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The Outcomes of Evolution
Macroevolution
What Is a Species?
• Biological concept of species uses breeding behavior
as basis for demarcating species. Species are groups
of actually or potentially interbreeding natural
populations that are reproductively isolated from
other such groups (Ernst Mayer).
What Is a Species?
• Some complications prevent universal use of this concept.
– Bacteria don’t have breeding behavior.
– Cannot always do breeding experiments to determine
species boundaries (there are over 1 million known species
in the world).
– Some species of plants (e.g., trees such as poplar, oak,
maple) hybridize naturally, even though each is readily
recognizable as a distinct species based on visible
characteristics.
How Do New Species Arise?
• Speciation.
– Single species can diverge into two species, the “parent”
species continuing while another branches off from it.
– What causes branching? Cessation of gene flow through critical
lack of interbreeding.
• Evolution within a population = a change in allele
frequencies. If two populations continue to breed with one
another, allele frequency does not change, and the two
populations will evolve together, remaining a single species.
How Do New Species Arise?
• Speciation.
– What causes branching? Cessation of gene flow through
critical lack of interbreeding.
• If migration stops between the two populations, they
cannot share allele frequency changes. Alterations in
form and behavior that accompany allele changes may
pile up over time so that even if the two populations are
reunited, they may no longer freely interbreed and are no
longer considered the same species.
How Do New Species Arise?
• Speciation through cladogenesis.
– Occurs when populations become reproductively isolated
from each other and thereafter evolve independently of each
other.
– Allopatric speciation—population first separated by
geographical barriers (populations cut into two by glacier,
river that changes course, landslide) followed by intrinsic
reproductive isolation.
How Do New Species Arise?
• Speciation through cladogenesis.
– Sympatric speciation—populations come to be
reproductively isolated (through intrinsic barriers)
even though they share the same geographical
area.
How Do New Species Arise?
• Mechanisms of intrinsic reproductive isolation.
– Ecological—gene flow restricted because populations come
to occupy different habitats.
– Temporal—individuals from two populations mate at different
times.
– Behavioral—populations develop differences in courtship
rituals or response to them, therefore are unable to mate
successfully.
How Do New Species Arise?
• Mechanisms of intrinsic reproductive isolation.
– Mechanical—morphological or anatomical differences
preclude successful mating.
– Gametic—biochemical or cellular changes create
incompatibility between gametes, so sperm cannot fertilize
egg.
– Hybrid inviability—offspring are infertile or otherwise
deformed or unable to reproduce, mule.
Essay: New Species through Genetic Accidents:
Polyploidy
• Incredibly rapid mechanism of speciation critical in
evolution of over 100,000 species of plants and
possibly all vertebrates.
• Hybrids are usually sterile because of mismatched
number of chromosomes, so they usually cannot
correctly undergo meiosis to produce eggs and sperm.
Essay: New Species through Genetic Accidents:
Polyploidy
• If the newly formed hybrid zygote failed to separate
duplicated chromosomes in early mitosis, there would
be pairs of chromosomes to separate in meiosis,
meaning the hybrid would no longer be sterile.
• If this were to occur then the hybrid would be unable to
breed with its parent species, but could breed with
itself and thus instantly become reproductively isolated
as a new species.
How Do New Species Arise?
• Sympatric speciation.
– Involves one or more of the previously listed mechanisms;
role in contributing to species formation has been
controversial, but some evidence for it in recent years. Case
of hawthorn fruit fly in North America.
– Introduction of apples by colonists led to appearance of new
form, the apple fruit fly, from the original populations of
hawthorn fruit flies.
How Do New Species Arise?
• Sympatric speciation.
– Population of hawthorn flies that began to emerge
earlier fed on apples, which bloom earlier than
hawthorns.
– Flies feeding on apples became reproductively
isolated from other hawthorn fly populations
(temporal barrier).
How Do New Species Arise?
• Speciation through hybridization: Iris hybrids able to have
fertile offspring by mating with the parental species that gave
rise to them.
When Is Speciation Likely to Occur?
• Process of speciation accelerates when new niches become
available.
• Niche is an exploitable zone that can be occupied, or a role
that can be played, by an organism in a specific habitat.
• Specialist species evolve to commandeer, and otherwise
exploit, the new habitat zone that becomes available;
generalists occupy diverse niches (feed on variety of food, for
example; as horseshoe crab does), and therefore evolve
relatively slowly.
When Is Speciation Likely to Occur?
• New environments (as produced by volcanic eruption, island
formation) create many new niches and therefore stimulated
speciation; this explosion in species diversity is known as
adaptive radiation.
• Some biologists (Gould, Eldredge) have proposed that new
species evolve not in a slow, steady manner, but in abrupt
burst of speciation interrupting periods of stasis (punctuated
equilibrium); theory hotly debated.
When Is Speciation Likely to Occur?
• Could be that different species evolve at different
rates; may be that genetic changes accumulate
slowly, but the resulting change in traits manifests
abruptly.
The Categorization of Earth’s Living Things
• In the eighteenth century, Carolus Linnaeus developed a
system of binomial nomenclature—each species was given a
two-part name consisting of the genus and the specific epithet.
• Domesticated dog is Canis familiaris; Canis lupus is name
given to wolves. Canus is genus name shared by all dogs and
gray wolves, familiaris and lupus are specific epithets unique
to those different species, although by some reckonings dogs
and wolves are the same species, since they will interbreed.
The Categorization of Earth’s Living Things
• Taxonomy, the science of classification, organizes all
species into eight basic categories: species, genus,
family, order, class, phylum, kingdom, and domain. A
domain is the most inclusive category, species the
least; organisms in each category make up a taxon
(plural for taxa).
The Categorization of Earth’s Living Things
• Systematics is a system of classification that
attempts to organize species into the above
categories based on their evolutionary relatedness
(phylogeny). Species in the same genus would have
had the most recent common ancestor; those in the
same family would share a common ancestor further
back in evolutionary time.
Constructing Evolutionary Histories: Classical
Taxonomy and Cladistics
• Classical Systematics.
– Put species into various groupings based on similarities in
visible characteristics or (more recently) biochemistry; these
similarities are used to judge the relatedness of organisms,
and become the basis for the classification scheme.
– Shared ancestry is inferred by a commonality in physical
structures (homology
homology),
homology for example, similarity in bone
anatomy of forelimb in gorilla, bat, and whale.
Constructing Evolutionary Histories: Classical
Taxonomy and Cladistics
• Classical Systematics.
– Convergent evolution complicates this approach; analogy is
presence of similar structures in evolutionarily divergent
lines of descent, because of similar environmental pressures
resulting in selection of similar structures in unrelated
species (convergent evolution)—for example, analogy in limb
structure of horses and extinct (unrelated) litopterans, both
taxa evolving in similar grassland habitat.
Constructing Evolutionary Histories: Classical
Taxonomy and Cladistics
• Cladistics.
– A new tool for deriving phylogenetic relationships; goal is to
establish lines of descent among related organisms, with
diversification of taxa represented as branching events in an
“evolutionary tree.”
– Cladogram is a graphical representation of the evolutionary
tree—order of branching (diversification) events is depicted.
Constructing Evolutionary Histories: Classical
Taxonomy and Cladistics
• Cladistics.
– Common ancestor is defined based on characteristics
shared by all members in the group.
– Descendants are clustered into ever more selective
groupings based on derived traits shared by all members of
each grouping (clade); derived traits used to create a clade
are unique to that clade and clades descended from it.
Constructing Evolutionary Histories: Classical
Taxonomy and Cladistics
• Cladistics.
– The more derived characters any two organisms
share, the more recently they will have shared a
common ancestor, compared to other organisms.
Constructing Evolutionary Histories: Classical
Taxonomy and Cladistics
• Discrepancies in classification can result between
cladistic analysis and classical systematics.
Cladistics focuses only on evolutionary relatedness,
while classical systematics uses phylogenetic
evidence along with other information and may give
disproportional weighting to some traits relative to
others.