Speciation
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
Genetic Drift in Snail Populations
Robert Selander studied Helix
aspersa
Collected snails from a two-
block area
Analyzed the allele
frequencies for five genes
Genetic Divergence in Snail Populations
Snail Speciation?
Will the time come when the snails from opposite sides of the street are so different that they can no longer interbreed?
If so, then they will have become two distinct species
Speciation & Natural Selection
Natural selection can lead to speciation
Speciation can also occur as a result of other microevolutionary processes Genetic drift Mutation Sexual Selection
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
Darwin's Explanatory Model of Evolution Through Natural Selection
Refer to Excel File
Variable Morphology
Grown in water Grown on land
Biological Species Concept
“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.”
Ernst Mayr
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
time A time B time C time D
daughter species
parent species
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
Ecological Isolation and Competitive Exclusion in Two Crayfish (Orconectes Virilis and Orconectes Immunis)
Ecology: Vol. 51, No. 2, pp. 225–236.
Richard V. Bovbjerg Abstract. The two crayfish, Orconectes virlis and O.
immunis, have similar ranges but are ecologically isolated within these ranges; the former species inhabits streams and lake margins, the latter inhabits ponds and sloughs. Field and experimental data suggest that the stream species, O. virilis, is excluded from ponds by summer drying and periodic low oxygen periods of those ponds.
O. Virilis is not tolerant of low oxygen levels and is more aggressive… “It (O. virilis) is not as competent a burrower as O.
immunis nor is its as tolerant of low oxygen levels. Neither current nor substratum excludes the pond species, O. immunis, from the stream. But both field and laboratory observations strongly indicate a competitive exclusion of O. immunis by O. virilis. The latter was demonstrated to be intrinsically more aggressive in interspecific contacts, and in other experiments, evicted O. immunis from crevices in the substratum. “
Sister Species
Orconectes virilis and Orconectes immunis
Streams and lake margins
Sloughs and ponds
Flowers and Temporal Isolation
“Populations may mate or flower at different seasons or different times of day. Three tropical orchid species of the genus Dendrobium each flower for a single day; the flowers open at dawn and wither by nightfall.”
http://www.britannica.com/eb/article-49883
Flowers Respond to the Same Meteorological Stimuli…
Flowering occurs in response to certain meteorological stimuli, such as a sudden storm on a hot day. The same stimulus acts on all three species, but the lapse between the stimulus and flowering is 8 days in one species, 9 in another, and 10 or 11 in the third.
Dendrobiums are Epiphytes
Behavioral Isolation “Behavioral isolation is another isolating
mechanism. This mechanism operates through courtship behavioral patterns. If one species displays a certain courtship pattern, it won’t be recognized by those of the other species. This is the mechanism which separates wolfs from dogs, their courtship patterns are different in the wild.”
Sonographs of Two Closely Related Tree Frogs
http://cas.bellarmine.edu/tietjen/images/speciation.htm
Mechanical Isolation
“Mechanical isolation deals with the actual mechanics of the reproductive organs. Mechanical isolation acts a prezygotic barrier by preventing sexual intercourse between two different species. “
http://ohoh.essortment.com/reproductiveiso_rcgu.htm
Mechanical Isolation Between Two Species of Sage
http://cas.bellarmine.edu/tietjen/images/mechanical_isolation.jpg
Gametic Mortality
Sperm are immobilized or destroyed before fertilization can take place.
Postzygotic Mechanisms
Zygotic mortality
Hybrid inviability
Hybrid sterility
Zygote Mortality
Hybrid Inviability
Hybrid Sterility
Hybrid sterility: Horse + Donkey -> mule!
http://cas.bellarmine.edu/tietjen/Evolution/Isolating%20MEchanisms.htm
Mechanisms of Speciation
Allopatric speciation
Sympatric speciation
Parapatric speciation
Allopatric Speciation
Speciation in geographically isolated populations
Some sort of barrier arises and prevents gene flow
Effectiveness of barrier varies with species
Allopatric Speciation in Wrasses
Isthmus of Panama arose and separated wrasses in Atlantic and Pacific
Since separation, genes for certain enzymes have diverged in structure
Divergence may be evidence of speciation in progress
Blue-Headed Wrasse
Sister Species – Florida and Western Scrub Jays
These
Aphelocoma coerulescens Aphelocoma californica
Common Ancestor
Species are separated by 1600 kmFlorida
Texas
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
Speciation on an Archipelago
1
23
4
1
23
4
1
2
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 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 1 by genetically different descendents of the ancestral species.
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
African Cichlids
Speciation by Polyploidy
Change in chromosome number (3n, 4n, etc.)
Offspring with altered chromosome number cannot breed with parent population
Common mechanism of speciation in flowering plants
Figure 18.9Page 299
Possible Evolution of Wheat
Triticum monococcum (einkorn)
T. aestivum (one of the common bread wheats)
Unknown species of wild wheat
T. turgidum(wild emmer)
T. tauschii (a wild relative)
42AABBDD14AA 14BB 14AB 28AABB 14DDXX
cross-fertilization, followed by a spontaneous chromosome doubling
Parapatric Speciation
Adjacent populations evolve into distinct species while maintaining contact along a common border
BULLOCK’S ORIOLE
BALTIMORE ORIOLE
HYBRID ZONE
Recall: Most products of crossbreeding, such as the mule, are sterile…
“New Mexico Whiptail, is an all-female species that is actually a mixture (hybrid) of … the Western Whiptail, which lives in the desert, and the Little Striped Whiptail, a denizen of grasslands.”
This is another example of parapatric speciation
http://www.amnh.org/exhibitions/expeditions/treasure_fossil/Treasures/Unisexual_Whiptail_Lizards/lizards.html?50
We’re All Related
All species are related by descent
Share genetic connections that extend back in time to the prototypical cell
Patterns of Change in a Lineage
Cladogenesis Branching pattern Lineage splits, isolated populations
diverge Stopped here 1/25
Anagenesis No branching Changes occur within single lineage Gene flow throughout process
Evolutionary Trees
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
extinction (branch ended before present)
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
Asteroid Imparter
Mass Extinction
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