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Chapter 27: Evolution of Life

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Evidence of Evolution

Evolution is all the changes that have occurred in living things since life began.

The earth is 4.5 billion years old, and prokaryotes evolved 3.5 billion years ago.

The eukaryotic cell arose 2.1 billion years ago, but there was no multicellularity until 700 million years ago.

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Evolution encompasses common descent and adaptation to the environment.

Due to common descent, all living things share common characteristics: they are made of cells, take chemicals and energy from the environment, respond to external stimuli, reproduce, and evolve.

Many fields of biology give evidence that evolution has occurred.

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Fossil EvidenceFossils are the remains of past life,

usually consisting of hard parts, such as shells, bones, or teeth.

Most fossils are found embedded in sedimentary rock.

Sedimentation causes rock formation as particles accumulate in layers; any given stratum (layer) is older than the one above it, and younger than those below.

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Paleontologists are biologists who study fossils.

Certain fossils serve as transitional links between groups.

Such fossils allow paleontologists to deduce the sequence in which certain groups evolved (i.e., fishes evolved before amphibians, which came before reptiles, which evolved before both birds and mammals).

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Transitional fossils

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Geological Time ScaleAs a result of studying strata across the

earth, scientists have divided earth’s history into eras, periods, and epochs.

There are two ways to date fossils:Relative dating provides an approximate

age based on position of the fossil within rock strata.

Absolute dating uses radioactive isotopes to measure the amount of radiation left in a fossil, yielding an actual age.

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Carbon 14 (14C) is the only radioactive isotope in organic matter.

The amount of radioactivity remaining in a fossil can be compared with that of a modern sample to determine the age of a fossil.

Radioactive isotopes decay at a known rate; the half-life of a radioactive isotope is the length of time it takes for half of the radioactive isotope to change into another stable element.

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Mass ExtinctionsExtinction refers to the death of every

member of a species.During a mass extinction, a large

percentage of species become extinct within a relatively short period of time.

Mass extinctions occurred at the ends of the Ordovician, Devonian, Permean, Triassic, and Cretaceous periods.

The Cretaceous mass extinction that led to the demise of dinosaurs was likely caused by an meteorite hitting the earth.

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Dinosaurs

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Biogeographical Evidence

Biogeography is the study of the distribution of plants and animals throughout the world.

The world’s six biogeographical regions have their own distinct mix of living things.

Continental drift refers to the changing positions of the continents over time.

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Two hundred twenty-five million years ago, all the present land masses belonged to one continent (Pangaea).

The distribution of plants and animals is consistent with continental drift.

Organisms, such as certain seed plant groups or reptiles, are widely distributed throughout the world.

Other groups, such as mammals that arose after the continents broke up, have great differences in species on different continents.

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Continental drift

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Anatomical EvidenceDespite dissimilar functions, all

vertebrate forelimbs contain the same sets of bones – this strongly suggests common descent.

Structures that are similar because they are inherited from a common ancestor are homologous structures.

Analogous structures are used for the same purpose but are not due to a common ancestor.

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Bones of vertebrate forelimbs

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Vestigial structures are anatomical features that are fully developed in one group but reduced or nonfunctional in other, similar groups.

Vestigial structures can be explained by common descent.

The homology shared by vertebrates extends to their embryological development; all vertebrates have a dorsal notochord and paired pharyngeal pouches at some point.

Evolution modifies existing structures.

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Significance of developmental similarities

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Biochemical Evidence

All organisms have certain biochemicals in common.

All use DNA, ATP, and many identical or nearly identical enzymes.

Organisms use the same triplet code and the same 20 amino acids in proteins.

This similarity is not necessary, but can be explained by common descent.

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Significance of biochemical differences

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Origin of Life

Under conditions present on the primitive earth, it is possible that a chemical evolution produced the first cells.

Chemical evolution refers to the reaction of inorganic chemicals to produce simple organic chemicals, that would later polymerize into macromolecules.

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Once a plasma membrane formed, a protocell could have come into existence.

Energy for the chemical reactions could have come from ultraviolet radiation, volcanoes, bombardment by comets, or from oceanic hydrothermal vents.

The early atmosphere lacked oxygen and also a shield of ozone; it was not until photosynthesis evolved that oxygen was present in earth’s atmosphere.

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Origin of the first cell(s)

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Evolution of Small Organic Molecules

Experiments by Stanley Miller in 1953 tested the hypothesis that small organic molecules were formed at the ocean’s surface.

The first atmospheric gases (methane, ammonia, and hydrogen) were placed into a closed system, heated, and circulated past an electric spark to simulate lightning.

A variety of amino acids and organic acids formed.

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Miller’s experiment

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Chemical evolution at hydrothermal vents

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MacromoleculesThere are three hypotheses concerning

how small organic molecules could give rise to macromolecules:

The RNA-first hypothesis suggests that only the macromolecule RNA was needed to progress toward the first cell.

RNA molecules (as ribozymes) can sometimes be both substrates and enzymes.

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The protein-first hypothesis, by Sidney Fox, suggested that amino acids collected in small puddles, and heat from the sun caused them to form proteinoids; when proteinoids were returned to water, they formed microspheres and had many properties of cells.

This hypothesis assumes that DNA came after proteins.

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The third hypothesis, by Graham Cairns-Smith, suggests that clay was helpful in causing polymerization of both proteins and nucleic acids at the same time.

Clay attracts small organic molecules and contains iron and zinc, which may have served as inorganic catalysts for polypeptide formation.

This hypothesis suggests that RNA and polypeptides arose at the same time.

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The ProtocellBefore the first true cell, there would have

been a protocell that had a lipid-protein membrane and used energy metabolism.

Fox has shown that if lipids are available to microspheres, the two form a lipid-protein membrane.

Other work by Alexandr Oparin has shown that concentrated mixtures of macromolecules form coacervate droplets that a semipermeable boundary may form around.

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Protocell anatomy

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The Heterotroph HypothesisThe protocell was likely a heterotroph,

absorbing small organic molecules from its environment.

Natural selection would favor cells able to extract energy from carbohydrates to transform ADP to ATP.

Fox has shown that microspheres have some catalytic ability, and Oparin found coacervates incorporate available enzymes.

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The True CellA true cell is a membrane-bounded

structure that can carry on protein synthesis to produce the enzymes that allow DNA to replicate.

It is possible that the sequence of DNA to RNA to protein developed in stages.

Once the protocells acquired genes that could replicate, they became cells capable of reproducing, and evolution began.

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Process of Evolution

Individuals do not evolve.

As evolution occurs, genetic changes occur within a population, and these lead to phenotypic changes that are commonly seen in that population.

Changes in gene frequencies in populations over time constitute microevolution.

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Population Genetics

A population is all the members of a species occupying a particular area at the same time; members of a population reproduce with each other to produce the next generation.

The various alleles of all the gene loci in all the members make up the gene pool for the population.

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Hardy and Weinberg used a binomial expression to calculate the genotypic and phenotypic frequencies of a population:

p2 + 2pq + q2 = 1This expression is used to determine

gene frequencies at a given time and to predict gene frequencies in the future.

If reproduction is completely random, the Hardy-Weinberg equation predicts the same gene pool frequencies generation after generation.

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Using the Hardy-Weinberg equation

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The Hardy-Weinberg LawThe Hardy-Weinberg law states that gene

frequencies will stay the same in a large population over time provided:

1) There are no mutations or mutations are balanced.

2) There is no genetic drift; changes in allele frequencies due to chance alone are insignificant.

3) There is no gene flow – no migration of individuals in or out of the population.

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4) Mating is random – individuals pair by chance and not by choice.

5) There is no selection – no selective force favors one genotype over another.

In real life, these conditions are rarely met, and microevolution, as seen by changing gene frequencies in Hardy-Weinberg equilibrium, occurs.

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Microevolution

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Five Agents of Evolutionary ChangeMutations

Mutations provide new alleles and therefore underlie all other mechanisms that produce variation.

Mutations alone are unlikely to cause evolution; selective agents acting on heritable variation cause evolution.

The adaptive value of a mutation depends on the environmental conditions.

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Genetic DriftGenetic drift refers to changes in allele

frequencies of a gene pool due to chance; genetic drift has a much larger effect in a small population.

The founder effect occurs when a few individuals leave the original population and begin a new population.

A bottleneck effect is seen when much of a population is killed due to a natural disaster, and only a few remaining individuals are left to begin a new population.

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Genetic drift

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Founder effect

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Gene FlowGene flow is the movement of alleles

between populations, such as when individuals migrate from one population to another.

Gene flow between two populations keeps their gene pools similar and prevents close adaptation to a local environment.

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Nonrandom MatingNonrandom mating occurs when

individuals pair up, not by chance, but according to genotypes and phenotypes.

Inbreeding is an example of nonrandom mating.

In a human population, inbreeding increases the frequency of recessive abnormalities.

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Natural SelectionNatural selection is the process by which

populations become adapted to their environment.

Evolution by natural selection requires:

Variation

Inheritance of the genetic difference

Differential adaptedness

Differential reproduction.

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Three types of natural selection are known:

Stabilizing selection – an intermediate phenotype is favored.

Directional selection – one extreme phenotype is favored.

Disruptive selection – both extreme phenotypes are favored over an intermediate phenotype.

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Stabilizing selection

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Directional selection

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Disruptive selection

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Maintenance of VariationAn example of sickle-cell disease shows

how genetic variation is sometimes maintained within a population.

Persons with sickle cell disease have sickle-shaped blood cells, which can lead to hemorrhage and death.

Persons without a sickle-cell gene are susceptible to malaria in parts of Africa.

But heterozygotes, with one sickle-cell gene and one normal gene, have only minor problems with blood cells and are resistant to malaria.

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Speciation

A species is a group of interbreeding subpopulations that share a gene pool and are isolated reproductively from other species.

Reproductive isolation can occur due to premating isolating mechanisms, in which reproduction is not attempted, or postmating isolating mechanisms that do not produce fertile offspring.

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Process of SpeciationWhenever reproductive isolation develops,

speciation has occurred.Allopatric speciation occurs when a

geographic barrier isolates two subpopulations from each other; when the barrier is removed, the two groups are no longer able to reproduce.

Sympatric speciation occurs when a single population suddenly becomes two reproductively isolated groups without geographic separation.

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Allopatric speciation

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Adaptive Radiation

The evolution of several species of finches on the Galapagos Islands is an example of adaptive radiation because each one has a different way of life.

Adaptive radiation occurs when a few individuals migrate to a new area, then natural selection promotes different feeding habits in different ecological habitats.

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The Galapagos finches

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The Pace of Speciation

Two hypotheses concern the pace of speciation:

Phyletic gradualism – suggests that change is slow and steady within a lineage before and after a divergence; few transitional links would exist.

Punctuated equilibrium – suggests that a period of no change is punctuated by period of rapid speciation.

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Phyletic gradualism versus punctuated equilibrium

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Classification

Classification involves the assignment of species to a hierarchy of categories: species, genus, family, order, class, phylum, kingdom, and domain.

Each species has a binomial scientific name including the genus and species.

Humans are Homo sapiens.

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Five-Kingdom System

The five-kingdom system of classification is based on structural differences and also on modes of nutrition among the eukaryotes.

The five kingdoms include:

Monera (prokaryotes)

Eukaryotic kingdoms of Protista, Fungi, Plantae, and Animalia.

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Five-kingdom system of classification

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Three-Domain System

The three-domain system recognizes three domains: Bacteria, Archaea, and Eukarya.

This system of classification is based on biochemical differences that show there are three vastly different groups of organisms.

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Three-domain system of classification

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The three domains of life

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Chapter Summary

The fossil record and biogeography, as well as comparative anatomy, development, and biochemistry all give evidence for evolution.

All organisms have certain biochemicals in common, and chemical similarities indicate the degree of relatedness.

The fossil record shows that mass extinctions occurred several times.

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Chemical evolution likely resulted in the first cells.

Inorganic chemicals derived from the primitive atmosphere reacted to form simple organic molecules.

The RNA-first and protein-first hypotheses seek to explain how the first protocell arose.

Eventually, the DNA → RNA → protein self-replicating system evolved, as did the first true cell.

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Evolution is a process that involves changes in gene frequencies in a population according to Hardy-Weinberg equilibrium.

Equilibrium is maintained unless disrupted by mutations, genetic drift, gene flow, nonrandom mating, or natural selection.

Speciation requires geographic isolation followed by reproductive isolation.

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There are two hypotheses regarding the pace of evolution – phyletic gradualism and punctuated equilibrium.

Classification involves the assignment of species to a hierarchy of categories: species, genus, family, order, class, phylum, kingdom, and domain.

The three-domain system recognizes three domains: Bacteria, Archaea, and Eukarya.