LECTURE 13 : LECTURE 13 :

PLANT EVOLUTION

“Absence of evidence is not evidence of absence.”

Early history of life• Solar system~ 12 billion years ago (bya)

• Earth~ 4.5 bya• Life~ 3.5 to 4.0 bya• Prokaryotes~ 3.5 to 2.0 byastromatolites

• Oxygen accumulation~ 2.7 bya photosynthetic cyanobacteriacyanobacteria

• Eukaryotic life~ 2.1 bya• Muticelluar eukaryotes~ 1.2 bya

• Animal diversity~ 543 mya• Land colonization~ 500 mya

The Origin of Life• Spontaneous generation vs. biogenesis (Pasteur)

• The 4-stage Origin of life Hypothesis:

1. Abiotic synthesis of organic monomers

2. Polymer formation

3. Origin of Self-replicating 3. Origin of Self-replicating molecules

4. Molecule packaging (“protobionts”)

Organic monomers/polymer synthesis• Oparin (Rus.)/Haldane (G.B.)

hypothesis (primitive earth): volcanic vapors (reducing atmosphere) with lightning & UV radiation enhances complex molecule formation (no O2)

• Miller/Urey experiment:– water, hydrogen, methane, ammonia – all 20 amino acids, nitrogen bases, & ATP

formed

• Fox proteinoid formation (abiotic• Fox proteinoid formation (abioticpolypeptides) from organic monomers dripped on hot sand, clay or rock

• Oparin (coacervates) protobionts(aggregate macromolecules; abiotic) surrounded by a shell of H2O molecules coated by a protein membrane

Abiotic genetic replication• First genetic material

• Abiotic production of ribonucleotides

• Ribozymes (RNA catalysts)

• RNA “cooperation”

• Formation of short polypeptides (replication polypeptides (replication enzyme?)

• RNA~ DNA template?

The Major Lineages of Life

EVOLUTION• Descent with Modification:

A Darwinian View of Life

What is Evolution?The kind we’re talking about is sometimes called organic evolution to distinguish it from non-biological changes over time.

Working definition: Evolution is the progressive change in organisms over time.

Evolution• Evolution: the change over time of

the genetic composition of populations

• Natural selection: populations of organisms can change over the generations if individuals having certain heritable traits leave more offspring than others (differential offspring than others (differential reproductive success)

• Evolutionary adaptations: a prevalence of inherited characteristics that enhance organisms’ survival and reproduction

November 24, 1859

Darwin’s Voyage of Discovery

A reconstruction of the HMS Beagle sailing off Patagonia.

The Voyage of the Beagle

Darwin’s Ideas Did Not Develop in a Vacuum Contributor’s to Darwin’s thinking included:1. Charles Lyell – uniformatarianism2. Georges Cuvier – species extinction.3. Thomas Malthus – struggle for existence4. Jean Baptiste de Lamarck – evolution by

acquired characteristics. (1) 1797-1875

Alfred Russel Wallace Independently

(2) 1769-1832(3) 1766-1834

(4) 1744-1829

Alfred Russel Wallace Independently Drew the Same Conclusions as Darwin

Papers from Wallace and Darwin were jointly presented (with little impact) to the Linnaean Society in 1858.

Darwin’s Observations and Inferences

Observation 1: Left unchecked, the number of organisms of each species will increase exponentially, generation to generation.

Observation 2: In nature, populations tend to remain stable in size.

Observation 3: Environmental resources are limited.

Inference 1: Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals, with only a fraction of offspring surviving in each generation.

Darwin’s Observations and Inferences

Observation 4: Individuals of a population vary extensively in their characteristics with no two individuals being exactly alike.

Observation 5: Much of this variation between individuals is heritable.

Inference 2: Survival in the struggle for existence is not random, but depends in part on the heritable characteristics of individuals. Individuals who inherit characteristics most fit for

Darwin’s Observations and Inferences

characteristics most fit for their environment are likely to leave more offspring than less fit individuals.

Darwin’s Observations and Inferences

Inference 3: The unequal ability of individuals to survive and reproduce leads to a gradual change in a population, with favorable characteristics accumulating over generations (natural selection).

Taken together, these three inferences are a statement of Darwin’s Theory of Evolution.

Evolutionary history• Linnaeus: taxonomy• Hutton: gradualism• Lamarck: evolution• Malthus: populations• Cuvier: paleontology

• Lyell: uniformitarianism• Darwin: evolution• Mendel: inheritance• Wallace: evolution

The Weak Link of Genetics and the Modern SynthesisA major problem in Darwin’s theory was the lack of a mechanism to explain natural selection.

How could favorable variations be transmitted to later generations?

With the rediscovery of Mendel’s work and its vast With the rediscovery of Mendel’s work and its vast extension in the first half of the 20th century, the missing link in evolutionary theory was forged.

Darwinian theory supported by genetics is known as the modern synthesis. Darwin in his early

years.

TOPICS OF DISCUSSION• Introduction

– Early history of life

– The Origin of Life

• Evolution• A Darwinian View of Life• Working definition• Evolutionary history• Evolutionary history• Darwin’s Observations and Inferences• Plant Evolution• N

Discomfort With EvolutionThe upheaval surrounding evolution began with publication of On the Origin of Species and continues nearly 150 years later.

An early disparaging view of evolutionary theory and its creator.

1925

Two of many arguments made against evolution are:

- Evolution is only a theory.- No one’s ever seen evolution.

The first point is true, but misses

Discomfort With Evolution

The first point is true, but misses the meaning of scientific theory, and the second point is easily countered.

Evidence for Evolution –The Fossil Record

Evidence for Evolution - Comparative Morphology

Why use the same skeletal plan for these very different appendages?

Evidence for Evolution - Comparative Embryology

Why do embryos of different animals pass through a similar developmental stage?

Recent discoveries of the conservation of molecular mechanisms of development are even more compelling.

Evidence of Evolution –Conservation and Diversification at the Molecular Level

Why should different organism possess related genes?

Why does the degree of relationship of genes match their degree of relationship established by other methods?

Evidence for Evolution –Evolution Observed

Evolution of pesticide resistance in response to selection.

Evolutionary Time Scales

Macroevolution: Long time scale events that create and eliminate species.

Evolutionary Time Scales

Microevolution: Short time scale events (generation-to-generation) that change the genotypes and phenotypes of populations.

We’ll begin our more intensive look at evolution with microevolution.

Evolution evidence: Molecular Biology

• Similarities in DNA, proteins, genes, and gene products

• Common genetic code

Plant EvolutionPlant Evolution

Adaptations• Cuticle• Alternation of generations• Specialized tissues

Cuticle• Waxy coating on surfaces resists drying out• Waxy coating on surfaces resists drying out• stomata exist to allow necessary gas exchange

Alternation of generations• Plants live part of their life in a haploid stage and part in a diploid stage– haploid portion is called the gametophyte generation because it produces the reproductive cells

– diploid portion called sporophyte generation because it produces spores

• The gametophyte plant produces male and female sex organs (male and female gametangium)– eggs are fertilized by sperm to form a zygote– embryo development (seed) occurs within the female gametangium

Specialized tissues• Other than bryophytes all other plants have Specialized tissues• Other than bryophytes all other plants have vascular tissues– xylem - conducts water and minerals– phloem - conducts food

Bryophytes• Only nonvascular plants (mosses,

liverworts)

• no ability to internally transport water and materialsmaterials

• require moist environment

• live in colonies, has rhizoids to anchor it

• important in soil formation

Ferns

• Seedless, flowerless, vascular plants

• have xylem and phloem

• has alternation of generation, the common familiar leafy plant is the sporophyte familiar leafy plant is the sporophyte generation

• has a rhizome and leaves

Ferns II• Spore production occurs on places on the

fronds, sporangia are formed in which meiosis occurs to form spores.

• Spores are often born in clusters called sori• spores are released and if they germinate • spores are released and if they germinate

will grow into the gametophytes• method of fertilization is primitive like

bryophytes

Ferns• Whisk ferns - extinct, no roots, no leaves

but did have vascular system above and below ground!

• Horsetails - roots, rhizomes and vertical • Horsetails - roots, rhizomes and vertical stems

Heterospory• Homospory - one type of spore produced by

bryophytes and many ferns• Heterospory - some ferns have two types of

spores produced microspores (male gametophytes) and macrospores (female gametophytes) and macrospores (female gametophytes)

• the development of heterospory leads two the two most successful kinds of plants

Gymnosperms• Vascular, seed bearing, flowerless plants

• means “naked seed”

• largest division is conifers - woody cone bearing gymnospermsbearing gymnosperms

• leaves are called needles

• most have male and female parts on same plant. Reproductive parts in cone.

Gymnosperm reproduction• Microspores and macrospores formed in

separate cones• Male cones smaller than female, on lower

branches• meiosis in male cone produces a male • meiosis in male cone produces a male

gametophyte, also called a pollen grain• carried by air current to female

gametophyte

Angiosperms• Flowering, vascular plants

• most successful plants

• fertilization in flowering plants called double fertilizationdouble fertilization

• 2 sperm involved - 1 fertilizes the egg, the other fuses with 2 cells in female gametophyte to form endosperm

2 classes of Angiosperms• Monocots - mostly herbaceous, long,

narrow leaves,

• parallel veination,

• flower parts occur in threes, • flower parts occur in threes,

• single cotyledon (embryonic leaf),

• endosperm present in seed

• vascular bundles scattered

2 classes of Angiosperms• Dicotyledons - herbaceous or woody,

• leaves variable in shape,

• netted veination,

• flower parts occur in 4 and 5, • flower parts occur in 4 and 5,

• 2 cotyledons,

• endosperm usually absent in mature seed

• vascular bundles arranged in a circle

Final words…...• “Absence of

evidence is not evidence of absence.”