Speciation

1
Speciation

• Chapter 17

2
Barriers to Gene Flow

• Whether or not a physical barrier deters gene
  flow depends upon
• Organisms mode of dispersal or locomotion
• Duration of time organism can move

3
Genetic Drift in Snail Populations

• Robert Selander studied Helix aspersa
• Collected snails from a two-block area
• Analyzed the allele frequencies for five genes

4
Genetic Drift in Snail Populations
5
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

6
Speciation Natural Selection

• Natural selection can lead to speciation
• Speciation can also occur as a result of other
  microevolutionary processes
• Genetic drift
• Mutation

7
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

8
Variable Morphology
Grown in water
Grown on land
9
Biological Species Concept

• Species are groups of interbreeding natural
  populations that are reproductively isolated from
  other such groups.
• Ernst Mayr

10
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

11
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

12
Genetic Divergence
populations of one species (gold)
populations of a daughter species (green)
time A
time B
time C
time D
13
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

14
Prezygotic Isolation

• Ecological Isolation
• Temporal Isolation
• Behavioral Isolation
• Mechanical Isolation
• Gametic Mortality

15
Postzygotic Mechanisms

• Zygotic mortality
• Hybrid inviability
• Hybrid sterility

16
Mechanisms of Speciation

• Allopatric speciation
• Sympatric speciation
• Parapatric speciation

17
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

18
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

19
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

20
Archipelagos

• Island chains some distance from continents
• Galapagos Islands
• Hawaiian Islands
• Colonization of islands followed by genetic
  divergence sets the stage for speciation

21
Speciation on an Archipelago
1
A few individuals of a species on the mainland
reach isolated island 1. Speciation follows
genetic divergence in a new habitat.
3
2
4
Later in time, a few individuals of the new
species colonize nearby island 2. In this new
habitat, speciation follows genetic divergence.
1
2
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.
1
3
2
4
22
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

23
Hawaiian Honeycreepers
FOUNDER SPECIES
24
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

25
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

26
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

27
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)
CROSS-FERTILIZATION, FOLLOWED BY A SPONTANEOUS
CHROMOSOME DOUBLING
X
X
42AABBDD
14AA
14BB
14AB
28AABB
14DD
28
Parapatric Speciation

• Adjacent populations evolve into distinct
  species while maintaining contact along a common
  border

BULLOCKS ORIOLE
BALTIMORE ORIOLE
HYBRID ZONE
29
Were All Related

• All species are related by descent
• Share genetic connections that extend back in
  time to the prototypical cell

30
Patterns of Change in a Lineage

• Cladogenesis
• Branching pattern
• Lineage splits, isolated populations diverge
• Anagenesis
• No branching
• Changes occur within single lineage
• Gene flow throughout process

31
Evolutionary Trees
extinction (branch ended before present)
new species
branch point (a time of divergence, speciation)
a new species
branch point (a time of divergence, speciation)
dashed line (only sketchy evidence of presumed
evolutionary relationship)
a single lineage
a single lineage
32
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

33
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

34
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

35
Adaptive Radiation
36
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

37
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