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 9images 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 16

Third base in codon is largely redundant

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 18

Boa constrictor(solid upper jaw)

Bolyerine snake(hinged upper jaw)

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)

Fossils & Evolution, Ch. 5 23

Stretched DC8Natalie Gulbis

Fossils & Evolution, Ch. 5 24

Macroevolution(= phylogenetic

evolution)

Fossils & Evolution, Ch. 5 25

Fossils & Evolution, Ch. 5 26

Fossils & Evolution, Ch. 5 27

Trend toward size increase in A, B, CTrend toward size decrease in D–F