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