Transcript Biological Science 2/e

The Origin of Species
Presented & supported
two hypotheses:
1) Evolution as descent
with modification
and
2) Natural selection as the
mechanism for adaptation
But
Didn’t really talk about the
origin of species!
Speciation = the divergence of a single ancestral
species into two descendent species.
It is the process responsible for all biodiversity.
http://orgs.unca.edu/tulula/images/biodiversity.jpg
What is a species? Do I know one when I see one?
(Morphological species concept = “looks different”)
Biological species concept:
Reproductive isolation between species.
Reproductive isolation = evolutionary independence.
Willow and Alder Flycatchers
Look identical! Don’t interbreed.
Phylogenetic Species Concept
The PSC looks for pattern of independent evolution.
Monophyletic group =
group that contains
all descendents of an
ancestor.
PSC= a species is the
smallest monophyletic
group.
Species concepts have conservation implications
Dusky seaside
VERY RARE
N=6 males
in 1980
Species concepts have conservation implications
Conservation
managers mated
these subspecies
to try to save the
dusky seaside
sparrow
The PSC recognizes two
species.
Matings should not be done
between the two.
Biological Species Concept focuses on
Reproductive Isolation.
Reproductive Isolation could be pre-or post-zygotic.
Examples of pre-zygotic RI include:
Habitat differences
Many examples!
Behavioral differences
Many examples!
http://www.ebertfest.com/two/grave_fireflies_bluebat.jpg
Examples of pre-zygotic RI include:
Genetic differences among species
Bindin protein on
urchin sperm is
species-specific.
http://www.luc.edu/depts/biology/dev/spersem.jpg
In post-zygotic reproductive isolation, offspring have
Either low fitness:
for example, stickleback with intermediate morphology.
Or low fertility:
Horse 2N=64; Donkey 2N = 62.
Mule has 2N = 63. Can’t make balanced gametes.
How is reproductive isolation established?
In Allopatric (different homeland) speciation,
geographic isolation leads to reproductive isolation.
Drift and selection can cause divergence.
Allopatric speciation can result from dispersal.
Speciation on islands is famous example.
Allopatric speciation can also result from vicariance.
Physical barrier divides species.
Barrier could be due to geologic changes such as
continental drift and volcanism
or to habitat changes due to climate change.
Ratites are flightless birds.
Gondwana 150 mya,
Began to split up 140 mya.
Summary of allopatric speciation.
Is sympatric (“same homeland”) speciation possible?
Gene flow can prevent
divergence, even
when selection would
favor different
adaptations.
Sympatric speciation may be occurring in soapberry bugs.
Sympatric speciation also thought to occur in fish:
Benthic and lotic forms of sticklebacks in Alaska
And in Rhagoletis flies that feed on apples instead of
hawthorne.
www.msu.edu/~jimsmith/research.html
Polyploidy in plants also leads to sympatric speciation.
Tetraploids are reproductively isolated from diploids.
Their triploid progeny are sterile.
What happens when recently diverged species
come into contact?
Sometimes R.I. is not complete, and fertile hybrids form.
Townsend’s warbler is overtaking the hermit warbler.
Some
Townsend’s
have
mtDNA of
hermits
suggesting
repeated
gene flow
from
Townsend’s
to hermits.
Humans create hybridization opportunities.
Lose distinct species through hybridization.
Humans create hybridization opportunities.
Create new invasive species through hybridization.
Phylogenetic context, but still somewhat typological. Notice that the degree of resolution is
far greater for non-Africans, and also that Europe is placed farthest from the root of the tree.
http://www.mcdonald.cam.ac.uk/genetics/images/MtDNA_DistributionMap.gif
Australia/New Guinea
mt DNA
Europe
~ 50,000 ybp
Asian/Native American
African
African
Ingman et al. (2000) Nature 408:708.