Plant Evolution and Diversity

Clade

a group of organisms that includes a single common ancestor and all descendants

Grade

 a group of organisms that does not include all descendants of a common ancestor 

Name the 5 land plant species (Embryophytes)

• Bryophytes (hornworts, liverworts, mosses)

• Lycophytes (club mosses) have microphylls, small leaves with a single vascular channel

• Pteridophytes

• Gymnosperms 

• Angiosperms

Mosses and other Nonvascular Plants spend the most of their lifecycle… haploid or diploid?

Haploid, branched

In Mosses which is dominant, and are they haploid or diploid (gametophyte, sporophyte)?

• Gametophyte (n): dominant 

• Sporophyte (2n): reduced, dependant on gametophyte for nutrition

In ferns and other seedless plants which is dominant, and are they haploid or diploid (gametophyte, sporophyte)?

• Gametophyte (n): reduced, Independent (photosynthetic and free-living)

• Sporophyte (2n): Dominant

What are the two seed plant taxa? What are the reproductive shoots of these taxa?

Gymnosperms and Angiosperms

In Angiosperms where is the gametophyte, and is it haploid or diploid (gametophyte, sporophyte)?

• Microscopic female and male  gametophyte inside flower parts (n)

• Sporophyte (2n)

In Gymnosperms where is the gametophyte, and is it haploid or diploid (gametophyte, sporophyte)?

• Microscopic female gametophytes (n) inside ovulate core 

• Microscopic male gametophytes (n) inside pollen cone

• Sporophyte (2n)

Phyllid

Leaf like structure on mosses

Sporophytes

(produce haploid spores) terminate in a single sporangium

Monilophytes

(ferns, including horsetails) produce fronds, large leaves that are photosynthetic and/or reproductive.

Which period showed the earliest angiosperm fossil?

Cretaceous

Which period showed the earliest conifer fossil?

Carboniferous

Which period showed the origin of seed plants?

Devonian

Which period showed the origin of land plants?

Ordovician

What are the two hypotheses of angiosperm evolution?

• One:

  • Early origin of angiosperms, followed by radiation later

• Two: 

  • Extinct relatives of the angiosperms (the search for intermediate ovule)

What regulates patterns of diversification in the angiosperms? (5pts)

• Modes of speciation 

• Biogeography 

• Major transitions in morphology, physiology, reproduction, etc…

• Interactions with animals and other plants 

• Genome evolution

Name 2 species domestication drastically changed?

Corn and sunflowers

What traits are selected under domestication? (The domestication syndrome)

• Seeds do not shatter (stay on plant)

• Determinate growth (seeds all in one place)

• Larger inflorescences (more seeds)

• Larger seeds

• Seeds do not require dormant period

Wild sunflowers with the BT transgene… do they have higher/lower fitness/herbivory?

• Wild sunflowers with the Bt transgene have reduced herbivory and higher fitness

Are species real? What are some reasons we believe this? (Give 2 reasons)

• Cluster analyses of morphological, behavioral and molecular (DNA) data indicate that species are discrete entities

• There is not continuous variation among species (field guides are not useless)

• Concordance of species groupings between cultures

Biological Species Concept

• Groups of interbreeding natural populations that are reproductively isolated from other such groups 

Systematic classification

• Represents the evolutionary history of species 

• Applies to the widest number of organisms

Enables understanding of how species arise

Allopatric Speciation

geographic isolation and divergence

Sympatric Speciation

divergence without geographical isolation ( strong selection or hybrid speciation)

Transgressive Segregation

can promote hybrid speciation if extreme phenotypes are able to colonize novel habitats 

What is a benefit of hybridization?

Can promote speciation and lead to novel adaptations

What happens to a species after a new location (island) is coloniozed? (5pts)

1. Ecological Release

• Colonisztion of a novel biotic environment which particular predators are absent 

• Can lead to relaxed selection causing… 

1. Loss of ‘unnecessary’ features (defenses) (dodo bird) 

2. Increased trait variance 

3. Exploitation of new niches 

2) Density Compensation 

• Species in habitats without competitors have higher densities, compensating for the number of missing competitors

Adaptive radiation

evolution of a group into a wide variety of types adapted to specialized modes of life (Darwin's finches) 

Why does SI (self incompatibility) breakdown? (2pts)

Advantages of ability to inbreed occur at two levels:

1. Individual level selection 

2. Group selection

The Automatic Transmission Hypothesis

 selfers can have higher fitness than outcrosser because they can transmit more gene copies to the next generation

Two Assumptions of Automatic Transmission Hypothesis

Assumption 1: Selfind does not reduce pollen export 

Assumption 2: Selfing does not reduce seed quality 

Why is homozygosity associated with reduced fitness in populations with deleterious mutations?

• Large populations maintain deleterious mutations at many loci 

• Deleterious mutations are usually only expressed in homozygotes 

• In a large, outcrossing population, these mutations are rarely expressed

Baker’s Law

• “With self-compatible individuals, a single propagule is enough to start a sexqually reproducing colony after long distance dispersal”

• Self compatible individuals are atleast twice as likely to be successful under long distance dispersal than SI individuals

Selfer’s have higher____________ rates, but also higher ____________ rates.

Selfers have higher speciation rates, but also higher extinction rates. Self-incompatible plants have higher net diversification rates. Species-level selection

Why is the evolution of selfing an evolutionary dead end? (4pts)

1. Selfing erodes genetic variation

2. Selfing increases chances that deleterious mutations are fixed

i) 1 + 2 = decreased ability to adapt to new environments therefore greater chance of population extinction

3. Outcrossing cannot evolve from selfing

4. ‘Irreversible collapse’ of variation at the S locus; so SI cannot re emerge after even a few generations of selfing

Give 3 examples of traits that have become irreversibly evolved

• Parthenogenesis 

• Selfing 

• Dioecy (separate sexes)

• Haplo-diploidy 

• Polyploidy 

• Muller’s ratchet 

• Heteromorphic sex chromosomes

How do plants avoid self-fertilization? 3pts

1. Temporal separation fo sex roles

2. Spatial separation of sex roles 

   1. Separate female and male flowers (monoecy)

   2. Separate female and male individuals (dioecy)

3. Self-incompatibility

   1. Gametophytic SI

   2. Sporophytic SI

Monoecy

Separate female and male flowers

Dioecy

Separate female and male individuals

4 Functions of Flowers

1. Attract pollinators  male and/or female role)

2. Disperse pollen (male role)

3. Receive pollen (male and/or female role)

4. Produce seeds and fruits (female role)

Two observations about flowers

Observation #1

Many species produce WAY more flowers than fruits 

Observation #2 

More flowers leads to greater visitation by pollinators

How does sexual selection in plants play a role?

• Males compete for access to mates (via pollinators)

• Males compete for access to ovules (among pollen grains)

In species with unisexual flowers (flowers that have stamens OR pistils), male flowers (petal sizes) are usually larger than female flowers

Darwin’s Coevolutionary race

Darwin believed interactions between plants and pollinators promoted evolutionary change

Generalist

Open floral morphology, easily accessed rewards(nectar/pollen)

Specialists

Floral Morphology restricts access to all but a smaller number of pollinators, floral rewards only attract specific pollinators

Give some examples of species which have convergently evolved to better their floral form for pollinators

• bees

• hummingbirds

• flies

• beetles

• bats

• moths

• butterflies

Pros and Cons of Specializing

Pro: plants with specialized pollinator associations do not compete with other plants and do not lose their pollen on stigmas of other species

Con: Plants with specialized pollinator associations are at a greater risk of pollination failure

Predictability of pollination - When to specialize vs when to generalize

• A plant should specialise on the most effective pollinator when it is abundant and reliable

• Generalization is favoured when the availability of the most effective pollinator is unpredictable

When to specialize vs when to generalize: Plant life-history

Long lived plants can risk specialization

short lived plants, reliant on seed production should be generalists

When to specialize vs when to generalize: Population Density

Plants in low-density populations might require a specialist that will seek out other members of the population

True or False: Plants with specialist pollinators do not lose pollen via transfer to other species

True. This increases their fitness

True or False: Evolutionary transitions between pollinators promotes reproductive isolation (speciation)

True

Two goals of plant, two goals of bees

Plant: disperse pollen to unrelated individuals and attract a large number of effective pollinators

Bees: collect nectar and pollen, and reduce competition with other animals

Pros and cons of 3 (Abiotic, bees, hummingbirds) pollination strategies

Abiotic → reliable but wasteful

Bees → moderately reliable but also moderately wasteful

Hummingbirds → efficient but unreliable

How do pollinator shifts occur?

In part of the species range an unrelated longer tongued pollinator becomes the predominant visitor. Rapid evolution of longer spurs owing to selection for improved pollen transfer. If pollinators cannot reach nectar, then they will not visit flowers and thus reversals to short spurs are less likely

True or False: a major shift in the ecology of the plant is needed to promote shifts in syndromes

True

Hemiparasites

Faculative and obligate parasites that photosynthesize during part of their life-cycle

Holoparasites

Non-photosynthetic, obligate parasite

Striga

• casuese large economic losses due to its parasitic nature, ruins millions of hectares of crop production, this loss is expected to increase by 15% annuall

Plant Carnivory

• morphological adaptations to attract, trap kill and digest animals. all these adaptations involve modified leaves

The escape and Radiate Model (5pts)

1) angiosperms reduce novel secondary metabolites through mutation or recombination

2) plants produce chemicals that produce resistance to insects

3) plants that escape herbivory are free to radiate

4) insects evolve resistance to these compounds (selection favors insects which resist the chemicals)

5) insects radiate to exploit the now common group of plants

Key Innovation required to enable radiation by insects

NSP allows Pierid butterflies to feed on members of the Brassicaceae

True or False: Genome duplication events are not important in contributing to the diversity of plants

False

Modes of Speciation

• geographic isolation and divergence (allopatric)

• divergence without isolation (sympatric)

  • strong selection

  • hybrid speciation

    • POLYPLOID FORMATION

    • homoploid speciation

Name a method in which polyploidy arises

Hybridization

Autopolyploids

Multiple chromosome sets derived from a single species

Allopolyploids

Polyploids formed through hybridization

True or False: Newly formed polyploid lineages easily remain within the population

False: They face strong selection against their survival

2 Advantages of Polyploidy

1) Heterosis → hybrid vigor

• fixes heterozygosity

2) Gene redundancy

• leads to the masking of deleterious alleles

How many cotyledons does a Monocot and Dicot have?

Monocot - 1

Dicot - 2

Annuals

Entire life cycle (seed germ to veg plant, flowering, and seed formation) occurs within one growing season

Biennials

two seasons are needed from seed ger to flowering and seed formation

Perennials

Veg structures live year after year and flower in multiple seasons.

True or False: Plants with shorter generation times might evolve more slowly than those with longer generation times.

False: they are more likely to accumulate more nucleotide substitutions per unit time

Evolutionary Fates of Duplicated Genes

1) Loss of function because mutations to the redundant gene are selectively neutral

2) Sub-functionalization: Degenerative mutations occur in both copies, and expression of both is required for normal function. This ‘job-sharing’ between gene copies can lead to acquisition of novel functions through further mutation.

3) Neo-functionalization: Acquisition of new functions through mutation of one gene copy (e.g., changes to recognition sites of target proteins

Pseudogenization

Loss of function

Neofunctionalization

Increases diversity

Subfunctionalization

Reduces pleiotropy increases specialization

The Benefits of retaining the megaspore on the sporophyte

1) Eliminates need for gametophyte to find its own source of water

2) Provides the opportunity for the sporophyte to protect and nourish the developing embryo (next gen of sporophytes)

3) Maternally-derived tissues that promote dispersal and desiccation tolerance

Two Solutions to Life on Land

Solution A:

• Gametophyte generation is free living and the sporophyte has determinate growth

• Gametophytes grow close to the ground, enabling sperm dispersal

• Ground-hugging growth form does not require a vascular system

Solution B:

• The sporophyte has indeterminate growth and becomes free living, with destructive consequences for the gametophyte

• Sporophytes grow in stature, enabling spore dispersal

• Increased height requires a vascular system

Constraints to Life on Land

1) The function of the gametophyte is to produce gametes, which need freely available water

2) The function of the sporophyte is to produce spores, which are tolerant desiccation

Advantages of Multicellularity

• size and simultaneous access to different resources in heterogenous environments

• competitive advantages

• tissues and division of labor

• size and homeostasis

• multiple rounds of reproduction

• pathogen defense

• resilience to herbivory