Principles of Evolution

1
Principles of Evolution

• Chapter 9
• Speciation
• James F. Thompson, Ph.D., MT(ASCP)

2
Speciation

• The evolutionary formation of new species in
  space or time, usually by the division of a
  single species into two or more genetically
  distinct ones.
• Species share the same gene pool, or the sum of
  all genetic codes possessed by members of that
  species.
• This process crosses the boundary between
  microevolution and macroevolution

3
Species Definitions

• Morphological or Typological Species A set of
  organisms sharing structural similarities between
  members and discontinuities in structure between
  different species
• Mayrs Biological Species Concept "Species are
  groups of interbreeding natural populations that
  are reproductively isolated from other such
  groups.

4
Species Definitions

• Ecological Species A set of organisms adapted
  to a particular set of resources, called a niche,
  in the environment.
• Genetic species A set of organisms exhibiting
  similarity of DNA.

5
Species Definitions

• Agamospecies A set of organisms in which sexual
  reproduction does not occur, represented
  typically as a collection of clones.
• Chronospecies / Paleospecies a species which
  changes in morphology, genetics, and/or ecology
  over time on an evolutionary scale such that the
  originating species and the species it becomes
  could not be classified as the same species had
  they existed at the same point in time.
• Note experts establish fine distinctions
  between chronospecies and paleospecies.

6
Species Definitions

• Phylogenetic (Cladistic) / Evolutionary Species
  A set of organisms that shares a common ancestor
  and maintains its integrity with respect to other
  lineages through both time and space.
• Ring Species A set of generally hybridizing
  species with a geographic distribution that forms
  a ring and overlaps without hybridization at the
  ends.

7
Problems Defining Species Through Time

• Morphospecies Viewed today, at one moment in
  time, species A, C, and E are clearly distinct
  species, demarcated by current natural
  discontinuities between them.

8
Problems Defining Species Through Time

• Paleospecies (chronospecies). Viewed
  historically, through time, discovered fossil
  intermediates (B and D) fill in the missing gaps
  above, giving us a more or less continuous series
  with no obvious morphological discontinuities
  between them. still an over-simplification

9
What Interests Us About Speciation?

• Speciation is evidence that evolution occurs.
• Speciation provides important insight into the
  mechanisms of evolution.
• Patterns of speciation provide insight into the
  distribution patterns of organisms.
• Speciation explains patterns in the ecology and
  reproductive biology of organisms.
• What are the causes of speciation?
• What are the rates of speciation and do they
  differ among different taxa?

10
The Process of Species Formation

• In the beginning, there is a single population
  with a common shared gene pool.
• A discontinuity develops among some
  subpopulations.
• Changes in allele frequencies develop at various
  loci in the gene pools (and, usually, changes in
  phenotypes) of the subpopulations.
• Separate evolution of subpopulations continues
  until one or both has diverged to the point that
  each subpopulation now meets one of the
  definitions of a species concept.

11
An Important Reminder

• Regardless of species definitions, a species, to
  be a biological entity, must exist in an
  ecological niche
• Can a population of organisms which no longer has
  a niche still be a biological entity?
• Only 400-500 Siberian tigers still exist within
  their range

12
SPATIAL ASPECTS OF SPECIATION

• Allopatric speciation a physical barrier
  divides a continuous population
• Peripatric speciation a small founding
  population enters a new or isolated niche
• Parapatric speciation a new niche found
  adjacent to the original niche
• Sympatric speciation - speciation occurs without
  physical separation inside a continuous population

13
SPATIAL ASPECTS OF SPECIATION
14
Allopatric Speciation

• Four steps lead to speciation.
• A single species is an interbreeding reproductive
  community.
• A barrier develops, or a dispersal event occurs,
  dividing the species.
• Separated into different habitats, the divided
  populations diverge through the accumulation of
  gene and trait differences.
• The separate populations become so different
  that, if and when the barrier disappears and they
  overlap again, interbreeding does not occur.

15
Allopatric Speciation

• The populations of Tamarin monkeys are separated
  on the sides of the Amazon River.
• Where the river tributary is wide and individuals
  on opposite banks do not interbreed, the
  populations are diverging toward separate
  species.
• Where the river tributary is narrow, the
  individuals still interbreed.

16
Allopatric Speciation

• (sweepstakes) dispersal provides complete
  geographic isolation

17
Allopatric Speciation

• Vicariance event provides complete geographic
  isolation for two robber fly genera

18
Incomplete or Peripheral Isolation

• Ernst Mayr and Theodosius Dobzhansky, speaking
  for the Modern Synthesis, emphasized that most
  that speciation was allopatric, i.e.,
  geographical isolation required
• Two modifications or variants have been proposed
  
• Peripatric speciation a small population enters
  a new or isolated niche
• originally proposed by Mayr, and related to the
  founder effect and genetic drift altering the
  isolates gene pool
• Parapatric speciation a new niche found
  adjacent to the original niche

19
Peripatric vs. Parapatric speciation

• Peripatric speciation is caused by being at the
  edge of the range and almost isolated
  geographically ? geographic isolation leads to
  genetic isolation.
• Parapatric speciation is by becoming genetically
  isolated which leads the population to become
  geographically isolated ? genetic isolation leads
  to geographic isolation.

Less common and more difficult to demonstrate
since small niche and habitat differences rarely
have fossil records
20
Peripatric and Parapatric Speciation

• Speciation triggered by partial isolation
  (peripatric and parapatric) are now argued as
  being as or more important in explaining
  speciation events than classic allopatric
  speciation
• Eldridge and Goulds Theory of Punctuated
  Equilibria is just one example of such
  advocation.

dispersal or vicariance
Systematists and palaeontologists needs more data
to resolve that technical debate
21
Peripatric Speciation
dispersal

• Potential peripatric speciation triggered by
  partial isolation in a Bolivian natural forest
  island isolated 3000 years from the larger
  continuous forest habitat.
• Divergence in song and certain alleles
  frequencies between the two populations
  (reproductive isolation) suggest that incipient
  speciation is under way.

22
Parapatric Speciation
dispersal

• Potential parapatric speciation in sweet vernal
  grass/buffalo grass, Anthoxanthum odoratum,
  triggered by adaptation to heavy metal
  contaminated soils in many locations globally.
• Divergence in flowering times (reproductive
  isolation) between the two populations suggest
  that incipient speciation is under way.

23
Ring SpeciesSalamanders

• The ensatina salamander (Ensatina eschscholtzii)
  occurs from Canada to Southern California with
  interbreeding between adjacent populations
  through this range.
• The Central Valleya dry, hot lowland areais
  divided into a coastal arm and inland arm.
• However, where these two arms of the species meet
  again in Southern California, interbreeding does
  not occur.
• Ring species are often considered examples of
  parapatric speciation.

24
Ring Species - Herring Gulls

• As glaciers retreated, herring gulls (Larus
  argentatus) were released out of a north Pacific
  refugia spreading one way across North America
  and into western Europe and spreading in the
  other direction across Alaska into Siberia.
• From Siberia, as the herring gull now extended
  its range further across Asia, it tended to
  differentiate, producing a subspecies (or species
  by some ornithologists) such as the vega gull
  (Larus vegae) and farther west the lesser
  blackbacked gull (Larus fuscus).
• Eventually its current circumpolar distribution
  became established (dashed lines).
• Adjacent subspecies interbreed (solid arrows),
  but where the ends of the circular range of the
  herring gull meet and overlap in Europe, there is
  very little interbreeding (dotted lines).
  (Simplified originally from Mayr, 1963)
• Ring species are often considered examples of
  parapatric speciation.

25
Sympatric Speciation

• Speciation without any geographical isolation
  within the continuous ancestral population
• Reproductive isolation develops through changes
  in behavior, microhabitat, seasonality of
  breeding, or chromosomal mutation or ploidy
  events
• Common in plants less common in animals
• Difficult to confirm sympatric origin

26
Sympatric Speciation

• Sympatric African Indigobirds are host specific
  nest parasites.
• Their hosts rear their young but their young do
  not destroy the hosts young, as cuckoos often do.

27
Sympatric Speciation

• Sympatric Neotropical butterfly species
  Heliconius cydno l and H. melpomene r are
  Mullerian mimics.
• Their common toxicity is cyanide derived from
  cyanoglucosides in various Passiflora host plants
  eaten by the larvae.

28
Sympatric Speciation

• Crater lakes and oceanic islands provide optimal
  locations for studying sympatric speciation
  because differentiation between sister taxa found
  at these locations is likely to have occurred in
  situ.
• Clockwise from top left, Amphilophus citrenellus,
  A. zaliosus cichlid fish, Howea forsteriana, H.
  belmoreana palms, Lord Howe Island, S. Pacific,
  Craterlake Apoyo, Nicaraugua.

29
Sympatric Speciation

• The composites, salsify plants, from eastern
  Washington include a tetraploid hybrid derived
  from two diploid species
• The many polyploid hybrids such as these are the
  best examples of sympatric speciation, including
  a few animals

30
Speciation Types Summary

• A population with common gene pool
• Discontinuity develops among subpopulations
• Different selection pressures applied in
  different niches drive evolutionary change
• Reproductive isolation develops as the new
  species evolve

peripatric
31
Reproductive Isolating Mechanisms (RIMs)

• Different types of mechanisms can prevent
  reproduction between individuals of different
  species.
• These may occur premating or postmating, as
  illustrated here with two species of salamander.
• RIMs are also referred to as prezygotic versus
  postzygotic mechanisms

32
Geographical (Reproductive) Isolation
Iguana iguana
Amblyrhynchus cristatus
Conolophus subcristatus

• The two Galapagos iguana genera are, themselves,
  ecologically isolated

33
Ecological (Reproductive) Isolation

• Water or cotton-mough moccasin is semi-aquatic,
  feeds on aquatic vertebrates, and is aggressive
• Copperhead is terrestrial, feeds on terrestrial
  vertebrates, and is less aggressive

Agkistrodon piscivorus
Agkistrodon contortrix
34
Behavioral (Reproductive) Isolation
Anolis trinitatis
Anolis garmani

• Members of the genus Anolis on Jamaica chose
  different perches and use different patterns of
  head bobbing to attract female anoles
• They also have separate ecological niches

Anolis opalinus
35
Temporal (Reproductive) Isolation

• Members of the genus Magicicada, exist in
  temporily separated populations, three species of
  17 year cicadas, and four species of 13 year
  cicadas
• There is also some geographical isolation with 17
  year cicadas in the northeastern US and 13 year
  cicadas in the southeastern US

a 13 year cicada
a 17 year cicada
36
Mechanical (Reproductive) Isolation

• Members of the genus Parafontaria, Japanese
  millipedes, differ in body size and in the size
  and shape of their reproductive gonopodia

37
Reproductive Isolation

• Prezygotic mechanisms Factors which prevent
  individuals from mating.
• Temporal isolation Ecological isolation
  Behavioral isolation Mechanical isolation ?
  already discussed
• Gametic incompatibility Sperm transfer takes
  place, but the egg is not fertilized.
• Postzygotic isolating mechanisms Genomic
  incompatibility, hybrid inviability or sterility.
• Zygotic mortality The egg is fertilized, but the
  zygote does not develop.
• Hybrid inviability Hybrid embryo forms, but is
  not viable.
• Hybrid sterility Hybrid is viable, but the
  resulting adult is sterile.
• Hybrid breakdown First generation (F1) hybrids
  are viable and fertile, but further hybrid
  generations (F2 and backcrosses) are inviable or
  sterile.

38
Post-Zygotic Isolation

• The four groups of leopard frogs resemble one
  another closely in their external appearance.
• But early tests of interbreeding produced
  defective embryos (hybrid inviability) in some
  combinations, leading biologists to suspect that
  these might be different subspecies or even
  different species.
• Research on males mating calls indicates that
  the various groups differ substantially, and that
  such prezygotic behavior separates and
  reproductively isolates members of each group,
  producing four species (1) Rana pipiens (2)
  Rana blairi (3) Rana utricularia (4) Rana
  berlandieri.

39
Hybrid Sterility
mule
hinny
liger
tigon
These hybrids have reduced, if not absent,
fertility, though they are often otherwise healthy
40
Reproductive Isolating Mechanisms (RIMs)

• Genetic and ecological isolation may be occurring
  at the same time, or before or after reproductive
  isolating mechanisms form
• Not all RIMs are required for any particular
  speciation event
• The sequence in which RIMs develops is also
  unique to each species

41
Speciation for Sexual Species

• If species reproduce asexually, reproductive
  isolation is inherent in their formation
  offspring form clones
• If species reproduce sexually, the degree to
  which species may hybridize varies greatly
• The ability to hybridize does not necessarily
  contradict the reality of species distinction
• Some sister species never have the opportunity to
  reproduce across populations for form hybrids in
  nature

42
Patterns of Speciation

• Regardless of species definitions, a species, to
  be a biological entity, must exist in an
  ecological niche
• Sometimes, the abiotic factors important in a
  species niche vary in a regular fashion across
  the range of the species
• If so, we can map those abiotic and then,
  sometimes, find patterns within the species
  itself, tracking the patterns in the abiotic
  factors of the niche

43
Clines

• The word cline comes from a Greek word meaning
  to lean think of incline
• A cline can be a gradual change or transition in
  the average aspect of an abiotic feature across
  some geographic range.
• A cline can also be a gradual change in a
  species gene pool for traits that are adapted to
  the transition in some abiotic factor in the
  geographic range of the species
• Clines are illustrated with contour plot maps

44
Abiotic Clines

• Thermocline by latitude, altitude, or oceanic
  depth
• Average annual sunlight by latitude, altitude, or
  oceanic depth
• Average partial pressure of oxygen by altitude,
  or oceanic depth
• Average annual rainfall by latitude or altitude

45
Average Solar Radiation
46
Global Photosynthesis (July 2000)
Note the greatest primary production is in the
temperate forests.
47
Global Ocean Temperatures
Data from the Atlantic Ocean at 8 deg. 15' N, 47
deg. 36' W on May 17, 1957.
48
Global Ocean Salinity

• Salinity map showing areas of high salinity (36
  parts/thousand) in green, medium salinity in blue
  (35 parts/thousand), and low salinity (34
  parts/thousand) in purple.

49
Annual Rainfall
50
Biotic Clines

• A biotic cline, in reference to population
  biology, is a gradual change of phenotype (trait,
  character or feature) and underlying gene pool
  allele frequencies in a species over a
  geographical area, often as a result of
  environmental heterogeneity.
• This meaning of "cline" was introduced by Sir
  Julian Huxley.

51
Species Richness ? Mammals

• The numbers of mammal species, from high
  latitudes (north) to low latitudes (south), are
  shown along the lines.
• Note the general increase in the number of
  species from north to south across the various
  latitudes.
• This is true for most organisms, not just mammals

52
Species Richness ? Birds and Plants
53
Species And Family RichnessAcross Biomes
54
What Explains Species Richness?

• The tropics have the greatest species richness.
  The tropics have the advantages of
• Longevity of ecosystems no ice ages to render
  the tropics practically uninhabitable
• Climate stability reduced extremes of
  temperature and rainfall
• Highest primary productivity the food webs are
  supported by a huge photosynthetic biomass at the
  base of the food pyramid

55
Biological Clines

• Bergmann's Rule (1847) is a generalization
  which states that within a species the body mass
  increases with latitude and colder climate, or
  that within closely related species that differ
  only in relation to size that one would expect
  the larger species to be found at the higher
  latitude (a latitudinal cline).
• German biologist, Christian Bergmann (1814-1865)
  formulated in reference to mammals and birds
  (endotherms), but some researchers have also
  found evidence for the rule in ectothermic
  species including Drosophila.

56
Bergmann's Rule

• Siberian Tiger (300 Kg) vs. Bengal Tiger (200
  Kg)
• Polar Bear (600 Kg) vs. Brown Bear (500 Kg)
• Snow Leopard (50 Kg) vs. African Leopard (70
  Kg) violates the rule
• Emperor Penguin (35 Kg) vs. Galapagos Penguin
  (2.5 Kg)

57
Clinal Variation

• In the leopard frog (Rana pipiens), tadpoles
  exhibit a range of temperature tolerances,
  generally enduring colder temperatures in higher
  (northern) latitudes and warm temperatures at
  lower (southern) latitudes.

58
Reproductive Success

• In a study by J. Moore in 1949 of the leopard
  frog (Rana pipiens), eggs from females in the
  north were fertilized with sperm from males
  progressively farther to the south.
• The degree of embryo or tadpole abnormalities was
  scored, from A (normal young) through
  progressively more abnormalities to F (high death
  rate).
• This study and others prompted biologists to
  divide leopard frogs into several different
  species.

egg mass
59
Biological Clines

• The flowering time of a plant may tend to be
  later at higher altitudes (an altitudinal cline).
• In species in which the gene flow between
  adjacent populations is high, the cline is
  typically smooth, whereas in populations with
  restricted gene flow the cline usually occurs as
  a series of relatively abrupt changes from one
  group to the next.

60
Clinal Variation in Yarrow

• Achillea, the yarrow, is a member of the
  Compositae along with daisies and sunflowers.
• Clausen, Keck, Hiesey (1948) is a classic study
  of genetic and environmental differences between
  populations of this flowering plant.

Achillea millefolium lanulosa
61
Clausen, Keck, Hiesey

• Clausen, Keck, Hiesey observed that in nature,
  low altitude populations of Achillea are taller
  on average than yarrow in high altitude
  populations.
• Yarrow will sprout from cuttings.

62
Variation in Height in Yarrow

• Plant height has an inverse relationship with
  altitude.

63
Different Ecotypes of Yarrow Differ in Different
Environments

• Three garden plots were selected at three
  different locations along the transect- sea level
  (Stanford), 4,600 feet (Mather), and at 10,000
  feet (Timberline).
• Yarrow seeds collected from five locations along
  this transect-San Gregorio, Knights Ferry, Aspen
  Valley, Tenaya Lake, Big Horn Lake-were planted,
  grown into young plants, and then divided into
  equivalent tufts, clones, planted at the three
  garden sites-Stanford, Mather, Timberline.

64
Different Ecotypes of Yarrow Differ in Different
Environments

• The resulting germination and growth of these
  five collected clones planted at these three
  garden sites is graphically indicated.
• Note especially that sea-level clones (from San
  Gregorio) at high elevations did poorly (died),
  and high-elevation clones (from Big Horn Lake) at
  low elevations still did not grow to large
  heights.

65
Clausen, Keck, Hiesey

• Clausen, Keck, Hiesey took two cuttings from
  each of seven yarrow at two locations.
• One set was planted in an experimental garden in
  Mather, CA.
• The other set was planted in an experimental
  garden in Stanford, CA.

Stanford ? Mather
66
Clausen, Keck, Hiesey

• The seven cuttings at each location grew side by
  side.
• They shared identical environments at Mather and
  Stanford, CA.
• Difference in height had to be due almost
  entirely to genetic variation.

67
High Heritability Within Populations Tells Us
Nothing About the Causes of Differences Between
Populations

• Each lettered plant pair (A-G) are identical
  twins (cuttings) in terms of genome.
• All plants were shorter at Mather, the higher
  altitude site.
• Not all genotypes responded to the environments
  in the same way.

68
High Heritability Within Populations Tells Us
Nothing About the Causes of Differences Between
Populations

• If you did not know that the gene pools of the
  two experimental populations (N 7) were
  identical
• You might conclude that the Stanford population
  was superior in regard to plant growth (height).
• That would be wrong!

69
More on Clausen, Keck, Hiesey (1948)

• Clausen, Keck, Hiesey (1948) also collected
  sample cuttings from multiple individuals of
  Achillea (yarrow) on a larger east-west transect.
• They grew all the cuttings under constant
  environmental conditions in a laboratory
  greenhouse.

70
More on Clausen, Keck, Hiesey (1948)

• The blue figures are histograms of the variation
  in height of individual plants.

• The upper part of the illustration shows the
  different appearances of Achillea lanulosa
  populations the green drawings represent the
  average of the plants' heights cultivated under
  identical standard conditions in climatic
  chambers.

71
More on Clausen, Keck, Hiesey (1948)

• These results also illustrate the complex
  interplay between gene pools and environments.

• The lower part of the illustration gives the
  natural geographic origin of the single
  populations by way of a profile of a west-to-east
  cross-section through California

72
Clausen, Keck, Hiesey (1948)

• Clausen, Keck, Hiesey (1948) illustrated that
  subpopulations of yarrow species are well adapted
  to their own microhabitats.
• This adaptive sorting of subpopulation gene pools
  suggests that speciation events could follow if
  selective forces increased on some of the
  populations.
• Speciation is driven by competitive reproductive
  success in particular niches!

73
Clausen, Keck, Hiesey (1948)

• Clausen, Keck, Hiesey (1948) illustrated that
  subpopulations of yarrow species are well adapted
  to their own microhabitats.
• This adaptive sorting of subpopulation gene pools
  suggests that speciation events could follow if
  selective forces increased on some of the
  populations.
• Speciation is driven by competitive reproductive
  success in particular niches!

74
After Speciation, What?

• Species evolve into other species in periods of a
  few dozen hundreds of thousands of years
• Over geological time, speciation has led to the
  evolution of all the higher taxa
• We discussed the general trends in Chapters 4 and
  5
• Speciation events lead to Adaptive Radiations

75
Adaptive Radiations

• An Adaptive Radiation is the evolutionary
  diversification of a species or single ancestral
  lineage into various forms that are each
  adaptively specialized to a specific
  environmental niche.
• Adaptive radiation generally proceeds most
  rapidly in environments where there are numerous
  unoccupied niches or where competition for
  resources is minimal.
• Adaptive radiations often also increase the
  variety of available niches over time within the
  ecological community or ecosystem

76
Adaptive Radiations

• Speciation events drive Adaptive Radiations into
  new niches!
• Adaptive Radiations drive Speciation in new
  niches!

77
Adaptive Radiation of Species
78
Adaptive Radiation of Higher Taxa
79
Adaptive Radiation of Higher Taxa
Orders and Families
80
Adaptive Radiation of Higher Taxa
Orders and Families of Dinosaurs
81
Adaptive Radiation of Higher Taxa
82
Adaptive Radiation

• Each species is unique
• Each niche is unique
• But habitats, ecosystems, and biomes have many
  similar features

Therefore, natural selection may find similar
solutions to adaptive challenges parallel and
convergent evolution (homoplasy).
83
Convergent EvolutionAdaptive Radiation of
Mammals

• Australian marsupials resemble placental mammals
  in the rest of the world.
• Within the relative isolation of Australia, the
  marsupials entered similar habitats to those
  available to the placentals elsewhere.
• Under similar selective pressures in similar
  biomes, similar features and ecological
  lifestyles evolved, but upon a marsupial theme.

84
Convergent Evolution ? Xeric Succulents

• Different families of desert plants have evolved
  similar adaptations to the deserts dry, hot
  conditions - namely, succulent shoots with
  spines.

85
Origin of Species Last Paragraph

• It is interesting to contemplate a tangled bank,
  clothed with many plants of many kinds, with
  birds singing on the bushes, with various insects
  flitting about, and with worms crawling through
  the damp earth, and to reflect that these
  elaborately constructed forms, so different from
  each other, and dependent upon each other in so
  complex a manner, have all been produced by laws
  acting around us. These laws, taken in the
  largest sense, being Growth with reproduction
  Inheritance which is almost implied by
  reproduction Variability from the indirect and
  direct action of the conditions of life, and from
  use and disuse a Ratio of Increase so high as to
  lead to a Struggle for Life, and as a consequence
  to Natural Selection, entailing Divergence of
  Character and the Extinction of less improved
  forms. Thus, from the war of nature, from famine
  and death, the most exalted object which we are
  capable of conceiving, namely, the production of
  the higher animals, directly follows. There is
  grandeur in this view of life, with its several
  powers, having been originally breathed by the
  Creator into a few forms or into one and that,
  whilst this planet has gone circling on according
  to the fixed law of gravity, from so simple a
  beginning endless forms most beautiful and most
  wonderful have been, and are being evolved.
  Charles Darwin (1859)

86
The Tangled Bank of the Napo
87
End Chapter 9
88
Variety

• Bird bill adaptations