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(ch 22)1A1
B. Natural Selection and Adaptation
1- All species produce more offspring than can possibly
survive
- Natural Selection can only amplify or diminish heritable
variations.
- If the environment changes, what is favorable also
changes.
2- Resources are limited, so organisms must compete
3- There is variation within a population (Darwin did not
know the source.)
4- Some variations make individuals more successful in the
environment
5- The individuals w/ the best variations will survive and
reproduce (Natural Selection)
ch23
-gene pool: total aggregate of genes in a population at any one
time. Consists of all the alleles at all gene loci in all individuals
of a population.
-An allele is said to fixed if all members are homozygous
for that allele
-Hardy-Weinberg equilibrium
If the gene frequencies are not altered over succeeding
generations then the population is said to be in Hardy-Weinburg
equilibrium.
-Hardy-Weinburg equation
p2 + 2pq + q2 = 1
frequency of
AA
Aa
aa
p + q = 1
- At genetic equilibrium there is no evolution (Barron’s p187)
1. No natural selection
2. No mutations
3. No gene flow
4. No genetic drift
5. Random mating
Problems:
1. If 9% of the population has blue eyes, what percent of the population is hybrid for brown
eyes? Homozygous for brown eyes?
2. Determine the percent of the population that is homozygous dominant if the percent of the
population that is homozygous recessive is 16%.
3. Determine the percent of the population that is hybrid if the allelic frequency of the
recessive trait is 0.5.
Genetic Drift ch23 1A3
Genetic Drift: a random increase or decrease of alleles. When
populations are small the effect of genetic drift can be very
strong and dramatically influence evolution.
a. Founder effect: occurs when allele frequencies in a group of
migrating individuals are, by chance, not the same as that of
their population of origin.
ex: Amish - polydactylism Polydactyly --
extra fingers or sometimes toes -- is one symptom of Ellis-van Creveld
syndrome. The syndrome is commonly found among the Old Order Amish
of Pennsylvania, a population that experiences the "founder effect."
(http://www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html)
b. Bottle neck: Occurs when the population undergoes a
dramatic decrease in size (natural disaster, predation, disease)
Evidence of Evolution 1A4
1. Biogeography (p448) : geographical distribution of species
-Islands have many native species which are closely related to
species on nearest mainland or neighboring island.
2. Fossil Record: is incomplete
Shows chronological appearance of vertebrates beginning with
fish, amphibians, reptiles, then birds and mammals.
3. Comparative Anatomy: Anatomical similarities among species
are a reflections of their common descent.
a. homologous structures: structures that are similar
because of common ancestry
ex) mammalian forelimbs: bat wing, cat leg, human arm,
whale flipper
- Vestigial organs: structures of marginal or no use
to an organism
- had important functions in ancestral forms
ex) appendix in humans, pointed canine teeth, tail bone
- remnants of leg bones in snakes
- eyes in bats and moles
4. Comparative embryology : Closely related organisms go
through similar stages in their embryonic development.
All vertebrates embryos go through a stage in which they
possess gill slits on the sides of their throats.
5. Molecular biology: An organism's hereditary background is
reflected in its genes and their protein products.
The more closely related two species are, the higher the
percentage of common DNA.
- Closely related species also have proteins of similar amino acid
sequence.
-best evidence
-common genetic code
ch 24 p474 (EK 1C2)
-Maintaining Reproductive Isolation: If species are not
physically separated by a geographic barrier, various
mechanisms exist to maintain reproductive isolation and prevent
gene flow.
A. Pre-zygotic Isolating Mechanisms: prevent fertilization
1) (Ecological) Habitat isolation: occurs when species
don't encounter one another.
2) Temporal isolation: occurs when species mate or
flower during different seasons or different times of day
3) Behavior isolation: occurs when a species doesn't recognize
another species as a mating partner because it doesn't perform
the correct courting rituals, release proper chemicals, etc.
4) Mechanical isolation: occurs when male and female
reproductive structures are incompatible or flower structures
select for different pollinators.
5) Gametic isolation: occurs when male gametes don't
survive in the environment of the female gamete or when
female gametes don't recognize male gametes.
B. Post-zygotic isolating mechanisms: Consists of mechanisms
that prevent the formation of fertile progeny.
A zebroid (also zedonk, zorse, zebra mule, zonkey, and zebrule) is
the offspring of any cross between a zebra and any other equine:
essentially, a zebra hybrid
1) Hybrid inviability: occurs when the zygote fails to
develop properly and aborts or dies before reaching
reproductive maturity.
2) Hybrid sterility: occurs when hybrids become functional
adults but are reproductively sterile
ex) mule
3) Hybrid breakdown: occurs when hybrids produce offspring
that have reduced viability or fertility.
Polyploidy: possession of more than the normal 2 sets of
chromosomes. Often occurs in plants. A tetraploid zygote can
be established when a diploid sperm fertilizes a diploid egg.
Since meiosis will continue to produce diploid gametes
reproductive isolation and speciation occurs in a single
generation.