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The Origin of Species
Biological Species Concept
Ernst Mayr, 1942
defines species as a population who have the
potential to interbreed.
•Must be able to interbreed in nature
•Produce viable, and fertile offspring (interfertility)
•Must occur in nature
Prezygotic Barriers
Factors that impede mating between species
or hinders fertilization.
1. Habitat Isolation: contact is rare b/w species
because of habitat location
2. Behavioral Isolation: attractor signals are not the
same between the species
3. Temporal Isolation: breeding occurs at different
times, seasons, years.
4. Mechanical Isolation: mating can be attempted but
not successful.
5. Gametic Isolation: fusion b/w gametes does not
Postzygotic Barriers
Barriers that prevent a fertilized egg from
developing into a viable, fertile adult.
1. Reduced Hybrid Viability: development is not
complete, or survivors are weak.
2. Reduced Hybrid Fertility: hybrids are sterile,
gametes are not formed by meiosis. (Mule).
3. Hybrid Breakdown: 1st generations are viable and
fertile, but next generations are feeble or sterile.
Modes of Speciation
Allopatric Speciation: a geographical barrier that
isolates populations and prevents gene flow.
Sympatric Speciation: chromosomal changes and nonrandom mating changes gene flow within a
Ring Species in Progress?
Conditions favoring Allopatric
1. Gene pools differ between the outer population
extreme clines created, differ from inner pop.
Outer pop small  founder effect
2. Genetic Drift occurs until outer pop. is large
enough. Some may become fixed, causing more
3. Evolution by Natural Selection may differ between
outer and inner pop.
Sympatric Speciation
Polyploidy: Creation of a new species when during
cell division extra sets of chromosomes are
generated. Occurs in plants species.
Autopolyploidy: failure of meiosis results in 4n. Can
self fertilize or mate with other 4n’s. Sterile
offspring made when fertilize a 2n.
Discovered by Hugo de Vries in 1900’s,
Allopolyploid: two species contribute to a polyploid
hybrid. Two different species interbreed.
Propagation by asexual reproduction. May be
more vigorous than parents, best of both.
Sympatric Speciation
• Reproductive Isolation
• Different Figs, different wasps
• Monkey & Fig
• Lake Victoria, East Africa
– Selective reproduction (blue back vs. Red back)
– Mate choice based on color is main repro.
– Can still interbreed……..if prezygotic barrier is
– Speciation recent.
Tempo of Speciation
•Punctuated Equilibrium
•Abrupt changes
•Period of stasis
•Slow divergence
•Slight changes
• Interdisciplinary science b/w evolutionary biology and
study of development.
• Genes regulate (zygote to adult)
– Control rate
– Timing
– Spatial pattern of change in form
• Allometric growth:
– Growth rates of different parts
– Changes form/structure
– Heterochrony: evolutionary change in the rate or timing of
developmental events
• At sexual maturity, species may retain body
features that were juvenile structures in ancestral
• Salamanders have larval stage to become adult.
• Some become mature but still have gills and other
structures (juvenile)
Evolution is not goal oriented.