Transcript Chapter 24

Chapter 24
The Origin of Species
Macroevolution is the origin of new taxonomic groups, as
opposed to microevolution, which is genetic variation
between generations.
A. What is a species?
1. Biological species concept
- A species is a population or group of populations whose
members have the potential to interbreed with one another
and produce viable offspring, but who cannot produce viable
offspring with other species.
Figure 24.3 (p. 473) – The biological species concept is based
on interfertility rather than physical similarity.
--> Speciation is the process by which a new species
originates and involves the creation of a population of
organisms that are novel enough to be classified in their
own group. There are two processes by which this can
occur:
- Anagenesis is the accumulation of heritable traits
in a population, that transforms that population into a new
species
- Cladogenesis is branching evolution, in which a
new species arises as a branch of from the evolutionary
tree. The original species still exists. This process is the
source of biological diversity.
For a new species to form, there needs to be isolation of
some members of a species as a separate population.
Forms of isolation, that interfere with breeding include
both..
2. Prezygotic and postzygotic barriers (Fig. 24.4, p. 474-5)
 Prezygotic barriers prevent mating or egg fertilization if
members of different species try to mate. Examples:
a. Habitat isolation
- Two species that live in the same area, but occupy
different habitats rarely encounter each other.
b. Behavioral isolation
- Signals that attract mates are often unique to a
species (e.g. different species of fireflies flash different
patterns).
c. Temporal isolation
- Two species breed at different times of the day or during
different seasons.
d. Mechanical isolation
- Closely related species attempt to mate, but are
anatomically incompatable. (Example: flowering plants with
pollination barriers; some plants are specific with respect to
the insect pollinator, often occurs with butterflies/moths)
e. Gametic isolation
- Gametes must recognize each other. (Example: fertilization
of fish eggs, chemical signals between sperm and egg allows
sperm to “recognize” the correct egg)
- Postzygotic barriers prevent a hybrid zygote from
developing into a fertile adult. Examples:
a. Reduced hybrid viability
- Abort development of hybrid at some embryonic stage.
b. Reduced hybrid fertility
- Meiosis doesn’t produce fertile gametes in vigorous
hybrids.
c. Hybrid breakdown
- First-generation hybrids are fertile, but they cannot produce
fertile offspring in the next generation (e.g. different species
of cotton).
Figure 24.4 Reproductive Barriers
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Habitat
isolation
Behavioral
isolation
Temporal
isolation
Individuals
of different
species
Mechanical
isolation
Mating
attempt
HABITAT ISOLATION
TEMPORAL ISOLATION
BEHAVIORAL ISOLATION
(b)
MECHANICAL ISOLATION
(g)
(d)
(e)
(f)
(a)
(c)
Gametic
isolation
Reduce
hybrid
fertility
Reduce
hybrid
viability
Hybrid
breakdown
Viable
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
GAMETIC ISOLATION
REDUCED HYBRID FERTILITY HYBRID BREAKDOWN
(k)
(j)
(m)
(l)
(h)
(i)
There is a problem with the idea of biological species
concept --> How do you get organisms to breed to see
whether viable offspring are produced? There are…
3. Alternative concepts of species
a. Ecological species concept
- Species are defined by their use of environmental
resources; their ecological niche (e.g. species that are
defined by their food source such as butterflies with certain
flowers)
b. Morphological species concept
- Takes into consideration factors such as body shape,
size, etc.
c. Paleontological species concept
- Species in the fossil record are characterized according
to a unique set of structural features.
d. Phylogenetic species concept
- Recognizes species are sets of organisms with unique
genetic histories. This idea is based often on molecular
analyses such as DNA sequences.
B. Modes of speciation  Figure 24.5 (p. 476)
1. Allopatric speciation
- Allopatric speciation describes speciation that
takes place in populations with geographically separate
ranges. Gene flow is interrupted and new species evolve.
2. Sympatric speciation
- Sympatric speciation describes speciation
that takes place in geographically overlapping
populations. Chromosomal changes and nonrandom mating
reduce gene flow.
Remember: Species arise when individuals in a population
become isolated one from the other.
Figure 24.5 Two main modes of speciation
(a) Allopatric speciation. A
(b) Sympatric speciation. A small
population becomes a new species
population forms a new
without geographic separation.
species while geographically
isolated from its parent
population.
Examples of Allopatric speciation:
Figure 24.6 (p. 477) – Allopatric speciation of
squirrels in the Grand Canyon. Animals like birds do not
show speciation like those animals that are barred from
breeding by the canyon.
Another place where adaptive radiation is apparent is on
island chains (e.g. Fig. 24.12). This example is illustrative
of what happened on the Hawaiian islands.
Would this example be allopatric or sympatric speciation?
Remember  Once geographic isolation has occurred,
there still must be changes that reproductively isolate
populations of individuals. If the populations evolve so
that they are now new species, they cannot interbreed to
produce fertile, viable offspring. In other words, they need
to be reproductively isolated!
Figure 24.12 Adaptive radiation
Dubautia laxa
1.3 million years
MOLOKA'I
KAUA'I
MAUI
5.1
million
years O'AHU LANAI
3.7
million
years
Argyroxiphium sandwicense
HAWAI'I
0.4
million
years
Dubautia waialealae
Dubautia scabra
Dubautia linearis
2. Sympatric speciation
- Sympatric speciation describes speciation
that takes place in geographically overlapping
populations. This can occur by chromosomal changes and
nonrandom mating. Both can reduce gene flow between
organisms and cause populations to evolve to new species.
Example:
- Reproductive barriers can arise by polyploidy
(greater than 2 sets of chromosomes). This mechanism is
most common in plants.
Figure 24.8 (p. 478) – Sympatric speciation by
autopolyploidy in plants.
Figure 24.8 Sympatric speciation by
autopolyploidy in plants
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
branch will be diploid.
Offspring with tetraploid
karyotypes may be viable
and fertile—a new
biological species.
2n
2n = 6
4n = 12
4n
Figure 24.9 One mechanism for
allopolyploid speciation in plants
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Species A
2n = 4
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
- Animals diverge mostly due to reproductive
isolation. Reproductive isolation is a result of genetic factors
that cause offspring to rely upon resources not used by
previous generations. (Example: switch to a new food
source)
An extremely good example of sympatric speciation in
animals occurred in Lake Victoria which has 200 closely
related species of Cichlids (fish) which probably all arose
from one ancestor with the driving force for speciation being:
Competition for a limited resource (food) within the lake, and
adaptation to new food sources. This gave rise to different
species that are kept from breeding with each other by
distinctive coloration patterns (Fig. 24.10)
Figure 24.10 Does sexual selection in cichlids
result in reproductive isolation?
EXPERIMENT
Researchers from the University of Leiden placed males and females of Pundamilia pundamilia and
P. nyererei together in two aquarium tanks, one with natural light and one with a monochromatic orange
lamp. Under normal light, the two species are noticeably different in coloration; under monochromatic orange
light, the two species appear identical in color. The researchers then observed the mating choices of the fish
in each tank.
Monochromatic
Normal light
orange light
P. pundamilia
P. nyererei
RESULTS
CONCLUSION
Under normal light, females of each species mated only with males of their own species. But
under orange light, females of each species mated indiscriminately with males of both species.
The resulting hybrids were viable and fertile.
The researchers concluded that mate choice by females based on coloration is the main
reproductive barrier that normally keeps the gene pools of these two species separate. Since
the species can still interbreed when this prezygotic behavioral barrier is breached in the
laboratory, the genetic divergence between the species is likely to be small. This suggests
that speciation in nature has occurred relatively recently.
C. From speciation to macroevolution
How then do we get from the mechanism of speciation to
evolution on a grand scale, i.e. macroevolution?
There are two models to describe the tempo of speciation:
-The Gradualism model suggests that change is gradual with
the accumulation of unique morphological adaptation.
- The Punctuated Equilibrium model suggests that rapid
change occurs, with a new species “erupting” from the
ancestral lineage and then staying the same thereafter.
-Fig. 24.13
However it does occur, we need to remember that 
Speciation occurs when divergence leads to reproductive
barriers between the new and the ancestral population.
And this probably takes vast amounts of time to occur.
 But how do evolutional novelties emerge? For
example, how did something as complex as the eye first
evolve? We need to remember that:
 Most evolutionary novelties are modified versions of
older structures. And an extremely good example is the
eye as shown in Figure 24.14 (p. 483) – A range of eye
complexity among mollusks.
The lesson from the eye example is that
 Existing structures can be modified for brand new
functions. These are called Exaptations: structures that
evolve for one purpose but become useful for another
function.
Finally, we should
** Remember that evolution is not goal
oriented. Differential reproduction is only a reaction of
individuals to their environment.
Figure 24.24 (p. 481) – The branched evolution of
horses. This figure can give the illusion of goal-oriented
evolution of the horse, but it is only an illusion.