Organic Evolution &
Natural Selection
Organic Evolution
► changes
in life through time
► development
of complex life forms
► development
of a variety of life forms
Natural Selection
► mechanism
responsible for evolution
► through mutation and selection
► survival of the
 fittest (for a niche)
 luckiest
 combination
Scientific Explanations
► must
be natural
► must be supported by available data
► must be testable & falsifiable
► subject to revision or refinement or
negation
► should be predictive
► OE
and NS meet all of these requirements
Theory & Hypothesis
► Hypothesis
 a working explanation or “educated guess”
 competing hypotheses are often
investigated simultaneously
► Theory
 the best explanation supported by a
preponderance of the evidence
 the best a scientist can hope for
Terminology
► species
 reproductively isolated group of living
organisms
► population
 group of organisms consisting of all the
members of a species living in an area
Organic evolution:
observations
► sedimentary
rocks
 deposited in layers
 oldest layers are on the bottom
 layers may be correlated with layers in
other areas
► conclusions
 relative time scale
 the Earth is very old
 confirmed with absolute dating
Organic evolution: observations
► fossil
record
 oldest rocks have only simple fossils
 younger rocks have more organisms
similar to those living today (at levels
from species to kingdom)
 fossils record includes appearances and
extinctions of many species
Organic evolution:
observations
► geographic
distribution of organisms
 many organisms are similar but unique
 they are confined to specific areas (islands,
continents, water bodies)
 includes modern and fossil organisms
 distribution has changed through time
Organic evolution:
observations
► anatomy
 cell structure is similar in all living
organisms
 embryology - embryos of mammals, birds,
and reptiles are very similar
 homologus organs - similar organs,
different functions
 vestigal organs - no purpose in one,
purpose in another
Organic evolution:
observations
► genetics
 structure of DNA and RNA is the same in
all living organisms
 similarity in genetic code varies between
organisms (some organisms are more
similar than others)
 mitochondrial and plasmid DNA
►separate
from nuclear DNA
►similar to some bacteria
Organic evolution: observations
► cladistic
analyses
 statistical examination using
►anatomy
►DNA
►behavior
 indicates strength of affinity between
species
Organic evolution: observations
► combining
data
 cladistic analyses of fossils and living species
 time data (relative & absolute)
 spatial relationships
(where fossils and living populations are found)
► reveals
phylogeny
(“tree of life through time”)
Organic evolution:
conclusions
► the
characteristics of populations of living
organisms have changed through time
 life has become more complex
 life has become more diverse
 all life is related
► this
is accepted as a factual observation
Evolutionary Trends
► divergence
& speciation
► extinction
► convergence
► phyletic
gradualism
► punctuated equilibrium
Organic evolution:
Missing links!!!
► the
“link” between two fossil species OR
between a fossil species and a living species
► PRESERVATION
► many
AS A FOSSIL IS RARE!
“links” found (it’s just a matter of time and effort)
► however: each “gap” filled creates two new
“gaps”
Question
► What
is the mechanism that resulted
in the evolution of life?
Natural selection:
observations
► populations
of organisms display a
variety of characteristics
 characteristics may be useful, not useful,
or detrimental
 the variety is reflected in an organisms
genes
Natural selection:
observations
► new
genes provided by
 mutation
 duplication
 exchange
► passed
to offspring during
reproduction
Natural selection: observations
► Symbiosis
 Living organisms in close proximity, at
least one of the symbionts cannot live
without the other
► Symbiotic
relationships can result in
new characteristics
Natural selection:
observations
► artificial
selection
 domesticated plants and animals can be bred to
favor certain characteristics
 ONLY individuals with desired characteristics are
allowed to breed
 descendant populations of plants and animals are
dominated by characteristics that are desired by
breeders…
…and thus favored their survival
Natural selection:
observations
► The
natural environment acts as a
selective breeder
 contains opportunities and stresses for
living organisms
 selection is enhanced by
►stress
on an existing niche
►a new niche
Natural selection: observations
► Populations
of a species may become
isolated by geography or behavior
► Isolation
 provides new opportunities and stresses
 isolates the population from the larger
gene pool
Conclusion
The variety of conditions in the
natural environment results in
natural selection of populations
which, in turn, is responsible for
biological evolution.
Natural selection: details
► organisms
with favorable characteristics for
their niche are more likely to thrive and
reproduce while organisms with unfavorable
characteristics are less likely to thrive and
reproduce
► over time, favorable characteristics can
accumulate in a population until the
members of the population can no longer
reproduce with the population from which
they became isolated
Natural selection: details
► genetic
changes
 favorable: more likely to be passed on to
offspring
 neutral: likely to be passed on to offspring
 unfavorable: less likely to be passed on to
offspring
► natural
selection is not random
Evolutionary trends
► divergence,
speciation, & extinction
► convergence
► homology & vestigal organs
► coevolution & symbiogenesis
► phyletic gradualism & punctuated
equilibrium
Divergence & Speciation
►a
population has a gene pool
► members of the population interbreed
► the population may become isolated from
others of a species
 development of niches & resource partitioning
 migration
 development of physical barriers
► populations
may be selected
 by stress
 by opportunity
► isolation
may result in genetic divergence
& speciation
Extinction
► stress
on limiting factors destroy a
population
► “divergent” evolution into subsequent
species (pseudo-extinction)
Convergence
► Characteristics
of a habitat favor certain
styles of adaptation
 e.g. streamlined shape, tooth shape
► Natural
selection results in organisms in
similar niches having similar forms
► Similar
forms resulting from convergence
usually do not have similar underlying
structures
Homology & Vestigial Organs
►a
slightly modified organ or organ system
my provide advantage in a niche
► when the modification results in selection
generation after generation, the
modification may become enhanced
► when
compared with closely related species,
the organ or organ system will have the
same parts used for different functions
(homology) and some parts may no longer
be in use (vestigial)
Coevolution & Symbiogenisis
►
A niche may be associated with another organism
►
The characteristics that favor interdependence may
be subject to continued selection (coevolution)
►
Selection that favors an “organic” niche may result
in dependence or interdependence (symbiosis)
►
Symbiotic relationships may become permanent
due to genetic exchange and/or incorporation
(symbiogenesis)
Phylogeny
► relationships
between organisms can
be determined using
 genetics
 anatomy & physiology
 fossils
Phyletic Gradualism &
Punctuated Equilibrium
► gradualism
 slow, steady change
► equilibrium
& punctuation
 long periods with little change
 short bursts of rapid change
How did it start?
►
Chemical evolution
 needs energy, but no oxygen
 simple compounds form(CO2, NH3)
 organic compounds form (amino acids, nucleic acids)
 organic compounds assemble (microspherules, bubbles,
clay particles, ice crystals)
►
First life: simple, wall-less, fermenting bacteria
►
Where?
 deep ocean vents, tidal pools, ice sheet, clay sediment
Randomness
► mutations
are random
► evolution is not random
► natural selection is not random
► favorable
mutations survive through
reproduction
End point
► Evolution
has no end