What Were the Main Accomplishments of Charles
Darwin and Alfred Wallace?
• Descent with Modification and Mutability (vs. Great Chain of Being)
– Darwin studied beetles in the Amazon, mockingbirds on Galapagos Islands,
other fauna and fossils in South America
– Darwin’s Origin of Species convinced most naturalists of evolution; added to
earlier concepts of “transmutation” (Buffon, Lamarck, Chambers, Lyell)
– Mutability of species: giant sloth and armadillo fossils; giant tortoise, marine
iguana of Galapagos Islands suggested that species could be transformed
• Adaptations
– Darwin wrote about adaptations observed during voyage of HMS Beagle,
and multiple functions of adaptations (ex., steamers using wings to row)
– Wallace especially interested in cryptic coloration and mimicry (ex., stick
insects)
• Biogeography
– Both noted similarities of island fauna to fauna of nearby continents
• Natural Selection: proposed jointly as the main mechanism of change
(“survival of the fittest” later coined by Herbert Spencer)
– Both influenced by Thomas Malthus’ “Tragedy of the Commons” thesis
– Darwin influenced by results of selective breeding (artificial selection)
– Reigning paradigm: Inheritance of Acquired Characteristics (Jean Baptiste
Lamarck); Darwin grudgingly accepted as the mechanism of inheritance; August
Weismann (1888) disproved by cutting tails off mice
Fig. 6.1
Fig. 6.4
Fig. 6.5
What are the Postulates of Darwin’s
Theory?
• Darwin’s Postulates (theory of natural selection as the major
cause of evolution – each postulate can be tested; each
potentially falsifiable)
1. Individuals within populations are variable
2. Variations among individuals are, at least in part, passed from parents to
offspring (Darwin was not aware of genetic mechanisms)
3. In every generation, some individuals are more successful at surviving and
reproducing than others
• Most juveniles die before reproducing (note biotic potential)
4. The survival and reproduction of individuals are not random; instead, they
are tied to the variation among individuals. The individuals with the
most favorable variations, those who are better at surviving and
reproducing, are naturally selected
• Fitness: measurement of organism’s ability to survive and reproduce
Fig. 6.32
What Factors Cause Evolution?
• Evolution (population genetics definition): change in
gene frequencies in a population (changes in gene
pool)
• Factors that can change the nature of a gene pool:
1. Natural selection: a strong force in evolution
2. Migration: especially strong in island populations
3. Mutation: a weak force in evolution, but the ultimate
source of novelty; mutations are generally mildly
deleterious (due to second copy of gene)
4. Non-random mate choice: sexual selection generally involves
female choice (among competing males)
5. Chance events: environmental changes and catastrophes;
“random” evolution called genetic drift
Fig. 6.28
What Evidence Supports the Modern
Theory of Evolution?
1. Direct observations of change through time
– Ex., changes in beak morphologies among Darwin’s finches
(long-term study at Galapagos Islands)
– Ex., change in beak lengths of soapberry bugs after
introduction of golden rain trees in Florida
2. Vestigial traits: functionless or rudimentary version of
functional feature in other, closely related species or
subspecies
– Examples: eye sockets in blind cave fishes; wings in flightless
birds; pelvic and leg bones (and spurs) in snakes
(similar situation with cetaceans); reduced tailbone
(coccyx) and arrector pili muscles in humans (
goosebumps; lift hair in other mammals)
What Evidence Supports the Modern
Theory of Evolution?
3. Evidence from the fossil record
– Extinction: in 1812, Cuvier provided strong evidence of
extinction with analysis of fossils (mammoths,
mastodons, and Irish elk)
– Law of Succession: general pattern of correspondence
between fossil and living forms from the same
locale; supported from wide variety of locations
and taxonomic groups (ex. marsupials of
Australia)
– Transitional forms: exhibit various characteristics seen
in ancestral species and other characteristics
seen in more recent descendents (the latter often
including important novel features)
•
Examples: Archaeopteryx; Basilosaurus; transitional tetrapods
Fig. 6.33
What Evidence Supports the Modern
Theory of Evolution?
4. Homology: the study of likeness (modern meaning: similarity
due to inheritance of traits from a common ancestor)
–
Structural and developmental homology
•
•
–
Ex., pattern of limb bones similar in all tetrapods
Ex., vertebrate embryos undergo similar developmental stages
before acquiring group-specific features (first noted by Karl
Ernst von Baer in 1828)
Molecular homology: shared genetic code for nearly all living
organisms; genes for critical enzymes with few
differences among groups; shared genetic flaws in
related species
5. Thousands of lab, field, and in silico studies that document
the importance of natural selection, sexual selection,
mutation, and migration in the evolution of populations
Fig. 6.14
Fig. 6.17
Fig. 6.23
What are Adaptations?
• Adaptation: a feature used for some function that has become
prevalent or is maintained in a population because of
natural selection for that function
– Multiple functions of single traits: many traits have multiple uses
(ex. functions of fish swim bladder include buoyancy, oxygen storage,
and sound production)
– Trade-offs: single traits may have off-setting benefits and detriments (ex.
fish swim bladder provides buoyancy, but is a good target for dolphin
echolocation)
– Key innovations: traits that are associated with large gains in evolutionary
success (ex. skeletal fin rays in bony fishes)
– Preadaptation: a feature already present in a population that fortuitously
serves a new function
• Examples: wings in ancestral insects likely selected for surface-skimming
performance; bird wings likely enabled uphill running, gliding, and/or
thermoregulation before birds obtained flight
How Does Speciation Occur?
• The Biological Species Concept: species are groups of actually or
potentially interbreeding populations, which are reproductively isolated from other such groups (Ernst Mayr, 1942);
emphasizes reproductive isolation (lack of gene flow); later
modified to account for existence of fertile animal hybrids
(animal hybrids are rare, and are typically sterile or exhibit
low fitness)
• Mechanisms of Speciation
– Speciation: origin of new species (process vs. event)
– Allopatric Mechanisms (physical isolation triggers
reproductive isolation)
• Via dispersal and colonization (ex., islands, edge of range)
• Via physical split of original range (ex., new mountain range or
isthmus, change in river’s course)
– Sympatric Mechanisms
• Genetic mechanisms: polyploidy (ex., wheat), mutations in regulator
genes
• Behavioral mechanisms: temporal separation, courtship displays
Fig. 6.22
What are Some Patterns of Macroevolution?
• Adaptive Radiation: ancestral species evolves into multiple descendent
species, with each exploiting a different available lifestyle in their
respective environment
– Darwin’s finches on Galapagos Islands
– African cichlids (very diverse family of fishes in African Great Lakes)
• Convergence: independent evolution of superficially similar traits (in
response to similar selection pressures)
– Streamlining in dolphins, penguins, tunas (reduces drag in water)
– Echolocation in bats and dolphins (swarmed, patchy food sources)
• Coevolution: reciprocal changes in two or more species in close association
with each other
– “Arms races” between predators and their prey
– Adaptations for pollination (insects/hummingbirds/bats and flowering plants)
• Gradualism: slow emergence of new species (Darwin emphasized)
• Punctuated Equilibrium: long periods of stasis interrupted by sudden
emergence of new species (Stephen J. Gould and Niles Eldridge, 1972)
Fig. 6.25
Fig. 6.12
What Traits Characterize Humans?
• Bipedal Locomotion
– Involves modifications of pelvis, femur, and backbone
– Various hypotheses regarding benefits, including free hands (vs. knuckle-walking
and brachiation of great apes)
• Large Brain Size
– Requires omega fatty acids – high levels in marine organisms, nuts, marrow
• The Making and Use of Complex Tools
– Earliest stone tools produced by chipping flakes from a rock; oldest dated at
~ 2.5 mya (Oldowan Stone Tools)
– Opposable thumbs allow fine control and tight grips; include thick metacarpals
with broad heads for muscle attachment; three muscles not found in
chimpanzees
• Language and Cultural Evolution
– Capacity for language is innate in humans (ex., children born deaf create
languages)
– Modifications of larynx allow more tongue movement and great diversity of
vocalizations (trade-off is greater chance of choking); hyoid bone of
Neanderthals virtually identical to that of present-day humans
– Memes: ideas that can be disseminated via oral or written means, subject to
modifications and non-genetic descent (impart information, cultural analogue to
genes)
What are Some Notable Fossil Hominids?
• Pre-australopithecine: ~ 7 - 5 million years ago (mya)
– Sahelanthropus tchadensis: 6-7 mya (Chad); found in lake deposits from
ancient Lake Chad
– Orrorin tugenensis: ~ 6 mya (Kenya); recent evidence for bipedalism based
on CT scan of femur neck (controversial)
• Australopithecines: projecting faces; braincases < 550 cm3
– Australopithecus africanus: ~ 2.8 - 2.4 mya; first hominid fossil found in
Africa (“Taung Child”); discovered in coastal South African cave by
Raymond Dart in 1925; more specimens found nearby in 1930s - 1940s
– Australopithecus afarensis (incl. “Lucy”): ~ 3.9 - 3.0 mya (Tanzania
and Kenya); footprints in volcanic ash dated at 3.6 mya
• Early Humans: apparently coexisted with robust australopithecines in Africa
– Homo erectus (= H. ergaster): ~ 1.8 - 0.4 mya; widespread through
Africa and Asia (ex. “Java man”); large braincase (> 850 cm3); most
likely ancestor of more modern humans
– Homo neanderthalensis: ~ 0.3 - 0.03 mya; found in Europe
– Early Homo sapiens (incl. Cro-Magnon): ~ 0.1 mya - present; present-day
average braincase = 1200 cm3; Cro-Magnon Man (30,000 ya) buried with
animal bones, jewelry, and tools
Fig. 28.34
Fig. 28.35