Download Chapter 22: Descent with Modification: A Darwinian View of Life

Chapter 22: Descent with Modification: A Darwinian View of Life
22.1: The Darwinian revolution challenged traditional views of a young Earth
inhabited by unchanging species
Overview: Endless Forms Most Beautiful
 Key observations about life:
o Ways in which organisms are suited for life in their environments
o Shared characteristics/unity of life
o Rich diversity of life
 Evolution = descent with modification
o Darwin used in proposing that Earth’s many species are descendants
of ancestral species that were different from present day species
o Change in genetic composition of a population from generation to
generation
 Two ways to view evolution:
o Pattern of evolutionary change revealed by data = facts/observations
about the natural world
o Process of evolution = mechanisms that produce observed pattern of
change representing natural causes of natural phenomena we observe
 Unifying theory of evolution gives ability to explain and connect observations
about the living world
Scala Naturae and Classification of Species
 Some Greek philosophers suggested life changed gradually over time
 Aristotle viewed species as fixed, arranged life forms on a ladder/scale of
increasing complexity = scala naturae
o Ideas consistent with Old Testament creation story
o Interpreted match of organisms to their environment as evidence that
a creator had specifically designed each species
 Carolus Linnaeus = Swedish physician/botanist who developed two part
format for naming species, still used today
o Nested classification system, grouping similar species into
increasingly general categories
o Attributed resemblances to pattern of creation, not to evolution
Ideas About Change over Time
 Darwin drew from studies of fossils = remains/traces
of organisms from past
o Found in sedimentary rocks from sand/mud at
bottom of water
o New layers of sediment cover older ones
compressing them into layers of rock = strata
 Paleontology = study of fossils, developed by Georges
Cuvier
o Noted that older the stratum, the more
dissimilar the fossils were to current life-forms
o From one layer to the next some species appeared while others
disappeared
o Cuvier opposed idea of evolution
o Instead advocated for catastrophism- idea that events in the past
occurred suddenly and were caused by mechanisms different from
those operating in the present
 Each boundary between strata represented a catastrophe that
destroyed many of the species living at that time
 Other scientists suggested that profound change could take place through
cumulative effect of slow but continuous processes
o James Hutton proposed that Earth’s geologic features could be
explained by gradual mechanisms still operating today
o Charles Lyell incorporated Hutton’s ideas into principle of
uniformitarianism = mechanisms of change are constant over time
 Same geologic processes are operating today as in the past, at
the same rate
 Hutton and Lyell influenced Darwin’s thinking, agreeing that if geologic
change results from slow, continuous actions then Earth must be much older
than believed
o Applied idea of gradual change to biological evolution
Lamarck’s Hypothesis of Evolution
 Jean-Baptiste de Lamarck proposed idea of how life changes over time
o Vilified in his own time by those who denied evolution
 Correctly recognized that evolutionary change explains patterns in fossils
and the match of organisms to their environments
o Compared living species to fossil forms finding several lines of
descent, a chronological series of older to younger fossils leading to a
living species
 Incorrectly hypothesized how evolution occurs
o Use and disuse = idea that parts of the body that are used become
larger and stronger and those that aren’t used deteriorate
o Inheritance of acquired characteristics = an organism could pass these
modifications to its offspring
 Thought evolution happens because organisms have an innate drive to
become more complex
o Darwin rejected idea
 In reality: traits acquired by use during an individual’s life are not inherited
22.2: Descent with modification by natural selection explains the adaptions of
organisms and the unity and diversity of life
Darwin’s Research
 The Voyage of the Beagle
o Left medical school to become a clergyman, paid to sail around world
o Mission of voyage was to chart poorly known stretches of South
American coastline
o Darwin spent most of his time on shore observing and collecting
thousands of South American plants and animals, noting that their
characteristics made them well suited to diverse environments
o Noted that fossils in South America were distinctly South American
o Also studied geology on journey supporting Lyell’s idea that physical
evidence didn’t support idea that Earth was a few thousand years old
o Fascinated by organisms found at the Galápagos = volcanic islands
 Darwin’s Focus on Adaptation:
o Adaptations = inherited characteristics of organisms that enhance
their survival and reproduction in specific environments
o Believed that adaptation to environment and origin of new species
were related
 New species could arise from gradual accumulation of
adaptations to a different environment
 Ex: Galápagos finches beaks adapted to different foods
o How adaptations arise centers on natural selection = process in which
individuals with certain inherited traits tend to survive and reproduce
at higher rates than others because of those specific traits
o Darwin wrote essay on descent with modification and natural
selection but didn’t want to publish because of uproar
 Alfred Wallace published first
 Darwin published book On the Origin of Species by Means of
Natural Selection (The Origin of Species)
 Convinced most scientists through plausible mechanism,
immaculate logic, avalanche of evidence
The Origin of Species
 Descent with Modification
o Organisms share characteristics = unity of life, attributed to descent of
all organisms from remote ancestor
o Descendants of that ancestor lived in various habitats, accumulating
diverse modifications/adaptations
o Led to rich diversity of life today
 Viewed history of life as a tree- branching to youngest twigs = diversity of
present organisms
o Forks = most recent common ancestors
o Many evolutionary branches are dead ends
o ~99% of all species that have ever lived are now extinct 
o Fossils of extinct species can fill in gaps
 Artificial Selection, Natural Selection, and Adaptation
o Artificial selection = humans modified other species over many
generations by selecting and breeding individuals with desired traits
o Saw connection between natural selection and capacity of organisms
to overreproduce
 Not all offspring survives- eaten, starved, diseased, unmated…

o Heritable traits can influence how well an organism’s offspring cope
with environmental challenges
 If an organism has a trait that gives its offspring an advantage,
such advantages can increase the number of offspring that
survive and reproduce  traits are favored and more likely to
appear at greater frequency in the next generation
 Over time natural selection can lead to an increase in the
proportion of favorable traits in a population
o Observation 1: members of a population often vary in inherited traits
o Observation 2: all species can produce more offspring than their
environment can support, and many of these offspring fail to survive
and reproduce
o Inference #1: individuals whose inherited traits give them a higher
probability of surviving and reproducing in a given environment tend
to leave more offspring than other individuals
o Inference #2: this unequal ability of individuals to survive and
reproduce will lead to the accumulation of favorable traits in the
population over generations
Natural Selection: A Summary
o Natural selection is a process in which individuals that have certain
heritable traits survive and reproduce at a higher rate than other
individuals because of those traits
o Over time, natural selection can increase the match between
organisms and their environment
o If an environment changes, or if individuals move to a new
environment, natural selection may result in adaptation to these new
conditions, sometimes giving rise to new species
o Individuals don’t evolve- the population evolves over time
o Natural selection can amplify or diminish only heritable traits that
differ among the individuals
 Even if a trait is heritable, if all the individuals in a population
are genetically identical for the trait evolution by natural
selection won’t occur
o Environmental factors vary from place to place and over time
 A trait that is favorable in one place/time may be useless or
detrimental in other places/times
22.3: Evolution is supported by overwhelming amount of scientific evidence
Direct Observations of Evolutionary Change
 Natural selection is a process of editing, not a creative mechanism
o A drug selects for resistant individuals already present- doesn’t create
resistant organisms
 Natural selection depends on time and place
o Favors characteristics that provide advantage to current local setting
 Natural Selection in Response to Introduced Plant Species
o What happens when plants that herbivores feed on change?
o Soapberry bugs: feed most effectively when their beak length matches
the depth of seeds found within fruit
 The Evolution of Drug Resistant Bacteria
o Problem with bacteria and viruses = resistant strains
o Staphylococcus aureus (MSRA): certain strains form pathogens 
pathogenic strain became more common with rise of penicillin
 Strains became resistant to penicillin
 Resistant strains emerged by bacteria learning to synthesize
proteins deactivated by medicine
o As species evolve it becomes increasingly difficult to develop
antibiotics against as pathogens become multi-drug resistant
Homology = similarity resulting from common ancestry
 As a result of natural selection related species can have characteristics with
underlying similarity but function differently
 Anatomical and Molecular Homologies
o Forelimbs of all mammals show same arrangement of bones even
though appendages have different functions
o Anatomical resemblances would be unlikely if structures had arisen
anew in each species
o Homologous structures = variations on structural theme that was
present in common ancestor
o Comparing early stages of development reveal additional anatomical
homologies not visible in adult organisms
 Eventually develop into structures with different functions
o Vestigial structures = “left over” structures of little importance
 Remnants of features that served function in organism’s
ancestors
o Similarities at molecular level
 Same genetic language/code of DNA and RNA
 Share genes inherited from distant common ancestor
 Some genes lost their function
 Homologies and “Tree Thinking”
o Homologous characteristics form nested pattern:
 Homologous characteristics shared by all species date to deep
ancestral past
 All life shares deepest layer
 Homologous characteristics evolved more recently are shared
only with smaller groups of organisms
 Each successive smaller group adds homologies to those
it shares with larger groups
o Pattern of descent from common ancestors and resulting homologies
in evolutionary tree = diagram reflecting evolutionary relationships
 Each branch point represents common ancestor of lineages
beginning there and to the right of it
 More closely related to organism with closer common ancestor
 Hypotheses summarize understanding of descent patterns
 Convergent evolution = independent evolution of similar features in different
lineages
o Why distantly related organisms can resemble each other
o A result of different populations evolving in similar environments
o Analogous = resemblance in cases of convergent evolution
o Ex: Australian marsupials and eutherians
The Fossil Record
 Documents pattern of evolution, showing that past organisms differed from
present-day organisms and that species have become extinct
 Show evolutionary changes that occurred in various groups of organisms
 Shed light on new groups of organisms, document steps in transition from
life on land to life in sea
o Document formation of new group of mammals: cetaceans
 Show that descent with modification produces increasingly large differences
among related groups of organisms, resulting in diversity seen today
Biogeography = geographic distribution of species
 Influenced by continental drift = slow movement of continents over time
o Pangaea = unit of earth’s landmasses 250 million years ago
 Can predict where fossils of different groups of organisms may be found
 Islands have species that are endemic = found nowhere else
o Still closely related to species from nearest mainland
o Explains why islands w similar environments in distant parts of world
are populated with species more related to mainland than each other
What is Theoretical About Darwin’s View of Life?
 Pattern of evolution = observation that life has evolved over time has been
documented directly and supported by evidence
 Process of evolution (natural selection) explains lots of data, observed and
tested in nature
 Theory = more comprehensive than hypothesis = has stood up to thorough
and continual testing
 Skepticism prevents it from becoming dogma