Descent With Modification

1
Descent With Modification

• Chapter 22

2
On the Origin of Species (1859)

• 2 major points made by Darwin
• many modern species are descendents of ancestral
  species
• the mechanism for this evolutionary process is
  natural selection
• a population can change over generations if
  individuals with certain heritable traits leave
  more offspring than others
• result evolutionary adaptation
• the accumulation of inherited characteristics
  that enhance an organisms ability to survive
  reproduce in a specific environment

3
Evolution

• change over time in the genetic composition of a
  population

4
Challenges to Traditional Beliefs

• paleontology (study of fossils)
• the deeper (older) the strata, the more
  dissimilar the fossils are from current life
• geology (study of the Earth)
• gradualism the idea that profound change can
  take place through the cumulative effect of slow
  but continuous processes

5
Lamarck (1744-1829)

• applied the concept of gradualism to
  biological evolution
• proposed 2 mechanisms for evolution (1809)
• use disuse parts of the body that are used
  become larger and stronger whereas parts that are
  not used deteriorate
• inheritance of acquired characteristics
  modifications acquired during ones lifetime can
  be passed to offspring

6
Darwin (1809-1882)

• descent with modification
• organisms are related through descent from a
  common ancestor
• as descendents of the ancestral organism
  encountered new habitats, they accumulated
  adaptations that fit them to specific ways of
  life
• natural selection
• differential success in reproduction among
  individuals that vary in their heritable traits

7
Natural Selection

• OBSERVATION 1 populations would grow
  exponentially if all individuals reproduced
  successfully
• OBSERVATION 2 but populations tend to stay
  stable in size
• OBSERVATION 3 resources are limited
• INFERENCE 1 production of more individuals
  than the environment can support leads to a
  struggle for existence within a population so
  only a fraction of individuals survive each
  generation

• OBSERVATION 4 variation exists among members
  of a population
• OBSERVATION 5 much of this variation is
  heritable
• INFERENCE 2 individuals with inherited traits
  that increase their chances of survival are more
  likely to reproduce and leave more offspring
• INFERENCE 3 the unequal ability of individuals
  to survive reproduce will lead to a gradual
  change in the population, with favorable
  characteristics accumulating over generations

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Examples of Natural Selection

• important conclusions
• natural selection selects for adaptations already
  in existence (it doesnt create new ones)
• natural selection depends on time place (what
  is advantageous in one situation may be useless
  or harmful in another)

• guppies
• Reznick Endler
• size at sexual maturity changes depending on
  which predator is present
• HIV
• of drug-resistant HIV viruses increases
  rapidly (in just a few weeks) in HIV patients

9
Darwins theory helps explain

• homology similarities between related species
  that are inherited from a common ancestor
• homologous structures (similar structures with
  different functions)
• anatomical similarities in vertebrate embryos
• vestigial organs (remnants of structures that
  served important functions in an organisms
  ancestors)
• genetic code
• genes proteins
• biogeography geographic distribution of species
• closely related species tend to be found in the
  same geographic region
• (ex) most island species are similar to species
  on the nearest mainland rather than to species on
  distant islands with similar environments
• fossil record
• succession of life forms is consistent with
  descent with modification

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Homologous Structures
11
Comparative Embryology
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Vestigial Structures
13
Molecular Similarities