Transcript The Origin of Species

Evolution and Systematics
Chapter 18
Diversity of Life
• Relevant fields of study
– Taxonomy
• Process of sorting and naming life forms
– Evolution
• Process by which living species change and new species
come into being
– Systematics
• Effort to find how modern life forms are related
• Look for evolutionary steps that led from ancient to modern
forms of life  phylogeny (“origin of groups”)
What is a Species?
• A group of organisms that are more
closely related to one another than to
organisms of any other kind
– May look more like one another
– Interbreed more freely with one another than
with organisms outside the group
What is a Species?
• Characters
– Traits of organisms ranging from shapes and colors of
body parts to DNA
– Used to define most currently known species
• Phenetic species
– Species that are defined by combinations of traits
– Example: citrus trees
– Characterized partly on distinctions between their
fruits
What is a Species?
• Type specimen
– An organism placed in museum or botanical
garden when species is first named
– Used for comparison
– Does not always reflect all members of that
species
What is a Species?
• Mating test
– If organisms from two populations mate and
produce fertile offspring under natural
conditions, then the two populations belong to
same species
• Biological species
– Species defined by mating test
What is a Species?
• Problems associated with mating test
– Does not apply to organisms that lack sexual
reproduction
– Many plant species can interbreed with
closely related species and produce offspring
that are weakly fertile
Taxonomy
• Need formal system for assigning names
for scientific communication
• Hierarchy of levels within levels
• Begun by Carolus Linnaeus
– 1753 published book
– Named about 6,000 species of plants
– Assigned them to 1,000 groups called genera
• Genus  group of species that are similar enough
to be obviously related
Taxonomy
– Wrote short description of each species
– Gave every species an abbreviated two-word
name  binomial
– Every species has a binomial, or species
name
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First word is genus (always capitalized)
Second word is specific epithet (never capitalized)
Both words are written in italics
Example: Zea mays
Taxonomy
Classification of common garden nasturtium (Tropaeolum majus)
Linnaean Rank
Name
Ending
Domain
Eukarya
-a
Kingdom
Plantae
-
Phylum (Division) Magnoliophyta
-ophyta
Class
Magnoliopsida
-opsida
Subclass
Rosidae
-idae
Order
Brassicales
-ales
Family
Brassicaceae
-aceae
Genus
Tropaeolum
-
Species name
Tropaeolum majus
-
Specific epithet
majus
-
Taxonomy
• Extra levels may be needed to divide up multiple
species
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Examples
Superfamily  group of several families
Subfamily  smaller division of family
Subspecies, varieties (races, among animals), forms
 divisions below species
• Important in cultivated plants
• Cultivar  equivalent to variety
– Used to describe products of human selection within a species
Taxonomy
• Taxon (plural, taxa)
– Taxonomic group at any level
– Examples: species, kingdom
Taxonomy
• Original taxonomic plan
– Two kingdoms
• Plant
• Animal
• Examples of problems
with this scheme
– Some microscopic
organisms have both plantlike and animal-like
characteristics
– Fungi have more in
common with animals than
plants
Kingdom
Animal
Plant
Description
Move actively and
consume prey
Do not move or
consume prey
Taxonomy
• Early 20th century
biologists divided
plant kingdom into
four new kingdoms
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–
Monera
Fungi
Plantae
Protista
Kingdom
Examples
Monera
Bacteria
Plantae
Green plants
Fungi
Fungi
Protista
Catch-all kingdom
composed of all
organisms that did not fit
into other kingdoms
Animalia
Animals
Taxonomy
• Mid 20th century
• Electron microscope provided information
showing bacteria have simpler cell
structure than other organisms
– No envelope around DNA
• Prokaryotic
– Cells of plants, animals, fungi, protists
• Most of DNA enclosed in membranous envelope
(true nucleus)
• Eukaryotic
Taxonomy
• Carl Woese
– Found prokaryotes included two distinct
groups of organisms
– Probably evolved separately
– Evidence came from analysis of ribosomal
RNA called rDNA
Taxonomy
• Needed higher level above kingdom to accommodate
new system of classification
– Domain  contains one or more kingdoms
• Three domains
– Bacteria
– Archaea
– Eukarya
• “Kingdom” Protista
– Questionable as to where many members belong
– Many smaller groups do not fit into the three established
kingdoms within Eukarya
Taxonomy
• Two eukaryotic groups have been
proposed for kingdom status
– Alveolates
– Heterokonts
• Remains to be seen how domains
Bacteria and Archaea will be divided into
kingdoms
Taxonomy
Domain
Cell Type
Description
Eukarya
Eukaryotic
Membrane bounded organelles, linear
chromosomes
Archaea
Prokaryotic
Found in extreme environments, cell
structure and differ from members of
Domain Bacteria
Bacteria
Prokaryotic on earth, play major role as decomposers
Ordinary bacteria, found in every habitat
Evolution
• Fossils
– Relics of life such as bones and leaves
embedded in stone
• Observation of how older fossils differ from
more recent ones challenged view that
species did not change
– 300 million years ago, horsetails were treesized and exhibited secondary growth and
wood
– Modern horsetails are herbs
Evolution
• Charles Darwin and Alfred Wallace
– English naturalists
– Came up with idea that hereditary
characteristics of species could change, or
evolve, over many generations
– Darwin’s ideas took shape during trip around
world
• Stop at Galápagos Islands made strongest
impression
– Examined finches on island that differed in many ways
from those he had seen in Ecuador
Evolution
• Darwin kept thoughts to himself until he
received letter from Wallace stating same
ideas
• Darwin
– 1859
– Published The Origin of Species
Evolution
• Darwin’s mechanism of evolution based
on following assertions
– Changes in heredity occur in the individuals of
a population, leading to varied progeny.
– Populations produce more progeny than the
environment can support. This leads to
competition among the progeny.
Evolution
– The progeny that are best adapted to the
*environment will reproduce most abundantly.
– Repeated over many generations, the
preceding three factors could lead to great
changes in heredity, and, hence, great
changes in the forms of life.
* Natural selection – Darwin’s term for effect of environment
Evolution
• Darwin’s ideas suggested
– No ideal body form for each species
– Forms can change as environment changes
Evolution
• In order for changes to be passed from
one generation to the next, changes must
occur in DNA
• Two main sources of change in DNA
– Mutation
– Recombination
Mutations
• Mutations
– Random changes in DNA
– Primary source of new hereditary information
– Base substitution
• Type of mutation in which wrong base is inserted in
DNA copying process
• Body heat keeps molecules in motion causing
collisions that sometimes cause this type of
mutation
Mutations
– Some mistakes are corrected
– Others are missed
– Errors occur at random locations
• When error occurs in DNA of reproductive cells,
altered gene can produce new hereditary
characteristics in progeny
Mutations
– Mutagens
• Agents that cause mutations
– Body heat
– High-energy radiations  dental X-rays, ultraviolet light from
sun, high-energy particles released from radioactive decay
– Chemicals
– Normal metabolism
– Most mutations have little or no effect on evolution
• Cause damage that leads to their elimination
– Occasionally a mutation helps organism, spreads
through population, contributes to evolution
• Example: appearance of antibiotic resistance in bacteria that
cause human disease
Mutations
• If mutation occurs at critical point in gene
for vital protein
– Cell makes copies of protein
– Leads to cell death
• If mutation damages proteins that control
cell division
– Cells multiply without limit
– Produce tumors and cancers (in animals)
Recombination
• Process that creates new combinations of
genes by joining parts of DNA molecules
from separate organisms
• Ways recombination occurs
– Transduction
• Viruses carry DNA of one host organism to another
Recombination
– Transformation
• Bacteria take up segments of DNA that are
released from decaying organisms
• Enzymes insert compatible portions of foreign DNA
into cell’s own DNA
– Conjugation
• Bacteria pass copy of their own DNA into another
bacterium of same species
• Enzymes exchange parts of host’s own DNA for
some of the transferred DNA
Recombination
– Sexual reproduction
• Occurs in cells of eukaryotes
• Most common source of recombination
• Meiosis
– Crossing over
» Happens at many random points along most
chromosomes
» No two gametes are likely to have same combination
of parental chromosome segments
Hybridization
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Mating between two different species
Process called hybridization
Progeny are called hybrids
Characteristics of hybrid plants
– Often cannot reproduce sexually
• Mismatch between chromosomes disrupts meiosis
– May be vigorous
– May multiply by asexual reproduction
Hybridization
• Introgression
– Process by which hybrid plants can transfer genes
between the two parent species
– Transfer requires back-crossing
• Biologists uncertain as to how often hybridization
occurs among plants on the whole
• Some fear hybridization and introgression may
allow genes from genetically engineered plants
to escape into wild populations
Endosymbiosis
• Cells of one species reside inside cells of
another species
• If endosymbiosis lasts for many
generations, DNA may pass from guest
species to the host species
– Adds to host’s nuclear DNA
– Leaves guest as a dependent organelle
– Examples: mitochondria and chloroplasts
Endosymbiosis
• Primary endosymbiosis
– Example: origin of mitochondria and
chloroplasts from bacteria
• Secondary endosymbiosis
– Example: eukaryotic predators gained
chloroplasts through endosymbiotic
partnership with eukaryotes that already had
chloroplasts
– Led to brown algae and certain other protists
Natural Selection
• Guides evolution
• Natural selective agents can be abiotic or
biotic
– Biotic factors
• Examples: Competing organisms, predators, prey
– Abiotic factors
• Examples: Climate, water supply, light
Directional Selection
• Adaptations – favorable hereditary traits
that enhance success in a particular
environment
• Leads to new adaptations
– Example: spines of cacti
• Spines
– Help plant collect rain water
– Dead at maturity
Directional Selection
Stabilizing Selection
• Maintains existing adaptations
• Selective forces act equally against
variations on both sides of the mean
– Example
• Each generation of adult cacti has same average
spine diameter as generation before
Stabilizing Selection
Diversifying Selection
• Natural selection that increases genetic
variation
• Can be caused by
– Disease agents
– Factors that favor two or most distinct types in
a population
• Example:
– Grass growing on mine tailings (rich in lead and zinc)
– Same species of grass growing on surrounding normal
soil
Diversifying Selection
– Plants that grow on mine tailings fail to thrive on normal
soil
– Plants that grow on normal soil fail to grow when
transplanted to mine tailings
– Presence of mine tailings beside normal soil permits lead
and zinc tolerant and intolerant plants to persist
simultaneously in population
Diversifying Selection
Types of Evolution
• Divergent evolution
– Increase in genetic differences among groups
• Convergent evolution
– Increase in similarity between two taxa
– Occurs when differing populations are
exposed to similar environments over many
generations
Types of Evolution
• Coevolution
– Interdependent evolution of two or more species
– Adaptations of interdependent species selected by
mutual interaction
– Can result in new species
– Example:
• Moth-pollinated plants produce nectar at base of long,
slender tubes
• Ideal for long tongues of moths but beyond reach of other
pollinators
Types of Evolution
• Pollen transfer more efficient because pollinator
visits just one plant species
• Pollinators get private food supply
• Mutual benefit suggests that moth pollination
favored evolution of long spurs in the flowers, as
well as long tongues in the moths
Population Genetics
• By 20th century, genetics was advanced
enough to show molecular basis of
evolution
• Question raised concerning heredity and
evolution
– Why do different versions of the same gene
(called alleles) persist in a population, even
though one allele is more abundant or is
expressed more strongly from the other?
Population Genetics
• G.H. Hardy and G. Weinberg
– 1908
– Simultaneously published model to answer
questions about population evolution
• Conditions that should apply to an ideal population
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Mutations do not occur
Organisms do not migrate between populations
Reproduction is limited to random sexual mating
There is no natural selection
The population is very large
Population Genetics
• Analysis by Hardy and Weinberg showed
under those ideal conditions
– Two alleles for same gene remain indefinitely
in population at fixed ratio, even if one allele is
dominant over the other
• Called Hardy-Weinberg equilibrium
• Became basis for new discipline known as
population genetics
– Integrates genetics and evolution
Population Genetics
Hardy-Weinberg
Equilibrium
Factors Leading to Evolution (Disequilibrium)
Mutations do not
occur.
Mutations convert one allele to another, and therefore
alter the ratio of alleles, unless forward and reverse
mutations exactly balance.
Organisms do not
migrate between
populations.
If many individuals enter or leave the population, the
allele ratio will change unless the migrating individuals
have alleles in exactly the same ratio as the overall
population.
Reproduction is
limited to random
sexual mating.
If mating is not random, some allele combinations may
be reproduced disproportionately often.
There is no natural
selection.
Natural selection favors the reproduction of individuals
with a certain allele combination over others.
The population is
very large.
If the population is very small, chance can determine
which individuals reproduce.
Population Genetics
• Population genetics
– Tool to predict changes and explore causes of
evolution
• Effects of chance on small populations
– Best-adapted individuals do not always leave
the most offspring
– Random accidents (fire, epidemic) in small
population may accidentally eliminate all
individuals that have best allele
Population Genetics
• Genetic drift
– Random change in allele ratio
• Founder effect
– Occurs when a few individuals from a large
population establish a small, isolated population
– Founders may have combination of traits that are
uncommon in old population
– May start new population on new path of evolution
– Often seen in studies of oceanic islands
• Island plants are related to mainland species, but traits differ
in many ways
Speciation
• Process which splits one species into two
• Involves the following processes
– Reproductive isolation and directional
selection
• Block to gene exchange
– Geographic isolation
• Geographical barriers prevent populations from
meeting to exchange genes
Speciation
– Polyploidy
• Possession of more than two chromosome sets
per cell
• Important source of new species in plants
• New polyploid plant is reproductively isolated
because it cannot exchange genes with its diploid
relatives
– Hybridization
• another source of reproductive isolation that can
lead to speciation
Speciation
• New hybrids often sterile
• Fertility can be restored if cell at tip of hybrid plant
becomes polyploid and initiates polyploid shoot
that forms gametes
Macroevolution
Microevolution
Consists of changes large enough to
represent the emergence of a new life
form
Consists of changes too small to
alter the fundamental nature of the
species
More difficult to observe
May
•Be sum of many microevolutionary
changes over long periods
•Involve larger abrupt changes, such
as chromosome rearrangements
Majority of modern biologists believe
macroevolution generated all modern
forms of life from microscopic forms
that first populated Earth some 3.8
billion years ago.
Rapid, easy to observe, easy to
produce artificially in the laboratory
Phylogenetic Systematics
• Recent developments making
phylogenetic systematics active field
– Cladistics
– Invention of fast, inexpensive computers to
make it practical to analyze large amounts of
data
– Invention of quick ways to read information
stored in DNA
Phylogenetic Systematics
• Phylogenetic tree
– Diagram showing evolutionary relationships
– Tips of branches
• Most recent products of evolution along each
branch
– Each branch point
• Act of speciation (where one species divides into
two)
Phylogenetic Systematics
• Some reasons for studying systematics
– Practical rewards for knowing how evolution
led to present-day species
• Search for new medicines
– Slow growing plant produces compound that cures colon
cancer
– Look for faster growing relatives of plant for alternative
sources of compound
• Ways to stop parasites that attack food plants
– Experiment with relatives of parasite that can be grown
without a host
Cladistics
• Cladistics
– Klados – “tree branch”
– Set of quantitative methods and concepts for
exploring evolutionary relationships among
taxa
– Compares modern species to determine most
probably point in evolution where each
species branched off from evolving group
Cladistics
– Clade
• Branch in tree of life
• Consists of an originating taxon and all its
descendant taxa
– Cladogram
• Phylogenetic tree produced by cladistics
• Rarely include more than a small sampling of
species that evolved from the ancestor
• Only species that contributed data to study are
listed
Cladistics
– Node
• Branch point where ancestral species split to
produce two new species
• Ancestor itself ceased to exist
• Oldest node called root of cladogram
Cladistics
• Types of cladograms
– Rooted
• Identify node in cladogram that occurred first
• Shows direction of evolution throughout clade
• Several different ways to draw cladogram to show
branching
• Reveals sequence in which important character
states evolved
Cladistics
– Unrooted
• Do not show which node is closest to the root
• Leave direction of evolution between each pair of
nodes unspecified
• Number of possible unrooted cladograms depends
only on the number of species
Cladistics
– Alternative cladograms
• Equally valid as long as they agree on number of
nodes that separate any two taxa
• Differences in orientation of branches unimportant
• Differ in how many steps of evolution stand
between each pair of species
Cladistics
• Cladistics compares species with respect to
various characters
– To be useful character must occur in all species being
considered
• Details called character state
– Morphological characters
• Related to body form
– Molecular characters
• Chemical traits
– Examples
» Structure of segment of DNA
» Ability to make a particular kind of molecule
Cladistics
• Homologous traits
– Alternative states of the same character
– Arose from the same ancestral trait
– Example
• Wings of bird and forelegs of horse
Cladistics
• Analogous traits
– Have similar form or function but evolved from
different structures
– Not alternate states of same character
– States of different characters
– Example
• Wings of insects and wings of birds
Cladistics
• Character matrix
– Prepared table that compares characters
among species
• Taxa listed along left margin
• Characters listed across top
• Boxes show state of each character for each
species
Cladistics
• Principle of Parsimony
– Postulates that the cladogram requiring the
fewest evolutionary events is most likely to be
correct
– Cladogram described as parsimonious
– Good hypothesis but can never be sure it is
correct
Cladistics
• Consensus tree or consensus cladogram
– Includes all the points of agreements
– Leaves points of disagreement unresolved as
nodes from which more than two branches
depart
Cladistics
• Finding root of cladogram
– Include data on additional taxa called
outgroups along with character data on the
ingroup (set of taxa that is target of the study)
Cladistics
• Ancestral and derived character states
– Derived character state
• Character that evolved later
– Use of terms ancestral and derived requires
care
• Judgment depends on point of view
Cladistics
• Cladistics reveals convergent evolution
– Similar character states sometimes arise
independently in two groups of organisms
– Cladogram can reveal which characters arose
through convergent evolution
Cladistics
• All formally named taxa should be
monophyletic
– True clade includes ancestor and all of its
descendants and nothing else
• Each currently accepted domain and
kingdom of life is believed to be
monophyletic
– Many traditional taxa at lower levels are still
not monophyletic