Chapter 24: The Origin of Species

1
Chapter 24The Originof Species
2
Species and Speciation
Speciation is the formation of a new species from
an older, immediately ancestral species
It is not enough to explain how adaptations
evolve in a population Evolutionary theory must
also explain how new species originate and
develop through the subdivision and subsequent
divergence of gene pools. p. 472, Campbell
Reece (2005)
3
Species Concepts

• Biological Species Concept
• Reproductive Isolation
• Not necessarily easy to apply
• Morphological Species Concept
• Phenotypic differences
• Paleontological Species Concept
• Fossil species
• Ecological Species Concept
• Filling of ecological niches
• Competition for resources
• Phylogenetic Species Concept
• Evolutionary lineages/genetic history
• Cohesion Species Concept
• Persistence of discrete phenotypes (bacteria)
• Pluralistic Species Concept
• Combinations of above as appropriate
• Conspecifics are members of the same species

Morphological similarity is not always a good
indicator of same species
4
Reproductive Isolation

• Absolute reproductive isolation means that genes
  (alleles) do not pass from one population to a
  second population, one with which the first
  population is reproductively isolated
• Note that reproductive isolation does not mean
  that individuals within two populations are not
  mating nor producing offspring within populations
• Instead, if there are offspring, those offspring
  are not contributing their alleles to either of
  the parental populations (e.g., because these
  hybrid offspring are sterile and/or do not
  survive to reproduce)
• Also note that reproductive isolation need not be
  100 it is possible for two populations to
  maintain a large degree of reproductive isolation
  with some small amount of gene exchange still
  occurring (a.k.a., introgression)

5
Biological Species Concept

• The biological species concept is a way of
  defining species that employs as its number one
  criteria the concept of reproductive isolation
• A biological species is a " population or group
  of populations whose members have the potential
  to interbreed with one another in nature to
  produce viable, fertile offspring, but who cannot
  successfully interbreed with members of other
  species that is genetically isolated from other
  such populations."
• Each species is circumscribed by reproductive
  barriers that preserve its integrity as a species
  by blocking genetic mixing with other species.
• "In the laboratory or in zoos, hybrids can often
  be produced between two species that do not
  interbreed in nature. This fact does not nullify
  the biological species concept.

6
Patterns of Speciation
Only via branching evolution can species increase
in number
Cladogenesis is branching evolution
7
Anagenesis
Anagenesis is the transformation of a single
ancestral species into a single descendant
species anagenesis is a mode of speciation
Anagenesis involves the extinction of the older,
ancestral species
8
Cladogenesis (Adaptive Radiation)
Cladogenesis is the transformation of one
ancestral species into more than one descendant
species cladogenesis is a mode of speciation
Cladogenesis does not (or, at least, does not
necessarily) involve the extinction of the
parental species
9
Ana- vs. Cladogenesis

• Cladogenesis is probably more common than
  anagenesis
• Anagenesis is probably just a special case of
  cladogenesis where the parental population
  either
• goes extinct coincident to the formation of the
  progeny species, or
• the parental species is driven to extinction by
  the progeny species soon after the latter's
  genesis
• these two scenarios are effectively the same
  thing so far as the fossil record is concerned
• The evolution of many diversely adapted species
  from a common ancestor is called adaptive
  radiation. (p. 471, Campbell Reece, 2002)

10
Cladogenesis Horses
Regardless of the lineage involved, or the
speciation concept one employs, the concepts of
species, speciation, and reproductive isolation
are irretrievably intertwined
11
Selection for Reproductive Barriers

• What if a mating between two individuals from
  different populations results in offspring which
  display a reduced Darwinian fitness relative to
  their parents?
• If that is the case, then there would be a
  fitness cost to the parents associated with
  mating outside of ones population, to
  hybridizing
• This fitness cost is realized given the
  investment of time and energy into mating,
  gestating, and raising offspring that otherwise
  display reduced fitness
• Such costs give rise to selection for genetically
  based barriers that prevent individuals from
  hybridizing and/or engage in behaviors that are
  costly in the course of attempting such
  hybridization

12
Reproductive Isolation

• Geographical Isolation
• Habitat Isolation
• Behavioral Isolation
• Temporal Isolation
• Mechanical Isolation
• Gametic Isolation
• Reduced Hybrid Viability
• Reduced Hybrid Fertility
• Hybrid Breakdown

The term zygotic refers to the product of
conception
13
Geographical Isolation
Geography supplies distance and other obstacles
to gene flow between populations
Geographical isolation is the only reproductive
barrier that we will consider that does not have
a large genetic component (other than the genetic
basis of inabilities to surmount geographical
barriers)
14
Habitat Isolation
Two species that occupy different habitats
within the same area may encounter each other
rarely, if at all, even though they are not
isolated by obvious barriers such as mountain
ranges.
15
Temporal Isolation
Species that breed during different time of the
day, different seasons, or different years cannot
mix their genes.
16
Behavioral Isolatoin
Courtship rituals that attract mates and other
behaviors unique to a species are effective
reproductive barriers, even between closely
related species.
17
Mechanical Isolation
Morphological differences can prevent successful
mating.
flowers often have distinct appearances that
attract different pollinators.
For animals, attempting to mate is not without
cost, e.g., susceptibility to predation during
the mating process
18
Gametic Isolation
The female does not lose gametes to hybridization
until conception has occurred
Sperm of one species may not be able to
fertilize the eggs of another species.
Gametic isolation separates certain closely
related species of aquatic animals such as sea
urchins which release their sperm and eggs into
the surrounding water, where they fuse and form
zygotes.
19
Gametic Isolation

• Both male and females bear the costs of mating,
  but with mating only the male initially wastes
  gametes
• The female does not lose gametes to hybridization
  until conception has occurred
• A female's eggs are typically a lot more
  expensive than a male's sperm, the female often
  is charged with the brunt of the cost of
  childrearing, and the female often is more
  limited in reproductive opportunity
• Thus, a female typically has more incentive to
  avoid conception than does a male
• Gametic isolation is conception avoidance
  following mating via either destruction of sperm
  or otherwise blocking of proper sperm-egg
  interaction prior to fertilization
• Gametic isolation additionally occurs when pollen
  is excluded by flowers

20
Reduced Hybrid Viability
The genes of different parent species may
interact and impair the hybrids development.
Im an example(?) of a frail hybrid salamander.
21
Reduced Hybrid Fertility
Even if hybrids are vigorous, they may be
sterile. If chromosomes of the two parent species
differ in number or structure, meiosis in the
hybrids may fail to produce normal gametes.
22
Hybrid Breakdown
Some first-generation hybrids are viable and
fertile, but when they mate with one another or
with either parent species, offspring of the next
generation are feeble or sterile.
With hybrid breakdown it is the grandchildren of
a hybrid cross that display reduced fitness
23
Reproductive Isolation
24
Reproductive Isolation
Note that prezygotic barriers proceed conception
Note that postzygotic barriers follow conception
and are costly (because gametes/time/energy is
wasted)
25
Reproductive Isolation
Speciation results from experiencing or
avoiding the cost of bearing reduced-fitness hybri
d progeny
26
Reproductive Isolation
All but geographical isolation, of reproductive
barriers, has a strong genetic component
Having a strong genetic underpinnings suggests is
product of natural selection
27
Reproductive Isolation
Mating is costly, even if there is no conception
E.g., energy and time wasted increased
susceptability to predation(?)
28
Increasing Fitness Costs

• Geographical Isolation
• Habitat Isolation
• Behavioral Isolation
• Temporal Isolation
• Mechanical Isolation
• Gametic Isolation
• Reduced Hybrid Viability
• Reduced Hybrid Fertility
• Hybrid Breakdown

Especially in blue represent increasing
costliness associated with attempting to produce
hybrid progeny
29
Modes of Speciation
30
Allopatric Speciation
At the point of mingling of previously
geographically isolated populations there is a
race between reconstitution of a single
interbreeding population and the formation of
effective barriers to reproductive isolation
31
Allopatric Speciation
Alternatively, one of the populations will drive
the other to extinction, giving rise to what
otherwise would appear (in the fossil record) to
be a case of anagenesis rather than cladogenesis
32
Geographical Barrier
Population broken into remnants by physical
barriers
Homologous population



potential for mating between individuals A and B
no potential for mating between individuals A and
B
33
Peripheral Isolates
Homologous population vs. population with
peripheral heterogeneity

"Status as a peripheral isolate merely gives a
lottery ticket to a small population. A
population can't win (speciate) without a ticket,
but there are very few winning tickets." Stephen
Jay Gould, p. 443, Campbell, 1996
34
Peripheral Isolation

• Peripheral populations
• may be different from the parental population
  before separating (because of different
  environments, extremes of range, clines, and
  natural selection for phenotypic divergence)
• may be founded by only a small number of
  individuals (due to founder effects, i.e.,
  genetic drift leading to divergence of
  populations)
• may not have an opportunity to increase in size
  over the medium term (due to bottleneck effect,
  i.e., genetic drift leading to divergence of
  populations)
• may find themselves in environments that differ
  from that of the parental population (natural
  selection for phenotypic divergence)

35
Allopatric Speciation
http//www.compusmart.ab.ca/kbush/peripheralisolat
e.jpg
"Whenever populations become allopatric, it is
possible for speciation to occur as the isolated
gene pools accumulate genetic differences by
microevolution. But an isolated population that
is small is more likely than a large population
to change substantially enough to become a new
species."
36
Hybrid Zone
37
Hybrid Zone

• When two formerly isolated populations come back
  into contact, the resulting zone of hybridization
  typically will not encompass the entire range of
  either population
• It is within the hybrid zone that the
  reproductive isolation of two populations is
  tested and evolves
• Two populations that come into contact at a
  hybrid zone will either evolve more-robust
  reproductive isolating mechanisms, e.g.,
  behavioral isolation, or will fail to, thus
  setting the stage for a melding of the two
  populations back into one population
• Two populations may be able to stably retain
  something resembling species status as a
  consequence of only limited gene exchange at the
  hybrid zone (i.e., some form of hybrid
  inviability or hybrid infertility)

38
Introgression
Introgression low level gene flow between
otherwise reproductively isolated populations
Even though F1 shows reduced fitness, that does
not mean a fitness of zero (thus mating and gene
exchange back to parental populations can still
occur)
39
Introgression
On the other hand, if the backcross has
sufficiently low fitness then introgression will
not occur
40
Scenario for Allopatric Speciation
41
Scenario for Allopatric Speciation

• Start with a single population
• Geographical barriers arise that separate that
  population into two or more smaller populations
• Note that the parental population may
• remain more or less intact while one or more
  peripheral populations may form, or
• the parental population may be broken up entirely
  into a number of remnant populations
• The peripheral populations
• may be different from the parental population
  before becoming separated
• may be founded by only a small number of
  individuals
• may not have an opportunity to increase in size
  over the medium term
• may find themselves in environments that differ
  from that of the parental population

42
Scenario for Allopatric Speciation

• Key is that the geographical barrier prevents
  gene flow between the peripheral population and
  the parental population
• Thus, the peripheral population is in the
  position to diverge genetically from the parental
  population
• Note that the fate of the majority of peripheral
  populations is extinction
• Note that the fate of the "parental" population,
  if it has been essentially broken up into a
  number of remnant populations, likely is
  extinction
• Note that regardless, speciation has not occurred
  until reproductive barriers are tested (at least
  according to the biological species concept)
• Testing of reproductive barriers occurs only
  should the geographical barrier fail thus
  allowing the peripheral population's range to
  come to overlap the range of the parental
  population

43
Scenario for Allopatric Speciation

• When the ranges of two isolated populations come
  to overlap, one of three things can result
• The two populations evolve effective reproductive
  barriers thus preventing significant allele
  exchange between populationsspeciation occurs
• The two populations exchange genes to a
  sufficient extent that speciation fails to occur
  and the two populations turn into one population
• One population can drive the other population to
  extinction
• Recall that costly postzygotic isolating
  mechanisms will drive the evolution of
  less-costly prezygotic isolating mechanisms

44
Scenario for Allopatric Speciation

• Note that should the formerly peripheral
  population succeed in driving the parental
  population to extinction, then that would appear
  (in the fossil record) as anagenesis
• Note that should speciation occur such that the
  formerly peripheral population and the parental
  population coexist, that would be an example of
  cladogenesis
• Note that should the parental population be
  reduced to remnant populations, two of which
  succeed in forming new species, i.e., ones that
  differ morphologically from the parental
  population, then this would appear in the fossil
  record as one species "suddenly" diverging into
  two (or more) different species

45
Allopatric Speciation on Islands
46
Long-Distance Dispersal (Seeds)
47
Adaptive Radiation
Adaptive radiation typically occurs when a few
organisms make their way to new, often distant
areas when environmental changes cause numerous
extinctions, opening up ecological niches for
their survivors. p. 480, Campbell Reece (2005)
48
Ring species
Subspecies may be populations caught in the act
of speciating

• Subspecies are morphologically distinct from
  other subspecies of the same species
• Members of subspecies are more likely to breed
  with their own subspecies than with other members
  of their species
• Subspecies are geographically localized

49
Sympatric Speciation
50
Sympatric Speciation
http//home.earthlink.net/snailstales2/speciation
.JPG
Sympatric speciation is the idea of speciation
events being initiated without a geographical
isolation of populations this may occur as a
consequence of isolation between
microenvironments (different trees in the same
forest, for example)
51
Sympatric Speciation
Alternatively it may involve the founding of new
populations that are reproductively isolated from
the parent population from day one
52
Habitat Isolation
These two garter snakes live in different
habitats in the same geographical area
Experiment described in your text
53
Isolation due to Autopolyploidy
Unable to successfully mate with parent
genotypes, but able to successfully mate with
others of same karyotype
54
Isolation due to Allopolyploidy
The product of both nondisjunction and
hybridization of two different species
55
Punctuated Equilibrium
56
Punctuated Equilibrium
The idea that populations do most of their
evolving in small, isolated populations forms the
basis of the concept known as punctuated
equilibrium
57
Punctuated Equilibrium

• Because most morphological change occurs in small
  populations and this change occurs over "only" a
  few 10s, 100s, or 1000s of generations, there is
  a reduced likelihood that fossilization will
  document these morphological changes step by step
  as they occur
• The "equilibrium" of the concept of punctuated
  equilibrium refers to the persistence of stable
  morphotypes in the fossil record over long
  periods (millions of years)
• The "punctuated" part of the concept refers to
  the "sudden" appearance of morphological change
  over a period of "only" a few tens of thousands
  of years
• In other words, the likelihood of fossilization
  is directly proportional to population size and
  population duration small, short-lived
  populations are poorly represented in the fossil
  record

58
Punctuated Equilibrium

• Suppose that a species survived for 5 million
  years, but most of its morphological alterations
  occurred during the first 50,000 years of its
  existence--just 1 of its total lifetime.
• Because time periods this short often cannot be
  distinguished in fossil strata, the species would
  seem to have appeared suddenly and then lingered
  with little or no change before becoming extinct.
• Even though the emergence of this species
  actually took tens of thousands of years, this
  period of change left no fossil record. p. 482,
  Campbell Reece (2005

59
Species Selection

• Various Microevolutionary processes have
  Macro-evolutionary counterparts
• Birth of individual (microevolution) ? Birth of
  species, i.e., Speciation (macroevolution)
• Death of individual (microevolution) ? Extinction
  of species (macroevolution)
• Genetic drift (microevolution) ? Species
  extinction due to random (not foreseeable) events
  (macroevolution)
• Natural selection (microevolution) ? Differential
  rates of speciation and extinction due to factors
  intrinsic to lineages, i.e., Species selection
  (macroevolution)
• These processes are analogous rather than
  identical (in the sense that the
  microevolutionary process does not necessarily
  give rise to the macroevolutionary process)

60
Species Selection

• Things that make a species more fit over the
  short term (e.g., specialization) do not
  necessarily give rise to greater rates of
  speciation or lower rates of extinction within
  lineages
• In other words, there is a reason that
  generalists persist despite the fact that
  specialists typically are more fit to the
  environments in which they evolved
• Other characteristics of a species might also
  make that species less susceptible to random
  changes in the environment (e.g., asteroid
  impact) such characteristics might include small
  size and wide range as well as a lack of specific
  dietary needs, etc.
• "The species that endure the longest and generate
  the greatest number of new species determine the
  direction of major evolutionary trends.
• Thus, to impact greatly on the evolution of the
  diversity of life, an organism must possess
  qualities that go beyond simply being highly
  adapted to life within a specific environment

61
How Morphological Change Happens
62
(No Transcript)
63
The End