biology ncea3 study notes

8/6/2019 Biology Ncea3 Study Notes

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Terms Definitions

Abscission Leaf or fruit fall because of death of cells in the abscission layer in the stalk

AggressionThreatening behaviour usually associated with competition. It can involve predationhowever. A more specific term agonistic behaviour refers to conflicts within species

but excludes predation

AllelopathyChemical inhibition of one species by another. Basically the same as antibiosis, except

that the definition emphasises the chemical connection.

Ambivalence When a gesture contains elements of dominance and submission

AntibiosisAn interspecific relationship where one organism is harmed, but the other is

unaffected.

Apical dominance A tree where there is one main trunk and much smaller side branches. A bush is whereall stems are equal.

Auxin A plant hormone that lengthens cells.

Batesian mimicry A harmless animal looks like a poisonous one.

Biological clocksInternal timing systems that continue without external clues, and control (to some

extent) the timing of activities of plants and animals.

Circa

The biological clocks can have somewhat different periods compared with the

geophysical cycles, so their names begin with 'circa' which means 'about'. Circa is used

to describe endogenous rhythms that either fall short of or exceed the geophysicalcycle. For example, circadian means about a day and describes an endogenous activity

period that falls short of or exceeds a 24-hour period.

Circadian A rhythm of about 24 hours.

Circatidal A rhythm of about 12.5 hours.

Circalunar A rhythm of about 1 month

Circaannual A rhythm of about 1 year.

Commensalism An interspecific relationship where one organism benefits, but the other is unaffected.

Crepuscular When an animal is active around sunrise and sunset.

Day Neutral Plant A plant that flowers independently of the day length or season.

Diurnal Active during the day.


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DormancyWhen seeds will not germinate unless certain conditions (such as cutting of the coat or

a long period of cold) happen.

Ecosystem All the living things in an area plus the physical factors.

Endogenous rhythm An internal rhythm that occurs when there are no external cues. It is caused by abiological clock.

Entrainment Forcing the free running period to follow an external pattern.

Exogenous Rhythm A rhythm that continues only when external cues are present.

Free Running

PeriodThe natural period of the rhythm if there are no external cues.

Gause's Competitive

Exclusion Principle

Theory stating that if two species have the same niche they cannot remain for long in

the same habitat. One will lose out and be eliminated (or at least reduced to a very

small population.)

HierachyWhen animals are ranked. A linear hierarchy is where every animal is above or below

another; there are no equals.

Home Range An area that an animal uses for food, but will not defend.

Homing The ability to find and return to the home site. In animals only.

IAA Indoleacetic acid, an auxin

Interspecific Between two different species.

Intraspecific Within one species.

KinesisA whole-body response of animals where the response is independent of the stimulusdirection, but may depend on the intensity of the stimulus.

Limiting FactorAny variable factor of the environment that limits the activity of an organism or

population.

MigrationAnnual mass movement of animals, from breeding areas to other non-breeding areasand then returning. In animals only.

Long Day Plant A plant that flowers with increasing day length, usually over 12 hours.

Mullerian Mimicry Where several poisonous species have similar colourations.

Nastic MovementA plant response that is independent of the direction of the stimulus. The response isnot a growth response, but usually involves cell water (turgor) pressure.


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ParasitismAn interspecific relationship where one individual is benefited and the other harmed. In

this type of exploitation, the organsim relies on its host for nutrition and habitat.

Period The length of a rhythm, how long it takes to repeat.

Personal Distance The close-up distance round an animal that is never invaded except for mating orfighting.

Phase Shift To change the start times of a rhythm, but not its period. (See Entrainment.)

Photoperiodism The response of plants to lengths of day (or night).

Phytochrome A plant pigment that controls the photoperiodic response.

Short Day Plant A plant that flowers in short days, during autumn or winter.

Stimulus Anything that causes an organism to react.

Sun CompassA biological clock that enables a migrating bird or insect to fly using the sun and

continuously adjust its angle to the sun while flying.

TaxisMovement of an animal or part of its body towards or away from a directional

stimulus.

Territory An area used by an animal for feeding or breeding, that the animal will defend.

Tropism A plant growth response to a directional stimulus.

Vernalisation Exposure of seeds to a period of cold to break the seeds' dormancy.

ZeitgeberTime signal for a biological clock. Eg sunrise and sunset, temperature, tidal movement,

day length.

Exploitation Parasitism and Predation are examples of this

Symbiosis Species which benefit from living together

Kin Selection Kin equals your own young, members of the group are genetically related

Polyandry One female and many males, males care for the offspring, e.g. bees

Polygyny One male completes for many females. Males do not help rear young.

Co-operative

Breeding

A social system in which individuals help care for young that are not their own, e.g.

lioness


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Aggressive

behaviour

A mood that arises from competition for resources of food, water, space, mates and

breeding sites.

Agonistic behaviourIs aggressive behaviour towards members of the same species involving threats or

fights. It is a contest to get resources.

Disruptive

colourationMarkings that hide the outline of an animal.

Cryptic colouration Colouration that matches the background, e.g. stick insect lying on a stick in the forest.

Batesian MimicryResemblance of a harmless organism to one that is dangerous or poisonous, e.g. the

viceroy butterfly (not poisonous) and the monarch butterfly (poisonous)

Mullerian Mimicry Several poisonous species that all have similar warning colours, e.g. Bees and wasps

Turgor Movement Movement of water inside a plant causes swelling to occur

Critical Day Lengththe period of daylight, specific for any given species, that triggers a long-day or a

short-day response in organisms

Group Co-operation When members of the same species work together to ensure survival

Social Behaviour When animals share duties like rearing of young or hunting

Parental CareThe amount of care given by parents; varies depending on how much is given before

and after birth

Cooperative food

gathering When animals work together to hunt or trap food by taking on different roles

Mating systemsA framework of social relationships within which the individuals find and compete for

a mate

Cooperative defence

and attackWhen animals work together to protect themselves from predators or to attack prey

Migrationregular or annual movement and return of animals from one breeding place to feeding

place

Orientation Relative position of an animal or plant to its surroundings

Evolution


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evolutionchange over time; process by which modern organisms descended from ancient

organisms


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theory well-tested, well-supported explanation that unifies a broad range of observations

fossil preserved remains of an ancient organism

natural variationdifferences among individuals of a species; results from mutation and sexual

reproduction

struggle for

existencecompetition between organisms for food and space

fitness ability of an organism to survive and reproduce in its environment

adaptation inherited characteristic that increases an organism's chance of survival

survival of the

fittest

individuals that are better suited to their environment survive and reproduce most

successfully; natural selection

natural selection individuals that are better suited to their environment survive and reproduce mostsuccessfully; survival of the fittest

common descent principle that all living things have a common ancestor

homologous

structurestructures that have different mature forms but develop from the same embryonic tissues

vestigial organorgan so reduced in size, it does not serve an important function; may be homologous to

structures in other organisms

gene pool combined genetic information of of all the members of a population

allele frequency how often a form of a gene appears in a gene pool

speciestwo organisms that are so similar they can interbreed in nature and produce fertileoffspring

speciation formation of a new species as a result of reproductive isolation

reproductive

isolation

separation of species that prevents them from interbreeding and producing fertile

offspring

behavioralisolation type of reproductive isolation in which two organisms have different mating rituals thatprevent them from interbreeding

geographic

isolation

type of reproductive isolation in which two populations are separated by geographic

barries like mountains or bodies of water

temporal isolation type of reproductive isolation in which two organisms reproduce at different times


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biodiversity variety of organisms that exist in the biosphere

taxonomy classification of organisms

binomial

nomenclature

two part scientfic name for an organism; its genus is listed first, followed by its species

genus first part of an organism's scientific name

kingdomsecond largest taxonomic group; there are six - animalia, plantae, protista, eubacteria,

archaebacteria, fungi

domainmost inclusive taxonomic group, larger than kingdom; three exist - bacteria, archaea,

eukaryota

Protista a single celled plant or animal, ex. amoeba, paramecia, euglena

molecular clock model that uses DNA comparisons to estimate how long two organisms evolved from acommon ancestor

phylogenetic treediagram showing evolutionary relationships of organisms with a common ancestor;

resembles a tree

cladogramdiagram that shows the evolutionary relationships among organisms based on derived

characters; resembles a timeline

divergent

evolutionpattern of evolution in which two species become more and more dissimilar

Fungi kingdom of heterotrophs that obtain nutrients through absorption, ex. mushrooms, yeasts

Eukaryota domain of organisms that contain nuclei, includes animals, plants, fungi, and protists

gene pool consists of all genes, including all the different alleles, that are present in a population

relative frequencythe number of times that the allele occurs in a gene pool compared with the number of

times other alleles for the same gene occur

single-gene trait traits controlled by a single gene that has two alleles

polygenic trait traits controlled by two or more genes of a polygenic trait usually with two or morealleles

directionalselection

form of natural selection in which the entire curve moves; occurs when individuals at one

end of a distribution curve have higher fitness that indivduals in the middle or at the

other end of the curve

stabilizing

selection

form of natural selection by which the center of the curve remains in its current position;

occurs when individuals near the center of a distribution curve have higher fitness than


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individuals at either end

disruptive

selection

form of natural selection in which a single curve splits into two; occurs when individuals

at the upper and lower ends of a distribution curve have higher fitness than individuals

near the middle

genetic drift random change in allele frequencies that occurs in small populations

founder effectchange in allele frequencies as a result of the migration of a small subgroup of a

population

Hardy-Weinberg

principle

principle that allele frequencies in a population will remain constant unless one or more

factors cause the frequencies to change

geneticequilibrium

situation in which allele frequencies remain constant

speciation formation of new species

reproductive

isolation

separation of species or populations so that they cannot interbreed and produce fertile

offspring

behavioral

isolation

form of reproductive isolation in which two populations have differences in courtship

rituals or other types of behavior that prevent them from interbreeding

geographic

isolation

form of reproductive isolation in which two populations are separated physically by

geographic barriers such as rivers, mountains, or stretches of water

temporal isolation form of reproductive isolation in which two populations reproduce at different times

Allopatric Speciation: The Great Divide

Allopatric speciation is just a fancy name for speciation by geographic isolation, discussed earlier. In thismode of speciation, something extrinsic to the organisms prevents two or more groups from mating with

each other regularly, eventually causing that lineage to speciate. Isolation might occur because of great

distance or a physical barrier, such as a desert or river, as shown below.


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Allopatric speciation can occur even if the barrier is a little porous, that is, even if a few individuals can

cross the barrier to mate with members of the other group. In order for a speciation even to be considered

allopatric, gene flowbetween the soon-to-be species must be greatly reducedbut it doesnt have to be

reduced completely to zero.

Sympatric Speciation

Unlike the previous modes, sympatric speciation does not require large-scale geographic

distance to reducegene flowbetween parts of a population. How could a randomly

mating population reduce gene flow and speciate? Merely exploiting a new niche may

automatically reduce gene flow with individuals exploiting the other niche. This may

occasionally happen when, for example, herbivorous insects try out a new host plant.

For example, 200 years ago, the ancestors of apple maggot flies laid their eggs only on hawthornsbut today, these flies

lay eggs on hawthorns (which are native to America) and domestic apples (which were introduced to America by

immigrants and bred). Females generally choose to lay their eggs on the type of fruit they grew up in, and males tend tolook for mates on the type of fruit they grew up in. So hawthorn flies generally end up mating with other hawthorn flies and

apple flies generally end up mating with other apple flies. This means that gene flow between parts of the population that

mate on different types of fruit is reduced. This host shift from hawthorns to apples may be the first step toward sympatric

speciationin fewer than 200 years, some genetic differences between these two groups of flies have evolved.

apple maggot flies apples hawthorns

Gene flow has been reduced between flies that feed on different food varieties,even though they both live in the same geographic area.

However, biologists question whether this type of speciation happens very often. In general, selection for specialization

would have to be extremely strong in order to cause the population to diverge. This is because the gene flow operating

inrandomly-mating population would tend to break down differences between the incipient species

Divergentevolution occurs when a group from a specific population develops into a new species. In order

to adapt to various environmental conditions, the two groups develop into distinct species due todifferences in the demands driven by the environmental circumstances. A good example of how divergent

evolution occurs is in comparing how a human foot evolved to be very different from a monkey's foot,

despite their common primate ancestry. It is speculated that a new species (humans) developed because

there was no longer was a need for swinging from trees. Upright walking on the ground required alterations

in the foot for better speed and balance. These differing traits soon became characteristics that evolved to

permit movement on the ground. Although humans and monkeysare genetically similar, their naturalhabitatrequired differentphysical traits to evolve for survival.
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If different selective pressures are placed on a particularorganism, a wide variety of adaptive traits may

result. If only one structure on the organism is considered, these changes can either add to the original

function of the structure, or they can change it completely. Divergent evolution leads to speciation, or the

development of a new species. Divergence can occur when looking at any group of related organisms. The

differences are produced from the different selective pressures. Any genus of plants or animals can showdivergent evolution. An example can involve the diversity of floral types in the orchids. The greater the

number of differences present, the greater the divergence. Scientists speculate the greater that two similarspecies diverge indicates a longer length oftimethat the divergence originally took place.

There are many examples of divergent evolution in nature. If a freely-interbreeding population on anisland

is separated by a barrier, such as the presence of a new river, then over time, the organisms may start to

diverge. If the opposite ends of the island have different pressures acting upon it, this may result indivergent evolution. Or, if a certain group ofbirds in a population of other bird of the same species varies

from their migratory track due to abnormal wind fluctuations, they may end up in new environment. If thefood source is such that only birds of the population with a variant beak are able to feed, then this trait will

evolve by virtue of its selective survival advantage. The same species in the original geographical location

and having the original food source do not require this beak trait and will, therefore, evolve differently.

Divergent evolution has also occurred in the red fox and the kit fox. While the kit fox lives in the desert

where its coat helps disguise it from its predators, the red fox lives inforests, where the red coat blends

into its surroundings. In the desert, theheat makes it difficult foranimalsto eliminate body heat. The ears

of the kit fox have evolved to have greater surface area so that it can more efficiently remove excess body

heat. Their different evolutionary fates are determined primarily on the different environmental conditions

and adaptation requirements, not on genetic differences. If they were in the sameenvironment, it is likely

that they would evolve similarly. Divergent evolution is confirmed byDNAanalysis where the species that

diverged can be shown to be genetically similar.

Normally, hybrids between two different species, even if offering beneficial traits, are sterile. And in many

cases, hybrids are not viable at all.

A mule (photo), the result of the mating of a horse and a donkey, is sterile.

Researchers at Cornell University have made the first discovery of a gene pair that provokes problems at

hybridization.Two genes from two fruit fly species (Drosophila melanogaster and D. simulans) interfere with each other,

preventing the production of male offspring.

The finding explains what causes lethality or sterility in hybrids and also offers clues to how species evolve

from common ancestors.

A

rare mutation in a D. melanogaster gene called "Hmr" (Hybrid male rescue) and a similar

mutation in a D. simulans gene called "Lhr" (Lethal hybrid rescue) make these genes nonfunctional.

When either of these genes is eliminated, the hybrid males survive.

"We have found the first example of two genes that interact to cause lethality in a species hybrid," said thepaper's senior author, Daniel Barbash, assistant professor in Cornell's Department of Molecular Biology
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and Genetics.

This confirms the Dobzhansky-Muller model, a theory from the 1930s that suggests hybrid

incompatibilities (such as death or sterility) are caused by genes that have evolved from a common ancestor

but diverged in each of the species.

In the common ancestor, these genes may have worked perfectly well together.

But, as each gene evolved in its own species, it began to code for proteins that no longer work in the other

species.

When genes from each species were compared with each other, the Hmr gene in D. melanogaster and the

Lhr gene in D. simulans each evolved much faster than most genes and diverged due to natural selection, a

genetic change due to a pressure that benefits the survival of a species.

The Dobzhansky-Muller model also proposes that these evolved genes depend on each other to cause

hybrid incompatibilities.

However, when Barbash and his colleagues cloned each gene and inserted an Lhr gene from D. simulans

into D. melanogaster, the two genes did not interfere with each other in the engineered D. melanogaster

strain even though the Lhr and Hmr genes interfere with each other in hybrids.

"This tells us there must be other things involved in the hybrid" that impacts the incompatible pairing of

these genes, said Barbash.

The scientists hope to determine whether the hybrids die because of additional genes like Hmr and Lhr, or

because of more subtle differences between the chromosomes of the species

Nondisjunction: Failure of paired chromosomes to disjoin (separate) during cell division so that both

chromosomes go to one daughter cell and none to the other. Nondisjunction causes errors in chromosome

number such as trisomy 21 (Down's syndrome) and monosomy X

Polyploidy is the process of genome doubling that gives rise to organisms with multiple sets of

chromosomes. The term ploidy (see glossary for this and other related terms) refers to the number of

complete genomes contained in a single cell. In general, polyploid organisms contain a multiple orcombination of the chromosome sets found in the same or a closely related diploid species. Polyploidy can

arise from spontaneous somatic chromosome duplication, or as a result of non-disjunction of the

homologous chromosomes during meiosis resulting in diploid gametes (for review see Ramsey and

Schemske, 2002). It can also be artificially induced by treatment with drugs, such as colchicine, which

inhibits cell division. Polyploidy can occur in all or most somatic cells of the organism or it can be

restricted to a specific tissue. In the latter case the preferred term is endopolyploidy. Some examples of

such specialized cells in animals include the salivary gland cells in Drosophila or liver cells in humans.

Historically, there has been much confusion over whether to classify polyploids by mode of origin criteria

or by cytological criteria. Here we follow Ramsey and Schemske (2002) and adopt mode of origin criteria:

if the chromosomes of one genome within an organism or species are simply duplicated, the resulting

polyploid is an autopolyploid. However, if genome duplication occurs during a cross of two different

species, the resulting organism is referred to as an allopolyploid.

Two main modes of origin of the polyploid condition are recognized somatic doubling in mitosis, and

nonreduction in meiosis (Heilborn, 1934; Grant, 1971). The mechanism of somatic doubling is exemplified

by polyploid Primula kewensis, and nonreduction was the mode of origin seen in polyploid

Rhaphobrassica. It used to be thought most that polyploids formed by hybridization followed by

chromosome doubling. However, Harlan and deWet (1975) argued that unreduced gametes played an


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important role. While agronomy researchers took notice of this (e.g. Peloquin, 19XX), textbooks did not

change. Recently, a lot of theoretical modeling (Rodriguez, 1996; Ramsey and Schemske, 1998, 2002) and

fieldwork (Husband 1999, 2000) has contributed to the view that unreduced gametes and triploid bridges

are a major source of polyploid formation. This is also a mechanism for how diploid and polyploid

genomes can interact (thus, the new polyploid species are not strictly sealed off from its diploid

progenitors).

During meiosis, homologous chromosomes pair and undergo crossing over resulting in the exchange of

parts of their chromosomes. In diploid hybrids derived from crosses of two species, chromosomes from the

two species may differ or one of the chromosomes may be absent. This can cause irregularities during

meiosis and may result in cell cycle arrest and subsequent embryo abortion (Fig. 1a). However, if the

chromosome number is doubled in the hybrid, allotetraploids are formed, which have four sets of

chromosomes. This can occur by crossing autotetraploids of the two species, or more likely in nature, by

the fussion of unreduced gametes. Allotetraploids generally will have pairing and crossing over only within

the two chromosomes of each original parent (the homologous chromosomes AA) and only rarely between

chromosomes from the two original parents (the homeologous chromosomes AA). This meiotic behavior

assures proper pairing of the chromosomes and the correct assortment into gametes (Fig. 1b).

How do polyploids become established?

The frequency of polyploid events is exceedingly rare (estimated to be 10-5 among offspring of diploids;

Ramsey and Schemske, 1998). Although the formation of unreduced (2n) gametes is considered to be rare

in general (McCoy, 1982), 2n gamete production is likely to play a major role in polyploid origins (Harlan

and deWet, 1975; Vorsa and Binghm, 1979). A number of factors genetic and environmental have been

shown to influence the frequency of 2n gamete formation (Sax 1937; Thompson and Lumaret 1991;

Ramsey and Schemske 2002). Genetic factors also control unreduced gamete formation (potato, Mok and

Peloquin, 1975; Veilleux et al. 1982, Peloquin, in press; alfalfa, McCoy, 1982; blueberry, Qu and Vorsa,

1999). Genes that control rates of unreduced gamete production could become fixed in small populations,

and enable rare polyploids to become more frequent. Environmental factors that affect 2n gamete formation

include sudden changes in temperature (heat or cold treatment), dehydration, x-rays, uv light, infections,

etc., and can induce chromosome doubling (Sax, 1937). Otto and Whitton ?? Mable?? Other factors that

can contribute to polyploid formation (aside from unreduced gametes) include superior vegetative (clonal)

growth, perennial life history, niche separation, assortative mating, and other fitness differences. Therefore,

broad generalizations may not apply to specific cases. As such, different species need to be characterized

and systematically analysed to determine what mechansims are responsible for bringing about the observed

polyploid frequencies and subsequent evolutionary patterns.

A critical first step in polyploid evolution is the establishment and subsequent persistence of theneopolyploid (Fowler and Levin, 1984). A new and therefore rare polyploid in a diploid population would

be at a major fertility disadvantage, since most pollinations of the polyploid will involve pollen from

diploids. The predominance of one cytotype excluding the rare cytotype from reaching high frequences is

known as the minority cytotype exclusion principle (Husband, 1999; 2000). A number of models have been

developed to determine how polyploids may become established in a diploid population (Fowler and Levin,

1984; Felber, 1991; Rodriguez, 1996; Husband, 2000). Parameters included in these models include the

production of unreduced (2n) gametes by the diploid cytotype, the frequency of tetraploids formed with


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each generation, multiple origins of polyploids over several generations (given perenials versus annuals).

Once 2n gamete production exceeds a certain threshhold, the tetraploids are able to replace diploids. The

threshold varies by modifying fertility/viability of the cytotypes. Frequency-dependent processes can be

overcome by reducing inter-cytotype matings, so rare cytotypes could become established despite the

minority cytotype disadvantage.

How do polyploids acquire variation?

Polyploids can acquire variation both through mechanisms of population genetics (gene flow with diploids

and multiple origins of polyploids), and through mechanisms that generate de novo variation such as

chromosomal rearrangements and epigenetic phenomena.

Polyploidy has long been considered an important example of instant or sympatric speciation, since

polyploid species are mostly reproductively isolated from their diploid progenitors (Stebbins 1950, 1971;

Levin 1983). An interesting aspect related to allopolyploidization or hybridization of different species is the

question of the species barrier when using a biological species concept. Members of the same biologicalspecies are commonly defined as related individuals of a population that can interbreed and whose

offspring are fertile. Thus, the horse and a donkey are considered separate species because their hybrid

offspring are viable but infertile. In plants, hybridization of different species is quite common and many of

the well-known crop plants are allopolyploids resulting from inter-species hybrids. Such allopolyploids

pose a challenge to phylogenetic species concepts, which define species on strict monophyletic criteria.

Over the last decade this challenge has taken on additional relevance as polyploid species have been

found to form repeatedly in close proximity to one another (Soltis and Soltis, 1993; 1999; 2000). The

polyphyly of polyploid species calls into question the very definition of species. Allopolyploids like

other organisms with reticulate evolutionary histories (e.g., eukaryotes, lichens) give biologists important

examples when theorizing about evolutionary entities. Aside from philosophical considerations about

species definitions, there are many implications for the multiplicity of origins for polyploids. Multiple

origins of polyploid species have been reported for mosses, ferns, and many angiosperms (reviewed in

Vogel et al., 1999; Soltis and Soltis, 2000).

Allopolyploidy presents a paradox because it is both a diversifying force and a genetic bottleneck

(Stebbins, 1971). However, the genetic bottleneck problem may be solved by the fact that population-level

genetic studies of polyploid plants and animals indicate that polyploidy is not a rare event leading to unique
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and uniform genotypes. Rather, the multiple independent formations of polyploid species from

heterozygous diploid progenitors may provide a significant source of genetic variation (reviewed in Soltis

and Soltis, 1993; 1999; 2000).

Many new polyploids also are genetically unstable, and the next section describes mechanisms that can

lead to novel variation.

allopolyploid Apolyploid organism, usually a plant, that contains multiple sets of chromosomes derived

from different species. Hybrids are usually sterile, because they do not have sets ofhomologous

chromosomesand thereforepairing cannot take place. However, if doubling of the chromosome number

occurs in a hybrid derived from two diploid (2n) species, the resulting tetraploid (4n) is a fertile plant. This

type of tetraploid is known as an allotetraploid; as it contains two sets of homologous chromosomes,pairing and crossing over are now possible. Allopolyploids are of great importance to plant breeders as

advantages possessed by different species can be combined. The species of wheat, Triticum aestivum, used

to make bread is an allohexaploid(6n), possessing 42 chromosomes, which is six times the original haploid

number (n) of 7. See alsoamphidiploid. Compare autopolyploid.

Plant Auxins - Phototropism & Geotropism

- Control of Growth & Development

As with animals, plants also use a variety of hormones to control their growth and development. A family

of hormones called auxins are commonly found in plants, and promote (and sometimes inhibit) growth.

Auxins

Auxins are produced in the meristems of plants (meristems are explained on successive pages).

Auxins are responsible in promoting cell elongation, a process that is required before differentiation of a

cell. It is able to this by promoting the intake of water, increasing the elasticity of the cell to cope with the

increase of water taken in by the cell.

One of the most common auxins is indole acetic acid.

Indole Acetic Acid (IAA)

Indole Acetic Acid affects the root and shoot tips of the plant, as described below.

Shoot Tip - No matter what the concentration, IAA promotes growth in the shoot area of a plant (though

higher concentrations promote growth more) .

Root Tip - High concentrations of auxin inhibit growth while small amounts are enough to promote

growth in the root with indole acetic acid.

Phototropism

Auxins also play a part in phototropism, an occurrence that involves plants bending or moving away from

light. The shoot tip is responsible for directional movement by the plant in response to sunlight, as this is

the area where auxins can be found.
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Sunlight eradicates auxin, meaning that the part of the shoot tip of the plant which is receiving direct

sunlight will have the least amount of auxin.

The extra auxin present on the shaded side promotes more cell division and elongation, causing the plant tobend towards the sunlight after this lop-sided growth.

Geotropism

Geotropism is a similar occurrence to phototropism where the plant exhibits directional growth in response

to gravity. The shoot tip illustrates negative geotropism (grows against force of gravity) while the root tip

exhibits positive geotropism (grows in the same direction as gravity).

Apical Dominance

The presence of auxins in the lateral areas of the plant (in between the root and shoot tip) prevent lateral

growth. If you cut off the shoot tip of a plant, the lack of 'diffusable' auxins means that they cannot inhibit

growth in these lateral areas. This is known as apical dominance.

Leaf Abscission

The presence of auxins in the lateral areas also prevents leaf abscission. In the colder months, auxin

concentrations and the rate of photosynthesis drops.

This lack of auxin in the lateral areas results in the forming of anabscissionlayer at the stalk of the leaf,

which weakens its connection with the plant and soon falls off it.

The next page looks at another family of growth hormones, the gibberellin family, with continuing pages

looking at the meristems, the sites of plant growth.

orienting or directing homeward or to a destination; "the homing instinct"; "a homing beacon"

Homing is the inherent ability of an animal to navigate towards an original location through

unfamiliar areas. This location may be either a home territory, or a breeding spot

Migration Examples

Humpback whales of the Pacific Ocean head south in the fall to give birth to their young in

subtropical waters off Hawaii, and then in late spring head north to spend the summer in the cold

waters off Alaska that are rich with food.

Salmon are reproductive migrants that start their lives in freshwater streams, move to the open

ocean for their adult lives, then return to their home stream to lay eggs.

Dall sheep of Noatak National Preserve are seasonal, altitudinal migrants that spend summers nearthe top of mountain ranges and then winter at lower elevations where there is less snow and food

easier to find.

Arctic terns are complete migrants that spend all year in summer by alternating subpolar regions in

the northern and southern hemispheres.

Golden eagles of Denali National Park and Preserve spend the summer in the north where there is

plenty of food, and head south for the winter when there is less food in the north and the
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temperatures drop far below zero. While all of the golden eagles of Denali do migrate, golden

eagles are considered partial migrants because those that live far enough south do not migrate.

Sea turtles return from ocean waters to the coast to lay eggs in the sand, where they hatch and

head to the open ocean until it is their turn to lay eggs. They are another example of reproductive

migrants.

Locusts change when they get too crowded and become more active and social creating large

groups of insects that move across the land in search of new places with plenty of food (and fewerlocusts). This adaptation to overcrowding is removal migration.

Great gray owls are an irruptive migrant, migrating southward only occasionally and in numbers

that vary greatly. Northern finches and crossbills are also irruptive migrants.

Migration cues

How do animals know when to migrate? That depends on the type of migration. For many types of

migration it is the change of seasons that spurs animals on. As summer becomes fall, days become shorter

and that can trigger animals to prepare for migration. Closer to the equator, the days don't change in length

and one theory is that animals become restless after too many days with a constant length.

Other migrations are initiated by seasonal conditions. Food availability can be a motivator for somelongitudinal and altitudinal migrators. For example, as plant foods in upper elevations become hidden

under snow, animals move down toward the valleys, and then in the spring as the plants come out again,

animals move back into the upper areas following the plants as they appear. Nomadic animals move to the

next feeding ground as they run out of food where they are. As ponds dry with seasonal changes, animals

will move to find available water supplies, and then return during with the seasonal rains.

In some species, migration happens when there are just too many animals too close together. Theovercrowding causes many of the individuals to leave in hopes of finding another habitat with less

competition. Or when there isn't enough food, not because of the changing seasons, but because the food

where they are has been eaten. Then the animals start moving in search of new food.

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