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EVOLUTION
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The Unifying Concept
• “Nothing in biology makes sense, except in the light of evolution.”
• Theodosius Dobzhansky
• Evolution: The descent of modern organisms with modification from
pre-existing life-forms; strictly speaking, any change in the
proportions of different genotypes in a population from one
generation to the next
All Life Shares a Common Ancestry
Are the these organisms...
…related?
EXAMPLE
• Some distant ancestor of today’s elephants is also the ancestor of the
extinct mammoths
• If we follow this line back far enough, it links up with other ancestors
of other organisms which ultimately links up with the HUGE tree of
life, linking ALL organisms
Darwin’s Conclusion 1
• There is competition for survival and reproduction. In each
generation, many individuals must die young, fail to reproduce,
produce few offspring, or produce less fit offspring that fail to survive
and reproduce in their turn
Conclusion 2
• The most well-adapted individuals in any generation tend to be the
ones that leave the most offspring
Conclusion 3
• Over many generations, differential or unequal, reproduction among
individuals with different genetic makeup changes the overall genetic
composition of the population
How do we know evolution has occurred?
• What do you think?
• Fossil record, comparative anatomy, embryology, biochemistry, and
genetics
The Fossil Record
• The fossil record shows a slow change over time
• We see a progressive series of fossils leading from an ancient,
primitive form, through intermediate stages culminating in the
modern form
The best example we have
• The fossil record of the horse is the best example of this
• That of elephants, giraffes and mollusks are good as well
• They all suggest that species evolved from and replaced previous
species
Comparative anatomy
• Appearance has long been used as an indicator of the relatedness of
organisms
• The elephant and the mammoth clearly have similar anatomies and
share a common ancestor
Examples of homologous structures
Figure 3.1.1. Comparison of the forelimbs of various relatives of modern birds. Forelimbs of (A) Ornitholestes, a theropod dinosaur,
(B) Archaeopteryx, (C) Sinornis, an archaic bird from the lower Cretaceous,
and (D) the wing of a modern chicken (modified from Carroll 1988, p. 340;
Vestigial structures
• These are structures that serve NO apparent purpose
• examples include: molar teeth in vampire bats, pelvic bones in whales
and snakes, the appendix, wings on ants
Embryology
• It was observed, in the 1800’s, that vertebrate embryos look quite
similar to each other in early development
• All vertebrates have gill slits at some point in their development. Only
fish retain them in adulthood
• The plausible explanation is that early forms had these traits and
passed the genes on but in some forms the genes get masked or
turned off during development
Examples
• ALL use DNA as the carrier of genetic information
• ALL use RNA, ribosomes and approximately the same genetic code to
translate genetic information into proteins
• ALL use the same 20 amino acids to build proteins
• ALL use ATP as the intracellular energy carrier
Some evidence that populations evolve by
natural selection
• Artificial Selection: The breeding of domestic plant and animal species
to produce specific, desirable features
• The reasoning is this, if humans can produce such a wide variety of,
say, dog breeds in a few hundred or thousand years it stands to
reason that nature could produce a wide variety of organisms in
several billion years
•