Biological Diversity
Turner College & Career High School  2016
Speciation & Homology
Descent with modification
 modified characteristic =
homology
Speciation produces “sister
species” descended from a
common ancestor.
Descendant species retain
characteristics of common
ancestor possibly in
modified form.
Genealogical Relationship
Evidence of common ancestry.
Same characteristics in 2 or more species
inherited from a common ancestor
= homology.
Possibly/probably modified since split from
sister species.
The common ancestor no longer exists, so
how can we tell what’s a homology and
what’s not?
Criteria for Homology
1. Position in relation to other body
structures.
2. Embryonic origin
3. Continuity: characteristics homologous
to another characteristic are homologous
to each other.
Criteria for Homology
Position in relation to other body structures.
 Forelimbs
 Humerus, radius & ulna, carpals, metacarpals, phalanges
Criteria for Homology
Position in relation to other body structures.
 Insect wings, legs
 Antennae
Criteria for Homology
Position in relation to other body structures.
 Flower parts
 Sepals
 Petals
 Stamen
stamen
Criteria for Homology
Embryonic origin similar developmental
origin.
 Pharyngeal arch: jaw
 Limb buds
 Lobes of brain
 Heart & arteries
 Gill slits
 Tail
Criteria for Homology
Continuity: characteristics homologous to
another characteristic are homologous to
each other.
 Dentary
 Inner Ear
Homology Provides
Evidence for Evolution
Comparative
Comparative
Comparative
Comparative
Anatomy
Embryology
Fossil Anatomy
Physiology & Biochemistry
Comparative Anatomy
Modification of existing
characteristics/organs for other functions.
 Wings in bat
 Whale flippers
 Human arm
 Turtle leg
Comparative Anatomy
Modification of existing
characteristics/organs for other functions.
 Teeth: fangs in rattlesnake
 Salivary gland: venom gland
Comparative Anatomy
Imperfection of adaptation.
 Panda's thumb: actually “radial sesamoid” bone.
 "Oh, my aching . . ."
 after 2-3 million years, we are still subject to
back strain.
Comparative Anatomy
Modification but without function.
 Vestigial organs: senseless signs of history.
 Rudimentary structure in organism
corresponding to a functional structure or organ
in ancestral organism.
Appendix in humans
Comparative Anatomy
Vestigial organs
 Pelvic bones in whales.
Comparative Anatomy
Vestigial organs
 Coccyx (tailbone) in humans.
Coccyx (tailbone)
Comparative Anatomy
Vestigial organs
 Nictitating membrane in humans.
 In animals, it is a clear eyelid can be drawn
across the eyeball for protection from debris,
prey, or the dryness of air, similarly to regular
eyelids.
Nictitating membrane
Comparative Embryology
Some traits seen only in embryos or
larvae.
Homology with larval or adult traits in other
organisms.
 Notochord in vertebrates.
 Notochord & dorsal nerve cord in tunicate larvae.
 Bilateral symmetry in echinoderm larvae
Comparative Embryology
Comparative Fossil Anatomy
The fossil record.
Homology relates fossils
to existing organisms.
 Change in lineages
 Extinction
 Example: horses
Comparative Fossil Anatomy
Homology relates fossils to existing
organisms.
 Change in lineages
 Extinction
 Example: trilobites
Comparative Physiology
& Biochemistry
Homology applies to physiological &
metabolic processes.
 Glycolysis and Krebs cycle
 Mechanisms of cell signaling
Homology applies to biochemicals.
 DNA as genetic code
 RNA to translate code into protein structure
 ATP as “energy currency” of cells
Other Factors in Evolution
Convergence
Evolution (adaptation) of dissimilar
organisms to common function,
superficially similar characteristics.
• Piercing sucking mouth in bugs, mosquitoes, fleas,
butterflies
• Wings in bird, bat, pterosaur, insect (airplane?)
Convergent characteristic NOT
present in common ancestor.
Convergence
Biogeography
Organisms have evolved independently in
different parts of the world.
Geographic distributions of species, genera,
families, etc.
Biogeography
Organisms have evolved independently in
different parts of the world.
 Endemic species, genera, etc. on islands.
 Galapagos finches
Biogeography
Biogeography
Used with fossil record to reconstruct
evolutionary history.
 Camels in Asia: camel, dromedary
 South America: llama, alpaca
 Camels in N. America? Fossils?
Biogeography:
 Alligator in SE USA and China/SE Asia.
 Hellbenders in SE USA and China.
 Cyclocosmia in SE USA and Malaysia.
Biogeography
Isthmus of Panama divided Caribbean from
Pacific about 3 million years ago.
 Previously a body of water existed.
Several species crabs, snails, fish, etc. have
nearest relatives on other side of land
barrier.
Natural Selection, Genetic Drift
& Gene Flow
Evolutionary Change
Three major factors alter allele frequencies
and bring about most evolutionary change:
Natural selection
Genetic drift
Gene flow
Natural Selection
Differential success in reproduction results
in certain alleles being passed to the next
generation in greater proportions.
For example, an allele that confers
resistance to DDT increased in frequency
after DDT was used widely in agriculture.
Natural Selection
Genetic Drift
The smaller a sample, the greater the
chance of deviation from a predicted result.
Genetic drift describes how allele
frequencies fluctuate unpredictably from
one generation to the next (chance).
Genetic drift tends to reduce genetic
variation through losses of alleles.
Genetic Drift
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
5
plants
leave
offspring
Genetic Drift
CRCR
CRCR
CRCW
CWCW
CRCR
5
plants
leave
offspring
CWCW
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
2
plants
leave
offspring
Genetic Drift
CRCR
CRCR
CRCW
CWCW
CRCR
5
plants
leave
offspring
CWCW
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
2
plants
leave CRCR
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Founder Effect
 The founder effect occurs when a few
individuals become isolated from a larger
population.
 Allele frequencies in the small founder
population can be different from those in
the larger parent population.
The Bottleneck Effect
 Sudden reduction in population size due to
a change in the environment.
 New gene pool may not reflect original.
 If the population remains small, it may be
further affected by genetic drift.
Original
Population
Bottleneck
Effect
Surviving
Population
Case Study:
Impact of Genetic Drift on the Greater Prairie Chicken
Loss of prairie habitat = reduction of
population.
The surviving birds had low levels of
genetic variation, and only 50% of their
eggs hatched.
Impact of Bottleneck Effect
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Pop. Size
Number of
Alleles/Locus
Percentage of
Eggs Hatched
1,000-25,000
<50
5.2
3.7
93
<50
Kansas
1998 (no b/n)
750.000
5.8
99
Nebraska
1998 (no b/n)
75,000-200,000
5.8
96
Location
Illinois
1930-1960
1993
Impact of Bottleneck Effect
 Researchers used DNA from museum specimens to
compare genetic variation in the population before
and after the bottleneck.
 The results showed a loss of alleles at several loci.
 Researchers introduced greater prairie chickens
from populations in other states and were
successful in introducing new alleles and
increasing the egg hatch rate to 90%.
Effects of Genetic Drift: A Summary
1. Significant in small populations.
2. Causes allele frequencies to change at random.
3. Can lead to a loss of genetic variation within
populations.
4. Can cause harmful alleles to become fixed.
Gene Flow
Gene flow consists of the movement of
alleles among populations.
Alleles can be transferred through the
movement of fertile individuals or gametes
(for example, pollen).
Gene flow tends to reduce variation among
populations over time.
Gene flow can decrease the fitness of a
population.
Gene Flow
 Consider, for example, the Parus major on the
Dutch island of Vlieland:
 Mating causes gene flow between the central
and eastern populations.
 Immigration from the mainland introduces
alleles that decrease fitness.
 Natural selection selects for alleles that
increase fitness.
 Birds in the central region with high
immigration have a lower fitness;
birds in the east with low immigration
have a higher fitness.
60
Survival rate (%)
50
Population in which the
surviving females
eventually bred
Central
Eastern
Central
population
NORTH SEA
Eastern
population
Vlieland,
the Netherlands
40
2 km
30
20
10
0
Females born
in central
population
Females born
in eastern
population
Parus major
Gene Flow
 Gene flow can increase the fitness of a population.
 Consider, for example, the spread of alleles for
resistance to insecticides:
 Insecticides have been used to target mosquitoes
that carry West Nile virus and malaria.
 Alleles have evolved in some populations that
confer insecticide resistance to these mosquitoes.
 The flow of insecticide resistance alleles into a
population can cause an increase in fitness.
 Gene flow is an important agent of evolutionary
change in human populations.