Intro: If
you are particularly concerned with or interested in evolution, there is
an informative packet entitled "The Evolution Packet" which is
available in the HBLL Reserve Library.
- Charles Darwin (1809 - 1882): Evolution
by natural selection
- Evolution (L. "to unfold"):
Change in allele frequencies in a population through time
...through successive generations.
More than 95% of all species that have lived on earth have gone extinct.
Allele frequencies: how common or uncommon an allele is among a species
(example: how common is the allele A? allele B?). Change in allele frequency
is brought about by natural selection, which we will be discussing shortly.
- A historical glimpse
- Creationism
We can't test the scriptural account of creation
scientifically.
- Scientific creationism
This term is a misnomer because 'scientific' implies
that creationism can be tested scientifically.
- Lamarck (1774-1829):
Inheritance of characteristics based on "need"  18.1
Two Theories
Lamarch just preceeded Darwin. He proposed that
organisms change because they acquire characteristics that they must use
to survive. (Example: the giraffe stretching its neck to reach higher leaves
causes the neck to lengthen.) This theory doesn't cut it.
- Natural selection (Charles Darwin/
Alfred Wallace)
As a youngster, Darwin was a naturalist by hobby.
His father was a physician, and Darwin eventually went to school to become
a doctor. He didn't take to that, so he went elsewhere and studied to become
clergy. His travels (1840's) and studies led him to develop his groundbreaking
ideas, which he presented to the Linnaen society. A man named Alfred Wallace
had done similar work, but conceded to Darwin's previous announcement.
The Origin of Species was published in 1859. Darwin died in 1882.
- The driving force behind evolution
18.2
Sieve Analogy
Mutations: new and unique portions of DNA. (More often than not, mutations
are not beneficial.) Natural selective agents: bioitic and abiotic forces
that select against certain alleles. (Examples: predation is a biotic selective
force: slow hares are easier to catch, and faster bobcats are better at
catching hares, so the faster hares and bobcats are more likely to survive
to pass on their alleles.) An adaptation is any trait or characteristic
in an individual that allows the organism to survive and reproduce.
Abiotic selective forces: fire (Yellowstone park example: fire causes cones
of lodgepole pines to open. Without fire, the Yellowstone ecosystem would
not be able to maintain itself. Temperature, water supply, etc. are other
examples of abiotic selective forces. Allele frequencies are changes by
these forces.
- Basic premises of Darwin's evolution
by natural selection:
Darwin wondered all the variation he beheld in nature.
Darwin's conclusions are based on the following ideas:
- Many more offspring are produced
than survive. Thus, natural populations remain fairly constant in numbers
Individuals that are better adapted to survive under
the prevailing conditions are more likely to survive and reproduce (pass
on their alleles).
- Much heritable genetic variation
exists among individuals in natural populations
This is clearly observable.
- Some genetic variants (individuals)
within a population are better adapted than others
Also a reasonable premise.
- Better adapted individuals leave
more offspring (DIFFERENTIAL REPRODUCTION)
Better adapted individuals are more likely to survive
and, therefore, reproduce, passing on their adaptations.
- Offspring inherit parental characteristics
Genetics.
- Natural selection defined: differential
reproduction among adapted vs. less-well adapted individuals of a population.
1 - Natural selection operates on phenotypes, not
genotypes. 2 - Natural selection occurs at the population level, not the
individual level; (in other words, allel frequencies change at the population
level, not the individual level). 3 - Natural selection does not change
the DNA; (it doesnt' CAUSE mutations in the DNA, but selects for or against
them once they have occurred.)
- Types of selection
18.3
Modes of Natural Selection
Allele frequencies can be changed in these ways:
- Stabilizing selection
Abiotic and/or biotic selective forces select against
both extremes, thus favoring the alleles of the population average. (Example:
human birthweight: extremely light and extremely heavy babies have a higher
mortality rate.)
- Directional selection
Selective forces favor one extreme over the other.
(Example: Peppered moth: On clean, white trees, white peppered moths were
more difficult for predators to spot, so were more numerous than their
dark counterparts. With the advent of the industrial pollution, soot would
darken trees, making the dar moths more difficult to spot. So there was
a shift in the frequency of the light and dark alleles in the population
of peppered moths, in the direction of the darker phenotype.)
(Another example: when we spray insects, and some are immune, those will
survive to reproduce. Eventually the spray will be useless because the
allele frequency in the insect population has shifted in the direction
of immunity. The same thing occurs with our overuse of antibiotics: survivors
reproduce and eventually, will all be immune to the antibioitics.)
- Diversifying selection
Selective forces favor a variety of phenotypes and,
thus become more diverse phenotypically. (Example: human blood types: it
appears that some types are better adapted to deal with a variety of diseases
(selective forces).
Another example: the swallowtail butterfly: all the males are yellow with
black (stabilizing selection), but their are three successful varieties
of females, each distinct in color from the others. Each of the three successful
varieties of female swallowtails is due to the mimicking of the appearance
of another species of butterfly, which birds don't like to eat. Those varieties
'in between' these three were selected against (eaten) because they were
unable to fool the predators into thinking they would taste nasty and make
them sick. Note: some varieties (in this case, 'color morphs' of the swallowtail
butterfly) may be more/less successful than others, success being measured
by the surviving population size.)
- Summary: Evolutionary significance
of genetic variation
18.4
Selective Forces
- Genetic variation: the raw material
from which natural selection shapes biological survival
- Sources of variation:
- Point mutations
(discussed in a previous lecture.)
- Chromosomal mutations
(discussed in a previous lecture.)
- Sexual reproduction (genetic recombination)
Also review: Sexual reproduction increases variation
(crossing-over, independent assortment) which will interact again with
selective forces.
- Polyploidy
Common in plants. Polyploidy is defined as having
more than two sets of chromosomes (there number of sets can vary). About
60% of plants in northern areas demonstrate polypoidy--due to climate factors:
short growing season.
- Genetic drift
Random changes in allele frequencies. Occurs within
smaller, more isolated populations. Example: If a small group of people
(or any organism) settle on an island, or in any reproductively isolated
area, their allele frequencies will be become more prevalent within the
future population, though they may not have been as prevalent within the
greater population from which they 'drifted'.
- Evolutionary significance of genetic
variation: Permits adaptation to a changing environment
As the environment changes, selective forces change,
which change the frequency of alleles in populations. So species adapt
to their changing environment.
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