Transcript You Light Up My Life - Las Positas College

Speciation
Chapter 17
Morphology & Species
• Morphological traits may not be useful in
distinguishing species
– Members of same species may appear
different because of environmental
conditions
– Morphology can vary with age and sex
– Different species can appear identical
Biological Species Concept
“Species are groups of interbreeding
natural populations that are
reproductively isolated from other
such groups.”
Ernst Mayr
Species
• Similar in morphology and behavior
• Able to reproduce under natural
conditions
• Produce fertile off-spring
Speciation & Natural Selection
• Natural selection can lead to speciation
• Speciation can also occur as a result of
other microevolutionary processes
– Genetic drift
– Mutation
Barriers to Gene Flow
• Whether or not a physical barrier
deters gene flow depends upon:
– Organism’s mode of dispersal or locomotion
– Duration of time organism can move
Reproductive Isolation
• Cornerstone of the biological species
concept
• Speciation is the attainment of
reproductive isolation
• Reproductive isolation arises as a
by-product of genetic change
Genetic Divergence
• Gradual accumulation of differences in
the gene pools of populations
• Natural selection, genetic drift, and
mutation can contribute to divergence
• Gene flow counters divergence
Genetic Divergence
populations of one species (gold)
time A
time B
time C
time D
populations of
a daughter
species (green)
Reproductive Isolating
Mechanisms
• Prezygotic isolation
– Mating or zygote formation is prevented
• Postzygotic isolation
– Takes effect after hybrid zygotes form
– Zygotes may die early, be weak, or be sterile
Prezygotic Isolation
Ecological Isolation
Temporal Isolation
Behavioral Isolation
Mechanical Isolation
Gametic Mortality
Postzygotic Mechanisms
• Zygotic mortality
• Hybrid inviability
• Hybrid sterility
Mechanisms of Speciation
• Allopatric speciation
• Sympatric speciation
• Parapatric speciation
Allopatric Speciation
• Speciation in geographically isolated
populations
• Probably most common mechanism
• Some sort of barrier arises and prevents
gene flow
• Effectiveness of barrier varies with
species
Extensive Divergence
Prevents Inbreeding
• Species separated by geographic
barriers will diverge genetically
• If divergence is great enough it will
prevent inbreeding even if the barrier
later disappears
Archipelagos
• Island chains some distance from
continents
– Galapagos Islands
– Hawaiian Islands
• Colonization of islands followed by
genetic divergence sets the stage for
speciation
1
Speciation
on an
Archipelago
A few individuals of a
species on the mainland
reach isolated island 1.
Speciation follows genetic
divergence in a new habitat.
Later in time, a few
1
individuals of the new
species colonize nearby
island 2. In this new
habitat, speciation follows
genetic divergence.
Speciation may also
follow colonization of
islands 3 and 4. And it
may follow invasion of
island a by genetically
different descendants
of the ancestral species.
3
2
4
2
1
3
2
4
Hawaiian Islands
• Volcanic origins, variety of habitats
• Adaptive radiations:
– Honeycreepers - In absence of other bird
species, they radiated to fill numerous
niches
– Fruit flies (Drosophila) - 40% of fruit fly
species are found in Hawaii
Hawaiian Honeycreepers
FOUNDER SPECIES
Speciation without a Barrier
• Sympatric speciation
– Species forms within the home range of the
parent species
• Parapatric speciation
– Neighboring populations become distinct
species while maintaining contact along a
common border
Sympatric Speciation in
African Cichlids
• Studied fish species in two lakes
– Species in each lake are most likely
descended from single ancestor
• No barriers within either lake
• Some ecological separation but species
in each lake breed in sympatry
Parapatric Speciation
Adjacent
populations
evolve into
distinct species
while maintaining
contact along a
common border
BULLOCK’S
ORIOLE
BALTIMORE
ORIOLE
HYBRID ZONE
We’re All Related
• All species are related by descent
• Share genetic connections that extend
back in time to the prototypical cell
Evolutionary Trees
extinction
(branch
ended
before
present)
new species
branch point
(a time of
divergence,
speciation)
a single
lineage
branch point
(a time of
divergence,
speciation)
a new
species
a single
lineage
dashed line
(only sketchy
evidence of
presumed
evolutionary
relationship)
Gradual Model
• Speciation model in which species
emerge through many small
morphological changes that accumulate
over a long time period
• Fits well with evidence from certain
lineages in fossil record
Punctuation Model
• Speciation model in which most changes
in morphology are compressed into brief
period near onset of divergence
• Supported by fossil evidence in some
lineages
Adaptive Radiation
• Burst of divergence
• Single lineage gives rise to many
new species
• New species fill vacant adaptive
zone
• Adaptive zone is “way of life”
Adaptive Radiation
Extinction
• Irrevocable loss of a species
• Mass extinctions have played a
major role in evolutionary history
• Fossil record shows 20 or more
large-scale extinctions
• Reduced diversity is followed by
adaptive radiation
Who Survives?
• Species survival is to some extent
random
• Asteroids have repeatedly struck Earth
destroying many lineages
• Changes in global temperature favor
lineages that are widely distributed
From Primates to Humans
“Uniquely” human traits
evolved through modification
of traits that evolved earlier, in
ancestral forms
Hominoids
• Apes, humans, and extinct species of their
lineages
• In biochemistry and body form, humans
are closer to apes than to monkeys
• Hominids
– Subgroup that includes humans and extinct
humanlike species
Adaptations to an
Arboreal Lifestyle
• During the Eocene, certain
primates became adapted to life in
trees
– Better daytime vision
– Shorter snout
– Larger brain
– Forward-directed eyes
– Capacity for grasping motions
First Hominids
• Earliest known is Ardipithecus ramidus
– Lived 4.4 million years ago in Africa
– More apelike than humanlike
• Numerous australopiths evolved during
the next 2 million years
– Large face, protruding jaw, small skull
– Walked upright
Humans Arise
• First member of
the genus Homo
is H. habilis
• Lived in
woodlands during
late Miocene
Homo erectus
• Evolved in Africa
• Migrated into Europe and Asia
about 1.5 million - 2 million years
ago
• Had a larger brain than H. habilis
• Was a creative toolmaker
• Built fires and used furs for clothing
Homo sapiens
• Modern man evolved by 100,000 years
ago
• Had smaller teeth and jaws than H.
erectus
• Facial bones were smaller, skull was
larger
Earliest Fossils Are African
• Africa appears to be the cradle of human
evolution
• No human fossils older than 1.8 million
years exist anywhere but Africa
• Homo erectus left Africa in waves from 2
million to 500,000 years ago
Multiregional Model
• Argues that H. erectus migrated to many
locations by about 1 million years ago
• Geographically separated populations
gave rise to phenotypically different races
of H. sapiens in different locations
• Gene flow prevented races from
becoming species
African Emergence Model
• Argues that H. sapiens arose in subSaharan Africa
• H. sapiens migrated out of Africa and
into regions where H. erectus had
preceded them
• Only after leaving Africa did phenotypic
differences between races arise
40,000 years ago
Fig. 24.30, p. 402