The Origin of Species

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The Origin of Species

• Chapter 24

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Basic Patterns of Evolution

• Anagenesis ? one species accumulates heritable
  changes, gradually the species becomes a
  different species
• Cladogenesis ? branching evolution, one species
  to several w/ potential for interbreeding

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Biological Concept of Species

• A population or group of populations whose
  members have the potential to interbreed with
  each other in nature to produce viable, fertile
  offspring, but who cannot produce viable, fertile
  offspring with members of other species

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Species are based on interfertility, not physical
similarity

• The eastern and western meadowlarks may have
  similar shapes and coloration, but differences in
  song help prevent interbreeding between the two
  species

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Humans have considerable diversity,but we all
belong to the same species because of our
capacity to interbreed
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How are Species kept separate?

• Reproductive Barriers ? prevents populations
  belonging to different species from
  interbreeding, even if their ranges overlap
• Reproductive barriers
• prezygotic
• postzygotic,

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Pre zygotic Barriers

• Impede mating between species or hinder
  fertilization of ova if members of different
  species attempt to mate
• Habitat isolation
• Behavioral Isolation
• Temporal Isolation
• Mechanical Isolation
• Gametic isolation

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Post zygotic Barriers

• Prevents the hybrid zygote from developing into a
  viable, fertile adult
• Reduced hybrid viability
• Reduced hybrid fertility
• Hybrid breakdown

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Habitat Isolation

• Two organisms that use different habitats even in
  the same geographic area are unlikely to
  encounter each other to even attempt mating
• Two species of garter snakes, in the genus
  Thamnophis, that occur in the same areas but
  because one lives mainly in water and the other
  is primarily terrestrial, they rarely encounter
  each other.

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Behavioral Isolation

• Many species use elaborate behaviors unique to a
  species to attract mates
• Visual Fireflies
• Bird plumage
• Red Stickleback
• Fiddler Crabs
• Fruit Fly
• Blue Footed Booby
• Olfactory?Scandinavian moth?Pheromones
• Auditory Bird songs, frog calls

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Temporal Isolation

• Two species that breed during different times of
  day, different seasons, or different years cannot
  mix gametes
• Flowers
• Skunk mating seasons
• Frogs

Wood frog 44o
Leopard Frogs 55o
Green Frog 60o
Bull Frog Above 60o
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Mechanical Isolation

• Closely related species may attempt to mate but
  fail because they are anatomically incompatible
  and transfer of sperm is not possible
• Flower structure of certain flowering plants
  prevents pollination by insects or other animals
• With many insects the male and female copulatory
  organs of closely related species do not fit
  together, preventing sperm transfer

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Gametic Isolation

• Gametes of two species do not form a zygote
  because of incompatibilities preventing fusion or
  other mechanisms
• Sperm/egg recognition
• Reproductive Tract unfavorable

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Post Zygotic Barriers

• Reduced hybrid viability
• Hybrids may be frail?hybrids between frogs in the
  genus Rana, which do not complete development and
  those that do are frail.

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Reduced Hybrid Fertility

• Hybrids may be vigorous but may be infertile and
  the hybrid cannot backbreed with either parental
  species
• Horse (2n 64)
• Donkey (2n 62)

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Reduced Hybrid Fertility

• Mule (which is sterile)
• Hence, donkeys and horses are separate species

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Hybrid Breakdown

• Some first generation hybrids are viable and
  fertile, but when the mate with one another,
  offspring are feeble

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Species Concept

• Biological Concept?Species are groups of
  actually or potentially interbreeding natural
  populations, which are reproductively isolated
  from other such groups. Ernst Mayr.
• Morphological species concept, the oldest and
  still most practical, defines a species by a
  unique set of structural features
• Ecological species defines a species in terms of
  its ecological niche, the set of environmental
  resources that a species uses and its role in a
  biological community

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Biogeography of Species

• Two ways in which speciation can occur.
• Allopatric speciation occurs when a gene pool is
  divided into two
• Sympatric speciation occurs without geographic
  separation

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Allopatric Speciation

• A geographic barrier isolates the population and
  species are maintained
• Squirrels on the N/S of the Grand Canyon

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Allopatric speciation of squirrels in the Grand
Canyon
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Sympatric Speciation

• Reproductive isolation without geographic
  isolation
• Autopoliploidy
• Allopolyploidy
• Non-random mating animals

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Autopoliploidy

• Single parent doubles chromosome
• Results in TETRAPLOIDS

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Allopolyploidy

• 2 different species are involved?polyploid hybrid
  is formed
• This is how modern wheat, oats, cotton, potatoes
  were developed
• More important in plant evolution

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Non-random mating animals

• Mate choice is based on coloration

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Adaptive Radiation

• Evolution of many diversely adapted species from
  a common ancestor upon introduction of new
  environmental opportunities

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Tempo of Speciation

• Niles Eldredge/Steven J. Gould
• Punctuated Equilibrium
• Gradualism

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Macroevolution

• Leads to new taxonomic groups
• Origin of mammals from reptiles
• Feathers and flight
• Increasing brain size of mammals
• Adaptive radiation of flowering plants

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Macroevolution

• Evolutionary novelties are modified versions of
  older structures
• Exaptations Preadaptations
• structures that evolve in one context but become
  co-opted for another
• Ex. Ancestral reptiles
• Hollow bones in birds
• Evolution is like modifying a machine while it
  is running!

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Evo-Devo

• The role evolution has in development
• Ex Shape of an organism depends on relative
  growth rate of its parts
• Changes in Rate and Timing
• Allometric Growth
• Heterochrony
• Paedomorphosis
• Changes in Spatial Patterns
• Homeotic Genes

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• Differential growth rate? expanded time of brain
  development

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Figure 24.19 Allometric growth

• Allometric growth proportional change

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Heterochrony
Longer time for foot growth results in longer
digits and less webbing
Foot growth ends sooner-shorter digits and more
webbing
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Paedomorphosis

• Juvenile traits extended to adult
• Salamanders that retain gills dont have to leave
  the water

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Homeotic Genes

• Control the 3D placement of structures
• HOX gene? organize the embryo in space
• Invertebrates? 1 set
• Vertebrates? several sets? more complex growth

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Figure 24.18 A range of eye complexity among
mollusks