3)What makes a species invasive? c) Micro-evolutionary change hypothesis Micro-evolutionary change hypothesis Frequent colonizing events are a central

3) What makes a species invasive?c) 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 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 environmentNew biotic environment – shifts in relative proportions of

competition vs. defense pressures




• 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


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


Case Study: 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


Case Study: 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


Case Study: 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


Case Study: 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


Case Study: 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


Case Study: Tamarix in US*Gaskin & Schaal (2002) PNAS 99:11256-11259• Native range: 54 species across Europe & Asia• Introduced range: originally 8-12 species into US for shade & for

erosion control2 species particularly invasive:Tamarix ramosissima & T. chinensis


Case Study: Tamarix in US*Gaskin & Schaal (2002) PNAS 99:11256-11259• In native ranges: T. ramosissima & T. chinensis overlap• But different genotypes from DNA sequencing

T. chinensis: Haplotype 2 (red); primarily homozygousT. ramosissima: Haplotype 1 (blue) most abundant, but also

others; fewer homozygotes, many heterozygotes• No T. ramosissima -T. chinensis hybrids (red-blue combinations)


Case Study: Tamarix in US*Gaskin & Schaal (2002) PNAS 99:11256-11259• In US:

~20% genotypes are T. ramosissima homozygotes (blue-blue)~20% genotypes are T. chinensis homozygotes (red-red)~20% genotypes are T. ramosissima -T. chinensis hybrids (red-

blue)• Also get novel genotypes plus hybrids with other Tamarix species


Case Study: Tamarix in US*Gaskin & Schaal (2002) PNAS 99:11256-11259• In US:

~20% genotypes are T. ramosissima homozygotes (blue-blue)~20% genotypes are T. chinensis homozygotes (red-red)~20% genotypes are T. ramosissima -T. chinensis hybrids (red-

blue)• Also get novel genotypes plus hybrids with other Tamarix species

Complicates biological control efforts in US


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

Documents

3)What makes a species invasive? c) Micro-evolutionary change hypothesis Micro-evolutionary change hypothesis Frequent colonizing events are a central