Download Chapter 22 Notes

Chapter 22 Notes
In the Origin of Species, Darwin proposed that species change through natural selection
Darwin’s Research
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Darwin was born in England
o Interested in nature as a child
o Studied at University of Edinburgh to study medicine
 Left and enrolled at Cambridge
 Studied under Henslow
The Voyage of the Beagle
o Left England in 1831 on the Beagle
o Mission of voyage to was chart unknown stretches of the South American coastline
 Spent his time on shore, collecting plants and animals
 He noticed that plants and animals in temperate regions resembled species living in SA
tropics, not temperate regions of Europe
 Fossils were distinctly South American
o Darwin experienced geologic change when a violent earthquake rocked the coast of Chile
o Inferred that the rocks containing fossils must have been raised by a long series of earthquakes
o Galapagos Islands
 Unusual organisms
 Different species of finches that were similar but different
 Some on the same island, some found on two islands
Darwin’s Focus on Adaptation
o Begin to perceive adaptation to the environment and the origin of new species as closely related
processes
o Biologists have concluded that a new species arise from an ancestral form by the gradual accumulation
of adaptations to different environments
 Ex: Galapagos finches
 Beaks and behaviors are adapted to the specific foods available on their home islands
o Early 1840s: Darwin worked out features of his theory of natural selection as a mechanism of evolution
 1844: Darwin wrote long essay on the origin of species and natural selection
 Reluctant to publically announce because of uproar it would cause
o Less uproar happened because he had lots of evidence
The Origin of Species
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Darwin developed two main ideas:
o Evolution explains life’s unity and diversity
o Natural selection is a cause of adaptive evolution
Descent with Modification
o In his book, did not use evolution until very end
 Instead called it descent with modification: phrase that summarized his view of life
o Thought all organisms are related through descent from an ancestral organism that lived in the past
 Over millions of years, ancestral organisms accumulated diverse modifications, that fit them to
specific ways of life
o History of life is like a tree
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Multiple branching from a common trunk
 Each fork represents an ancestor of all the lines of evolution
o Most branches of evolution are dead ends
 About 99% of all organisms that ever lived are extinct
Natural Selection and Adaptation
o Three inferences based on five observations:
 1: production of more individuals than the environment can support leads to a struggle for
existence among individuals of a population, with only a fraction of their offspring surviving each
generation
 2: survival depends in part on inherited traits. Individuals whose inherited traits give them a
high probability of surviving and reproducing in a given environment have higher fitness and are
likely to leave more offspring than less fit individuals.
 3: this unequal ability of individuals to survive and reproduce will lead to a gradual change in a
population, with favorable characteristics accumulating over generations
o Darwin perceived a connection between natural selection and the capacity of organisms to overproduce
 In each generation, environmental factors filter heritable variations, favoring some over others
 Organisms with these traits favored by the environment tend to produce more offspring
than do organisms without those traits
o Artificial Selection
 Artificial selection: modifying of species over many generations by selecting and breeding
individuals that possess desired traits
 As a result, crop plants and animals bred as livestock or pets often bear little
resemblance to their wild ancestors
o Summary of Natural Selection
 Natural selection is the differential success in reproduction among individuals that vary in their
heritable traits. These reproductive differences emerge as each individual interacts with its
environment.
 Over time, natural selection can increase the adaptation of organisms to their environment.
 If an environment changes over time, or if individuals of a species move to a new environment,
natural selection may result in adaptation to these new conditions, sometimes giving rise to new
species in the process.
 Three subtle but important points about evolution by natural selection:
 Although natural selection occurs through interactions between individual organisms
and their environment, individuals do not evolve; population is the smallest unit that
can evolve
 Natural selection can amplify or diminish only heritable traits (traits passed from
generation to generation);
 Environmental factors vary from place to place and from time to time; a trait that is
favorable in one situation may be useless or harmful in different circumstances
22.3
Darwin’s theory explains a wide range of observations
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The power of evolution as a unifying theory is its versatility as a natural explanation for a diversity of data from
biology’s main subfields
Natural Selection in Action
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Differential Predation and Guppy Populations
o Guppies: small freshwater fish
o Observed significant differences between populations in average size and age at which they reach sexual
maturity
 Changes correlated with the type of predator most active on that population
 Guppies preyed on by pike-cichlids (ate mature fish) begin reproducing at a younger age
and are smaller at maturity
 Guppies preyed on by killfish (ate young fish)
o Designed experiment to test correlation with type of predator and maturity
 Wanted to see if differences were due to natural selection
 Introduced guppies from “pike-cichlid” pools to new pools that contained killfish but no guppies
 Over 11 years, compared age and size of maturity of guppies that were transplanted
with the original population
o After 30-60 generations, transplanted guppies were 14% heavier at maturity
 Average age of maturity also rose
 Supports hypothesis that natural selection caused the
difference in the populations
Evolution of Drug-Resistant HIV
o Drug-resistant pathogens cause issue in humans
 Due to quick rates of reproduction
 A variation that makes individuals resistant to a particular drug can increase in
frequency very quickly
o Ex: HIV
 Numerous drugs made to combat HIV
 3TC—picked up by reverse transcriptase in HIV cells; causes DNA to no longer elongate and can
no longer reproduce
 3TC-resistant strains carry slightly different versions of reverse transcriptase that are
able to discriminate between the drug and normal C-bearing nucleotides
o These examples highlight two important key points about natural selection
 Natural selection is more a process of editing than a creative mechanism
 Natural selection depends on time and place
Homology, Biogeography, and the Fossil Record
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Homology
o Darwin’s concept of descent with modification can explain why certain characteristics in related species
have an underlying similarity, even though they may have very different function
 Such similarity resulting from common ancestry: homology
o Anatomical Homologies
 View of evolution as a remodeling process accounts for the findings of comparative anatomy
 Forelimbs of all mammals show the same arrangement of bones from shoulder to tips of
digits, even though they have different functions
 Homologous structures: represent variations on a structural theme that was present in their
common ancestor
 Comparative embryology: comparison of early stages of development
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 Shows that mammals have pharyngeal pouches and tails
 Vestigial organs: structures of marginal importance to the organism
 Remnants of structures that served important functions in the organism’s ancestors
o Ex: snakes; appendix; human knee joints
o Molecular Homologies
 Similarities at molecular level
 All forms of life use DNA and RNA
 Genetic code is universal
o Likely that all species descended from a common ancestor
 Bacteria and humans share many genes that have been inherited from a distant common
ancestor
o Homologies and the Tree of Life
 Darwin’s evolutionary tree can explain the homologies that researchers have observed
 Ex: genetic code
 Ancestral resemblances among species are generally reflected in their molecules
 Ex: hemoglobin
o Same pattern of evolutionary relationships that they find when comparing other
proteins
Biogeography
o Biogeography: the geographic distribution of species
o Closely related species tend to be found in the same geographic region
o The same ecological niches in distant regions are occupied by very different species
 Ex: marsupials and eutherians
 Even though they may look similar and have adapted to similar environments in similar
ways, they evolved independently from different ancestors
o Endemic: found nowhere else in the world
 Ex: finches
 Most island species of finches are closely related to species from the nearest mainland
or neighboring island
o This explains why two islands with similar environments in different parts of the
world are populated not by closely related species but by species that resemble
those of the nearest mainland, where the environment is quite different
The Fossil Record
o Succession in forms observed in the fossil record is consistent with other inferences about the major
branches of descent in the tree of life
 Oldest know fossils are prokaryotes
o Darwinian view of life predicts that evolutionary transitions should leave signs in the fossil record
 Such fossils have been discovered
o Darwin’s theory endures because it explains so many different kinds of observations: anatomical,
molecular, biogeography, fossil record
 Natural selection can also explain how similar adaptations can evolve independently among
distantly related species
What is Theoretical about the Darwinian View of Life?
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What is theoretical about evolution?
o Theory: comes close to what scientists mean by a hypothesis
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Accounts for many observations and data and attempts to explain and integrate a great variety
of phenomena
Does not become widely accepted unless its predictions stand up to thorough and continual
testing