2. Defining Species: ReviewDefining Species: Review
• Evolution involves populations, not
Individuals
• Species is a population of organisms
whose members can interbreed under
natural circumstances and reproduce
fertile (viable) offspring
3. How Do New Species Arise?How Do New Species Arise?
• Two fundamental processes give rise to new species:
• Cladogenesis: The splitting off of one species into two
clades, usually because of geographical isolation, but
also because of reproductive isolation.
• Two kinds of species develop by cladogenesis:
• Sympatric Species: Those whose speciation is the
product of geographical isolation
• Allopatric Species: Those whose speciation is the
product of reproductive isolation of population in the same
region.
• Anagenesis: The replacement of an ancestral species by
a daughter species over time; the ancestral species
become extinct.
4. Speciation: Evolution ThroughSpeciation: Evolution Through
CladogenesisCladogenesis
• Cladogenesis:
• Time I: Genes flow freely in region
• Time II: Barrier separates two
populations
• Time III: Mutations change
genotype and phenotype of 2
populations
• Time IV: Two populations cannot
interbreed even with removal of
barrier
• Definition: Branching of one
species into two
• From clade (“branch”) or group
with common evolutionary ancestry.
5. Speciation: Allopatric SpeciesSpeciation: Allopatric Species
• Allopatric speciation
occurs when two populations
are separated by a
geographical barrier (river,
mountain range)
• In this example, three
species of fish have evolved
in separate zones
6. Speciation: Sympatric SpeciesSpeciation: Sympatric Species
• Sympatric species are those
that are separated by a
reproductively isolation
mechanism
• Speciation occurs among
three populations of fish even
though the different species
occupy the same region
• There are several ways for
subspecies to become
reproductively isolated
7. Modes of Reproductive Isolation InModes of Reproductive Isolation In
SympatrySympatry
• Ecological Isolation: Different populations are separated by
occupy a slightly different niche
• Seasonal Isolation: The breeding season of two closely related
populations do not match.
• Sexual Isolation: One or both sexes of a species initiate mating
behavior that does not act a stimulus to the opposite sex of a closely
related population
• Mechanical isolation: Populations do not mate because of an
incompatibility of the male and female sex organs of the individuals
(extreme example: wolves and Chihuahuas)
• Gamete Isolation: Incompatibility of sex cell with bodily
environment
• Hybrid Infertility or Sterility: Hybrids do not survive or reproduce
(mules)
8. Speciation: Role ofSpeciation: Role of
MicromutationMicromutation
• Micromutation: Mutations with
• extensive or important phenotypic
results
• Example: Axolotl (species of
salamander)
• This salamander starts life as
tadpole-like larvae, as do other
salamanders
• Axolotl, however, never grows up—
doesn’t sprout mature legs, keeps
its gills, remains aquatic existence.
• Injection of a hormone enables
maturity and to live on land, so that
one mutation can and does create
major change
9. Adaptive Radiation DefinedAdaptive Radiation Defined
• Definition: Evolution and spreading out of
related species into new niches
• Niche: An environment in which an organism is
found and its adaptive response to that
environment
• Generalized Adaptive Radiation: The
adaptation of a species to a wide range of
niches. Homo sapiens is an example.
• Specialized Adaptive Radiation: The
adaptation of a species to a narrow range of
niches.
10. Adaptive Radiation: FavorableAdaptive Radiation: Favorable
ConditionsConditions
• Absence of similar and therefore
competing species
• Occurrence of extensive extinction,
thereby emptying an environment of
competitors
• Adaptive generalization of new group of
related species which enable it to occupy
several niches and displace species
already there.
11. Adaptive Radiation: Absence ofAdaptive Radiation: Absence of
Competing SpeciesCompeting Species
• Example: Darwin’s finches on Galápagos
Islands who were blown there by winds from
mainland Ecuador
• Niches opened up for 13 varieties with different
bills, including those that feed on cactus or eat
specific insects in trees
• Others use twig or cactus spine to probe for
insects
• A vampire finch sucks blood from larger birds
12. Finches and Their AdaptiveFinches and Their Adaptive
RadiationRadiation
• Ground finches (Geospiza) who are seed and cactus eaters;
• Tree finches (Camarhynchus), who are insect and bud eaters
• Warbler finches (Certhidea) who vary by color.
13. Adaptive Radiation: SpecializationAdaptive Radiation: Specialization
• Definition: Adaptation of a species to a narrow range of
environmental niches
• Example: Again, some species of Darwin’s finches on
Galápagos Islands are examples.
• Medium ground finch was nearly wiped out in the 1977
drought
• Sudden change could eliminate this or others of these
genera and species of finches
• Example: prosimians adapt on in habitats afforded by
Madagascar and are close to extinction.
14. Adaptive Radiation: GeneralizationAdaptive Radiation: Generalization
• Definition: Adaptation of a species to a wide range of
environmental niches
• Examples:
• Mammals spread after the disappearance of dinosaurs 65
m.y.a. and occupied innumerable niches, from grassland
(ungulates) to trees (bats)
• Monkeys with a mixed diet occupied diverse arboreal
(tree) habitats; they displaced the prosimians
• Humans: from frozen north to tropical rainforest or desert
—thanks to culture—are the most generalized primate
15. Types of Evolution: Darwinian orTypes of Evolution: Darwinian or
Phyletic GradualismPhyletic Gradualism
• Definition:
• Slow, step-by-step changes over time
• Intermediate forms assume “missing links”
• Darwin postulated this model
• Examples: From monkeys to apes; apes
to hominins (e.g. Lucy); and from early
hominins to modern Homo sapiens
16. Problems with GradualismProblems with Gradualism
• Fossil record does not reveal fine gradations from one lifeform
to a descendant life form: no “missing links.”
• Bipedalism occurred quickly as the fragmentary fossil record
shows.
• Reproductive advantage: do slight changes bestow this
advantage?
• Continuum question: at which point does a population become
two species?
• Sometimes, change can take place rapidly, either through
oscillating selection or punctuated equilibrium
17. Types of Evolution: OscillatingTypes of Evolution: Oscillating
SelectionSelection
• Definition: Adaptive variation around a norm rather than
direction in response to environmental variation
• Example: Medium and small ground finch lacked a bill
strong enough to crack tough seeds
• Occurrence of drought selected plants whose seeds had a
tough exterior
• Survival of large, longer-billed finches
• Smaller, shorter-billed finches returned after the climate
returned to normal,
• Shifting bill size and lengths reflected the oscillation of the
environmental conditions.
18. Types of Evolution: PunctuatedTypes of Evolution: Punctuated
EquilibriumEquilibrium
• Definition: Species tend to
remain stable over time, then,
evolutionary changes occur
suddenly (in terms of centuries or
millennia)
• Causation: Populations may
become fragmented and isolated,
and from there new forms arise
• Small, new populations may
invade a region, and through the
founder effect and better
adaptation, create and spread a
new species
• Example: Archaeopteryx (ancient
bird), a dinosaur with feathers:
suddenly appears and may have
created a new class known as
Aves (birds)
19. Phyletic Gradualism andPhyletic Gradualism and
Punctuated EquilibriumPunctuated Equilibrium
• A summary of gradualism and punctuated equilibrium
20. Pseudoscience and CreationismPseudoscience and Creationism
• Pseudoscience consists of scientifically
testable ideas in form that are taken on
faith even after they are proven as false
• (Scientific) Creationism is the belief in a
literal biblical interpretation of the creation
of earth in six days 6,000 to 10,000 years
ago
• The claim is testable, has been tested,
and has been demonstrated to be false.
21. Counterevidence to CreationismCounterevidence to Creationism
• Existence of strata, such as the Grand Canyon,
accumulated over 2 billion years falsifies the claim that the
earth is only a few thousand years old
• Presence of extinct lifeforms, from fossil fish to dinosaurs,
demonstrate that other forms existed at one time but are
now extinct
• Presence of ancient hominins establish extinct humanlike
creatures that look like us but are not us.
• Both kinds of evidence are abundant
22. ConclusionConclusion
• Species is unit of evolution
• Evolutionary change is more random than
progressive
• Speciation is the basic process of evolutionary
change
• Changes may be gradual or rapid
• Scientific rule: follow the evidence
• Evidence for evolution is overwhelming in the
form of geological strata and fossil lifeforms
