Speciation is the process by which new species form. It occurs when populations become isolated from each other and diverge to the point that they can no longer interbreed. Isolation can be geographic, such as when physical barriers separate populations, or reproductive, such as when changes prevent successful mating. Speciation rates may be gradual over millions of years or punctuated by rapid changes following events like natural disasters. The evolutionary history and relationships between species can be modeled in cladograms or phylogenetic trees.

1. SPECIATION
Chapter 16
Section 3

2. Formation of Species
• Speciation: process of
forming new species
• Results in many related
populations
• May remain similar or
become quite different

3. Concepts of “Species”
• 1) Morphological: species defined by appearance
• Does not account for individual variations
• 2) Biological: species defined by reproduction
patterns
• Does not explain interbreeding species
• 3) Modern: species are similar in appearance and
can interbreed to produce fertile offspring
• Concept currently used in Biology
Manchester
Terrier
Dandie
Terrier

4. Isolating Mechanisms
• Speciation occurs when members of a species stop
interbreeding and split
• Become isolated from each other
• 2 Types Isolating Mechanisms:
• 1) Geographic
• 2) Reproductive

5. Isolating Mechanisms
• Geographic Isolation: the
physical separation of
members of a population
• i.e.: by canyons, mountains,
rivers, etc.
• Once physically separated,
will continue to diverge until
they can no longer mate
• ALLOPATRIC SPECIATION

6. Isolating Mechanisms
• Reproductive Isolation:
members of a population no
longer able to mate successfully
• i.e. due to disruptive selection,
changes in mating habits, etc.
• Once separated, will continue to
diverge
• Prezygotic vs postzygotic isolation
• SYMPATRIC SPECIATION

7. Rates of Speciation
• Gradualism: some species evolve gradually & at
stable rate
• Although speciation may take millions of years, it is not
always gradual

8. Rates of Speciation
• Punctuated
Equilibrium: when
species evolve in rapid
spurts of great change
• Involves long periods of
no change between
“growth spurts”
• Usually due to natural
disasters

9. Phylogeny
• Phylogeny: the evolutionary history
of an organism (how that organism
evolved)
• Can be used to create a:
• A) Cladogram
• Shows how species diverged from
one another as they evolved from a
common ancestor
• Closer together on cladogram =
more recent common ancestor
• B) Phylogenetic Tree
• Shows hypothetical relationships
among organisms
• Tree of Life