Transcript Evolution- Speciation (Zygotic) Barriers PPT Lecture

Evolution and Zygotic
Barriers (Part 5)
Ms. Gaynor
AP Biology
Chapter 24: The Origin of
Life
Practice Problem #2
Green color (G) is dominant to white
color (g) in turtles.
 In a population of 200 turtles, 13% are
white.
 What are the allele frequencies?
 What percentage of each genotype are
in this population?

ANSWER KEY
G= Green g= white
 White = 13% = 0.13 = gg= q2
 √0.13=√q2
 q = 0.36 then p = 0.64
 GG = p2= (0.64)2=0.4096= 40.96%
 Gg = 2pq= 2(0.64)(0.36)=0.4608= 46.08%
 gg = q2= (0.36)2= 0.1296= 12.96%

How are new species
created?
1.
2.
3.
4.

Geographic isolation
Reproductive barriers
Change in chromosome numbers
through mutation
Adaptive radiation (example of
divergent evolution)
Speciation = formation of NEW
species
Hello over
there 
A. harrisi
A. leucurus
Geographic Isolation
REVIEW ANIMATIONS
http://www.hippocampus.org/Biology
See “Origin of Species” videos #1 through 6 under
“Biology for AP*”
Speciation and geographic
separation
Speciation can occur in two ways:
– Allopatric speciation (means “other”)
• a genetic isolation WITH a
geographical barrier; new group
isolated from its parent population
– Sympatric speciation (means “together”)
• genetic isolation WITHOUT a
geographical barrier; a
reproductive barrier isolates
population in SAME habitat
http://bcs.whfr
eeman.com/th
elifewire/cont
ent/chp24/240
2001.html
Allopatric speciation
Sympatric speciation
http://www.pbs.org/wgbh/nova/evolution/evolution-action-salamanders.html
Reproductive Isolation
 biological
factors (barriers) that
stop 2 species from producing
viable, fertile hybrids
 Two types of barriers
– Postzygotic “after the zygote”
• Zygote can NOT develope
– Prezygotic “before the zygote”
• Sperm and egg can not fuse
Pre-Zygotic
Barriers
Habitat Isolation

2 species encounter each other
rarely, or not at all, because they live
in different habitats, even though not
isolated by physical barriers
Temporal Isolation

Species that breed at different times
of the day, different seasons, or
different years cannot mix their
gametes
Late Summer
Late Winter
Behavioral Isolation

Courtship rituals and other behaviors
unique to a species are effective
barriers
http://wps.aw.c
om/bc_campbel
l_biology_7/26/
6661/1705356.
cw/index.html
Mechanical Isolation
• Morphological
differences can
prevent successful
mating
• Related species
may attempt to
mate but CAN’T
anatomically
incompatible
• Sperm = transfer
Mating organs don’t fit
Gametic Isolation
Sperm of one species may not be able
to fertilize eggs of another species
 Ex: specific molecules on egg coat
adhere to specific molecules on
sperm

Post-Zygotic
Barriers
Reduced Hybrid Viability
Genes of the different parent species
may interact and impair the hybrid’s
development
 Hybrids are very weak and/or
underdeveloped

Salamander
hybrid shows
incomplete
development
Reduced Hybrid Fertility

Even if hybrids may live and be
strong, they may be sterile
Hybrid Breakdown

Some F1 hybrids are fertile, but when
they mate with another species or
with either parent species, offspring
of the next generation (F2) are weak
or sterile
Cultivated rice
have different
recessive
mutant alleles
F2 accumlates
many of these
alleles
Polyploidy

Polyploidy is presence of EXTRA
sets of chromosomes due to
accidents during cell division
– ex: “nondisjunction”
It has caused the evolution of some
plant species
 More common in plants than in
animals

An autopolyploid is an individual with more than two
chromosome sets, derived from one species
Failure of cell division
in a cell of a growing
diploid plant after
chromosome duplication
gives rise to a tetraploid
branch or other tissue.
Gametes produced by
flowers on this
tetraploid branch are
diploid.
Offspring with
tetraploid karyotypes may be viable
and fertile—a new
biological species.
2n
2n = 6
4n = 12
4n
• An allopolyploid is a species with multiple
sets of chromosomes derived from different
species
Unreduced gamete
with 4 chromosomes
Species A
2n = 4
Hybrid with
7 chromosomes
Unreduced gamete
with 7 chromosomes
Viable fertile hybrid
(allopolyploid)
Meiotic error;
chromosome
number not
reduced from
2n to n
2n = 10
Normal gamete
n=3
Species B
2n = 6
Normal gamete
n=3
Changes in Spatial Pattern
 Substantial
evolutionary change
can also result from alterations in
genes that control the placement
and organization of body parts
Homeotic
genes
– determine such basic features as
where wings and legs will develop
on a bird or how a flower’s parts
are arranged
Hox Genes
 One
class of homeotic genes are
called Hox genes
 The products of Hox genes provide
positional information in the
development of fins in fish and
limbs in tetrapods
 Evolution of vertebrates from
invertebrate animals was
associated with alterations in Hox
genes

Tetrapod
evolution
– Fish; Hox
gene leads
to fin
development
– Chicken;
same Hox
gene leads
to leg
development