BIOLOGY 1A: VARIATION

Why do offspring resemble their parents?

• Parents pass on information to their offspring through genes (hereditary unit)

• Genes contain sequences of DNA that code for different enzymes and proteins, producing traits (eg. eye colour)

• Genes are passed on by packages known as chromosomes

• Each species has a characteristic number of chromosomes (eg. humans have 46, fruit flies have 8 and sheep have 54)

• Chromosomes are passed on through DNA replication, which produces copies of genes that can be passed down from parents to offspring

When are parents and offspring genetically identical?

• When:

  • Reproduction is asexual

  • A single individual is the sole parent

• The offspring will inherit all of the parent’s genes through mitotic division (mitosis)

• The parent and offspring may not be phenotypically identical because factors such as environment can influence both of their phenotypes

Draw (with labels) a diagram showing the process of mitosis

When does genetic variation arise in asexual reproduction?

• Through mutations

Does sexual reproduction generate new genetic variation?

• No

  • It rearranges pre-existing genetic variation to produce new combinations of genes

  • Only mutations introduce new genetic variation

What are the 3 mechanisms of genetic variation in sexual reproduction?

• Independent assortment of chromosomes (during meiosis)

  • Chromosomes are assorted randomly into gametes, resulting in the gametes having a random mix of maternal and paternal chromosomes

• Crossing-over (during meiosis)

  • Chromosomes cross over, resulting in them having a mix of maternal and paternal genetic material

  • These chromosomes are known as recombinant chromosomes

• Random fertilization (during fertilization)

  • The gametes (1 from each parent) that fuse during fertilization is random

Draw (with labels) a diagram for the process of independent assortment of chromosomes

Draw (with labels) a diagram for the process of crossing-over

What is the Mendelian law of segregation?

• States that: during gamete formation in each parent, their alleles segregate and end up in different gametes

What is dependent assortment?

• The inheritance of an allele from one gene is dependent on the inheritance of an allele from another gene (the alleles of the genes are inherited together)

• The loci are close to each other on the same chromosome

• Produces the phenotypic ratio: 3:1

What is independent assortment?

• The inheritance of alleles of different genes are independent of each other (alleles can be inherited separately)

• The loci are far apart on the same chromosomes or on different chromosomes

• Produces the phenotypic ratio: 9:3:3:1

What are the probability laws that govern Mendelian inheritance?

• Probability of 2 independent events occurring = probability of event 1 occurring X probability of event 2 occurring

• Probability of 2 mutually exclusive events occurring = probability of event 1 occurring + probability of event 2 occurring

What is Hardy-Weinburg equilibirum?

• States that: if a population is not evolving, genotype and allele frequency will remain constant through the generations

• p + q = 1

  • Where p = frequency of dominant alleles and q = frequency of recessive alleles

What is the Hardy-Weinburg equation?

• p² + 2pq + q² = 1

  • Where p² is the frequency of homozygous dominant individuals in a population, 2pq is the frequency of heterozygous individuals in a population and q² is the frequency of homozygous recessive individuals in a population

What are the conditions for Hardy-Weinburg equilibrium?

• No mutations

• Mating is random

• No selection

• Extremely large population size

• No gene flow

What are complex traits?

• Known as quantitative, continuous or polygenic traits

• Traits that are dependent on more than one gene, where no allele is completely dominant over another allele

What is polygenic inheritance?

• The inheritance of quantitative or complex traits

• Gives rise to a wider variety of phenotypes

What is heritability?

• Proportion (from 0 to 1) that indicates how much of phenotypic variation is due to genetic differences

• Can be calculated by:

  • h² = additive genetic variation (Va)/phenotypic variation (Vp)

  • h² = 2(slope) of a parent-offspring regression

What is a parent-offspring regression?

• Shows the relationship between variation in a parent’s trait (x-axis) and variation in offsprings’ trait (y-axis)

• Steeper slow indicates that the trait is more heritable and genes are more important in determining the variation of a trait

• A near-horizontal slope indicates that the trait is less heritable and variation of the trait is mostly due to environment

What is natural selection?

• Production of more individuals than an environment can support leads to a struggle for existence between the individuals

• Individuals with traits better suited for the environment are more likely to survive and reproduce, passing on their favourable alleles to their offspring

• Over generations, the more favourable traits become more common and the less useful traits may disappear

• Natural selection only acts on complex traits

Conditions for natural selection

• There must be variation in traits within a population

• Some of the variation in traits has to be heritable and based on genetic differences

• The trait must influence an individual’s ability to survive and reproduce, affecting the genetic contribution to the next generation

What is homology?

• Similarities in different species due to a shared common ancestor

• Types of homology:

  • Developmental homology: many different species have similar early developmental stages

  • Molecular homology: the core of metabolism for all organisms is the same, though the details may differ

  • Homology in the genetic code: all organisms (except a few viruses) uses DNA as genetic information

What is fitness?

• Measure of how much a genotype contributes to the next generation

What is relative fitness (W)?

• The fitness of a genotype relative to the fittest genotype in the population

What is coefficient of selection (s)?

• Measure of how strongly natural selection acts against a genotype

• Calculated by: 

  • s = 1 - W

• If s = 0, the genotype has the highest fitness or has the same fitness as the fittest genotype

How does differences in fitness between genotypes affect evolutionary change?

• Bigger difference in phenotypes = more rapid evolutionary change

• If the fitness of a genotype decreases, the coefficient of selection increases

• Natural selection acts more strongly against the less-fit genotype and more strongly for the fitter genotype

• The fitter genotype contributes more of its alleles to the next generation (increase in allele frequency)

• Due to selection pressure, the less-fit genotypes contribute less of their alleles and diminishes over time

What determines how the speed of alleles spreading throughout a population?

• The relative fitness of the genotypes in the population

• Whether the allele is dominant or recessive

What are continuous traits?

• Known as complex, quantitative or polygenic traits

• Traits that depend on more than one gene and the environment

What are the types of natural selection?

• Directional selection

  • Favours one extreme of a trait and acts against the other extreme of the trait

• Stabilizing selection

  • Favours the intermediate or average traits and acts against the 2 extremes of the trait

  • Keeps the intermediate or average traits the same but reduces variation

• Disruptive selection

  • Favours the 2 extremes of the trait and acts against the intermediate or average traits

  • Results in the population having different morphs

What is the relationship between natural selection and the parent-offspring regression?

• The less steep the slope, the less heritable the trait and the less likely the trait will respond to natural selection

• An entirely horizontal graph indicates that the trait is not heritable at all and natural selection cannot occur 

  • Due to one of the conditions for natural selection: some of the variation must be heritable and based on the differences in genes

How does mutation occur?

• It is a random process

• Whether a particular mutation occurs is not dependent on how useful it would be in the environment

  • Organisms do not produce mutations just because they need them

What is the significance of mutations in evolution?

• Mutations are the source of all genetic variation

• When there are no mutations, there is no genetic variation in a population and natural selection cannot occur, meaning that there is no evolution

What are the rate of mutations dependent on?

• Species

• Environment (eg. some bacteria increases their rate of mutations in response to stress)

What are the effects of mutations on phenotype?

• Mutations in non-coding regions of the genome may have no effect on the phenotype

  • Mutations in introns do not get translated into proteins

  • Splicing gets rid of the introns (unnecessary parts of the mRNA) before the mRNA is translated into proteins

• Mutations in exons may not affect the protein being produced

  • Multiple codons code for the same amino acid

  • The 3rd base of the codon is redundant and any mutation occurring there does not affect the amino acid and thus the protein

  • Known as synonymous mutations

• Mutations in the exons may also affect the protein being produced

  • If the mutations occur at the 2nd base of the codon, it will affect the amino acid and thus the protein

  • Known as non-synonymous mutations

• Mutations that affect the protein sequence have phenotypic effects that are hard to predict

What are deleterious mutations?

• Mutations that affect an a individual’s phenotype and has a negative effect on fitness

What are the factors that affect the effects of mutations on fitness?

• Can be displayed on a phenotype space

• How well adapted a population is

  • Well adapted (old environment): a greater proportion of the mutations will bring the fitness level of the population away from the optimal fitness (deleterious mutations)

  • Not well adapted (new environment): a greater proportion of the mutations will bring the fitness level of the population close to the optimal fitness (positive effect)

• How big the effect of the mutations are

  • Bigger effect: more likely to bring the fitness of the population away from the optimal fitness (deleterious mutations)

  • Smaller effect: more likely to bring the fitness of the population closer to the optimal fitness (positive effect)

    • Most adaptation in a population is due to mutations of small effect

Why is there so much genetic variation?

• Even though natural selection acts to reduce genetic variation in a population by increasing the frequency of alleles with high fitness (to fixation) and removing the alleles with low fitness, there is still genetic variation 

• Reasons include:

  • Neutral mutations

    • Neutral mutations have no effect on phenotype and fitness

    • Not affected by natural selection and instead by random processes such as genetic drift

    • Neutral alleles can persist in a population for long periods of time without being fixed

    • Genetic variation is maintained because natural selection does not act to remove the neutral alleles

  • Mildly deleterious mutations

    • Results in alleles that reduce fitness by a very small amount

    • Natural selection is weak when the mutations have small effects

    • Genetic variation is maintained because of mutation selection balance, where mildly deleterious mutations are constantly being introduced but natural selection is inefficient at removing these mutations

  • Disruptive selection

    • Natural selection favours 2 extremes of a trait, leading to diverse morphs within a population

  • Heterozygous advantage

    • Heterozygous alleles are fitter than either the homozygous dominant or homozygous recessive alleles

    • Since both alleles are required to produce the fittest genotype (eg. Aa), natural selection does not act against either one

    • This causes both alleles to remain at an intermediate equilibrium, with neither going to fixation of being lost

  • Frequency dependent selection

    • The fitness of each phenotype depends on their frequency

    • None of the alleles are able to fully replace others and all the alleles will be maintained

Why is there genetic variation between populations?

• Local adaptation

  • The environment varies in different populations, causing different phenotypes to be favourable in different populations

  • Variation occurs when different alleles are fixed in different populations

• Random divergence

  • The frequency of neutral alleles are determined by random processes such as genetic drift instead of natural selection

  • Random divergence will operate independently in each population, causing the frequency of alleles to differ among populations

  • The alleles will start off with relatively similar frequencies but they will diverge as time passes (eg. alleles in some populations may be fixed while those in other populations may be lost)

  • Most important for neutral alleles and may affect non-neutral alleles

    • Effects on non-neutral alleles will be very weak unless the alleles have very small effects on fitness

• Migration

  • The movement of individuals from one population to another, introducing gene flow between populations

  • Homogenises populations genetically, making different populations more genetically similar than they were before

    • Reduces genetic diversity between populations

    • May increase genetic diversity within a population

  • The more migration occurs, the more similar the frequency of alleles are between the populations, with the trends following a similar trajectory

What is the impact of migration on local adaptation?

• Alleles that are favoured in one population will flow to the other population, where they may be less fit due to the differing environments of both populations

What is a species?

• Defined as a group of organisms that commonly interbreed in the wild (do not interbreed with other similar species and are reproductively isolated from them)

How do we determine if a group of organisms are from the same species or different species?

• Can use different concepts

• Biological species concept

  • Same species:

    • Fertilization can occur

    • Offspring are viable and fertile

  • Different species:

    • Fertilization cannot occur

    • Offspring are not viable and sterile

• Morphological species concept

  • Same species:

    • Have the same physical characteristics (eg. size, shape)

  • Different species:

    • Have different physical characteristics

• Physiological species concept

  • Same species:

    • Have the same bodily systems or enzyme functions

  • Different species:

    • Have different bodily systems or enzyme functions

• Ecological species concept

  • Same species:

    • Lives in the same environments and uses the same resources

  • Different species:

    • Lives in different environments and uses different resources

• Phylogenic species concept

  • Same species:

    • Clustered together on a phylogenic tree

  • Different species:

    • Clustered apart from each other on a phylogenic tree

  • The phylogenic tree can be generated by extracting genetic information from a group of individuals and observing their similarities and differences

  • Most accurate concept

What are the 2 ways that 2 species can be reproductively isolated from one another?

• Pre-zygotic reproductive isolation (before fertilization)

  • Prevents mating or fertilization from occurring

  • Eg. different species have different mating dances and live in different habitats

• Post-zygotic reproductive isolation (after fertilization)

  • Prevents the hybrid offspring from developing properly (hybrid inviability) or reproducing (hybrid sterility)

What are the limitations of the biological species concept?

• Only applies to sexual organisms

• Requires a substantial amount of work to demonstrate

• Does not always work

What are the 2 kinds of evolutionary change?

• Anagenesis

  • Selection acting within a species and causes it to change

  • The ancestral species gradually transforms into another species and ceases to exist

• Cladogenesis

  • The ancestral species splits into 2 daughter species

  • The ancestral species continues to exist alongside the descendent species

What are the different processes that allow reproductive isolation to evolve/occur?

• Geographical isolation (allopatric isolation)

  • Main mechanism of selection in animals

  • Can stop gene flow for long enough for reproductive isolation to evolve

    • The population is geographically split into 2 different populations

    • Each population evolves independently and there is no gene flow between them

    • The populations are geographically brought back together but now they are different (eg. difference in mating displays, morphology)

    • The individuals that were from different populations during the geographical split are now reproductively isolated from one another

    • The population is now diverged with individuals with different characteristics, increasing biodiversity

• No geographical isolation (sympatric isolation)

  • Can occur through polyploidy in plants but are very rare in animals

    • The diploid cell, which has 2 copies of each gene, becomes a polyploid cell, which has 4 copies of each gene

    • This results in the formation of a diploid gamete that is able to self-pollinate (does not need another parent)

    • These diploid gametes are now reproductively isolated from the species of their parent cells

    • This stops gene flow, reducing biodiversity

What are the 2 types of natural selection in animals?

• Viability selection

  • Natural selection acts on the variation of traits that affect the viability of individuals

  • Eg. ability to avoid predators and efficiency at foraging for food or resources

• Sexual selection

  • Natural selection acts on the variation of traits that affect the mating opportunities of individuals

• Viability selection and sexual selection often acts against each other

  • Eg. male peacocks have large, colourful tails that make them more successful at attracting mates but the same tail may make it easier for the male peacock to be seen by preadators

What are the 2 types of sexual selection?

• Intrasexual sexual selection

  • Involves active competition amongst one sex

  • Eg. males competing against other males for mating opportunities

• Intersexual sexual selection

  • Involves interactions between the 2 sexes

  • Eg. females choosing their mates based on certain desirable traits

In a species where one sex competes for mates, it is usually the male, why is this so?

• Due to anisogamy (males and females producing gametes of different sizes, with females producing larger gametes than the males)

• Female reproductive success is limited by the number and quality of the offspring she can produce

  • Since each egg the female produces is an investment, females tend to be more selective in choosing mates

• Male reproductive success is limited by the number of females he can mate with

  • Since males produce a lot of cheap gametes (sperm), they are able to father many offspring with many females

  • This results in males competing with each other for access to females to increase their reproductive success

What are the different reproductive strategies used by males to compete with other males?

• Harem defense

  • Males fight to dominate groups of females

  • Develops when females are distributed in defendable groups

• Territory defense

  • Males defend spawning sites (sites where females must visit to lay her eggs)

• Sperm competition

  • Some females mate with many males during one reproductive cycle

  • Males who produce more sperm are more likely to have his sperm fertilize the egg

• Sneaky males

  • Territory holders: males need to be big and strong to hold territories

  • Sneaks: males hover around boundaries of territories and rush in to fertilize the eggs when the territorial male is distracted

  • Female mimics: males look like females so territorial males do not try to keep them off their territory, allowing the female mimics to enter and fertilize the eggs

• Extra-pair copulations

  • Males in socially monogamous species may attempt to get extra matings

Why do females choose their mates?

• To obtain:

  • Good resources

    • Females mate with males who can be a good provider for her offspring, has a good territory and can provide her with nuptial gifts (resources given from the male to the female during courtship or mating)

  • Good genes

    • Females mate with the biggest or most attractive male

    • In socially monogamous species, females may encourage extra-pair copulations from males with better genes so that her offspring can have better genes

What is sex role reversal?

• In species where males invest more than females, sex roles can be reversed

• Eg. 

  • In seahorses

  • Male reproductive success is limited by the capacity of his brooding pouch

  • Female reproductive success is limited by the number of males she can mate with

  • Sex roles are reversed and the females are the ones competing for males, with the males become the more selective sex

What are the costs of sex?

• Direct costs

  • Individuals have to find a mate, defend a mate or attract a mate

• Mixing up genes (recombination load)

  • An individual that has survived to reproduce likely has a good set of genes (likely to have survived natural selection)

  • There is no guarantee that combining their genes with a mate will produce offspring that are high-quality with genes that are equally good or better than them

• Cost of males

  • Females need to invest at least half of their resources in producing male offspring, which do not directly reproduce themselves 

What are the benefits of sex?

• Sex allows a parent to produce offspring that are genetically better than they are

  • Sex brings together beneficial mutations

    • If both parents have one beneficial mutation each, it is possible that their offspring inherits both beneficial mutations

    • These offspring can then be selected by natural selection

  • Sex gets rid of bad mutations

    • If both parents have one deleterious mutation each, it is possible that their offspring does not inherit either of the deleterious mutations

    • These offspring can be selected by natural selection and the deleterious mutations can be purged

How does sexual reproduction affect the rate of adaptation?

• Sexual reproduction allows natural selection to fix beneficial mutations and remove deleterious mutations more efficiently

• This means that sexual species adapt and evolve faster than similar asexual species, making them less likely to go extinct

What is the first photosynthetic bacteria?

• Cyanobacteria

How did eukaryotes evolve to have chloroplasts?

• Through endosymbiosis

• A eukaryotic host cell engulfs a cyanobacteria (the first photosynthetic bacteria) and keeps it alive instead of digesting it

• The cyanobacteria then becomes a chloroplast (a type of plastid)

What are the evidence for the theory of plastids via endosymbiosis?

• Plastids all have a double-membrane

• Plastids have similar structures to cyanobacteria

• Plastids have their own DNA

• The DNA structure of plastids and cyanobacteria are the same (ring structure instead or chromosomes in eukaryotes)

• Plastid DNA is related to cyanobacteria DNA

Why are some genes from the cyanobacteria lost after being engulfed by the eukaryotic cell?

• Some genes are not needed anymore (eg. genes for motion and senses)

• Some genes are transferred to the nucleus of the eukaryotic host cell

• Some genes are redundant because the nucleus of the eukaryotic host cell also contains the same gene

• The cyanobacterium and the initial plastid have around 1.1 million base pairs in their DNA, but DNA gets reduced and modern plastids now have around 0.15 million base pairs

Why are leaves green?

• There are 2 types of chlorophyll that absorbs light

  • Chlorophyll A: absorbs more red light and less blue light than chlorophyll B

  • Chlorophyll B: absorbs more blue light and less red light than chlorophyll A

• When white light hits a leaf, the red light and blue light is absorbed but the green light is reflected back, making leaves look green

Why are red algae red even though they use the same chlorophyll?

• Red algae are found in the depths of the ocean

• Blue and green light are able to penetrate deeper into the water than red light, causing red algae to receive the blue and green light but not the red light

• When light hits the red algae, it absorbs the blue light and reflects the green light

• The blue light that is absorbed is not enough and hence red algae needs red accessory pigments (phycoerythrin) to expand the range of wavelength that can be absorbed 

• The red accessory pigment absorbs light around the green wavelength and transfers it to chlorophyll

• Chlorophyll then converts the light energy into chemical energy

How did photosynthesis spread around the evolutionary tree of life?

• During secondary endosymbiosis

• Another eukaryotic cell (eg. ciliate) engulfs the photosynthetic eukaryote from the primary endosymbiosis

• The eukaryotic cell digests the nucleus and cytoplasm of the photosynthetic eukaryote and keeps the chloroplasts (known as “stolen” chloroplasts or kleptoplasts)

• The eukaryotic cell becomes the new host

• During tertiary endosymbiosis, yet another eukaryotic cell (eg. dinoflagellate) can engulf the photosynthetic eukaryote from the secondary endosymbiosis and the process repeats

What are mixotrophs?

• Photosynthetic predators with permanent chloroplasts (chloroplasts that are never digested)

What is a colonial lifestyle?

• Cells remain attached after mitosis (by cytoplasm or gelatinous matrix)

• Properties:

  • All cells are equal and have the same roles

  • All cells are metabolically independent (can survive alone in good conditions)

  • The cells can cooperate with each other to occupy the best habitat

  • The cells are usually not attached to a substrate (a surface)

  • All cells are capable of sex

What is a multicellular lifestyle?

• Cells remain attached after mitosis (by cytoplasm or a gelatinous matrix)

• All cells are not equal and have specialised roles

• Some or all of the cells are not metabolically independent (eg. depends on others for food)

• The cells can occupy 2 or more environments (eg. in a plant, the root cells are in the soil and the leaf cells are in the air)

• The cells are usually attached to a substrate (a surface)

• Not all cells are capable of sex (only the reproductive cells are)

What are the advantages of a colonial lifestyle?

• The cells are able to move more effectively through water

• The cells are less likely to be eaten by predators due to its increased size

What are the 3 types of life cycles in algae?

• Diplont: the multicellular generation is the diploid one

• Haplont: the multicellular generation is the haploid one

• Haplo-diplont: the multicellular generation is both the diploid and the haploid one

Describe the alternation of generations in bryophyte (non-vascular) plants?

• Sporophytes (2n) undergo meiosis to produce spores (1n) in the sporangium

• The spores are dispersed and land somewhere with water

• The spores grow into gametophytes 

• The gametophytes (1n) undergo mitosis to produce gametes (male gametes: sperm, female gametes: egg)

• The sperm has to swim through the water to reach the egg (retained by the mother gametophyte)

• The sperm fuses with the egg during fertilization, producing a zygote (2n)

• The zygote grows into a sporophyte which is attached to and dependent on the mother/female gametophyte

What is the difference in life cycle for bryophytes (non-vascular plants) and vascular plants?

• Bryophytes:

  • The sporophyte (2n) undergoes meiosis to produce spores (1n) in the sporangium

  • The spores grow into gametophytes

  • The gametophytes undergoes mitosis to form gametes (male gametes: sperm, female gametes: egg)

  • The sperm has to swim through the water to reach the egg (retained by the mother/female gametophyte)

  • The sperm and egg fuse during fertilization to form a zygote (2n)

  • The zygote grows into a sporophyte, which is attached to/dependent on the mother/female gametophyte

  • The sporophyte grows upwards and depends on the mother/female gametophyte for nutrients and water

  • The sporophyte produces spores and then withers and dies (the mother/female gametophyte lives on)

• Vascular plants:

  • The sporophyte (2n) undergoes meiosis to produce spores (1n) in the sporangium

  • The spores are dispersed and lands somewhere with water

  • The spores grow into gametophytes (1n)

  • The gametophytes undergo mitosis and produces gametes (male gametes: sperm, female gametes: egg)

  • The sperm has to swim through water to reach the egg (retained by the mother/female gametophyte)

  • The sperm and egg fuse during fertilization to produce a zygote (2n)

  • The zygote grows into a sporophyte, which is attached to/dependent on the mother/female gametophyte

  • The sporophyte grows upwards as well as downwards, where it puts down a root

  • The sporophyte then begins to take in its own nutrients and water and performs photosynthesis by itself, outliving the mother/female gametophyte

    • As the sporophyte grows bigger, it branches out, with each branch having its own sporangium

    • Each sporangium produces its own spores and so the number of spores per zygote increases

What does heterosporous mean?

• The sporophyte produces 2 types of spores (male and female)

• The female spores grow into female gametophytes and the male spores grow into male gametophytes

• The zygote can only be produced on the female gametophytes

What does homosporous mean?

• The sporophytes produce 1 type of spore (bisexual)

• The bisexual spores grow into bisexual gametophytes

• The zygote can be produced on every single gametophyte

What is the advantage of heterosporous sporophytes?

• Even though only half of the spores (the female spores) grow into female gametophytes which zygotes are produced on, heterosporous sporophytes make sexual dimorphism possible

What is the difference between seed plants and seedless plants?

• Seedless plants:

  • Sporophytes produce spores

  • The spores are dispersed and are free-living, growing into gametophytes

• Seed plants:

  • Sporophytes produce spores

  • The spores are not dispersed and remain on the sporophyte, where it grows into gametophytes

  • The pollen (male gametophyte) is then dispersed and the lucky ones will land on a plant containing ovules (encloses the female gametophyte)

  • A pollen tube is formed and the male gametophytes produce gametes (sperm) which travel down the pollen tube to reach the female gametes (egg) produced by the female gametophytes

  • A zygote is formed when fertilization occurs and the sperm and egg fuses

How do the female gametophyte phases get reduced in seedless plants, gymnosperms and angiosperms?

• Seedless plants

  • The gametophyte is independent and free-living

  • The gamete (egg) is produced on the surface of the female gametophyte and is exposed to the environment

  • The gametophyte contains many cells

  • Has archegonia

    • Female reproductive structure

    • It produces the egg and is the site for fertilization

• Gymnosperm (non-flowering seed plants)

  • The gametophyte is enclosed ONCE by the ovule

  • The gametophyte contains many cells

  • Has archegonia

• Angiosperm (flowering seed plants)

  • The gametophyte is enclosed TWICE, first by the ovule and then by the ovary wall (later turns into a protective layer such as the flesh of a fruit)

  • The gametophyte contains 7 cells (includes the egg cell and 2 polar nuclei)

  • No archegonia

What are the defining features of angiosperms?

• Enclosed ovules

• Female gamtophytes with 7 cells

• Flowers with petals

• Endosperm (from double fertilization)

Explain the evolution of plants

How does fertilization occur in angiosperms?

• The sporophyte produces spores

• The spores do not get dispersed but instead remain on the sporophyte, where it grows into gametophytes

• The male gametophyte (pollen) is dispersed and the lucky ones land on a plant with ovules (encloses the female gametophyte)

• The pollen contains 2 nuclei (generative nucleus and tube cell nucleus)

• The tube cell nucleus creates the pollen tube and dies right after

• The generative nucleus divides into 2 (one fuses with the egg to form a zygote and the other fuses with 2 polar nuclei, forming the endosperm)

• Both the zygote and endosperm grows and expands

• Some species (eg. coconut) have small cotyledons, so the endosperm grows very large to feed the seedling after germination (endosperm is the main nutrient source)

• Other species (eg. peas) have larger cotyledons, so the endosperm gets used up very quickly and the cotyledon feeds the seedling after germination (cotyledon is the main nutrient source)

What are the groups of flowering plants and their traits?

• Monocots

  • Have one seed leaf

  • Does not have a primary root (has fibrous roots instead)

  • Flowers have bulb structures

  • Usually have petals in multiples of 3

  • Have vascular bundles that are distributed haphazardly (stem is not rigid)

  • Stems cannot have a secondary growth (allows a stem to widen and form wood)

  • Leaves are narrow with parallel veins

• Dicots (eudicots and archaedicots)

  • Have 2 seed leaves

  • Has a primary root

  • Usually have petals in multiples of 4 or 5, but can be any number

  • Have vascular bundles that are arranged in a ring (makes the stem rigid)

  • Steams can have a secondary growth (allows the stem to widen and form wood)

  • Leaves are wide with branched veins

Compare the pollen structures of the monocots, archaedicots and eudicots

• Monocots

  • Pollen has 1 aperture (monoaperture)

• Archaedicots

  • Pollen has 1 aperture (monoaperture)

• Eudicots

  • Pollen has 1 or 3 apertures (triaperture)

Compare the populations of monocots, archaedicots and eudicots

• Eudicots (most)

• Monocots (middle)

• Archaedicots (least)

What is the first flowering plant (angiosperm) to have branched off from gymnosperms?

• Amborella

What are water lilies?

• The second flowering plant to have branched off from the gymnosperms

• The first seed plant to be aquatic and herbaceous (non-woody)

What are some criminal tendencies that have evolved in plants?

• Parasitism (theft)

  • Refers to plants that steal sugar and energy from other plants

  • Evolution of pure parasitic strategy:

    • Non-parasite: plant makes its own food

    • The non-parasite evolves (difficult stage: attaching its roots to another plant) into a semi-parasite

    • Semi-parasite: makes its own food but steals from other plants too

    • Full parasite: plant does not make its own food at all, it is all stolen from others

• Cruelty to animals

  • Eg. carnivorous plants that eat insects

    • Plants do not eat insects to get energy (since most of their energy is obtained through photosynthesis)

    • Plants eat insects to get other nutrients that are not present in the soil that they are growing on

    • There are no land plants that only eat insects because insects do not contain enough energy

• Kidnapping

  • Eg. aristolochia

    • The flower produces a smell of rotting meat, attracting flies to enter it

    • However, upon finding out that there is not actually any meat in the flower, the fly attempts to leave but is trapped

    • The flower releases pollen, which sticks onto the fly

    • Only then is the fly able to leave the flower

    • The fly then smells another flower giving off the smell of rotting meat, so it enters it and ends up trapped again

    • The pollen from the fly is transferred to the flower, successfully pollinating it

    • Only then is the fly able to leave the flower

• Murder

  • Eg. strangling fig

    • The strangling fig first acts as a parasite

    • It grows on the branch of a tree and its roots begin to grow, stretching down along the trunk of the tree until it reaches the soil

    • After reaching the soil, its roots continue to grow, branching out and wrapping around the tree trunk

    • The strangling fig multiplies and more of its roots wrap around the tree trunk, squeezing the tree trunk until its xylem and phloem tubes are cut off

    • The tree is unable to get the nutrients it needs and ends up dying

    • The strangling fig then takes the place of the tree, acting as a parasitoid (an organism that acts as a parasite for part of its life before killing off the host)

• Fraud/deception

  • Mimic a food source

    • Eg. rafflesia

      • Rafflesia produces the smell of rotting meat instead of sweet smells (to attract flies instead of bees)

      • This is because rafflesia are very rare, and there may be a huge distance between 2 rafflesia in the rainforest

      • If rafflesia produces sweet smells, bees will be attracted to it, but since there is an abundance of other sweet-smelling flowers in the rainforest, the bees do not need to travel the huge distance to another rafflesia just to get access to another sweet-smelling flower

      • If rafflesia produces the smell of rotting meat, flies will be attracted to it, and it will travel the huge distance (past all the sweet-smelling flowers) just to get to another bad-smelling rafflesia, effectively pollinating it

  • Mimic another insect

    • Eg. bee orchids

      • Orchids have structures that look and smell like female bees, deceiving the male bees into thinking that they are a female bee and trying to mate with them

  • Mimic a rewarding flower

    • Eg. some flowers have yellow structures that look like pollen

• Drug dealing

  • Many plants produce narcotics which is usually used as a defense mechanism

    • Eg. poppy plants produce opium

• Grievous bodily harm (GBH)

  • Eg. orchids 

    • Some male orchids will punch bees so that the bees would be deterred from approaching other males

    • Instead, the bees would approach the different-looking females and bring the pollen from that male to her for pollination

• Adultery

  • Some plants can cross-breed with other species (though it is rare) to form a hybrid species

Monocots do not act as parasites on other plants, what does it act on instead?

• Monocots act as parasites on fungi

• Eg. orchids

  • Orchid seeds are small and do not contain food

  • A parasitic fungi invades the orchid seed but ends up being digested by the seed

  • The seed uses the fungi to grow

  • The fungi continues trying to invade the seed and continues being digested

  • Some orchids remain partially dependent on the parasitic fungi even as it becomes a mature plant

  • Other orchids completely switch to using chlorophyll to obtain food

  • This makes orchids semi-parasites

Why can parasitic plants survive without chlorophyll?

• In very rare cases, plant seeds may contain mutations that knocks out chlorophyll and disables photosynthetic abilities

• Non-parasitic plants would die as chlorophyll is its only way of obtaining food

• Parasitic plants would survive by obtaining all of its food from the plant or fungi it is acting on

What are the advantages and disadvantages of parasitism?

• Advantages:

  • Stealing resources from others allows parasitic plants to be extravagant (eg. rafflesia is a parasite on trees and it uses the tree’s resources to grow a very large flower)

• Disadvantages:

  • The seed of the parasitic plant has to find a suitable host to attach to

    • Eg. birds eat mistletoe seeds, but since they are too sticky to swallow, the bird has to wipe it off on the nearest branch (the mistletoe seed has been successfully deposited on another branch)

How do parasitic striga destroy maize crops and what solution have scientists developed to prevent it?

• Striga prevents maize from producing seeds because it takes away its energy and resources

• The striga seed senses chemicals from the maize roots and it grows a root, attaching to the maize root and drawing nutrients from it

• Scientists found out that striga seeds are very attracted to desmodium roots, even more so than it is to maize roots

• The solution was to plant desmodium seeds around the mazie plants

• The striga seed would grow a root and seek to attach to the demosdium root, but it is unable to do so

• The striga seed ends up not being able to draw nutrients from the desmodium root and so it starves and dies

• Desmodium also helps to repel pests because it smells like their predators