What Were the Main Accomplishments of Charles Darwin and Alfred Wallace? Descent with Modification and Mutability (vs. Great Chain of Being) – Darwin studied.

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 variable2. 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 evolution2. Migration: especially strong in island populations3. 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 Florida2. 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 reproductive-ly 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 australo-pithecines 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