Evolution by Natural Selection

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Evolution by Natural Selection

• Chapter 23

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Darwins Theories
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Charles Darwin

• While on the voyage of the HMS Beagle in the
  1830s, Charles Darwin went onshore and collected
  thousands of specimens of fossils and living
  things
• By studying these specimens he observed
• similarities between living and fossil organisms
• the diversity of life in the world including the
  Galápagos Islands, where there was great diversity

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Charles Darwin

• Darwin became convinced that the Earth was old
  and continually changing
• He concluded that living things also change, or
  evolve over generations
• He also stated that living species descended from
  earlier life-forms descent with modification

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Darwins Theories

• Natural selection- populations change over
  generations if individuals that possess certain
  traits have differential breeding success
• Result is evolutionary adaptation where
  accumulated traits enhance an organisms ability
  to survive
• Results in evolution, where a change in a
  populations genetic composition occurs over time

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Pre Darwin Theories

• Until Darwin came up with his theories, most
  ideas viewed species as fixed and unchanging
• Bible states that all animals were made perfect
  by God and never change
• Carolus Linnaeus
• Interpreted organismal adaptations as evidence
  that the Creator had designed each species for a
  specific purpose
• Was a founder of taxonomy, classifying lifes
  diversity for the greater glory of God

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Some Theories that Darwin Built Upon

• Taxonomy influenced Darwin and helped to
  reinforce Darwins theories
• The study of fossils also helped to lay the
  groundwork for Darwins ideas
• Usually found in sedimentary rock, which appears
  in layers or strata
• The theory of gradualism is the idea that
  profound change can take place through the
  cumulative effect of slow but continuous
  processes
• Helped Darwin show how old the earth could be
• Lamarck used observations in nature to develop
  his theories of acquired characteristics

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Darwins Voyage

• In 1831, after college, Darwin was accepted on
  board the HMS Beagle, which was about to embark
  on a voyage around the world
• During his travels on the Beagle Darwin observed
  and collected many specimens of South American
  plants and animals
• Darwin observed various adaptations of plants and
  animals that inhabited many diverse environments

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Darwins Voyage
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The Origin of Species

• After the voyage, in 1844, Darwin wrote a long
  essay on the origin of species and natural
  selection
• But he was reluctant to introduce his theory
  publicly, anticipating the uproar it would cause
• In June 1858 Darwin received a manuscript from
  Alfred Russell Wallace
• Who had developed a theory of natural selection
  similar to Darwins
• Darwin quickly finished The Origin of Species
• And published it the next year

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The Origin of Species

• Darwin made two major points in his book
• He presented evidence that the many species of
  organisms presently inhabiting the Earth are
  descendants of ancestral species
• He proposed a mechanism for the evolutionary
  process, natural selection

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Descent with Modification

• Darwin believed
• There was a unity to life
• That all organisms were related through descent
  with modification from an ancestor that lived in
  the remote past
• Called descent with modification
• Organisms developed diverse modifications to
  adapt to specific ways of life

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Descent with Modification

• In the Darwinian view, the history of life is
  like a tree
• With multiple branches from a common trunk to the
  tips of the youngest twigs that represent the
  diversity of living organisms

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Evidence of Change Over Time
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Evidence for Changethrough Time

• Fossils are traces of organisms that lived in the
  past
• Fossils found and described in the scientific
  literature make up the fossil record

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Evidence for Changethrough Time

• Most fossils are found in sedimentary rocks
• Layers of sedimentary rock are associated with
  different intervals in the geologic time scale
• A relative time scale based upon fossil content

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Carbon 14 Dating

• Use radioactive isotopes
• to assign absolute ages
• Carbon 14
• Uranium
• Find out the amount of the product left
• Earth is about 4.6 billion years old
• Earliest signs of life at about 3.85 billion
  years old

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Fossils

• Many fossils provide evidence for extinct species
  unlike any known living organisms
• May link present organisms to past organisms

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Transitional Forms

• The law of succession means that extinct fossil
  species are typically succeeded, in the same
  region, by similar species
• Darwin saw this as evidence that species change
  over time

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Transitional Forms

• Transitional forms have been discovered with
  traits that are intermediate between older and
  younger species

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Vestigial Traits

• Functionless structures that are similar to
  functioning structures in related species
• Evidence that trait structure and function change
  over time

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Evidence That Species are Related
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Decent with Modification

• Darwin found many types of mockingbird
• Superficially similar, but different islands had
  distinct species
• Descended with modification from a common ancestor

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Decent with Modification
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Decent with Modification
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Structural Homologies

• Morphological traits that are similar
• Same general limb structure in vertebrates

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Developmental Homologies

• Similarity in embryo morphology and/or pattern of
  tissue differentiation
• All vertebrates have gill pouches and tails early
  in embryonic development

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Developmental Homologies

• Observed at two levels
• Overall morphology of embryos
• Fate of particular embryonic tissues
• Pattern exists because the common ancestor of all
  vertebrates was a fishlike animal with gill
  pouches and a tail

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Genetic Homologies

• Similarity in the DNA sequences of genes from
  different species
• Result in structural homologies
• Reflected in their molecules, their genes, and
  their gene products

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Genetic Homologies
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Homology vs Analogy

• Homology - similarities due to common descent
• Analogy similarities not due to descent
• Due to environment
• Convergent evolution occurs when natural
  selection leads to similar solutions to the
  challenges posed by a particular habitat

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The Process of Evolution
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How does natural selection work?

• A population experiences natural selection
  whenever the following four conditions apply
• (1) individuals vary in their traits
• (2) some of these variations are heritable
• (3) some individuals survive and reproduce better
  than other individuals
• (4) differential survival and reproduction

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Heritable Traits

• Some traits help an individual survive or
  reproduce
• These traits will, over time, increase in
  frequency in the population, causing evolution
• An adaptation is a heritable trait that increases
  an individual's fitness in a particular
  environment

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Evolution and Natural Selection

• Evolution is the result of natural selection
• Natural selection is the mechanism for evolution
• Individuals do not change during natural
  selection
• Causes them to produce more offspring
• Causes a change in the genetic makeup of the
  population
• Shifts gene frequency

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Evolution

• Evolution is not progressive, there are no higher
  or lower forms
• There is no ultimate being to achieve
• Forms can be older or younger
• Adaptations are for a particular environment

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Evolution
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Evolution

• Adaptation is not perfect
• Not all traits are adaptive
• Some give no effect
• Structures that currently function in organisms
  may be subject to genetic or historical
  constraints
• Subject to time and place

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Genetic and Historical Constraints

• Genetic outcomes are limited to alleles in the
  population
• Can be constrained by lack of genetic variation
• Can only evolve from preexisting traits
• Mutations of existing traits

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