Chapter 21 The Mechanics of Evolution Biology 101 Tri-County
Technical College Pendleton, SC Slide 2 Defining Darwin Two main
points from Origin of the Species define Darwin Species are NOT
immutable, but change (adapt) over TIME Agent that produces the
change is natural selection Important to note that the environment
selects Darwin never used term survival of the fittest nor did he
write that man descended from the apes (monkeys) Slide 3 Importance
of Variation Offspring tend to resemble parents but are NOT
identical to one another or parents Slight variations among
individuals affect chance of survival and reproductive success
Darwin called this differential reproductive success of individuals
natural selection Tied to survival and selection for mating **Key
remains that the environment does the selecting for the favored
traits Slide 4 Population Evolution **Population is group of
organisms coexisting at same time and same place and capable of
interbreeding with one another and producing fertile offspring
Biological evolution is change over time in genetic composition of
a population Agents of evolution act on the phenotype Genotype is
not observable (well, in a sense) Slide 5 Population, cont.
Heritable trait is characteristic that is at least partly
influenced by organisms genes **Populations evolve when individuals
with different genotypes survive and reproduce at different rates
Best gene for best trait ever and sterile or impotent = zilch
Single individual has only some of the alleles found in population
to which it belongs Slide 6 Population, cont. **Sum of all the
alleles found in population constitutes its gene pool Gene pool
contains the variation producing different phenotypes on which
agents of natural selection act Reproductive contribution of
genotype or phenotype to subsequent generations relative to
contribution of other genotypes or phenotypes in same population is
called fitness Slide 7 Bless her heartwherever she is Relative is
critical concept Absolute number of offspring by individual DOES
NOT influence allele frequency in gene pool Changes in absolute
numbers of offspring responsible for > or < in size of
population Relative success among genotypes within population is
what leads to changes in allele frequency (evolution) Slide 8 So
whats the bottom line? Discussion on evolution centers on survival
and reproductive success These rates determine how many genes
different individuals contribute to subsequent generations FITNESS
of genotype determined by average rates of survival and
reproduction of individuals with that genotype According to Darwin,
ye ole professor is NOT very fit Slide 9 Have to be Hardy to endure
Hardy-Weinberg theorem states the allele frequencies in population
remain the same from generation to generation unless some agent
acts to change them Assumes certain conditions are met Five
essential assumptions required for Hardy-Weinberg 1. Mating is
random Slide 10 Hardy, cont. 2. Population size is very large 3.
There is NO migration between populations 4. Mutation must be
ignored 5. Natural selection does NOT affect the alleles under
consideration **These stringent conditions rarely exist in natureso
when we see population evolvingwe can look to see which
assumption(s) are not being met Slide 11 Show me the math!! p 2 +
2pq + q 2 where p is dominate allele and q is recessive allele
Overheads and chalk talk Lab topic but wanted to introduce the
concept Slide 12 Allele Frequency Visual Slide 13 Hardy-Weinberg
Visual Slide 14 Consequences of the Theorem Explains why dominant
alleles do NOT replace recessive alleles in population If
conditions met, frequencies of alleles at locus remain constant
from generation to generation Patterns of deviations from
equilibrium tells which assumptions are being violated and allows
identification of evolutionary change Slide 15 Usefulness of Model
**Most important message is allele frequencies remain the same from
generation to generation unless some agent acts to change them
Shows what distribution of genotypes to expect for population in
genetic equilibrium at any value of p and q If change in allele
frequencies are occurring, evolution is occurring and can be
studies and documented Slide 16 Microevolution Evolutionary agents
are forces that change allele and genotype frequency in population
Will cause deviations from Hardy-Weinberg equilibrium Such changes
in gene pool constitute small scale evolutionary changes, they are
referred to as microevolution If allele and genotype frequencies
are changing, there will be WINNERS and there will be LOSERS Slide
17 Applying the Model Few populations are completely isolated from
other populations of same species Usually some migration between
populations takes place Gene flow occurs when migrating individuals
breed in their new location Immigrants add new alleles to gene pool
of population or may change frequencies of alleles already present
if they come from population with different allele frequencies
Slide 18 Applying, cont. Genetic Drift defines chance events that
alter allele frequencies Time for an Estesismyeehaw!!! Has greater
effect on small populations If only few individuals contribute
genes to next generation, alleles they carry NOT likely to be in
same proportion as alleles in gene pool Slide 19 More applying
Mutation (germ line) is origin of variation Appear to be random
with respect to adaptive needs of organism Most mutations are
HARMFUL or neutral This condition never strictly met, but rate of
mutation usually so small it can be ignored Nonrandom mating
describes situations where individuals with certain alleles mate
more often with individuals of either same or different genotypes
THAN would be expected on random basis Slide 20 Enough applying
already Natural selection is strong contributor to microevolution
Some individuals live longer and are selected for much reproductive
success causing allele frequencies to change Natural selection can
produce different results Preserve; Change (1 direction); Change (2
directions) Slide 21 Natural Selection Visual Slide 22 Founding
fathers or mothers, whatever!! Many factors/circumstances can
affect microevolution Founder effect occurs when few pioneering
individuals colonize new region and resulting population will NOT
have all the alleles found among members of source population
Resulting pattern of genetic variation is equivalent that in large
population reduced by bottleneck Remember Mutiny on the Bounty?
Slide 23 Bottleneck Visual Slide 24 Directions, what are
directions? Most traits influenced by alleles at more than one
locus Size of organism likely controlled by many different loci
Polygenic If many loci influence size and there is no selection,
distribution of sizes in population should approximate bell-shaped
curve Slide 25 Directions, cont. Stabilizing selection occurs when
those closer to average (mean) contribute majority of offspring to
population Directional selection occurs when individuals at ONE
extreme contribute more offspring to next generation If occurs over
many generations, evolutionary trend within population results
Slide 26 Directions, cont. Diversifying (disruptive) selection
simultaneously favors individuals at BOTH extremes of the
distribution When disruptive selection operates, individuals at the
extremes contribute more offspring than those in the center (mean),
producing two peaks in distribution of a trait Slide 27 Directions
Visual
