THE ORIGIN OF SPECIES
Microevolution explains
evolutionary changes within a
population.
Macroevolution considers the
origin of new taxonomic groups
SPECIATION
• The origin of a new species
• Anagenesis (phyletic evolution)- describes
the transformation of one species into a new
species
• Cladogenesis (branching evolution)- new
species arise from parent species that
continues to exist
An evolutionary lineage,
illustrating cladogenesis
(lineage splitting at nodes
A, B, C, D & E), anagensis
(evolutionary change
between these points), and
extinction (termination of
secondary lineages arising
from nodes A, C, & D). The
hatched and shaded
portions represent a
lineage that splits at node
B by cladogenesis and
diverges therafter by
anagenesis as two
separate lineages. (after
Minkoff 1983)
SPECIES
• Biological species concept was
developed by Ernst Mayr in 1942.
• It states that a species consist of
individuals that can interbreed in nature
and produce viable, fertile offspring.
Ecological species concept
• Defines species on their ecological niche,
the role they play and the resources they
use in the specific environments in which
they live.
Pluralistic species concept
• Includes reproductive barriers and niche
Morphological species concept
• Based on physical characteristics
Genealogical species concept
• Based on evolutionary lineages
Please note:
• Biological species concept does
not apply to asexual species
• Extinct species cannot be
grouped on the basis of
interbreeding
MODES OF SPECIATION
• SPECIATION REQUIRES THE
INTERRUPTION OF GENE FLOW
BETWEEN POPULATIONS
Reproductive barrier
• A Mechanism that prevents two species
from producing viable, fertile offspring;
thereby preserving the genetic integrity of a
biological species
There are two types of
reproductive barriersPrezygotic and Postzygotic
• Prezygotic barriers function before the
zygote is formed
• Postzygotic barriers prevent zygote from
developing into a fertile offspring
Prezygotic barriers include:
• Habitat isolation• Temporal isolation- breed at different times
• Behavioral isolation- courtship rituals and
behavioral signals are species specific
• Mechanical isolation- anatomical
incompatibility
• Gametic isolation gametes fail to fuse
Lions and tigers are a good example
of habitat isolation. They once
covered the same territory, however
they became isolated as lions
remained on the open savanna and
tigers kept to the jungles. Eventually,
they became distinct enough from one
another to be unable to produce fertile
offspring.
Rana aurora - breeds January - March
Rana boylii - breeds late March - May
e.g. - Rana aurora (Red-legged frog) breeds in
fast-moving, ephemeral streams, whereas its
relative Rana catesbiana (Bullfrog) breeds in
permanent ponds. (The metamorphosis times of
the tadpoles are correspondingly different.)
Postzygotic barriers
• Reduced hybrid viability- zygote fails to
survive
• Reduced hybrid fertility- hybrid individual
is sterile
• Hybrid breakdown- hybrids are viable and
fertile, but their offspring are feeble or
sterile
Allopatric speciation
• Two populations are geographically
separated
Conditions for Allopatric
speciation:
• Isolation of populations may occur due to a
geographic barrier
• Or colonization of new areas
• Or a “Ring Species” may develop- meaning
members of a population spread around a
geographic barrier and may evolve enough
that they can no longer interbreed with the
original pop.
• Or Adaptive radiation my occur on island
chains when small founding populations
evolve in isolation and under different
environmental conditions
• Adaptive Radiation- evolution of numerous,
variously adapted species from a common
ancestor.
Sympatric speciation
• Biological barriers prevent gene flow
between overlapping populations
Conditions for Sympatric
speciation
• Sympatric speciation may occur due to
polyploidy
• An Autopolyploid has more than two sets of
chromosomes that have all come for the
same species (tetraploids 4n)
• Tetraploids can mate with themselves or
other tetraploids but no longer with diploids
from the parent population
• An Allopolyploid is an interspecific hybrid
Auto- And Allopolyploidy of Cultured Plants
basic
number (x)
species
number of
chromosomes
(2n)
AUTOPOLYPLOIDY
potato (Solanum
tuberosum)
coffee (Coffea
arabica)
banana (Musa
sapientum)
alfalfa (Medicago
sativa)
peanut (Arachis
hypogaea)
sweet potato
(Ipomoea batata)
12
48
11
22, 44, 66, 88
11
22, 33
8
32
10
40
15
90
ALLOPOLYPLOIDY
tobacco (Nicotiana
tabacum)
cotton (Gossypium
hirsutum)
wheat (Triticum
aestivum)
oats (Avena sativa)
sugar-cane
(Saccharum
officinarum)
plum (Prunus
domesticus)
strawberry
(Fragaria
grandiflora)
apple
(Malus
sylvestris)
pear (Pyrus
communis)
12
48
13
52
7
42
7
42
10
80
8
16, 24, 32, 48
7
56
17
34, 51
17
34, 51
• Polyploidy has been very important in the
evolution of new plants. Many of our
agricultural plants are polyploids and plant
geneticists now hybridize plants and induce
meitoic and mitotic errors to create new
species
Sympatric speciation in animals
• May evolve from different resource usage
• Non random mating (sexual selection)
Punctuated Equilibrium
• Long periods of stasis are punctuated by
episodes of relatively rapid speciation and
change. ( A few thousand years)
Gradualism
• Species continuously evolve over long
periods of time
EXAPTATION
• STRUCTURES THAT HAD EVOLVED
AND FUNCTIONED IN ONE SETTING
AND WERE THEN CO-OPTED FOR A
NEW FUNCTION
Now what is this exaptation stuff? Exaptation is the evolution of new
adaptations from adaptations that evolved in a different context. For
example, leaves which evolved mainly as organs that carry out
photosynthesis, become thorns in cacti and become adaptations that
protect the cacti from grazing animals.
Trap jaw ants
Evo-devo
• Combination of the fields of evolution and
developmental biology
• Explores how slight changes in
developmental genes can result in major
morphological differences between species
Allometric growth
• The differing rates of growth of various
parts of the body lead to the final shape of
the organism. A small genetic change that
affects allometric growth can produce a
very different proportioned adult form
This study examines differences in fighting strategies between small and large male crayfish, Orconectes rusticus. Due to allometric growth of claws, fighting
weapons are of disproportionate size in large crayfish compared to those in smaller individuals. Presumably, such differences in the prominence of claws are
reflected in differences in the likelihood of injuries, and we thus explored fighting in size-matched pairs of small or large crayfish and assessed associated
strategies in situations of conflict. Although fighting reached the highest intensities in a similar proportion of instances in small and large pairs, differences in
fighting strategies were evident. Small crayfish escalated more rapidly, fights were settled more quickly, and were resolved overall at lower intensities. This may
be explained by lower risks of injury compared to encounters among larger males due to proportionally smaller claws. Larger males thus appear to spend
considerably more time in assessing their opponent's fighting ability before each escalation event.
•
Heterochrony
• An evolutionary change in the rate or timing
of development
Several heterochronies have been described in humans,
relative to the chimpanzee. Chimpanzee brain and
head growth in the fetus starts at about the same
developmental stage and have a growth rate similar to
humans. However chimp brain and head growth stops
soon after birth. Humans continue their brain and
head growth several years after birth. This particular
type of heterochrony is called neoteny and involves a
delay in the offset of a developmental process in later
stages of development. Human are known to have
about 30 different neotonies in comparison to the
chimp.
Paedomorphosis
• The retention in the adult of juvenile traits
of the ancestral organism. Can occur when
genetic changes speed up the development
of reproductive organs relative to the
development of body form.
The Mexican axolotl opposite is a
famous example of paedomorphosis,
retaining in maturity the feathery gills
that related species lose in infancy.
Find the meaning of these roots
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Allo
Ana
Auto
Clado
hetero
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Macro
Paedo
Post
sym