Development, Evolution
and classification
AP Biology
From Single Cell to Multicellular
Organism
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Genetic analysis and DNA technology
have revolutionized the study of
development
Researchers use mutations to deduce
developmental pathways
They apply concepts and tools of
molecular genetics to the study of
developmental biology
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Researchers select model organisms
that are representative of a larger
group, suitable for the questions under
investigation, and easy to grow in the
lab
- Fruit flies, zebra fish, mice, C. elegans
Video: C. elegans Crawling
Embryonic development involves
cell division, cell differentiation,
and morphogenesis
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In embryonic development of most
organisms, a single-celled zygote gives rise
to cells of many different types, each with a
different structure and corresponding
function
Development involves three processes: cell
division, cell differentiation, and
morphogenesis (“creation of form”)
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Through a succession of mitotic cell
divisions, the zygote gives rise to a large
number of cells
In cell differentiation, cells become
specialized in structure and function
Morphogenesis encompasses the processes
that give shape to the organism and its
various parts
LE 21-4
Animal development
Cell
movement
Zygote
(fertilized egg)
Eight cells
Gut
Blastula
Gastrula
Adult animal
(cross section) (cross section)
(sea star)
Cell division
Morphogenesis
Observable cell differentiation
Seed
leaves
Plant development
Zygote
(fertilized egg)
Two cells
Shoot
apical
meristem
Root
apical
meristem
Embryo
inside seed
Plant
Different cell types result from
differential gene expression in cells
with the same DNA
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Differences between cells in a multicellular
organism come almost entirely from gene
expression, not differences in the cells’
genomes
These differences arise during development,
as regulatory mechanisms turn genes off
and on
Evidence for Genomic
Equivalence
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Many experiments support the
conclusion that nearly all cells of an
organism have genomic equivalence
(the same genes)
A key question that emerges is
whether genes are irreversibly
inactivated during differentiation
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Totipotency
One experimental approach for testing
genomic equivalence is to see whether a
differentiated cell can generate a whole
organism
A totipotent cell is one that can generate a
complete new organism
Cloning is using one or more somatic cells
from a multicellular organism to make a
genetically identical individual
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The Stem Cells of Animals
A stem cell is a relatively unspecialized cell that
can reproduce itself indefinitely and differentiate
into specialized cells of one or more types
Stem cells isolated from early embryos at the
blastocyst stage are called embryonic stem cells
The adult body also has stem cells, which replace
nonreproducing specialized cells
Embryonic stem cells are totipotent, able to
differentiate into all cell types
Adult stem cells are pluripotent, able to give rise
to multiple but not all cell types
LE 21-9
Embryonic stem cells
Totipotent
cells
Adult stem cells
Pluripotent
cells
Cultured
stem cells
Different
culture
conditions
Different
Liver cells
types of
differentiated
cells
Nerve cells
Blood cells
Transcriptional Regulation of Gene
Expression During Development
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Cell determination precedes differentiation and
involves expression of genes for tissue-specific
proteins
Tissue-specific proteins enable differentiated
cells to carry out their specific tasks
Cytoplasmic Determinants and CellCell Signals in Cell Differentiation
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Maternal substances that influence
early development are called
cytoplasmic determinants
These substances regulate expression
of genes that affect the cell’s
developmental fate
Animation: Cell Signaling
LE 21-11a
Unfertilized egg cell
Sperm
Molecules of another
cytoplasmic determinant
Molecules of a
Nucleus
cytoplasmic
Fertilization
determinant
Zygote
(fertilized egg)
Mitotic cell division
Two-celled
embryo
Cytoplasmic determinants in the egg
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The other important source of
developmental information is the
environment around the cell,
especially signals from nearby
embryonic cells
In the process called induction, signal
molecules from embryonic cells cause
transcriptional changes in nearby
target cells
LE 21-11b
Early embryo
(32 cells)
NUCLEUS
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
Induction by nearby cells
Pattern formation
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Pattern formation is the development of a
spatial organization of tissues and organs
It occurs continually in plants, but it is
mostly limited to embryos and juveniles in
animals
Positional information, the molecular cues
that control pattern formation, tells a cell its
location relative to the body axes and to
neighboring cells
The Life Cycle of
Drosophila
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Pattern formation has been extensively
studied in the fruit fly Drosophila
melanogaster
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After fertilization, positional information
specifies the body segments in Drosophila
Positional information triggers the formation
of each segment’s characteristic structures
Sequential gene expression produces
regional differences in the formation of the
segments
Axis Establishment
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Maternal effect genes encode for
cytoplasmic determinants that initially
establish the axes of the body of
Drosophila
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These maternal effect genes are also
called egg-polarity genes because they
control orientation of the egg and
consequently the fly
Segmentation Pattern
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Segmentation genes produce proteins
that direct formation of segments after
the embryo’s major body axes are
formed
Positional information is provided by
sequential activation of three sets of
segmentation genes: gap genes, pairrule genes, and segment-polarity
genes
Identity of Body Parts
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The anatomical identity of Drosophila
segments is set by master regulatory
genes called homeotic genes
Mutations to homeotic genes produce
flies with strange traits, such as legs
growing from the head in place of
antennae
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Cell signaling is involved in apoptosis,
programmed cell death
In vertebrates, apoptosis is part of
normal development of the nervous
system, operation of the immune
system, and morphogenesis of hands
and feet in humans and paws in other
mammals
LE 21-19
Interdigital tissue
1 mm
Widespread Conservation of
Developmental Genes Among
Animals
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Molecular analysis of the homeotic
genes in Drosophila has shown that
they all include a sequence called a
homeobox
An identical or very similar nucleotide
sequence has been discovered in the
homeotic genes of both vertebrates
and invertebrates
LE 21-23
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse
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Related genetic sequences have been found
in regulatory genes of yeasts, plants, and
even prokaryotes
In addition to developmental genes, many
other genes are highly conserved from
species to species
Sometimes small changes in regulatory
sequences of certain genes lead to major
changes in body form, as in crustaceans
and insects
Evolution and
classification
Gene Pools and Allele
Frequencies
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A population is a localized group of
individuals capable of interbreeding and
producing fertile offspring
The gene pool is the total aggregate of
genes in a population at any one time
The gene pool consists of all gene loci in all
individuals of the population
Individuals don’t evolve—
populations do
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Each gene exists in two or more forms
called alleles
Variation in a species results from one or
more of the following: mutations, crossing
over during meiosis 1, independent
assortment of alleles, fertilization, changes
in chromosome structure or number
Only mutation creates new alleles
Microevolution
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Change in relative allele frequency
over time; if allele frequency changes
evolution occurs
Causes of microevolution
– Genetic drift: change in a small gene pool
due to chance
– Bottleneck event: population size is
drastically reduced, leaving only the
alleles of the survivors in the gene pool
– Founder effect: the small group starting a
new colony contribute only their alleles to
the new population
– Gene flow: gain or loss of alleles through
immigration or emigration
– Non-random mating: organisms tend to
mate with neighbors although they are
capable of mating with any member of
their species anywhere on earth
Figure 23.4 Genetic drift
LE 23-8
Original
population
Bottlenecking
event
Surviving
population
Hardy-Weinberg
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Their formulas are used to establish allele
frequencies at genetic equilibrium (no
evolution is occurring)
The following conditions must all be fulfilled
–
–
–
–
–
The population is very, very large
There is no migration of individuals
No mutations
Mating is completely random
All members survive and reproduce
successfully
The formulas
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Allele frequency: fraction of that particular
allele in the population
The sum of all the allele frequencies = 1
p = frequency of the dominant allele
q = frequency of the recessive allele
p + q = 1; use for single alleles
To figure the frequency of each genotype
use: p2 + 2pq + q2 = 1; use for genotypes,
phenotypes or individuals
Figure 23.3a The Hardy-Weinberg theorem
Figure 23.3b The Hardy-Weinberg theorem
Sample problem
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In mice brown coat color, B, is
dominant to white, b. The frequency
of the dominant allele is .6 and the
frequency of the recessive allele is .4.
What is the probability of producing
each genotype of offspring?
Sample problem 2
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Given the following gene pool:
R r r r r
r r r r r
R R r r r
R R r r R
What is the value of p? Of q2? Of 2pq? Of p2?
Natural selection
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The major microevolutionary process
that results in differential survival and
reproduction
– There is variation among individuals
– More are born than can survive
– There is competition for resources
– Those individuals that are most fit for
their environment survive, reproduce and
pass on their alleles.
Types of selection
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Directional selection favors individuals at
one end of the phenotypic range
Disruptive selection favors individuals at
both extremes of the phenotypic range
Stabilizing selection favors intermediate
variants and acts against extreme
phenotypes
Frequency of
individuals
LE 23-12
Original
population
Evolved
population
Directional selection
Original population
Phenotypes (fur color)
Disruptive selection
Stabilizing selection
Sexual Selection
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Sexual selection is natural selection for
mating success
It can result in sexual dimorphism,
marked differences between the sexes
in secondary sexual characteristics
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Intersexual selection occurs when
individuals of one sex (usually
females) are choosy in selecting their
mates from individuals of the other
sex
Selection may depend on the
showiness of the male’s appearance
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Speciation, the origin of new species, is at
the focal point of evolutionary theory which
explains how new species originate and how
populations evolve
Microevolution consists of adaptations that
evolve within a population, confined to one
gene pool
Macroevolution refers to evolutionary
change above the species level
Limitations of the Biological
Species Concept
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The biological species concept does not
apply to
– Asexual organisms
– Fossils
– Organisms about which little is known
regarding their reproduction
Isolating mechanisms
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Prezygotic barriers: prevent mating or
fertilization
– Habitat isolation: live in the same place but
never meet
– Temporal isolation: reproduce at different times
– Behavioral isolation: mating rituals
– Mechanical isolation: parts have to fit to deliver
the gametes
– Gametic isolation: species specific proteins and
receptor sites
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Postzygotic barriers: prevent the
hybrid zygote from developing into
fertile adults
– Reduced hybrid viability-meet, mate, no
offspring produced
– Reduced hybrid fertility-meet, mate,
offspring are sterile
– Hybrid breakdown-meet, mate offspring
mate but their offspring die
Figure 24.3 Courtship ritual as a behavioral barrier between species
Allopatric speciation
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In allopatric speciation, a physical
barrier cuts off the gene flow between
2 or more populations
If conditions are different in areas of
the 2 populations, they can become
reproductively incompatible and
cannot interbreed, making them into 2
different species
Sympatric speciation
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In sympatric speciation species arise
from within the range of existing
species, in the absence of physical or
ecological barriers
Paripatric speciation occurs at borders
of populations
Figure 24.6 Two modes of speciation
LE 24-6
A. harrisi
A. leucurus
Figure 24.8 Has speciation occurred during geographic isolation?
Tempo of speciation
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Gradulism: tree diagrams have
branches at slight angles showing slow
steady change over time
Punctuated equilibrium: tree has
short, horizontal branches that show
rapid periods of change followed by
stable periods
Figure 24.17 Two models for the tempo of speciation
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Two basic patterns of evolutionary
change:
– Anagenesis (phyletic evolution)
transforms one species into another
– Cladogenesis (branching evolution) is the
splitting of a gene pool, giving rise to one
or more new species
Animation: Macroevolution
LE 24-2
Anagenesis
Cladogenesis
Adaptive radiation
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Burst of microevolutionary activity that
result in the formation of new species
in a wide range of habitats
A small group of founders start a new
colony; if there are available niches
and enough variation in the genes of
the population they can evolve into
many different species over time
(finches)
Figure 24.11 A model for adaptive radiation on island chains
Evidence for evolution
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Fossil record and biogeography
– Similar fossils in South America and Africa
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Comparative morphology
– Homologous structures
– Analogous structures
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Embryological development
– vertebrates
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Biochemical comparisons
– Proteins
– Nucleic acids
The 6 kingdoms
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Monera: now are 2 kingdoms:
eubacteria and archaebacteria
Protista
Fungi
Plantae
Animalia
Taxonomy
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Taxonomy is the ordered division of organisms into
categories based on characteristics used to assess
similarities and differences
In 1748, Carolus Linnaeus published a system of
taxonomy based on resemblances.
Two key features of his system remain useful
today: two-part names for species and hierarchical
classification
Binomial Nomenclature
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The two-part scientific name of a species is
called a binomial
The first part of the name is the genus
The second part, called the specific epithet,
is unique for each species within the genus
The first letter of the genus is capitalized,
and the entire species name is latinized
Both parts together name the species (not
the specific epithet alone)
Hierarchical Classification
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Linnaeus introduced a system for
grouping species in increasingly broad
categories
Domain, Kingdom, Phylum, Class,
Order, Family, Genus, Species
Animation: Classification Schemes
LE 25-8
Panthera
pardus
Species
Panthera
Genus
Felidae
Family
Carnivora
Order
Mammalia
Class
Chordata
Phylum
Animalia
Kingdom
Domain
Eukarya
Linking Classification and
Phylogeny
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Systematists depict evolutionary
relationships in branching phylogenetic
trees
Each branch point represents the
divergence of two species
“Deeper” branch points represent
progressively greater amounts of
divergence
Species
Mephitis
mephitis
(striped skunk)
Lutra lutra
(European
otter)
Genus
Panthera
Mephitis
Lutra
Felidae
Order
Panthera
pardus
(leopard)
Family
LE 25-9
Mustelidae
Carnivora
Canis
familiaris
(domestic dog)
Canis
lupus
(wolf)
Canis
Canidae
Figure 25.12 Cladistics and taxonomy
LE 22-15
Pharyngeal
pouches
Post-anal
tail
Chick embryo (LM)
Human embryo
LE 22-16
Species
Percent of Amino Acids That Are
Identical to the Amino Acids in a
Human Hemoglobin Polypeptide
Human
100%
Rhesus monkey
95%
87%
Mouse
69%
Chicken
54%
Frog
Lamprey
14%
LE 25-UN497
Leopard
Domestic cat
Common ancestor
Wolf
Leopard
Domestic cat
Common ancestor
Phylogenetic trees are made
based on shared characteristics
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A cladogram depicts patterns of shared
characteristics among taxa
A clade is a group of species that includes
an ancestral species and all its descendants
Cladistics studies resemblances among
clades
Cladistics
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Clades can be nested in larger clades, but
not all groupings or organisms qualify as
clades
A valid clade is monophyletic, signifying that
it consists of the ancestor species and all its
descendants
A paraphyletic grouping consists of an
ancestral species and some, but not all, of
the descendants
A polyphyletic grouping consists of various
species that lack a common ancestor
LE 25-10a
Grouping 1
Monophyletic
LE 25-10b
Grouping 2
Paraphyletic
LE 25-10c
Grouping 3
Polyphyletic
Shared Primitive and Shared
Derived Characteristics
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In cladistic analysis, clades are defined
by their evolutionary novelties
A shared primitive character is a
character that is shared beyond the
taxon we are trying to define
A shared derived character is an
evolutionary novelty unique to a
particular clade
Outgroups
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An outgroup is a species or group of species
that is closely related to the ingroup, the
various species being studied
We compare each ingroup species with the
outgroup to differentiate between shared
derived and shared primitive characteristics
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The outgroup and ingroup share
primitive characters that predate the
divergence of both groups from a
common ancestor
The focus is on characters derived at
various branch points in the evolution
of a clade
LE 25-11
Leopard
Turtle
Salamander
Tuna
Lamprey
Lancelet
(outgroup)
TAXA
CHARACTERS
Hair
Amniotic (shelled) egg
Four walking legs
Hinged jaws
Vertebral column
(backbone)
Character table
Turtle
Leopard
Hair
Salamander
Amniotic egg
Tuna
Four walking legs
Lamprey
Hinged jaws
Lancelet (outgroup)
Vertebral column
Cladogram
Trait
Shark
Frog
Kangaroo human
Vertebrae X
X
X
X
2 pairs of
limbs
Mammary
glands
Placenta
X
X
X
X
X
X