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CHAPTER 27: EVOLUTION OF LIFE
LECTURE OUTLINE
27.1 Origin of Life
The common ancestor for all living things was the first cell or cells. The very first living thing
had to have come from nonliving chemicals.
Evolution of Small Organic Molecules
Most chemical reactions take place in water, and the first protocell undoubtedly arose in
the ocean. The Miller-Urey experiment supports the hypothesis that small organic
molecules were formed at the ocean’s surface.
Macromolecules
Once formed, the first small organic molecules gave rise to still larger molecules and then
macromolecules. The RNA-first hypothesis, protein-first hypothesis, and Cairns-Smith
hypothesis help explain this stage of life.
The Protocell
After macromolecules formed, something akin to a modern plasma membrane was
needed to separate them from the environment. Thus, before the first true cell arose,
there would likely have been a protocell.
The Heterotroph Hypothesis
It has been suggested that the protocell likely was a heterotroph.
The True Cell
A true cell is a membrane-bounded structure that can carry on protein synthesis to
produce the enzymes that allow DNA to replication. The three main hypotheses about
the origin of life have varying explanations of how this cell arose.
27.2 Evidence of Evolution
Evolution is all the changes that have occurred in living things since the beginning of life due to
differential reproductive success. Evolution is defined as “common descent.”
Fossil Evidence
Fossils are the remains and traces of past life or any other direct evidence of past life.
Geological Timescale
As a result of studying strata, scientists have divided Earth’s history into eras,
and then periods and epochs. The absolute dating method relies on radioactive
dating techniques to assign an actual date to a fossil.
Mass Extinctions
Extinction is the death of every member of a species. During mass extinctions, a
large percentage of species become extinct within a relatively short period of
time. So far, there have been five major mass extinctions.
Biogeographical Evidence
Biogeography is the study of the distribution of species throughout the world.
Anatomical Evidence
The fact that anatomical similarities exist among organisms provides further support for
evolution via descent with modification.
Biochemical Evidence
When the degree of similarity in DNA nucleotide sequences or the degree of similarity in
amino acid sequences of proteins is examined, the more similar the sequences are,
generally the more closely related the organisms are.
27.3 The Process of Evolution
Evolution occurs at the population level. Change in gene frequencies within a population over
time is defined as microevolution.
Population Genetics
A population is all the members of a single species that occupy a particular area at the
same time and that interbreed and exchange genes. The total number of alleles at all the
gene loci in all the members make up a gene pool for the population.
The Hardy-Weinberg Principle
The Hardy-Weinberg principle states that allele frequencies in a gene pool will
remain at equilibrium after one generation of random mating in a large, sexually
reproducing population as long as five conditions are met: 1) no mutations, 2) no
genetic drift, 3) no gene flow, 4) random mating, and 5) no selection.
Five Agents of Evolutionary Change
Mutations
Mutations are genetic changes that provide the raw material for evolutionary
change.
Genetic Drift
Genetic drift refers to changes in the allele frequencies of gene pool due to
chance. The founder effect and the bottleneck effect are both examples of
genetic drift.
Gene Flow
Gene flow is the movement of alleles between populations, as occurs when
individuals migrate from one population to another.
Nonrandom Mating
Nonrandom mating occurs when individuals pair up, not by chance, but
according to their genotypes or phenotypes.
Natural Selection
Natural selection is the process by which some individuals produce more
offspring than others. Evolution by natural selection requires individual
variation, inheritance, overproduction, and differential reproductive success.
Directional Selection
Directional selection occurs when an extreme phenotype is favored and
the distribution curve shifts in that direction.
Disruptive Selection
In disruptive selection, two or more extreme phenotypes are favored over
any intermediate phenotype.
Maintenance of Variation
Sickle cell disease illustrates how genetic variation is sometimes maintained in a
population despite deleterious effects.
27.4 Speciation
A species is defined as a group of subpopulations that are capable of interbreeding and are
isolated reproductively from other species. Prezygotic isolating mechanisms are in place before
fertilization while postzygotic isolating mechanisms are in place after fertilization.
The Process of Speciation
Speciation has occurred when one species gives rise to two species. One type of
speciation, called allopatric speciation, usually occurs when populations become
separated by a geographic barrier and gene flow is no longer possible.
Adaptive Radiation
Darwin’s finches provide an example of adaptive radiation or the proliferation of a
species by adaptation to different ways of life.
The Pace of Speciation
Currently there are two hypotheses about the pace of speciation: phyletic gradualism and
punctuated equilibrium.
27.5 Classification
When species are classified, they are placed in a hierarchy of categories: species, genus, family,
order, class, phylum, and kingdom. This text uses one further classification category, called a
domain. Phylogeny is the evolutionary relationship among organisms.
Five-Kingdom System
The five-kingdom classification system consists of Plantae, Animalia, Fungi, Protista,
and Monera.
Three-Domain System
Molecular data based on the sequencing of rRNA suggest that there are three domains:
Bacteria, Archaea, and Eukarya.
Phylogenetics
Phylogeneticists arrange species and higher classification categories into clades. Clades
may be represented on a diagram called a cladogram. A clade contains a most recent
common ancestor (presumed but unidentified) and all its descendant species.