Transcript Evidence For Evolution

Speciation
What came first, the adult
alligator or the alligator
embryo in an egg?
Anagenesis

Speciation in Time


Directional selection causes
species to change so much their
ancestors would be considered
different species (they would no
longer recognize their ancestors
and breed with them).
No net increase in species number.
Cladogenesis

Two Species Descend from
One


Usually two populations get
separated and evolve
differences.
 Sometimes one stays the
same, or similar, and another
changes especially if a few
individuals end up in a
different place.
Occasionally speciation can
happen in an intact population.
Speciation
Allopatric
Cladogenesis

Speciation by
Geographic Isolation
 Two populations
separated by a
barrier evolve in
separate
directions until
they can no longer
interbreed
 Two species
result.
Allopatric Speciation
Tree frogs are a single
population.
Allopatric Speciation
The formation of a river
may divide the frogs
into two populations.
Allopatric Speciation
Over time, the divided
populations may
become two species
that may no longer
interbreed, even if
reunited.
Sympatric Cladogenesis


No geographic
isolation
Not many animal
examples


ring species
Polyploidy in plants
Sympatric
Speciation

Ring species
Ensatina in
California
The Tempo of Speciation


Gradualism - slow
and steady change in
species.
Punctuated
equilibrium - slow
evolution punctuated
by short events of
rapid evolution.
Polyploidy


A duplicate chromosome in humans
result in conditions like Down’s
Syndrome.
In plants it doesn’t seem to matter much
at all and can sometimes result in a new
species with twice or more the number
of chromosomes as before.

Haploid, Diploid, Triploid, Polyploid
Polyploidy - The Spartina Story


Before 1830 Spartina
maritima found in
British salt marshes.
S. alterniflora arrived
in 1829 from boats
from America.
Polyploidy - The Spartina Story


Two species hybridised produced a
sterile diploid plant S. townsendii.
Two chromosomes from different parent
species could not pair up in meiosis.
Spread widely through asexual
reproduction via underground stems.
Polyploidy - The Spartina Story

1829 chromosome doubling occurred
(complete non-disjunction) producing a
tetraploid plant S. anglica.


With two sets from each of the original
parents now possessed by the plant, meiosis
and sexual reproduction became possible
again.
It is very fertile and vigorous, and has
replaced nearly all of the other species
across England.
Polyploidy

Bread Wheat
selectively bred
by crossing two
species.
Allometry


Rate of growth
of body parts.
Can change
due to single
mutations.
Allometry
Allometry - Paedomorphosis

Sexually
mature
juveniles.
Allometry - Paedomorphosis

Or juvenile
appearing
adults?
Homeotic gene mutations

Hox gene
mutations can
change phenotype
dramatically.