15'1: Speciation

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15.1 Speciation Biodiversity
Key Terms biological species concept
macroevolution speciation reproductive
isolation geographic isolation adaptive
radiation punctuated equilibrium

• Objectives
• Describe the biological species concept.
• Distinguish between microevolution and
  macroevolution.
• List types of reproductive barriers between
  species.
• Explain how geographic isolation and adaptive
  radiation contribute to species diversity.
• Summarize models for the tempo of speciation.

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In 1928, a young biologist named Ernst Mayr led
an expedition into the remote mountains of New
Guinea to study the wildlife.
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What is a species?

• Mayr found a great variety of birds, eventually
  identifying 138 species based on their different
  appearances.
• He was surprised to learn that his list of bird
  species agreed almost exactly with the species of
  local birds recognized by the local natives.
• To Mayr, the experience was evidence that species
  represent recognizably distinct forms of life.
• How do today's biologists identify species? How
  do species arise?

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Biological Species Concept

• So, a species can be described as a distinct
  form of life.
• Many biologists use the biological species
  concept, which defines a species as a
  population, or group of populations, whose
  members have the ability to breed with one
  another in nature produce fertile offspring.
  (Fertile offspring are capable of mating and
  producing offspring.)
• Members of one species cannot successfully
  interbreed with members of other species.

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Appearance isnt everything Organisms may appear
to be alike and be different species. For
example, Western meadowlarks (Sturnella neglecta)
and Eastern meadowlarks (Sturnella magna) look
almost identical to one another, yet do not
interbreed with each otherthus, they are
separate species according to this definition.
The Western meadowlark (left) and the Eastern
meadowlark (right) appear to be identical, and
their ranges overlap, but their distinct songs
prevent interbreeding.
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• Organisms may look different and yet be the same
  species. For example, look at these ants. You
  might think that they are distantly related
  species. In fact, they are sisterstwo ants of
  the species Pheidole barbata, fulfilling
  different roles in the same colony.

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• The biological species concept has limitations.
  For example, organisms that only reproduce
  asexually (produce offspring from a single
  parent) are not included.
• Fossils, of course, are no longer reproducing, so
  they cannot be evaluated by this definition
  either.
• Even with these exceptions, the biological
  species concept is useful. This species concept
  helps biologists understand the origin of new
  species.

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From Microevolution to Macroevolution

• Microevolution refers to change in the allele
  frequencies within a population. It includes
  mechanisms such as gene flow, genetic drift, and
  small-scale natural selection.
• Microevolution and adaptation explain how
  populations evolve. But if that were all that
  happened, Earth would be inhabited only by a
  highly adapted version of the first form of life.

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• The term macroevolution encompasses dramatic,
  drastic biological changes (many of which are
  evident in the fossil record).
• These changes include
• the origin of different species
• the extinction of species
• the evolution of major new features of living
  things, such as wings or flowers. The origin of
  new species is known as speciation .

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Speciation is the main focus of the study of
macroevolution because with speciation comes
biological diversity.
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• If one species evolves into two or more surviving
  species, diversity increases.
• This figure shows a simple example of how
  speciation can lead to an increase in the number
  of species. In this case, the ancestral
  (original) species branches into two separate
  species, increasing the diversity of life.

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Reproductive Barriers Between Species

• Clearly, a fly will not mate with a frog or a
  fern. But what about species that are not so
  different? The inability to interbreed marks
  species as separate. If so, what keeps existing
  species that are similar and closely related from
  interbreeding?
• For example, the western spotted skunk and the
  eastern spotted skunk are so similar that only
  other spotted skunks and expert biologists can
  tell them apart. Where the skunks' ranges overlap
  in the Great Plains region, individuals from
  these two species do not mate.
• Why not? Some kind of reproductive barrier keeps
  the two species from interbreedinga condition
  known as reproductive isolation.

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• Some of the barriers that contribute to
  reproductive isolation include the following
  circumstances
• Timing Two similar species may have different
  breeding seasons. The skunks fit this category.
  Western spotted skunks breed in the fall, but the
  eastern species breeds in late winter. The timing
  of their breeding seasons keeps these species
  separate even where they coexist in the Great
  Plains.
• Behavior Two similar species may have different
  courtship or mating behaviors. For example,
  eastern and western meadowlarks are almost
  identical in shape, coloring, and habitat. Like
  the skunks, the ranges of these birds in the
  central United States overlap. Yet they remain
  separate species because their courtship rituals
  differ, including the songs that attract mates.
• Habitat Some species remain reproductively
  isolated because they are adapted to different
  habitats in the same general location. For
  example, certain lakes in British Columbia,
  Canada, contain two different species of
  three-spined stickleback fish. One species is
  adapted to living along the lake bottom, feeding
  on small snails. Fish of the other species spend
  most of their lives in the open water, filtering
  plankton (small floating organisms). The two
  species' preferences for different habitats help
  maintain their isolation.
• These are all mechanisms of prezygotic
  reproductive isolation.

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Other Reproductive Barriers

• In addition to timing, behavior, and habitat,
  other barriers can keep species reproductively
  isolated. For instance, two seemingly similar
  species may be unable to mate because their
  reproductive structures are physically
  incompatible.

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These damselfly penises illustrate just how
complex insect genitalia may be.
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Or, as in the case of some plants, the insects or
other animals that transfer flower pollen may do
so only among plants of a single species.
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• Some reproductive barriers come into play after
  fertilization takes place (postzygotic). A hybrid
  zygote may fail to develop. Or, some hybrid
  offspring may mature into adults, but they are
  infertile.
• In most cases, reproductive isolation results
  from a combination of two or more barriers. Such
  barriers often come about as "side effects" of
  other adaptations. For example, the different
  breeding seasons of the eastern and western
  spotted skunks probably were individual
  adaptations of each skunk species. These
  adaptations likely arose when the ancestral
  populations of the two species were isolated in
  different locations. If reproductive isolation
  keeps species separate after the species arise,
  then the origin of these barriers is the key to
  the origin of new species.

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Reproductive Barriers

• Prezygotic Barriers
• Geographic isolation-species occur in different
  places
• Ecological isolation-species utilize different
  resources
• Behavioral isolation-species perform different
  courtship behavior
• Temporal isolation-species breed at different
  times
• Mechanical isolation-structural differences
  prevent sperm or pollen transfer
• Gametic isolation-sex cells incompatible no
  fusion of egg sperm
• Postzygotic Barriers
• Hybrids do not develop properly
• Hybrids do not survive in environment
• Hybrids have reduced or no fertility

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Geographic Isolation and Speciation

• Geologic processes constantly change and
  rearrange Earth's features. Such change can
  separate different populations of one species. A
  mountain range may gradually emerge, slowly
  splitting a population of organisms that cannot
  cross it. A creeping glacier may slowly divide a
  population. In other cases, populations become
  separated when a small group disperses from the
  main population and colonizes an isolated
  location, such as an island.
• Separation of populations as a result of
  geographic change or dispersal to geographically
  isolated places is called geographic isolation.

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• How well a geographic barrier keeps populations
  apart depends on the ability of organisms to move
  about.
• For example, biologists hypothesize that two
  species of antelope squirrels near the Grand
  Canyon evolved from geographically separate
  populations. These species live on opposite rims
  of the canyon. Harris's antelope squirrel
  (Ammospermophilus harrisii) lives on the south
  rim. Just a few miles away on the north rim is
  the closely related white-tailed antelope
  squirrel (Ammospermophilus leucurus).
• Such small rodents may find a deep canyon or wide
  river too daunting to cross. In contrast, birds,
  mountain lions, and coyotes can navigate mountain
  ranges, rivers, and canyons. The windblown pollen
  of pine trees or the seeds of plants carried on
  animals also move back and forth.

Common ancestor
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• The separation of a small "splinter" population
  from its main population is a crucial event in
  the origin of species. Once separate, the
  splinter population may follow its own
  evolutionary course. Genetic driftchange in a
  gene pool due to chanceplays a key role in
  microevolution. Changes in allele frequencies
  caused by genetic drift and natural selection can
  accumulate in the splinter population, making it
  less and less like the main population.
• For each small, isolated population that becomes
  a new species, many more simply perish. Life in
  some environments is harsh, and most colonizing
  populations probably fail to survive in their new
  location. Even if such populations survive and
  adapt to their local environments, they do not
  necessarily evolve into new species.

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• The arrows symbolize populations that become
  geographically separated, then come together
  again at a later time.
• Speciation has occurred only if one population
  can no longer breed with the other population,
  even if the two populations should come back into
  contact.
• 2 possible outcomes for populations that meet
  again after having been geographically separate
• the changes do not prevent interbreeding, and the
  populations are still one species.
• the two populations have evolved in ways that
  prevent them from interbreeding. They have become
  two distinct species.

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Adaptive Radiation

• Since Darwin's time, islands have served as
  living laboratories for studying speciation.
  Islands often have species found nowhere else.
  The isolation and diverse habitats of some
  islands create conditions that seem to favor
  speciation.
• Only a few organisms manage to be the first to
  colonize new islands. Those that do, enter a
  diverse, "empty" environment. The small
  populations of colonizing species may undergo
  evolutionary change. Some of these organisms may
  move on to other islands in the chain, where the
  process repeats itself.
• New and varying species may evolve through
  genetic drift and adaptation to the different
  habitats. Such evolution from a common ancestor
  that results in diverse species adapted to
  different environments is called adaptive
  radiation.

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• Adaptive radiation on an island chain may lead to
  several new bird species evolving from one
  founding population.

In this example, one species is the common
ancestor of several new species that arise on the
islands. After migrating from the mainland,
species A may have undergone significant change
in its gene pool and become species B. Later, a
few birds of species B may have migrated to a
neighboring island. This population could have
evolved into species C. Some of these birds could
later move back to the first island. They might
coexist with species B if reproductive barriers
keep the two species separate. Species C could
also move among other islands where the same
evolutionary processes might continue
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• The Hawaiian Islands are one of the world's great
  showcases of evolution. The islands are about
  4,000 kilometers from the nearest continent, and
  each island is itself physically diverse. A range
  of altitudes and differences in rainfall on each
  island create multiple environments. Originally,
  the islands were uninhabited. New lava flows
  continually increased the amount of vacant land
  (and still do). These conditions supported
  repeated instances of adaptive radiation. Most of
  the thousands of native species on the islands
  are found nowhere else in the world.

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The Tempo of Speciation

• On the time scale of the fossil record, species
  often seem to arise abruptly. A new fossil
  species may appear rather suddenly (in geological
  terms) in a layer of rock, and persist for
  thousands or millions of years without noticeable
  change. Then, it may disappear from the fossil
  record as suddenly as it appeared.

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• Over the past 30 years, some evolutionary
  biologists have developed a model to address
  these observations. Now known as punctuated
  equilibrium, the model suggests that species
  often diverge in spurts of relatively rapid
  change. Then many newly formed species may remain
  mostly unchanged, at least in ways that are
  evident in the fossil record.
• The term punctuated equilibrium comes from the
  idea that long periods of little change
  (equilibrium) in a species are broken, or
  punctuated, by shorter times of speciation.

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• In contrast to a more gradual model of
  evolution, punctuated equilibrium suggests that a
  new species changes most as it buds from a parent
  species. There is little change for the rest of
  the time the species exists.

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Given a model of gradual adaptation through
natural selection, how could species have sudden
bursts of change? Speciation can sometimes be
quite rapid. In just a few hundred to a few
thousand generations, genetic drift and natural
selection can cause significant change in a small
population that is occupying a challenging new
environment.
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• The fossil record indicates that successful
  species last, on average, about one to five
  million years.
• A particular species may have accumulated most of
  its unique changes in its first 50,000 years.
  Though this time span may seem long on a human
  scale, it only represents a hundredth of the
  lifetime of a typical species and a short
  interval of time on the scale of the fossil
  record.
• This explanation would account for the punctuated
  equilibrium that scientists often observe in the
  fossil record. Remember, too, that the best
  candidates for speciation are small populations.
  Fossils from such populations are rare. By the
  time a new species grew in number and became
  widespread enough that it might leave a fossil
  record, its distinctive features would have
  already evolved.

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• Keep in mind that punctuated equilibrium does not
  contradict or weaken Darwin's theory. The theory
  of natural selection can account for observations
  of punctuated equilibrium in the fossil record.
  Natural selection and adaptation still happen,
  but mostly during that time when a species is
  "young."
• The addition of punctuated equilibrium to
  evolutionary biology demonstrates the principle
  of refining a scientific theory to reflect new
  evidence

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Concept Check 15.1

• 1. Why are donkeys and horses considered
  different species?2. What is macroevolution?3.
  Give an example of a reproductive barrier that
  may separate two similar species.4. Describe
  conditions that could make a new island a likely
  place for adaptive radiation.5. How does
  punctuated equilibrium relate to Darwin's theory
  of natural selection?

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15.1 Online Review

• Ernst Mayer 1, 2, 3
• Biological Species Concept 1, 2, 3
• Macroevolution 1, 2, 3, 4
• Speciation 1, 2, 3
• Punctuated Equilibrium 1, 2, 3
• Macroevolution/Speciation Quiz 1, 2, 3, 4, 5, 6,
  7

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