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Macroevolution and
-David Bowie
What is a species?
• Species means “kind” or “type”
Older way of looking at this question:
• Morphological Species Concept
– Classified organisms into separate species based on
structural / physical differences.
– Used by ____________
Carl Linnaeus (Swedish doctor & naturalist,
c.1735) but not adequate today.
What is a species?
With the help of population genetics, in 1940 Ernst Mayr
came up with the:
• Biological Species Concept
– A species consists of groups of populations
whose members are capable of ____________
fertile offspring AND
in nature to produce ______
who do not interbreed with members of
different species.
(this last part is called ____________________)
reproductive isolation
Defining a species
Biological Species Concept issues:
– Still this definition is not perfect: what about
asexually reproducing organisms like
bacteria or fungi ?!
• These are classified by structural and biochemical
in E.coli
Defining a species
Biological Species Concept issues:
– Additionally, some different species that do not
interbreed in nature can be made to do so in
an artificial environment (ranch, circus, zoo,
aquarium, or laboratory).
Ex) Zebroids, interspecies hybrids:
a cross between
a horse and a zebra
Zebroid, Mt. Kenya
Game Ranch and
Animal Orphanage,
How do you create a new species?
Usually through geographic isolation (separation)
of some members of a population.
The mechanisms of genetic drift, mutation, & natural
selection all act upon the newly founded population
and the original population. Over time, these now
separate populations evolve independently.
If the two populations are brought
back together and are capable of
successfully reproducing in
nature, we say they are still the
same species. If they are NOT
capable of this, then we declare
them different species.
Species boundaries are maintained
through geographic and/or
reproductive isolation
• What separates two species whose space /
range overlaps? Reproductive barriers
usually keep species boundaries intact.
• Reproductive barriers can be put into two
– Prezygotic barriers: prevent fertilization
– Postzygotic barriers: prevent hybrid from
developing into viable, fertile adult
Prezygotic Barriers to Species
• Ecological / Habitat isolation
– Two species occupy different
microenvironments (in same area)
and thus do not interbreed
Ex) Garter snakes: water v. land
• Temporal isolation
– Reproduce at different times (of day,
year, season, etc.)
Ex1) flowers: open/pollinated at
different time of day
Ex2) frogs: mate at different time of
season (temp dependent)
• Behavioral isolation
– Different courtship rituals / mating
Ex1) blue-footed boobies “dance”
Ex2) frog “calls” --> different “songs”
among different species
More Prezygotic Barriers to Species
• Mechanical isolation
– Differences in anatomy / physical structure do not permit
Ex) flowers: adapted for specific (insect) pollinators
• Larger flowers only pollinated by larger bees, smaller bees cannot
• Gametic isolation
– Egg & sperm of two species biochemically incompatible
(different recognition proteins on surface usually)
– This especially important to externally fertilizing aquatic organisms
or organisms with wind-borne gametes
Ex1) flowers: will not accept/recognize “foreign” pollen
Ex2) fish that fertilize externally, in open water
Postzygotic Barriers to Species
• Hybrid inviability
– Aborted development, usually at early embryonic stage
Ex1) bullfrog eggs & leopard frog sperm
Ex2) different species of irises
• Hybrid sterility
– Hybrid offspring survive (& may be
strong) but are sterile (often due to
abnormal gametes of hybrid)
Ex) female horse (2N = 64) and male
donkey (2N = 62) results in sterile mule
offspring (2N = 63).
• Different diploid number prevents
proper matching up of homologous
chromosomes in meiosis
• Hybrid breakdown
– F1 hybrid can successfully reproduce with other F1 hybrids or a Parental
individual but F2 hybrid unable to reproduce.
Ex1) sunflower species
Ex2) cotton species
Speciation and Adaptive Radiation
• Adaptive radiation refers to an evolutionary pattern in
which one species gives rise to many (also referred to as
divergent evolution)
• This has happened several times in the history of the
evolution of species… Evidence? Homologous structures
(similar structure, different function)
Adaptive radiation most often
follows mass extinctions or
when a species moves into a
new, unoccupied area. In both
these cases, there is usually
an abundance of available
ecological niches, that can be
filled by the new “daughter”
Speciation in honeycreepers
The islands of Hawaii were colonized by a single species of finch-like
birds. Eventually populations were isolated from each other on
separate islands. The diagrams show how they evolved into the
various modern honeycreeper species with beak shapes which adapt
them for different feeding methods.
Allopatric Speciation
The term Allopatric
Speciation is used to describe
a speciation event in which the
new species results due to
geographic isolation and
subsequent adaptation of the
emigrants to a new
environment. When the two
species are brought together
again, they do not interbreed.
Sympatric Speciation
The term Sympatric Speciation is used to describe a speciation event
in which reproductive isolation arises within the boundaries/range of
the parent species (without geographic isolation). This is most
common among plants, and usually results due to an increase in a
plant’s chromosome number. The resulting plant is thus considered to
be polyploid.
Image Sources
Budding yeast cells:
Snail speciation & Honeycreeper Beaks:
Frog Mating Behavior:
Adaptive Radiation:
Boobie Mating Dance:
Linnaeus portrait & Cinchona watercolor:
Ernst Mayr:
Honeycreepers #2: