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What is a species?
Evolution theory must also explain
how species originate.
Darwin’s “Mystery of mysteries”
In the Galápagos Islands Darwin
discovered plants and animals
found nowhere else on Earth
Speciation, the origin of new
species, is at the focal point of
evolutionary theory
 Evolutionary theory must explain
how new species originate and
how populations evolve

Fig. 24-1
How did this flightless
bird come to live on
the isolated Galápagos
Islands?
Microevolution consists of
adaptations that evolve within a
population, confined to one gene
pool
 Macroevolution refers to
evolutionary change above the
species level

 Example
– the appearance of
feathers during the evolution of
birds from one group of dinosaurs

1. Anagenesis
 ana
= new, genos = race
 Accumulation of heritable
changes that gradually
transforms a species into a
species with different
characteristics

2. Cladogenesis
 klados
= branch, genos = race
 Branching evolution in which a
new species arises from a
population that buds from a
parent species

Species is a Latin word meaning
“kind” or “appearance”
 Biologists compare morphology,
physiology, biochemistry, and
DNA sequences when grouping
organisms
Fig. 24-2a
Similarity between different species
Fig. 24-2b
(b)
Diversity within
a species
Diversity
within
a species

How many species of African
Violets are here?
•
•
The biological species concept states
that a species is a group of populations
whose members have the potential to
interbreed in nature and produce
viable, fertile offspring; they do not
breed successfully with other
populations
Gene flow between populations holds
the phenotype of a population together
Fig. 24-3c
Does gene flow
occur between
widely separated
populations?
Grey-crowned babblers
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Reproductive isolation is the
existence of biological factors
(barriers) that impede two species
from producing viable, fertile
offspring
Hybrids are the offspring of crosses
between different species
Reproductive isolation can be
classified by whether factors act
before or after fertilization

Prezygotic barriers block
fertilization from occurring by:
 Impeding different species
from attempting to mate
 Preventing the successful
completion of mating
 Hindering fertilization if
mating is successful

Postzygotic barriers prevent
the hybrid zygote from
developing into a viable, fertile
adult:
 Reduced hybrid viability
 Reduced hybrid fertility
 Hybrid breakdown

Habitat isolation: Two species
encounter each other rarely, or not
at all, because they occupy
different habitats, even though not
isolated by physical barriers
Water-dwelling
Thamnophis
Terrestrial
Thamnophis


Populations live in different
habitats or ecological niches.
Ex – mountains vs lowlands.

Temporal isolation: Species that
breed at different times of the
day, different seasons, or different
years cannot mix their gametes
Eastern spotted skunk
Western spotted skunk
(Spilogale putorius)
(Spilogale gracilis)


Breeding
seasons or
time of day
different.
Ex – flowers
open in morning
or evening.

Behavioral isolation: Courtship
rituals and other behaviors unique
to a species are effective barriers
Courtship ritual of bluefooted boobies



Mating or
courtship
behaviors
different.
Different sexual
attractions
operating.
Ex – songs and
dances in birds.

Mechanical isolation: Morphological
differences can prevent successful
mating
Bradybaena with shells spiraling
in opposite directions

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Structural differences that prevent
gamete transfer.
Ex – anthers not positioned to put
pollen on a bee, but will put pollen on a
bird.

Gametic isolation: Sperm of one
species may not be able to fertilize
eggs of another species
Sea
urchins
Gametes fail to attract each
other and fuse.
 Ex – chemical markers on egg
and sperm fail to match.

Zygote fails to develop or
mature.
 Ex – when different species of
frogs or salamanders
hybridize.

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Reduced hybrid viability: Genes of the
different parent species may interact
and impair the hybrid’s development
Ensatina hybrid
Reduced hybrid fertility: Even
if hybrids are vigorous, they
may be sterile
 Hybrids are viable, but can't
reproduce sexually.
 Chromosome count often “odd”
so meiosis won’t work.
 Ex – donkeys and horses produce
mules

Fig. 24-4m
(i)
Donkey
Donkey
Fig. 24-4n
(j)
Horse
Fig. 24-4o
(k)
Mule (sterile hybrid)
‘Heaven
Scent’
an F1 hybrid
between 2
species, but
sterile.

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Offspring are fertile, but can't
compete as well as the “pure
breeds”.
Ex – many plant hybrids

Hybrid breakdown:
Some firstgeneration hybrids
are fertile, but
when they mate
with another
species or with
either parent
species, offspring
of the next
generation are
feeble or sterile
Hybrid cultivated rice
plants with
stunted offspring (center)

The biological species concept
cannot be applied to fossils or
asexual organisms (including all
prokaryotes)
Other species concepts emphasize the
unity within a species rather than the
separateness of different species
 The morphological species concept
defines a species by structural
features
 It applies to sexual and asexual
species but relies on subjective
criteria


Organisms with very similar
morphology or physical form.

Where does extensive
phenotype variation fit?
The ecological species concept views a
species in terms of its ecological niche
 It applies to sexual and asexual
species and emphasizes the role of
disruptive selection
 The phylogenetic species concept:
defines a species as the smallest group of
individuals on a phylogenetic tree
 It applies to sexual and asexual
species, but it can be difficult to
determine the degree of difference
required for separate species


How many species of African
Violets are here?


1950-60’s –20 species described
70,000 cultivars or human created hybrids

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
2007 – lumping occurred to 6
species and 10 subspecies
2008 – AVSA now recognizes 9
species and 8 subspecies
point – species definitions can be
fluid and change