The Origin of Species How species emerge

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The Origin of Species How species emerge

• AP Biology Chapter 24

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Introduction

• The beginning of new forms of life the origin
  of species - is the focal point of evolution
  
• It is the creation of new species that leads to
  diversity
  
• Cant just explain how adaptations evolve in
  populations (microevo.)
  
• Evolutionary theory must explain MACROEVOLUTION.

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Some Terms

• Macroevolution the origin of new taxonomic
  groups (new species, genera, families,
  kingdoms).
  
• Speciation origin of new species. The keystone
  process because any higher taxon (genus, family,
  etc.) originates with a new species that is
  different enough to be the first member of a new
  taxon

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Two Main Processes in Speciation

• Revealed by fossil record
• Anagenesis accumulation of changes that
  transforms one species into another
  
• Cladogenesis branching evolution building of
  one or more species from a parent species that
  continues to exist. Only this type promotes
  diversity increases number of species.

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What is a Species?

• Biological Species Concept
• Population or group of populations whose members
  have the potential to interbreed in nature to
  produce fertile, viable offspring.
  
• Cannot produce fertile, viable offspring with
  members of another species.
  
• Exception to rule stems from species being
  defined in natural environments.
  
• In labs and zoos some fertile hybrids CAN be
  produced, but species would NOT interbreed in
  nature.

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What is a Species?

• Biological species concept is based on fertility
  rather than physical similarity.
  
• Eastern and western meadowlarks appear similar,
  but represent different species because they do
  not interbreed in nature.
  
• Humans seem diverse, but belong to one species
  because we can interbreed.

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What is a Species?

• Biological species concept hinges on reproductive
  isolation.
  
• Thus, there must be BARRIERS that isolate the
  gene pools of biological species.

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Reproductive Barriers

• Barriers to reproduction lead to reproductive
  isolation between species.
  
• No single barrier may completely isolate one
  species from another, but many species are
  separated by more than one reproductive barrier.
  

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What Makes a Reproductive Barrier?

• Only biological barriers to reproduction are
  considered
  
• That means barriers that are INTRINSIC to the
  organisms involved.
  
• Geographical barriers dont count.
• Obviously, if two species are geographically
  separated, they cannot interbreed.
  
• Reproductive isolation, however, prevents
  different species from interbreeding even if
  their ranges overlap.

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2 Main Types of Repro. Barriers

• Prezygotic reproductive barriers
• Prevent mating between species OR
• Prevent fertilization of egg if mating does
  occur
  
• Postzygotic reproductive barriers
• IF fertilization does occur between species,
  these barriers prevent the fertilized egg from
  developing into a viable fertile adult

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Prezygotic Barriers

• Habitat isolation
• Two species may live in the same geographic area,
  but in different habitats and thus encounter each
  other rarely.
  
• Example Garter snakes from the genus
  Thamnophis occur in the same area, but one lives
  in water while the other is terrestrial.
  
• Parasites, while living in the same geographic
  area are often confined to different hosts.

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Prezygotic Barriers

• Behavioral isolation
• Signals and rituals that attract mates and are
  unique to a species.
  
• May be most important barrier in species that are
  closely related.
  
• Example fireflies of different species attract
  mates by blinking their lights in particular
  patterns
  

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Prezygotic Barriers

• Behavioral isolation, continued
• Example Eastern and Western Meadowlarks
• Nearly identical in shape coloration, habitat and
  ranges overlap.
  
• Remain separate species because of difference in
  songs that allow them to recognize individuals of
  their own kind

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Prezygotic Barriers

• Behavioral isolation, cont.
• Example Courtship rituals are specific to
  species.
  
• Blue-footed boobies will mate only after a
  specific ritual of courtship displays
  
• Part of the ritual calls for the male to
  high-step which advertises his bright blue feet
  to the female.

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Prezygotic Barriers

• Temporal isolation
• two species that breed during different times of
  day, different seasons, or different years cannot
  mix their gametes.
  
• Example Western and Easter spotted skunk ranges
  overlap, but one mates in late summer and the
  other in late winter
  
• Example 3 similar orchid species (genus
  Dendrobium) in the same rain forest do not
  hybridize because they flower on different days.
  Pollination is limited to only ONE day because
  flowers open in the morning and wither that
  evening.

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Prezygotic Barriers

• Mechanical isolation
• Closely related species may attempt to mate, but
  fail because they are anatomically incompatible.
  
• Common among flowering plants pollinated by
  specific insects or other animals.
  
• floral anatomy is often adapted to certain
  pollinators that transfer pollen only among
  plants of the same species.

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Bird Pollinated Flowers
Bird Beak Shape
Flower
egg
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Bird Pollinated Flowers
Bird Beak Shape
Flower
egg
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Prezygotic Barriers

• Gametic isolation Even mating occurs, sperm and
  egg rarely fuse to form a zygote.
  
• Sperm of different species often cannot survive
  in female reproductive tract long enough to
  fertilize egg.
  
• Gamete recognition - likely based on the presence
  of specific molecules on the coats around the egg
  which adhere only to complementary molecules on
  the sperm of the same species.
• This is often the case with flowers. They can
  discriminate between pollen of same and different
  species.

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Postzygotic Barriers

• Reduced Hybrid Viability
• IF hybrid zygotes form, genetic incompatibility
  may abort development of the hybrid at some
  embryonic stage.
  
• Example Frogs of genus Rana are often in same
  regions and habitats and occasionally hybridize.
  
• Hybrids do not usually complete development. If
  they do they are frail.

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Postzygotic Barriers

• Reduced Hybrid Fertility Even if two species
  mate and produce a hybrid offspring, isolation
  remains in tact if that offspring is infertile.
  
• One cause is failure of meiosis to produce normal
  gametes in the hybrid offspring if the two
  parents have chromosomes of different number or
  structure.
• Example Horse Donkey Mule

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Postzygotic Barriers

• Hybrid Breakdown
• First generation of hybrids viable and fertile.
• When hybrids mate with each other or with parent
  species the next generation is feeble or
  sterile.
  
• Example Different cotton species can produce
  fertile hybrids, but breakdown occurs in the next
  generation
  
• Offspring of hybrids die as seeds or grow into
  week and defective plants.

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Summary of Reproductive Barriers
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Biological Species Concept Doesnt Always Work

• Extinct life forms
• Fossils must be classified according to
  morphology
  
• Asexual life forms
• Even some sexual life forms
• Coyotes can interbreed with wolves and dogs, yet
  all three remain distinct.
  
• Certain groups of subspecies may not directly
  breed with each other, but their genes may get
  passed to each other by breeding with other
  subspecies that do breed with each other.

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Alternative Species Concepts other ways of
defining species

• Biological species concept already discussed.
  Emphasizes reproductive isolation
  
• Morphological species measurable anatomical
  differences. Most species already classified
  were classified by this method.
  
• Recognition species concept
• Cohesion species concept
• Ecological species concept emphasizes species
  niches in environment
  
• Evolutionary species concept emphasizes
  evolutionary lineages and ecological roles.

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Modes of Speciation

• There are two main ways that speciation occurs.
• These modes are based on how gene flow between
  populations is interrupted.
  
• Allopatric speciation some geographical barrier
  physically isolates populations and initially
  blocks gene flow.
  
• Sympatric speciation involves some factor
  intrinsic to the organism altering gene flow
  between populations. The populations become
  isolated even though their ranges overlap.
• Chromosomal changes
• Nonrandom mating

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2 Modes of Speciation
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Allopatric Speciation

• Geographical barriers
• mountain range emerges and gradually splits a
  population.
  
• Land bridge (Isthmus of Panama) may form and
  separate marine life on either side.
  
• Creeping glacier may divide a population
• If individuals colonize a new isolated area, the
  colonizing population may become isolated from
  the parent population

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Allopatric Speciation - Example

• A geographical feature that is a barrier to one
  species may not be to some other.
  
• Grand Canyon is easily crossed by hawks, but is
  impossible for small rodents to cross.

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Allopatric Speciation - Example

• Another example Death Valley Pupfish
• Death valley was once rainy and had a system of
  interconnected lakes and rivers
  
• Drying trend began 10,000 years ago
• Left only isolated springs that vary in temp and
  salinity no more than a few meters across.

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• In these springs live pupfishes
• Each spring has its own species found no where
  else.
  
• Various pupfishes probably descended from one
  ancestor whose range was broken up when the
  region became arid.
  
• Once separated, the ancestral populations
  diverged from each other to create all the
  species found today.
  

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Allopatric Speciation Favorable Conditions

• Conditions that favor allopatric speciation
• an isolated population that is small is more
  likely than a large population to change enough
  to become a new species.
  
• In fact, geographical isolation of a small pop.
  usually occurs at the fringe of the parent
  population. WHY?

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Allopatric Speciation Fringe Populations

• Why fringe populations are more likely to undergo
  allopatric speciation
  
• Gene pool of the fringe population probably
  differs from that of the parent population from
  the outset.
  
• Living near the border of the range means the
  population represents the extremes of any clines
  that were present
  

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Allopatric Speciation Fringe Populations

• Why fringe populations are more likely to undergo
  allopatric speciation
  
• If fringe population is small, founder effect
  comes into play.
  
• May cause neutral variation to become fixed by
  chance.
  
• Causes divergence from parent population.

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Allopatric Speciation Fringe Populations

• Why fringe populations are more likely to undergo
  allopatric speciation
  
• Differences in Natural Selection Factors
• Because it inhabits a frontier, the fringe
  environment is somewhat different from the
  parents. The fringe population will probably
  encounter selection factors different from those
  affecting the parent population.

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Allopatric Speciation Success of Fringe
Populations

• Just because a fringe population gets isolated,
  doesnt mean it will survive to become a new
  species
  
• a fringe species has a lottery ticket
• Lots of lottery tickets get issued, but only a
  few win. Stephen Jay Gould

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Allopatric Speciation - Islands

• Islands are HUGELY important in the study of
  speciation.
  
• Organisms stray from parent populations and found
  new populations on different islands that evolve
  in isolation.

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Allopatric Speciation - Islands

• Example Galapagos Finches
• single dispersal event may have brought a small
  population of ancestral finches to one island
  
• This fringe population formed a new species
• A few individuals of this island species may have
  reached neighboring islands where geographical
  isolation permitted additional speciation
  episodes.
• After diverging on a different island, a new
  species could even recolonize the original island
  and coexist with the parent species.

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Allopatric Speciation - Islands
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Allopatric Speciation - Islands

• ADAPTIVE RADIATION evolution of many diversely
  adapted species from a common ancestor.

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Sympatric Speciation

• New species arise within the range of parent
  population
  
• How can reproductive isolation occur without
  separation of species geographically?
  
• Example a new species can be generated in a
  single generation if a genetic change results in
  a reproductive barrier between the mutants and
  the parent population.

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Causes of Sympatric Speciation

• Polyploidy
• accident during cell division that results in an
  extra set of chromosomes
  
• Two ways that polyploidy can occur
• Autopolyploid arise from a single species
• Allopolyploid arise from two different species

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Causes of Sympatric Speciation

• Types of Polyploidy
• Autoploidy
• An autopolyploid is an individual that has more
  than two chromosome sets, all derived from a
  single species
  
• Failure of meiosis during gamete production can
  double chromosome number from the diploid count
  2n to 4n
  
• The tetraploid that results can then fertilize
  itself or mate with other tetraploids
  
• It CANNOT mate with parents because offspring
  would be 3n and sterile because unpaired
  chromosomes result in abnormal meiosis
  
• Thus, reproductive isolation is achieved.
• Common in PLANTS.

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Causes of Sympatric Speciation

• Autoploidy

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Causes of Sympatric Speciation

• Polyploidy
• Allopolyploid
• More common than autopolyploid condition
• Results when TWO different species combine their
  chromosomes.
  
• Hybrid offspring usually sterile
• Haploid set from one parent cannot pair during
  meiosis with haploid set from second parent.
  
• However, hybrid may be more vigorous than its
  parents AND propagate asexually
  
• AND there are mechanisms that can transform
  sterile hybrids to fertile polyploids!
  

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Causes of Sympatric Speciation

• More on Allopolyploids
• Especially vigorous
• Combine best qualities of both parents?
• Speciation of polyploids (allopolyploids
  especially) accounts for 25-50 of plant
  species!
  

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Causes of Sympatric Speciation

• Example of Allopolyploid
• New species of salt-marsh grass in England
• Derived from
• European sp. (Spartina maritima)
• American sp. (Spartina alternaflora)
• Invasive American sp. hybridized with European
  sp.
  

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Causes of Sympatric Speciation

• Results of marsh grass allpolyploidy
• A third species (Spartina anglica) arose from the
  two different parent species
  
• Chromosome numbers are consistent with
  allopolyploid origin
  
• S.maritima, 2n 60
• S. alternaflora, 2n 62
• S. anglica, 2n 122
• New species is very successful and has become
  somewhat of a pest.

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More Plant Polyploids

• Oats
• Cotton
• Potatoes
• Tobacco
• Wheat
• Scientists work to create more plant polyploids
  with desirable traits.

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Sympatric Speciation in Animals

• Sympatric speciation may occur in animals, though
  mechanisms are different than the c-some
  doublings of plants.

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Sympatric Speciation in Animals

• Conditions required
• Genetic factors cause a population within a
  parent population to become fixed on some
  resource(s) not used by the parent population.

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Sympatric Speciation in Animals

• Example of Sympatric Speciation in Animals
  Wasps that pollinate figs
  
• Each fig species is pollinated by a different
  species of wasp

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Sympatric Speciation in Animals

• Fig Wasps, cont.
• Wasp mates and lays eggs in figs
• Genetic change that caused wasp to select a
  different fig would segregate mating individuals
  of this group from the parent population sets
  stage for divergence.

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How Fast is Speciation?

• Traditional view Gradualism
• Speciation is the product of gradual divergence
  over a long span of time.
  
• Problem
• Gradual transitions of fossils seldom found.
• Instead, species tend to
• appear as new forms suddenly
• Persist unchanged for their time on Earth
• Disappear from fossil record as suddenly as
  they came

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How Fast is Speciation?

• Punctuated Equilibrium
• Addresses the nongradual appearance of species in
  fossil record.
  
• Says species diverge in spurts of relatively
  rapid change NOT slowly and gradually
  
• Remember, we are talking rapid in geologic time
  terms!
  

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How Fast is Speciation?
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The Origin of Evolutionary Novelty

• What processes cause large-scale evolutionary
  changes?
  
• Ex. Flight adaptations of birds?
• Progressive changes like increased brain size
  during human evolution

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Most Evolutionary Novelties are Modifications of
Older Structures

• Exaptation a structure that evolved in one
  context becomes co-opted for another function
  
• Example lightweight honeycombed bones in birds
  aid in flight

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Birds From Dinosaurs?

• Fossil evidence indicates that birds evolved from
  a lineage of earthbound dinosaurs. Exaptation
  played a roll.
  
• These dinosaurs also had lightweight honeycombed
  bones
  
• This feature did not evolve in the
  anticipation of flight
  
• Instead, lightweight bones must have been an
  advantage for the original land bound dinosaurs.
  
• Made them lighter and more agile in catching
  prey
  
• Wing-like forelimbs and feathers are also
  observed in fossils of these dinosaurs
  
• Again, this feature did NOT evolve in
  anticipation of flight.
  
• Instead, winglike forelimbs and feathers must
  have been adaptive in some other way perhaps
  providing a large surface area for trapping prey
  or in courtship display (or both).

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Birds From Dinosaurs?

• The first flights may have been only extended
  hops in pursuit of prey or escape from a
  predator.
  
• However, once these hops/flights became an
  advantage, natural selection would have remodeled
  feathers and wings to better fit this function.

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Genes that Control Development Play a Major Role
in Evolutionary Novelty

• Sometimes just a few changes in the genome of an
  organism can result in major structural
  modifications.
  
• Genes that program development of an organism
  control the changes in an organisms form as it
  goes from zygote to adult. These genes control
  
• Rate of changes
• Timing of changes
• Spatial patterns of changes

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Genes that Control Development Play a Major Role
in Evolutionary Novelty

• Allometric Growth
• A difference in the relative rates of growth of
  various parts of the body
  
• Helps shape an organism
• If relative growth rates of body parts are
  changed even slightly, the adult form is changed
  substantially.

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Genes that Control Development Play a Major Role
in Evolutionary Novelty

• Allometric Growth Example
• Different allometric patterns contribute to the
  contrasting shapes of human and chimpanzee
  skulls
  
• Thus, a small change in some developmental genes
  causes profound changes in the adult organism.

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Genes that Control Development Play a Major Role
in Evolutionary Novelty

• Genetic changes can also alter timing of
  developmental events This is called
  Heterochrony
  
• Example - Paedomorphosis
• In some species, a sexually mature adult retains
  features that were juvenile structures in its
  ancestors
  
• Some salamanders retain gills and other juvenile
  features in the sexually mature adult form
  
• This small genetic change in developmental timing
  results in an a very different adult form.

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Genes that Control Development Play a Major Role
in Evolutionary Novelty

• Another example of Heterochrony
• Human brain is proportionately larger than the
  chimpanzee brain
  
• This is because the growth of the brain in a
  human is switched off much later in a human
  than in a chimpanzee
  

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Genes that Control Development Play a Major Role
in Evolutionary Novelty

• Homeosis
• Alteration in the basic body design or spatial
  arrangement of body parts.
  
• Homeotic genes
• Relatively small sets of genes that function as
  developmental master switches

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Genes that Control Development Play a Major Role
in Evolutionary Novelty

• Example of Homeotic Genes
• Homeotic genes initiate developmental events that
  determine such basic features as where a pair of
  wings and a pair of legs will develop on a bird.
  
• Mutations in homeotic genes create drastic
  changes in the body plan of an organism

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Genes that Control Development Play a Major Role
in Evolutionary Novelty

• The Hox complex
• A duplication of a cluster of homeotic genes
  (called the Hox complex) that occurred about 520
  million years ago may have been the event that
  caused the rise of vertebrates
• Vertebrates have multiple sets of the homeotic
  genes in this complex while invertebrates only
  have a single cluster of the Hox complex genes.

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Vertebrate Evolution an Hox genes
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Evolution as a bush, not a tree
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Evolution is not goal oriented

• Species as individuals
• Birth speciation
• Death extinction
• Offspring new species
• The longer lived a species, the more new species
  will arise from it and have more influence on
  major evolutionary trends
  
• HOWEVER, the appearance of a trend should not be
  interpreted as a drive toward some particular
  phenotype.
  

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Evolution is not goal oriented

• Evolution is the result of the interactions
  between organisms and their current environment
  not some future one.
  
• If environmental conditions change, an apparent
  trend can cease or even reverse itself.