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Species concepts, Reproductive barriers, speciation, taxonomy
How Biological Diversity Evolves
What is a species?
Latin: meaning “kind” or “appearance”
Traditionally, morphological differences have been used to distinguish species
Are organisms truly divided into the discrete units we called species, or is
this classification an arbitrary attempt to impose order on the natural world?
The biological species concept
 Ernst Mayr (1842). Emphasizes reproductive isolation
 “A population or group of populations whose
members have the potential to breed with each other
in nature to produce viable, fertile offspring, but
who cannot produce viable, fertile offspring with
members of other species”
 Led an expedition into New Guinea
and found a great diversity of birds.
 Species are based on interfertility, not physical
similarity
 Based on his work with birds, he
derived the biological species concept
(BSC).
Diversity within a species. As diverse as we may be in appearance, all humans
belong to a single biological species (Homo sapiens)
Hybrid sterility: Interbreeding is
possible but offspring (mule) is sterile
horse
mule
=
+
donkey
Spatial isolation: They can interbreed (e.g. Tiger and
Lions can interbreed and have fertile offspring but they
do not cross naturally in nature)
Female tiger
liger
Male lion
+
=
Behavioral isolation
Eastern (Sturnella
magna, top) and western
meadowlark (Sturnella
neglecta, bottom):
behavioral reproductive
isolation
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Species concepts, Reproductive barriers, speciation, taxonomy
Limitations of the biological concept
What do we need in the first place to apply this concept?
 Sexual reproduction!. Not applicable for asexual organisms
 There is no way to corroborate reproductive isolation in fossils
 Many organisms about which little is known regarding their reproduction
Which definition is better?
Other definitions of species
The morphological species concept
Species are differentiated considering key
structural characteristics: body shape, size,
and other structural features
The paleontological species concept
 Focuses on morphologically
discrete species known only from
the fossil record
The ecological species concept
peacocks
 Views a species in terms of its
ecological niche (functional role
within the ecosystem)
female
The phylogenetic species concept
male
 Defines a species as a set of
organisms with a unique genetic
history
How do species become reproductively isolated?
Prezygotic and postzygotic barriers isolate the gene pools of biological species
Reproductive barriers between species
Impede mating between species or hinder the fertilization of ova if members of
different species attempt to mate
(1) Avoiding sexual encounters
Temporal
Spatial isolation
Behavioral
Monkey flowers:
distinct flowers attract
different pollinator s
The areas of the Western
and Eastern spotted
skunks overlap, but they
breed in different seasons
Garter snakes living in the
same area: One is mainly
aquatic and the other terrestrial
Mechanical or
Morphological
Blue-footed booby: males
perform a high-step dance
that calls the female’s
attention to the male’s
bright blue feet!
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Species concepts, Reproductive barriers, speciation, taxonomy
(2) fertilization
Molecular
recognition
mechanisms
Gametic Isolation
Sea urchins release
their sperm and eggs
into the surrounding
water, gametes of
different species are
unable to fuse
(3) viability
Genetic
incompatibility
Reduced Hybrid
Viability
(4) block fertility in the offspring
Problems in meiosis because of differences in
chromosome number or structure
Reduced Hybrid
Fertility
Hybrid
Breakdown
Strains of cultivated rice
have accumulated different
recessive alleles. Hybrids
between them are fertile
and vigorous, but plants in
the next generation carry
Salamanders: Some
subspecies living in the
same area may
hybridize, most hybrids
do not complete
development or grow
fragile individuals
Horse and donkey can breed,
but the offspring is infertile
(mule)
too many recessive genes
and are small and sterile
Similar species can still interbreed!
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Species concepts, Reproductive barriers, speciation, taxonomy
Mechanisms of speciation
 or how new species appear…
a) With geographic separation:
Allopatric Speciation
A. harrisi
A. leucurus
1) Interbreeding population
of one species
2) The population becomes divided by a physical barrier such as
water, mountains, desert. This can happen when some of the population
migrates, or when the geography changes catastrophically (e.g.
earthquakes, floods) or gradually (erosion, continental drift)
3) Genetic Diversity: If the two environments are different  the two
populations will experience different selection pressures and will
evolve separately. If the environments are similar  the populations
may change by means of some of the factors that affect the genetic pool
4) Reproductive isolation: if the barrier is removed and the two
populations meet again, they are now so different that they can no
longer interbreed
b) Without geographic separation: Sympatric Speciation
 Speciation takes place in geographically overlapping populations
 This involves chromosomal number changes, called…
Polyploidy
 Is the presence of extra sets of chromosomes in
cells due to accidents during cell division
 Has caused the evolution of some plant species
Two species of anemone flower and their chromosomes
b1) Autopolyploidy in plants
 More than two chromosome sets, all derived from the same species
Failure of cell division in a cell of a
growing diploid plant after
chromosome duplication gives rise to
a tetraploid branch or other tissue.
Gametes produced
by flowers on this
branch will be diploid.
Tetraploid (4 sets) offspring
may be viable and fertile - a
new biological species.
2n
2n = 6
4n = 12
b2) Allopolyploidy in plants
4n
 Multiple sets of chromosomes derived from different species
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Species concepts, Reproductive barriers, speciation, taxonomy
Geographic separation is needed, so the
populations can evolve in different ways
reproductively isolated one from the other
Polyploidy is the mechanism here
Extra sets of chromosomes due to abnormal cell
divisions
Examples of polyploid plants
Many grasses
Potatoes
Cotton
Sugarcane
Wheat
Many garden flowers
CLASSIFYING THE DIVERSITY OF LIFE
 Systematics focuses on:
 Classifying organisms
 Determining their evolutionary relationships
Taxonomy

Identification, naming, and classification of organisms
grouping them into ordered categories based on similarities
and differences
It is estimated that only 10% of the species that exist have been named!
Trout
Why may be necessary to
give a specific name to a
species?
Largemouth black
bass
Trout (South of US)
 Common names for the same species can vary
from town to town depending on the region!
 Fish: Starfish, Crayfish, Jellyfish, Silverfish
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Species concepts, Reproductive barriers, speciation, taxonomy
 Aristotle
was the first person who started naming organisms in a logical
classification system
The modern method of classification started with Carolus Linnaeus
(1758).

Binomial System
Each organism gets two Latin names, a genus and a species name.
A genus is made of many
closely related organisms
Species is made of organisms
that can interbreed.
Homo sapiens or Homo sapiens
The genus name
is capitalized
The species name is written
in lower case
Hierarchical Classification System
 Linnaeus placed organisms into different groups, from more general (more inclusive)
to more specific groups (least inclusive), the largest being kingdom.
Originally there were two
kingdoms, the animal and the
plant kingdoms
Domain
Bacteria
Kingdom
--
Archaea
Eukarya
•
•
•
•
-Plantae
Animalia
Fungi
Protista
More specific
Now: Domain is the first
category, above kingdom
There are further subdivisions
for Kingdom, which are:
Phylum, class, order, and family,
genus, species
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