits• Plants from Chile (invaded range) were larger with no

competitors

Leger and Rice 2003. Ecology Letters


SUMMARY: Micro-evolutionary change hypothesis• Likely that most (if not all) invasive species go through founder

events, experience genetic bottlenecks & drift, and undergo selectionIn other words micro-evolutionary changes

• Good evidence for hybridization being beneficial


SUMMARY: Micro-evolutionary change hypothesis• Likely that most (if not all) invasive species go through micro-

evolutionary changes• Good evidence for hybridization being beneficial

But• Have evidence of micro-evolutionary changes for only a limited

number of species• For only a subset of these, have evidence that micro-evolutionary

changes have been beneficial• A species that undergoes micro- (or even macro-) evolutionary

changes does not automatically become invasive

3) What makes a species invasive?g) Escape from biotic constraints hypothesis

aka “Escape from enemy” hypothesis“Enemy release” hypothesis

Basic concepts:• Species in their native range are suppressed by natural

enemies



Basic concepts:• Species in their native range are suppressed by natural enemies• Alien species are immigrants to a new area• Aliens often arrive as seeds



Basic concepts:• Species in their native range are suppressed by natural enemies• Alien species are immigrants to a new area• Aliens often arrive as seeds

In other words, they arrive without the grazers, insect pests, diseases, parasites, etc. of their native range – their “enemies”



Basic concepts:• Species in their native range are suppressed by natural enemies• Alien species immigrate without enemies• Hence, alien species “escapes” from their enemies and are no

longer affected by biotic constraintsThus, alien growth and success is much greater in new range



Basic concepts:• Species in their native range are suppressed by natural enemies• Alien species immigrate without enemies• Aliens lack biotic constraints• However, alien success will depend on potential enemies in

new range:Are potential enemies generalists or specialists?




new range:Are potential enemies generalists or specialists?Are population sizes of potential enemies large or small?




new range:Are potential enemies generalists or specialists?Are population sizes of potential enemies large or small?Do potential enemies feed on foliage or seeds?




new range:Are potential enemies generalists or specialists?Are population sizes of potential enemies large or small?Do potential enemies feed on foliage or seeds?Are there similar hosts for potential enemies in new area?

What makes a species invasive? f) Micro-evolutionary change hypothesis

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