Speciation
• “It is really laughable to see what different
ideas are prominent in various naturalists’
minds, when they speak of ‘species’. It all
comes, I believe, from trying to define the
indefinable”.
• Charles Darwin
What is a species?
Using the BSC
• "a species is an array of populations which are
actually or potentially interbreeding, and which are
reproductively isolated from other such arrays under
natural conditions."
(Ernst Mayr)
• Reproductive isolation is the failure of populations to
interbreed or to form viable or fertile hybrids
• Speciation = generation of a reproductive isolating
mechanism
Fig. 24-4
Prezygotic barriers
Habitat Isolation
Temporal Isolation
Individuals
of
different
species
(a)
Postzygotic barriers
Behavioral Isolation
Mechanical Isolation
Gametic Isolation
Mating
attempt
(c)
(d)
(e)
(f)
Reduced Hybrid Viability
Reduced Hybrid Fertility
Hybrid Breakdown
Viable,
fertile
offspring
Fertilization
(g)
(h)
(i)
(j)
(b)
(k)
(l)
Fig. 24-4a
Prezygotic barriers
Habitat Isolation
Temporal Isolation
Individuals
of
different
species
(a)
Mating
attempt
(c)
(d)
(b)
Mechanical Isolation
Behavioral Isolation
(e)
(f)
Fig. 24-4i
Prezygotic barriers
Gametic Isolation
Postzygotic barriers
Reduced Hybrid Viability
Reduced Hybrid Fertility
Hybrid Breakdown
Viable,
fertile
offspring
Fertilization
(g)
(h)
(i)
(j)
(k)
(l)
Speciation is often gradual, so we expect to see some “gray
areas”
(are they different species or not?)
Speciation is often gradual, so we expect to see some “gray
areas”
(are they different species or not?)
Canis lycaon
Canis latrans
Canis lupus
Speciation is often gradual, so we expect to see some “gray
areas”
(are they different species or not?)
Canis latrans
Canis lycaon
Canis Soup
Canis lupus
The BSC can only apply to a small portion of life
The PSC
• the smallest set of organisms that share a
common ancestor and can be distinguished
from other such sets.
• Or
• the smallest diagnosable cluster of individual
organisms within which there is a parental
pattern of ancestry and descent
(Cracraft 1983)
• Speciation = cladogenesis
Eastern and Western Northern
Flickers
• Morphological differences evolved in allopatry
• Differences don’t function as premating RIM’s (they
hybridize in the Great Plains)
• Don’t seem to be any postmating RIM’s (hybrids fit
and reproduce)
Moore et al., 1991, Mol. Biol. Evol. 8(3):327-344
Types of speciation
• Anagenesis—draw it
Types of speciation
• Anagenesis—draw it
• Cladogenesis—draw it
Types of speciation
• Anagenesis—draw it
• Cladogenesis—draw it
• Reticulate speciation—draw it
Bold = species thought to have
formed as hybrids
Red arrows = mtDNA source
Koblmüller et al. BMC
Evolutionary Biology 2007 7:7
doi:10.1186/1471-2148-7-7
3 hybrid species of
sunflower that
thrive where the
parental species
cannot