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CHAPTER 22-24 OUTLINE part 1
EVOLUTION by Natural Selection depends on FIVE factors:
 More offspring are produced than can survive to reproduce
 The characteristics of living things differ among individuals of same species.
 Many differences are the result of heritable genetic differences
 Some differences affect how well adapted an organism is.
 Some differences in adaptedness are reflected in the number of offspring
successfully reared.
EXCESS PROGENY: Ie. the elephant…
VARIATION:
Genetic Variation: THREE main sources:
1 crossing over in meiosis
2 sexual recombination
3 mutation
1, 2 as discussed in chapters 12, 16 do NOT result in new alleles.
Mutations, both point and exon recombination, is a potential source of
entirely new alleles.
-Phenotypic Variation: Variation is irrelevant to selection, unless it is
expressed in the phenotype. For example, a recessive allele that NEVER
occurs in the homozygous condition is totally protected from Natural
Selection.
-Variation produced by somatic mutations is not part of evolutionary change.
For example, mutations in development of nervous tissue in the embryo:
These cells do not give rise to gametes, so they cannot result in evolutionary
change.
Jean-Baptiste Lamarck:
For example the giraffe:
HOW DO NEW GENES AND ALLELES ARISE:
-small steps… both point and gene duplication and recombination are methods…
Three Major observations:
 Eucaryotic chromosome contain introns, exons, junk DNA (remember almost
90% of DNA is ‘junk’).
 The fetal immune system actively rids of redundant gene segments, meaning
the genome is “active” and not so stable. (ch.15)
(and remember transposons in ch. 10)
 Uniqueness of immune system genetic makeup: MHC proteins and T-cell
receptors (ch. 15). This suggests that the exons (from parts of antibodies) arose
from the duplication and rearrangement of some prehistoric gene sequence.
Perhaps CAMS (cell adhesion molecules)
IDEA OF DUPLICATION: of genes/exons followed by small scale
evolutionary change is more likely to lead to functional genes with new
properties than random changes alone would allow.
FOR EXAMPLE
EVIDENCE:
 Example: genes of myoglobin and hemoglobin: conformational similarities
 ANOTHER EXAMPLE: HORIZONTAL TRANSMISSION: import a gene
from another species (lysogenic and retroviruses)
EXON RECOMBINATION: Gilbert and Blake (See 17.6)
HOW DO MUTATIONS SPREAD IN POPULATIONS?
Rare mutations become common alleles in two ways
GENE POOL: sum total of all genes possessed by all the individuals in a
population.
Gene pool is characterized by the gene FREQUENCY.
For example: A and a, only two forms of gene A. If A occurs in 90%
of population and a in 10% of the population.
SO…the frequencies are .9 and .1
IF THE FREQUENCIES CHANGEOVER TIME, IT WOULD BE
EVOLUTIONARY.
THEREFORE: what produces these shifts in gene frequency?
GENETIC EQUILIBRIUM: read short section in middle of page 454. It simply
explains how these numbers remain stable…both the genotypic frequencies and
the allelic frequencies (easy math). This is in LARGE populations.
EVOLUTION CAN OCCUR ONLY WHEN SOMETHING DISTURBS
THIS GENETIC EQUILIBRIUM.
HARDY-WEINBERG LAW. (1908) Under certain conditions of stability
both phenotypic and allelic frequencies remain constant from generation to
generation in sexually reproducing populations.
OR: variability and heritability, cannot alone cause evolution.
GENETIC EQUILIBRIUM: MUST HAVE:
1.
2.
3.
4.
5.
OR: variability and heritability, the two foundations of natural selection
cannot alone cause evolution…
BUT EVOLUTION DOES OCCUR, because the above five conditions can
never be met. So genetic equilibrium does not occur…over time
By looking at these five conditions, we can isolate the cause of the evolution:
CONDITION ONE: Chance is always a factor.
GENETIC DRIFT: Change in the gene pool as a result of CHANCE.
CONDITION TWO: no mutations or mutational equilibrium
CONDITION THREE: no emigration or immigration
CONDITIONS FOUR and FIVE: Random mating and reproductive success
THEREFORE: Natural selection is always working in populations, hence,
there is always SELECTION PRESSURE acting to disrupt the H-W
equilibrium and CAUSE evolution.
Although Natural Selection is the most important factor in evolutionary change, it
is not the only factor.
Refer again to the Giraffe:
NATURAL SELECTION:
How Selection Changes Allelic Frequencies:
 We’ve seen how MUTATIONS can affect allelic frequencies…NOW, natural
selection can affect allelic frequencies…
 SUPPOSE natural selection works against the dominant phenotype…
 EXAMPLE of penicillin-
Directional Selection of Polygenic Characters:
 Most characters on which Natural Selection act, are influenced by MANY
genes, which have multiple alleles.
 The expression of these many characters is influenced by the environment
which is why characters like “height” vary so greatly.
 Ie. Plant growth can be genetically determined…but influenced by rainfall.
 Directional Selection: changing environmental conditions which cause
the population to evolve in a particular way. (ie. plants above)
How Selection Can Create Novel Phenotypes:
 In Plant example
 HOW DID IT ARISE? Read explanation on p. 462.
 This process was illustrated by the Univ. Of Illinois in corn kernels and oil
content.
Disruptive Selection:
 Two directional selections favoring the extremes of the distribution. This
“disrupts” the smooth curve of phenotypes in a population. See 17.14
Ie. beaks of birds…
Or GAMETE Size:
Stabilizing Selection:
 But most often, when there are two or more opposing directional selection
pressures, the pressures select AGAINST the two extremes. This is “stabilizing
selection”.
 For example: Winter storms
Costs and Benefits: Net Selection
 Some characteristics may benefit the organism and some may harm the
organism.
 Guppy Showiness:
 Stabilizing Selection
“Whether an allele increases or decreases in frequency is determined by whether
the sum of the various selection pressures favoring it is greater or smaller than the
sum of the selection pressures acting against it.”
Example: Sickle Cell and Malaria resistance:
The equilibrium frequency of the sickle cell allele is determined by at least
four separate selection pressures:
1.
2.
3.
4.
Balanced Polymorphism:
 Definition:
 These are distinct, and are called: Discontinuous Phenotypes, intermediates are
rare. (ie. Mendel’s pea flowers: red or white NOT pink)
 Balanced Polymorphism: when the relative frequencies of the morphs are stable
over time.
 Ie. heterozygous superiority
Playing the Odds: Frequency Dependent Selection:
Read about scale eating Cichlids
The rarer phenotype enjoys higher success in attacking prey.
Sexual Selection:
 Contests:
 Between members of the same sex, for access to the other sex
 Male specific features include: large size, weapons, defensive structures (lion’s
mane to protect v. biting).
 Choices:
Other male features exist to only attract females, or ‘selection of males’:
ADAPTATION: any genetically controlled characteristic that increases an
organism’s fitness. (fitness: individuals probable genetic contribution to
succeeding generations.)
May be structural, physiological, or behavioral.
EXAMPLE: effects of predation on male guppies
Another good example of grasses is given on p. 470.
EXAMPLES OF ADAPTATION:
For Pollination: flowering plants
More Examples:
For Defense:
 Cryptic appearance:
 Helps animal to escape predation
Example: Industrial melanism in moths
As industry increase, trees got darker with soot, selection favored the
darker moths, and increased their frequency.
 In the mid 1950’s, controlled studies were done in England with BOTH
scenarios, and predators (birds).
 Warning Coloration:
The above animals wish NOT to be seen, some animals WANT to
be seen, they have coloration in contrast to their backgrounds, so
they are clearly visible.
 Aposematic
Ie.
 Mimicry:
 This can be adaptive, the mimics may suffer little predation.
 Batesian Mimicry:
 Mullerian Mimicry
Ie.
SYMBIOTIC ADAPTATIONS: ‘living together’ See Table 17.2, p. 479
 Commensalism:
 Mutualism:
 Parasitism:
 Term SPECIALIZATION is used: NOT what we would think of it
terms of increase structure complexity…
Enslavement: