fossils & evolution, ch. 51 chapter 5—key concepts the “modern synthesis” or...
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Fossils & Evolution, Ch. 5 1
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 Neo-Darwinists 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 2
Chapter 5—Key terms
• Neo-Darwinism (= modern evolutionary synthesis)
• Microevolution (phyletic evolution)
• Macroevolution (phylogenetic evolution)
• Structural genes
• Regulatory genes
Fossils & Evolution, Ch. 5 3
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
Fossils & Evolution, Ch. 5 4
Macroevolution = phylogenetic evolution (branching; evolution above the specieslevel)
Microevolution = phyletic evolution (evolution at the population/species level)
Conventional meaning of microevolution and macroevolution
Fossils & Evolution, Ch. 5 5
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
Fossils & Evolution, Ch. 5 6
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.
Fossils & Evolution, Ch. 5 7
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
Fossils & Evolution, Ch. 5 8
Exceptional evidence for Darwinian evolutionin the deep-sea fossil record
images courtesy of Ocean Drilling Program
Fossils & Evolution, Ch. 5 10
a
b c
Ap-W Ap-H
Ch-Height
Radius
Ch-Width
Diam.
Axis
Baselength
Um-angle
planktonic foraminiferaTurborotalia
image courtesy of Paul Pearson, Cardiff University
Fossils & Evolution, Ch. 5 11
Diameter (m)
350 400 450 500 550
34
36
38
40
42
44
46
P16
P15
P14
P13
P12
P11
Age
(M
a)
image courtesy of Paul Pearson, Cardiff University
a
b c
Ap-W Ap-H
Ch-Height
Radius
Ch-Width
Diam.
Axis
Baselength
Um-angle
Turborotalia Morphometrics
52 successive populationsspanning ~12 million years
(~0.25 m.y. increments)
Fossils & Evolution, Ch. 5 12
0.35 0.40 0.45 0.50 0.55 0.60
34
36
38
40
42
44
46
Aperture aspect ratio
Ag
e (
Ma
)
P16
P15
P14
P13
P12
P11
Baselength / diameter
0.60 0.65 0.7 0.75
34
36
38
40
42
44
46
Ag
e (
Ma
)
P16
P15
P14
P13
P12
P11
Chambers in final whorl
3.0 3.5 4.0 4.5 5.0 5.5
P16
P15
P14
P13
P12
P11
Aspect ratio of final chamber
0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62
34
36
38
40
42
44
46
34
36
38
40
42
44
46
P16
P15
P14
P13
P12
P11
Coiling axis / diameter
0.55 0.60 0.65 0.70 0.75 0.80
34
36
38
40
42
44
46
P16
P15
P14
P13
P12
P11
Ag
e (
Ma
)
Ag
e (
Ma
)
Ag
e (
Ma
)
0 % 20% 40% 60% 80% 100%
34
36
38
40
42
44
46
P16
P15
P14
P13
P12
P11
Dextral coiling (%) Diameter (m)
350 400 450 500 550
34
36
38
40
42
44
46
P16
P15
P14
P13
P12
P11
0.35 0.375 0.40 0.425 0.45
Posterior radius / diameter
34
36
38
40
42
44
46
P16
P15
P14
P13
P12
P11
0.78 0.80 0.82 0.84 0.86 0.88
Roundness
Ag
e (
Ma
)
Ag
e (
Ma
)
Ag
e (
Ma
)
34
36
38
40
42
44
46
P16
P15
P14
P13
P12
P11
Ag
e (
Ma
)
22 24 26 28 30 32 34
34
36
38
40
42
44
46
P16
P15
P14
P13
P12
P11
Ag
e (
Ma
)
Umbilical angle (degrees)
evolutionary trend
images courtesy of Paul Pearson, Cardiff University
Fossils & Evolution, Ch. 5 13©Cushman Foundation for Foraminiferal Research T. frontosa
T. pomeroli
T. cerroazulensisch
rono
spec
ies
Fossils & Evolution, Ch. 5 14
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
Fossils & Evolution, Ch. 5 15
nucleotide bases:A = adenineC = cytosineG = guanine
U = uracil
phosphate
sugar
DNA double helix
Fossils & Evolution, Ch. 5 17
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 19
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 20
Horses still possess structural genes of their three-toedancestors. When regulatory genes fail to suppress thesestructural genes, a three-toed horse develops
Fossils & Evolution, Ch. 5 21
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)
Fossils & Evolution, Ch. 5 22
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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