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1

Simple cells could have been produced on early earth through 4 stages

1. abiotic(nonliving) synthesis of small organic molecules
2. joining of small molecules into macromolecules
3. packaging of molecules into protocells
4. origin of self-replicating molecules which made inheritance possible

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protocells

droplets with membranes that maintained an internal chemistry different from that of their surroundings

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Evidence for abiotic synthesis of small organic molelcules

experiments done by scientists such Urey-Miller show that organic molecules could have formed in hypothesized early earth's reducing, neutral, and volcanic atmospheres
-meteorites may have been source of organic molecules; Murchison meteorite contained many amino acids and organic molecules that could not have been contaminants of Earth

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early earth atmosphere

-thick with water vapor
-contained various compounds released by volcanic eruptions
-part of the atmosphere may have been reducing (electron adding) environment where organic molecules could have formed simple cells
-little or no oxygen, primarily carbon dioxide, water vapor, and nitrogen

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Evidence for stage 2: Joining of small molecules into macromolecules

A 2009 study demonstrated that abiotic synthesis of RNA monomers can occur spontaneously from simple precursor molecules

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Stage 3: Packaging of molecules into protocells

life cannot persist without organism reproduction and energy processing(metabolism).
Necessary conditions may have been met in vesicles

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Vesicles

fluid filled compartments bounded by a membrane-like structure
-abiotically produced vesicles can exhibit some properties of life
- simple reproduction and metabolism, internal environment is different from surroundings
- montmorillonite, a clay thought to be common on early earth, increases the abiotic self assembly of vesicles and may have had particles with RNA or organic molecules attached that it could have absorbed

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Evidence for stage 4: Origin of self-replicating molecules

-RNA likely first genetic material
-Natural selection has produced ribozymes capable of self-replication in lab (RNA best able to replicate itself will produce most copies)
- similar selection could have occurred on early Earth, allowing RNA molecules to replicate and store info about the vesicles that carried them
-Once RNA sequences that carried genetic info appeared in protocells, other changes, like the assembly of DNA, were possible.

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Stromatolites

layered rocks that form when prokaryotes bind thin films of sediment together
- provide earliest direct evidence of life, dating to about 3.5 years ago
-single celled organisms not complex enough to form Stromatolites originated earlier, around 3.9 billion years ago.

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Photosynthesis

when oxygenic photosynthesis first evolved, free O2 dissolved in water but eventually began to gas out into the atmosphere
- gradual rising oxygen levels brought about by cyanobacteria

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Oxygen Revolution

-about 2.3 billion years ago atmospheric oxygen levels shot up
-probably doomed many prokaryotic groups
-diverse adaptations, like cellular respiration, evolved
- possibly followed the evolution of eukaryotic cells containing chloroplasts

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First Eukaryotes

emerged about 2.1 billion years ago
more complex than prokaryotes: have a cytoskeleton which allows them to engulf other cells

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endosymbiant theory

says that mitochondria and plastids were formerly small prokaryotes that began living within larger cells
-originally parasites or prey but became permanently part of a mutually beneficial relationship

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serial endosymbiosis

supposes that mitochondria evolved before plastids as a sequence of endosymbiotic events

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Evidence for endosymbiotic origin of mitochondria and plastids

-inner membranes have enzymes and transport systems homologous to those in the plasma membranes of prokaryotes
- similar replicating process to some prokaryotes
- contain single circular DNA molecules
-capable of transcribing and translating DNA into proteins
- ribosomes are in some ways similar to those of prokaryotes

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Earliest Multicellular Eukaryotes

-common ancestor lived about 1.5 billion years ago
-larger and more diverse soft-bodied organisms appeared around 575 years ago
-one hypothesis supposes that a series of ice ages around 750 to 580 m years ago confined life to areas where the water was not covered by ice; limiting the size and diversity of multicellular eukaryotes

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Cambrian explosion

sudden appearance of fossils of larger animals early in the Cambrian period
-before: all large animas where soft-bodied, after: many with claws, body armor etc appeared
-fossil discoveries of embryos suggests that animals similar to those living today were present 10s of millions of years before explosion

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Colonization of Land

-larger forms of life colonized and around 500 millions years ago when adaptations made it possible to reproduce on land and prevented dehydration
-small plants and fungi first grew on land bout 420 m years ago
-arthropods first animals to colonize land 420 m years ago

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Three Domains

-bacteria: contains most currently known prokaryotes
-archaea: prokaryotic organisms that inhabit a wide variety of environments
-eukarya: consists of all organisms that have cells with nuclei
-system highlights that much of the history of life has been about single-celled organisms

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horizontal gene transfer:

process in which genes are transferred from one genome to another through mechanisms like the exchange of transposable elements and plasmids, viral infections, fusion of organisms
-explains why trees built using different genes can give inconsistent results

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Evolution

a change in the frequency of an allele in a population over many generations

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Examples of Early Cells

methanogens: single-celled organisms that produce methane
thermophiles: live at very high temps, believed to be some of first cells on earth
halophiles: live in places with high concentrations of salt
cyanobacteria photosynthetic prokaryotes that live in water, most abundant bacteria, release large amounts of oxygen into air

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Common structures and functions shared by all living organisms

o All organisms have cells which contain info, stored in DNA and RNA, which directs the function of cells
-General coding and structure shared, many share specific genes
-All cells share similar processes for replicating DNA and transcribing and translating the code
o All organisms share similar organic and inorganic molecules as well as common metabolic processes like cellular respiration
o Structural evidence supports the relatedness of all eukaryotic cells.

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How did the appearance of protocells represent a key step in the origin of life

The segregation of groups of molecules into membrane-bound systems could have concentrated organic molecules together and assisted important biochemical reactions necessary for the development of life.

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Why did the appearance of free oxygen in the atmosphere trigger a massive wave of extinctions among the cells living at the time?

Free oxygen can attack chemical bonds, damage cells, and inhibit some enzymes. As a result, many prokaryotic cells that thrived in the anaerobic environment may have struggled to survive and reproduce as the atmosphere's oxygen levels increased.

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alleles

variations of a given gene
- offspring inherit one allele from each parent
-often dominant (P) or recessive (p)

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homozygous dominant

individual has two dominant alleles (PP)

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homozygous recessive

both alleles are recessive

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heterozygous

one dominant and one recessive allele (Pp)

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codominance

organisms express a combination of both traits

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phenotype

trait exhibited in an organism's appearance

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genotype

organisms genetic makeup

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adaptations

inherited characteristics of organisms that enhance their survival and reproduction in specific environments
-Darwin related adaptations to new environments to the origin of new species
-categories: physical appearance, physiological function, or behavior

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natural selection

a process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates because of those traits
-overtime can increase match between organisms and environment
- if environment changes or species moves to new environment, natural selection may result in adaptations and new species
-can amplify or diminish only heritable traits that differ in a population
-which traits are favorable depends on the context in which a species lives

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Natural Selection Explains

-Unity of Life: Darwin attributed unity of life to descent from common ancestor
Diversity of Life: descendants accumulated adaptations to different habitats over millions of years

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artificial selection

process of modifying a species over generations by selecting and breeding individuals with desired traits

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Darwin's Observations and Inferences

Observations:
#1 Members of populations often vary in inherited traits
#2 all species can produce more offspring than their environment can support, many offspring fail to survive and reproduce
Inferences:
#1 individuals whose inherited traits give them a higher probability of surviving and reproducing tend to leave more offspring than others
#2 unequal abilities to survive and reproduce leads to favorable traits in population over generations

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Genetic Variation

differences among individuals in the composition of their genes or other DNA segments
-necessary for evolution to occur
-only genetically determined part of phenotypic variation can have evolutionary consequences

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Variation within a population

discrete characters: can be classified on an either-or-basis
quantitative characters: vary along a continuum within a population (results from 2+ genes on single phenotype character)

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average heterozygosity:

the average percentage of loci that are heterozygous
- quantifies gene variability

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Variation Between Populations

geographic variation: differences in the genetic composition of separate populations
cline: a graded change in a character along a geographic axis

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Sources of Genetic Variation

-originates when mutation, gene duplication, or other processes produce new alleles and new genes
-sexual reproduction causes existing genes to be arranged in new ways

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Formation of New Alleles

mutations
-for multicellular organisms, only mutations in cell lines can be passed to offspring
-limits animals more than plants
point mutations can have significant impact on the phenotype and are often harmless

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mutation

a change in the nucleotide sequence of an organism's DNA

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point mutations

change of one base in a gene

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locus

a specific place along the length of a chromosome where a given gene is located

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Altering Gene Number or Position

- chromosomal changes that delete, disrupt, or rearrange many loci at once are usually harmful
- in some cases can be beneficial or neutral
-duplication of genes due to errors in meiosis, slippage during DNA replication, or activities of transposable elements are an important source of variation
- gene duplications without severe effects can persist over generations, mutations accumulate, new genes can take on new functions

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Rapid Reproduction

-prokaryotes typically have short generations spans so mutations can quickly generate genetic variation in these population

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Sexual Reproduction

-rearranges existing alleles into fresh combinations each generation, providing much genetic variation
- in meiosis, homologous chromosomes trade some alleles by crossing over. Chromosomes distributed at random into gametes, Fertilization brings together gametes that are likely to have different genetic backgrounds

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population

a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
-populations evolve, individuals do not

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gene pool

consists of all copies of every type of allele at every locus in all members of the population
-a diverse gene pool is important for the survival of a species in a changing environment

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The Hardy-Weinberg Principle

states that the frequencies of alleles and genotypes in a population will remain constant(in equilibrium) from generation to generation, provided only Mendelion segregation and recombination of alleles is at work
-describes the gene pool of a population that is not evolving

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Hardy Weinberg equilibrium equation for a locus with 2 alleles

p^2 +2pq+ q^2=1
(%PP+%Pp+%pp=1)
p+q=1
p= frequency of dominant allele
q= frequency of recessive allele

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Conditions for Hardy-Weinberg Equilibrium

1. No mutations
2. Random mating
3. no natural selection
4. Extremely large population size
5. No gene flow
(departure from these conditions usually results in evolutionary change)

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Mechanisms for Evolution

Natural selection is only mechanism that consistently causes adaptive evolution
-consistently increases frequencies of alleles that provide reproductive advantage, leading to adaptive evolution

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relative fitness

the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals

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Three modes of natural selection

-directional selection
-disruptive selection
-stabilizing selection

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Directional selection

occurs when conditions favor individuals exhibiting one extreme of a phenotypic range
-shifts a populations frequency curve for the character in one direction or another
-common in new or changing environments

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disruptive selection

occurs when conditions favor individuals at both extremes of phenotypic range over individuals with intermediate phenotypes

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stabilizing selection

acts against both extreme phenotypes and favors intermediate variants
-reduces variation, maintains the status quo

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Sexual Selection

a form of selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates
-can result in sexual dimorphism

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sexual dimorphism

a difference between two sexes in secondary sexual characteristics.

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intrasexual selection

individuals of one sex compete directly for mates of the opposite sex

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intersexual selection

mate choice; individuals of one sex (usually females) are choosy in selecting mates, often based on showiness
-females seem to prefer male traits that are correlated with "good genes" or good health

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neutral variation

differences in DNA sequences that do not confer a selective advantage or disadvantage

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The preservation of genetic variation

The tendency for directional and stabilizing selection to reduce variation is countered by mechanisms that preserve or restore it.
-diploidy and balancing selection

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diploidy

-in diploid eukaryotes, recessive alleles that are currently disadvantageous can persist in heterozygous individuals
- maintains a pool of alleles which could bring new benefits if the environment changes

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balancing selection

occurs when natural selection maintains 2 or more forms in a population
types: heterozygote advantage and frequency-dependent selection

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heterozygote advantage

exhibited if individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes
-maintains two or more alleles at that locus that more variety

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frequency-dependent selection

the fitness of a phenotype depends on how common it is in a population

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Why Natural Selection Cannot Fashion Perfect Organisms

1. Selections can act only on existing variations
2. Evolution is limited by historical restraints (only operates on the traits an organism already has)
3. Adaptations are often compromises
4. Chance events, natural selection, and changing environments interact.

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Homology

similarity resulting from common ancestry

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homologous structures

structures in different species that are similar because of common ancestry
ex. arms, forelegs, flippers, and wings of various of mammals
-similarities between vertebrate embryos (tales and throat pouches)

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vestigial structures

remnants of features that served a function in an organism's ancestors
-ex. vestiges of pelvis and legbones in snakes

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evolutionary tree

a diagram that reflects evolutionary relationships among groups of organisms
-all life shares deepest layer of similarity and each successive group adds its own homologies to those it shares with larger groups

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convergent evolution

the independent evolution of similar features in different lineages

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analogous features

share similar function but not common ancestry
-results from convergent evolution

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The Fossil Record

-shows there have been great changes in the kinds of organisms on Earth at different points in time
-fossils have to be created under right conditions, can be destroyed
-favors species that existed for a long time, where widespread in a certain environment, or had hard body parts

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Biogeography

the geographic distribution of species
-influenced by continental drift,
-earth was united into a single continent (Pangea) 250 million years ago

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endemic

species found only on a specific location (like an island) but no where else in the world
-animals often related to those in nearby places

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Genetic Drift

chance events that cause allele frequencies to fluctuate unpredictably from one generation to the next (especially in small populations)

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founder effect

occurs when a few individuals become isolated from a larger population and establish a new population whose gene pool differs from the source population

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bottleneck effect

occurs when there is a severe drop in population size. By chance, alleles may be over represented, under represented, or be absent

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effects of genetic drift

1. significant in small populations
2. Can cause allele frequencies to change at random
3. Can lead to a loss of genetic variation within populations(can eliminate alleles from a pop)
4. Can cause harmful alleles to become fixed

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Gene flow

the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes
-can result in two populations combining into one

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comparative embryology

the study of similarities and differences in the embryonic development of organisms.