Transcript The Origin of Species

The Origin of
Species
Chapter 24
Basic Patterns of Evolution
Anagenesis  one species accumulates
heritable changes, gradually the species
becomes a different species
 Cladogenesis  branching evolution, one
species to several w/ potential for
interbreeding

Biological Concept of Species

A population or group of populations
whose members have the potential to
interbreed with each other in nature to
produce viable, fertile offspring, but
who cannot produce viable, fertile
offspring with members of other
species
Species are based on interfertility, not
physical similarity
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The eastern and western meadowlarks may
have similar shapes and coloration, but
differences in song help prevent interbreeding
between the two species
Humans have considerable diversity,
but we all belong to the same species because
of our capacity to interbreed
How are Species kept separate?
Reproductive Barriers  prevents
populations belonging to different
species from interbreeding, even if
their ranges overlap
 Reproductive barriers
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 prezygotic
 postzygotic,
Pre zygotic Barriers

Impede mating between species or
hinder fertilization of ova if members
of different species attempt to mate
 Habitat
isolation
 Behavioral Isolation
 Temporal Isolation
 Mechanical Isolation
 Gametic isolation
Post zygotic Barriers

Prevents the hybrid zygote from
developing into a viable, fertile adult
 Reduced
hybrid viability
 Reduced hybrid fertility
 Hybrid breakdown
Habitat Isolation

Two organisms that use different
habitats even in the same geographic
area are unlikely to encounter each
other to even attempt mating
 Two
species of garter snakes, in the genus
Thamnophis, that occur in the same areas
but because one lives mainly in water and
the other is primarily terrestrial, they
rarely encounter each other.
Behavioral Isolation
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Many species use elaborate behaviors
unique to a species to attract mates
Visual Fireflies
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Bird plumage
Red Stickleback
Fiddler Crabs
Fruit Fly
Blue Footed Booby
OlfactoryScandinavian
mothPheromones
Auditory: Bird songs, frog calls
Temporal Isolation

Two species that
breed during
different times of
day, different
seasons, or
different years
cannot mix gametes
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Flowers
Skunk mating seasons
Frogs
Wood frog 44o
Leopard
Frogs
55o
Green
Frog
60o
Bull Frog
Above 60o
Mechanical Isolation
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Closely related species may attempt to
mate but fail because they are
anatomically incompatible and transfer
of sperm is not possible
Flower structure of certain flowering
plants prevents pollination by insects or
other animals
 With many insects the male and female
copulatory organs of closely related
species do not fit together, preventing
sperm transfer
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Gametic Isolation
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Gametes of two species do not form a
zygote because of incompatibilities
preventing fusion or other mechanisms
Sperm/egg recognition
 Reproductive Tract unfavorable
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Post Zygotic Barriers
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Reduced hybrid
viability
 Hybrids
may be
frailhybrids
between frogs in the
genus Rana, which do
not complete
development and
those that do are
frail.
Reduced Hybrid Fertility

Hybrids may be vigorous but may be infertile and
the hybrid cannot backbreed with either
parental species
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Horse (2n = 64)
Donkey (2n = 62)
Reduced Hybrid Fertility
 Mule (which is
sterile)
 Hence, donkeys and
horses are separate
species
Hybrid Breakdown
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Some first
generation
hybrids are viable
and fertile, but
when the mate
with one another,
offspring are
feeble
Species Concept
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Biological Concept“Species are groups of
actually or potentially interbreeding natural
populations, which are reproductively isolated
from other such groups.” Ernst Mayr.
Morphological species concept, the oldest and
still most practical, defines a species by a
unique set of structural features
Ecological species defines a species in terms
of its ecological niche, the set of
environmental resources that a species uses
and its role in a biological community
Biogeography of Species
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Two ways in which
speciation can
occur.
 Allopatric
speciation occurs
when a gene pool is
divided into two
 Sympatric
speciation occurs
without geographic
separation
Allopatric Speciation
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A geographic barrier isolates the
population and species are maintained
 Squirrels
on the N/S of the Grand Canyon
Allopatric speciation of squirrels in the
Grand Canyon
Sympatric Speciation
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Reproductive isolation without
geographic isolation
 Autopoliploidy
 Allopolyploidy
 Non-random
mating animals
Autopoliploidy
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Single parent doubles chromosome #
Results in TETRAPLOIDS
Allopolyploidy
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2 different species are involvedpolyploid
hybrid is formed
This is how modern wheat, oats, cotton,
potatoes were developed
More important in plant evolution
Non-random mating animals
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Mate choice is based
on coloration
Adaptive Radiation
Evolution of many diversely adapted species from a
common ancestor upon introduction of new
environmental opportunities
Tempo of Speciation
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Niles
Eldredge/Steven J.
Gould
Punctuated
Equilibrium
Gradualism
Macroevolution
Leads to new taxonomic groups
 Origin of mammals from reptiles
 Feathers and flight
 Increasing brain size of mammals
 Adaptive radiation of flowering plants
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Macroevolution
Evolutionary novelties are modified versions of older
structures
Exaptations: Preadaptations
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structures that evolve in one context but become co-opted
for another
Ex. Ancestral reptiles
Hollow bones in birds
‘Evolution is like modifying a machine while it is running!’
Evo-Devo
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The role evolution has in development
 Ex:
Shape of an organism depends on
relative growth rate of its parts
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Changes in Rate and Timing
 Allometric
Growth
 Heterochrony
 Paedomorphosis
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Changes in Spatial Patterns
 Homeotic
Genes
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Differential growth rate expanded
time of brain development
Figure 24.19 Allometric growth
Allometric growth: proportional change
Heterochrony
Longer time for foot growth
results in longer digits and less
webbing
Foot growth ends soonershorter digits and more
webbing
Paedomorphosis
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Juvenile traits extended to adult
Salamanders that retain gills don’t have to
leave the water
Homeotic Genes
Control the 3D placement of structures
 HOX gene organize the embryo in
space
 Invertebrates 1 set
 Vertebrates several sets more
complex growth
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