Chapter 5—Key concepts
• The “modern synthesis” or “Neo-Darwinism” emerged in the 1930’s
and 1940’s when population genetics was linked with natural selection
as the primary mechanism of evolutionary change. Extreme NeoDarwinists believe that all evolution is “microevolution” (i.e., gradual
changes, added up over hundreds of generations, to produce new
anatomical features, new species, and new fundamental body plans).
• Beginning in the 1970’s, the concept of “macroevolution” has
challenged extreme Neo-Darwinism. According to macroevolution,
large-scale evolutionary changes do not occur as a result of gradual,
stepwise processes, but more abruptly. Macroevolution is supported by
the discovery of regulatory genes.
• A slightly different meaning of “macroevolution” involves
phylogenetic evolutionary trends: i.e., trends above the species level.
Fossils & Evolution, Ch. 5
1
Chapter 5—Key terms
• Neo-Darwinism (= modern evolutionary
synthesis)
• Microevolution (phyletic evolution)
• Macroevolution (phylogenetic evolution)
• Structural genes
• Regulatory genes
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2
Microevolution vs. Macroevolution
• These terms mean different things to
different people
• Textbook author considers microevolution
to mean gradual, generation upon
generation change; and macroevolution to
mean abrupt, saltational change potentially
resulting in new anatomical features, new
species, even new body plans
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3
Conventional meaning of
microevolution and macroevolution
Macroevolution = phylogenetic evolution
(branching; evolution above the species
level)
Microevolution = phyletic evolution
(evolution at the population/species level)
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4
Darwinian evolution
• Evolution = descent with modification
– In natural populations, more offspring are produced
than can be sustained in the environment
– There is variation among individuals (because of
mutations and genetic recombination during sexual
reproduction)
– Natural selection results in the differential survival of
variants
– Variation is heritable: therefore, advantageous traits are
likely to be passed on from generation to generation
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5
Neo-Darwinism
• Population genetics = use of mathematical
models to determine how mutation and
natural selection can cause changes in gene
frequencies in populations
• When population genetics was coupled with
Darwinian selection, the so-called modern
evolutionary synthesis emerged.
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6
Neo-Darwinism
• A tenet of Neo-Darwinism is the belief that all
evolutionary changes can be attributed to repeated
rounds of microevolution
– Microevolution = small scale changes that occur in a
few generations in response to natural selection
– Phyletic evolution = Gradual changes, added up over
hundreds of generations, can produce new anatomical
features, new species, new body plans
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7
Exceptional evidence for Darwinian evolution
in the deep-sea fossil record
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8
images courtesy of Ocean Drilling Program
images courtesy of Ocean Drilling Program
Fossils & Evolution, Ch. 5
9
planktonic foraminifera
Turborotalia
Ch-Width
Ch-Height
Baselength
Ap-W
Ap-H
a
Diam.
Um-angle
c
b
Radius
image courtesy of Paul Pearson, Cardiff University
Fossils & Evolution, Ch. 5
Axis
10
Diameter (m)
350
400
450
500
550
34
P16
36
P15
Age (Ma)
38
P14
40
P13
42
P12
44
52 successive populations
spanning ~12 million years
(~0.25 m.y. increments)
P11
46
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11
image courtesy of Paul Pearson, Cardiff University
evolutionary trend
80%
Diameter (m)
100%
350
34
400
450
500
44
P14
P13
42
P12
44
Aperture aspect ratio
0.50
0.55
0.60
34
0.65
0.7
P13
42
P12
3.0
P15
P15
P13
42
P12
44
Age (Ma)
P14
40
P14
P13
42
P12
44
P11
P13
42
P12
P13
42
P12
P11
46
0.50
0.52
0.54
0.56
0.58
0.60
Coiling axis / diameter
0.62
0.55
34
P13
42
P12
44
0.65
0.70
0.75
P16
P15
P15
38
P14
40
P13
42
P12
44
P11
P14
40
P13
42
P12
44
P11
46
images courtesy
of Paul
Pearson, Ch.
Cardiff
Fossils
& Evolution,
5 University
0.80
36
38
P14
0.60
34
P16
36
40
46
P14
40
44
P15
P11
46
P14
0.48
38
40
34
P15
Aspect ratio of final chamber
5.5
36
38
32
38
P16
36
38
5.0
30
P11
34
P16
36
4.5
28
36
46
4.0
26
P16
44
3.5
24
34
40
Chambers in final whorl
0.75
34
P16
Age (Ma)
P14
40
Baselength / diameter
0.60
22
P15
46
Age (Ma)
0.45
0.88
P11
46
0.40
Umbilical angle (degrees)
0.86
38
P11
46
0.84
36
44
P11
0.82
P16
38
40
0.80
P15
Age (Ma)
Age (Ma)
Age (Ma)
P12
0.78
36
38
42
Roundness
0.45
34
P15
38
P13
0.425
P16
36
P14
0.40
34
P15
40
0.375
P16
36
46
0.35
34
P16
0.35
Posterior radius / diameter
550
Age (Ma)
60%
Age (Ma)
40%
Age (Ma)
20%
Age (Ma)
Dextral coiling (%)
0%
P11
46
12
T. cerroazulensis
T. pomeroli
Fossils & Evolution, Ch. 5
©Cushman Foundation for Foraminiferal Research
13
T. frontosa
Neutralism
• An initial challenge to “extreme Neo-Darwinism”
occurred when it was discovered that many genes
are not expressed
– If so, then how can natural selection act upon them?
• Much of the genetic code is “junk”
– Amino acids are coded for by groups of three
nucleotides (4 possible in each of three positions)
– 64 possible combinations
– Yet, only 20 amino acids!
– Third position in codon is largely redundant
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14
DNA double helix
nucleotide bases:
A = adenine
C = cytosine
G = guanine
U = uracil
sugar
phosphate
Fossils & Evolution, Ch. 5
15
Third base in codon is largely redundant
Fossils & Evolution, Ch. 5
16
Macroevolution
• Macroevolution has emerged as a model for
explaining the origin of major adaptive
features
– Not just repeated rounds of microevolution
• Example: hinged upper jawbone in
bolyerine snakes (related to boas)
– Hinge must have evolved abruptly as a novel
feature, not gradually from an unhinged
ancestral condition
Fossils & Evolution, Ch. 5
17
Boa constrictor
(solid upper jaw)
Bolyerine snake
(hinged upper jaw)
Fossils & Evolution, Ch. 5
18
Macroevolution
• Macroevolutionary changes are supported
by the discovery of regulatory genes
– Structural genes code for individual proteins
and structures
– Regulatory genes are like “switches” that turn
on or off the expression of other genes
• Small mutation in a regulatory gene can radically
alter or silence the expression of a string of
structural genes that it controls
Fossils & Evolution, Ch. 5
19
Horses still possess structural genes of their three-toed
ancestors. When regulatory genes fail to suppress these
structural genes, a three-toed horse develops
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20
Macroevolution
• Homeotic genes are special regulatory
genes. Mutations to homeotic genes can
cause abrupt transformations:
– Antennipedia in flies (appendage grows on
head instead of antenna)
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Macroevolution
• Hox complex of homeotic genes controls
basic segmentation of body in arthropods
and vertebrates
– mutations to Hox genes can cause “stretched
DC8”-type transformations)
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Stretched DC8
Natalie Gulbis
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Macroevolution
(= phylogenetic
evolution)
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24
Fossils & Evolution, Ch. 5
25
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26
Trend toward size increase in A, B, C
Trend toward size decrease in D–F 27
Fossils & Evolution, Ch. 5