The Origin of Species

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The Origin of Species
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Patterns of Speciation

• Fossil record indicates two patterns
• anagenesis (phyletic evolution) - an unbranched
  lineage
• cladogenesis -branching evolution
• most commot form
• promotes greater diversity by increasing the
  number of species

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Patterns of Speciation

• Fossil record indicates two patterns
• anagenesis (phyletic evolution) - an unbranched
  lineage
• cladogenesis -branching evolution
• most commot form
• promotes greater diversity by increasing the
  number of species

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Two Concepts of Species

• Morphospecies - defined by their anatomical
  features
• Biological Species - population whose members
  have the potential to interbreed in nature to
  produce fertile offspring but who cannot
  successfully interbreed with other species

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Limitations of the Concept of Biological Species

• Some species do not have sexual reproduction
• some species do not contribute 1/2 of genetic
  makeup (bacteria)
• species definition doesnt work for extinct forms
  of life
• geographical separation doesnt allow us to
  determine if they are the same species

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Case of Peromyscus maniculatus

• There are 4 populations, phenotypically distinct
  (called subspecies due to geographical
  separation)
• overlapping of territories does occur and some
  interbreeding takes place
• therefore, according to definition of biological
  species they should be considered the same
  species.

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• Permomyscus maniculatus artemisiae and Peromyscus
  maniculatus nebrascensis
• their ranges overlap but they dont interbreed
• they do interbreed with the other two subspecies
  where they overlap, and thus there is gene flow
• if the corridors for gene flow were eliminated by
  extinction of the other two species, P. m.
  artemisiae and P. m. nebrascensis would be
  considered separate species.

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So What Is A Species?

• Probably no single definition can be used to
  cover all cases
• morphospecies concept is most practical for
  taxonomic purposes

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Reproductive Barriers

• Prezygotic - prevent mating or fertilization
• habitat isolation
• temporal isolation
• behavioral isolation
• mechanical isolation
• gametic isolation

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Habitat Isolation

• Not necessarily geographic isolation
• aquatic vs terrestrial, ie. Aquatic vs
  terrestrial garter snakes
• host specific, ie. Two parasites living on
  separate hosts

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Temporal Isolation

• Different times of the day, ie diurnal vs
  nocturnal
• different seasons, ie western and eastern spotted
  skunk
• western breed in late summer
• eastern breed in late winter
• different years

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Behavioral Isolation

• Different signals
• songs
• flashes of light
• courtship ritual

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Mechanical Isolation

• Anatomical incompatibility
• penis structure in primates
• length of pollen tubes or styles in flowers

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Gametic Isolation

• Chemical incompatibility
• improper receptors on surface of sperm and egg
• different pollen, different pollen tube lengths,
  different growths

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Reproductive Barriers

• Postzygotic - prevents the development of viable,
  fertile adults
• hybrid invariability
• hybrid sterility
• hybrid breakdown

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Hybrid Invariability

• Once the egg is fertilized, offspring either is
  not brought to term or is too weak or too frail

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Hybrid Sterility

• Exampe is the mule
• cross between horse and donkey
• mules cannot backbreed with either

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Hybrid Breakdown

• Hybrid is fertile, but when hybrids mate with
  each other or their parents, offspring are feeble
  or sterile

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Introgression

• When alleles do slip by all reproductive
  barriers, the transplantation of alleles between
  species is called introgression.

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How Does Speciation Occur?

• Geographical Speciation
• allopatric speciation
• sympatric speciation
• Genetic Speciation
• divergence
• peak shifts

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Allopatric Speciation

• Geographic barrier physically isolates the
  population
• mountain ranges
• creeping glaciers
• land bridges
• lakes and rivers
• canyons

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Found on South rim of Canyon
Found on North rim of Canyon
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Conditions Favoring Allopatric Speciation

• Gene pool of the peripheral isolate differs from
  that of parent population from the beginning
• until the peripheral isolate becomes a large
  population, genetic drift will continue to change
  the gene pool at random
• evolution by natural selection may take a
  different direction in the peripheral isolate
  rather than in parent population

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• There is evidence allopatric speciation is faster
  in small popluations rather than large ones
• Adaptive Radiation - the emergence of numerous
  species from a common ancestor that spreads to
  new environments, ie Darwins finches.

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Adaptive Radiation
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Sympatric Speciation

• New species arise within parent population
  reproductive isolation occurs with geographical
  isolation
• many plant species have origins due to accidents
  in cell division
• autopolyploid
• allopolyploid

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Autopolyploids

• By accident, a species doubles its chromosomes
  from (2n) to tetraploid (4n)
• tetrapoloids can mate with other tetraploids or
  themselves
• cannot breed with original diploids

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Allopolyploids

• Two different species form a polyploid plant
• the resultant hybrid is infertile but may be more
  vigorous than parents and propagate asexually
• from these may arise fertile hybrids

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Speciation by Peak Shifts

• Adaptive landscapes by Sewell Wright
• adaptive peaks - equilibrium state where gene
  pool allele frequencies maximize average fitness
• to change to a different gene pool allele that
  maximizes average fitness, population must go
  into a valley
• then population has to work itself to a new
  adaptive peak

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• Peak shfits are not triggered by new physical
  environment but by nonadaptive changes in the
  genetic system
• can be caused by found effect or bottleneck
  effect
• genetic drift can cause peak shifts

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Models for Tempo of Speciation

• Gradualism - diverge more and more over time
• punctuated equilibrium - new species changes most
  as it buds from parent species and then changes
  little after that

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Speciation by Divergence

• Two populations, over time, adapt to disparate
  environments and accumulate differences in
  frequencies of genotypes and phenotypes
• from this adaptive divergence of two gene pools,
  reproductive barriers between the populations may
  evolve coincidentally, differentiating the two as
  two species.