Download Evolution Teacher notes 2012

Evolution
A. Definition – theory that a population of organisms changes as the generations pass
- descent from ancestral forms with modification
B. Historical Background
1. Jean Lamarck
a. use and disuse
b. inheritance of acquired traits
2. Charles Darwin
a. 1830’s voyage – 5 yrs. trip
(1) Galapagos Islands
(2) evidence for theory
b. 1859 – The Origin of Species
c. main points ( 3 observable facts and 2 deductions )
(1) fact- reproduction occurs in a geometric manner (overpopulate)
(2) fact- capacity never realized
(3) deduction- competition, struggle for existence
(4) fact- all species exhibit variations
(5) deduction- variations which give survivable ability are passed on to
offspring, natural selection
d. what Darwin didn’t know–
(1) what caused variations
(2) how variations are passed on to offspring
e. synthesis of Darwin’s original work and modern genetic theory – Neodarwinism
C. Genetic Composition of Populations
1. individuals don’t evolve – populations/species do
2. population – group of individuals belonging to same biological species that live in the
same area and interbreed, producing fertile offspring
3. gene pool – sum of all alleles in a population
4. allele frequency – relative occurrence of an allele in a population
(a) expressed numerically
(b) example: 8 out of every 100 African Americans are sickle cell carriers for (SA)
one out of every 500 is SS
1/500 = SS
459/500 = AA
- 40 out of 500 – SA
40/500 = SA
- of the 500 people: 1000 alleles; 42/1000 S, 958/1000 A
4.2% or .042 allele frequency for S
95.8 % or .958 allele frequency for A
(c) overall differences are very small in a population
- if all people became extinct but one area of Africa, 85% of all human variability
still present in reconstituted species
D. Factors That Cause Gene Frequencies to Change
1. evolution: changing of allele or gene frequency over time
2. 1908 Hardy, British mathematician and Weinberg, German biologist – discovered
under ideal conditions allele frequencies remain constant from generation to generation
- Hardy-Weinberg Law
- populations not changing (retain allele frequency) are in genetic equilibrium
3. ideal conditions (5)
a. absence of mutation
b. individuals cannot migrate into or out of population
c. population must be large, random changes don’t affect it
d. all equal chance of survival, equal viability
e. mating at random, no preference
4. factors which disrupt genetic equilibrium
a. mutation
b. gene flow- addition or removal of alleles, migration
c. genetic drift- changes that occur solely by chance, small populations
d. natural selection- some phenotypes better suited to survive
e. nonrandom mating- phenotypes that make them more likely to be selected as
mates
5. Hardy-Weinberg Law or Principle
a. express as expanded binomial
b. allele frequencies add up to one
p = frequency of dominant allele
p+q=1
q = frequency of recessive allele
(p + q)2 = 12
p2 + 2 pq + q2 = 1
c. if % of homozygous recessive is known other % distribution can be determined.
6. mutations - source of new alleles; may be silent with no observable phenotypic effect,
or result in new phenotype
a. must be in gametes to be passed on
b. some beneficial, detrimental, “neutral”
c. all humans carry 7 to 8 lethal recessive genes
d. environment determines what’s beneficial etc., change in environment important
e. example- insect immunity to DDT, probably “neutral” & became beneficial when
DDT spray discovered
7. gene flow – transfer of alleles through interbreeding
a. immigrants in, might add new alleles
b. emigrants out might reduce frequencies
c. resistant individuals spread resistance – conferring alleles into new area
d. 70% of alleles for cystic fibrosis traced to single ancestor
8. genetic drift
a. chance change in frequency in small populations
b. bottlenecks- large population reduced to smaller # (natural catastrophes, etc.)
c. founder effect- generate populations whose allelic frequencies may be different
from those of the original population from which the founders arose
(1) colonists on Tristan da Cunha & blindness from retinitis pigmentosa
(2) Amish community- inbreeding, highest % of dwarfism & polydactylism
(from emigrant who had it)
9. natural selection- driving force behind adaptation
a. of all forces that influence evolution, only natural selection generates populations
whose members are better adapted to their environment
o driving mechanism of evolution
o fitness, the number of surviving offspring left to produce the next generation,
is a measure of evolutionary success
b. example - change in coloration of peppered moths (Biston betularia)
-environment selects which variants will survive to reproduce
c. example- heterozygotes for sickle cell anemia
(1) frequency in Africa (malaria area) much higher for hetero up to . 12 from
.042
(2) heterozygotes have selection preference, partial resistance to malaria
d. patterns of natural selection (3)
(1) stabilizing – extreme characteristics die or fail to reproduce
(a) results in populations with intermediate features
(b) most common in unchanging environments
(c) over time, phenotypes closer to the optimum are retained
(2) directional- phenotypes at one extreme die or fail to reproduce while those at
other extreme leave higher #
(a) frequency distribution shifts toward direction of favored
(b) occurs when change in environment
(c) examples: peppered moths and insects & DDT
(3) disruptive- extreme phenotypes become more frequent
(a) phenotypes which are intermediate die or fail to reproduce
(b) promotes dimorphism or polymorphism (two or several forms of a trait)
(c) example: African Swallowtail Butterflies
[1] females mimic appearance of foul-tasting butterflies
[2] not eaten as readily
[3] intermediate phenotypes easily eaten
[4] vary widely in appearance
10. nonrandom mating – choose mates on basis of phenotypes
a. sexual selection – natural selection for mating success
(1) antlers of deer & tail feathers in peacock (attract mates)
(2) often leads to sexual dimorphism (difference in males & females)
b. inbreeding – often in small populations
(1) increases likelihood of homozygous in recessive alleles to occur
(2) decreases variation
(3) example: cheetah population—low genetic variation due to bottleneck
11.microevolution & macroevolution
a. microevolution
(1) change in a population or individual species’ gene pool from one generation to the
next
(2) contributing factors: genetic drift, natural selection, gene flow
(3) small scale
b. macroevolution
(1) large scale
(2) evolutionary change above the species level over extended periods of geologic time
(3) appearance of new species, families or kingdoms
(4) example: appearance of feathers
E. Speciation: the origin of species
1. definition: the process to form new species
2. approx 5 million species; only 1.8 million described
--all arose from a single ancestor 3.5 billion years ago
3. biological species concept—species are groups of actually or potentially interbreeding
natural populations which are reproductively isolated from other groups. Must be able to
produce viable, fertile offspring (not necessarily true in plants—hybrids can interbreed)
4. reproductive isolation mechanisms (two types)
a. prezygotic isolating mechanisms—prevents zygote from forming
examples
(1) temporal isolation—mating or flowering occurs at different seasons/times of day
(2) habitat isolation—populations live in different habitats & don’t meet (example: 2
different garter snake species—1 lives in water, other on land)
(3) behavioral isolation—little or no sexual attraction between different species
(example: different species of fireflies sending wrong blinking signals)
(4) mechanical isolation—structural difference in genitalia or flowers prevent
copulation or pollen transfer
(5) gametic isolation—male and/or female gametes die before uniting or fail to unite
(example: fertilization doesn’t occur in sea urchins of different species due to species
specific molecules on surface of egg)
b. post zygotic isolating mechanisms—eliminates crosses as they occur
examples
(1) hybrid inviability—hybrids don’t survive example: genus Rana frogs
(2) hybrid sterility—female horse + male donkey produce sterile mule
5. paths of speciation
a. allopatric speciation—most common
(1) physical barrier, ie. mountain range, river, etc.
separates population
(2) gene flow between groups stopped
(3) examples: various species of Darwin’s finches & species of fruitflies on Hawaiian
Islands
b. parapatric speciation—in populations adjacent to one another
(1) gene pools diverge because environment varies sufficiently in the different locales
(2) example: grasses growing close to mines & exposure to toxic mine wastes
c. sympatric speciation—individuals continue to live among one another
(1) some type of biological difference has divided the members into reproductive groups
(2) best example: polyploidy in plants from accidents in cell division resulting in extra
sets of chromosomes. Can self fertilize, or breed with other tetraploids
(3) over 40% of flowering plant species are polyploids;
d. hybridization—two species form hybrids from cross-mating. How most polyploid plants
arise
(1) not common in animals
(2) more common in plants (more tolerant of polyploids); requires hybridization of 2
parent species, and cell division error in the resultant hybrid
(3) example: common wheat: diploid ancient stock had 7 prs of chromosomes (2n=14);
crossed with a wild grass with 7 prs of chromosomes (2n=14). Error in cell division
and self fertilization produced new species with 14 prs of chromosomes (2n=28).
Then, that hybrid with 14 prs of chromosomes (2n=28) hybridized with goat grass
with 7 prs of chromosomes (2n=14); leading to modern wheat which has 21 prs of
chromosomes (2n=42).
F. Patterns of Evolution
1. divergent (most common pattern)
a. two or more species evolve from a common ancestor
b. adaptive radiation—many diverse environments, new species form rapidly & diverge.
Ex. range of diversity in Australia (marsupials-kangaroos--koalas)- all arose from small
opossum like marsupial ancestor
2. convergent evolution—species with different ancestors colonize similar habitats
a. streamlined body & paddlelike front limbs
ichthyosaur, penguin, seal, dolphin
b. mollusk eye & vertebrate eye
3. parallel evolution—two species from same ancestor remain similar over periods of time
because they independently acquire the same evolutionary adaptations.
a. species adapt to similar environmental changes in similar ways
b. example: ancestral arthropod (segmented body with a pair of legs in each segment)
all three groups (crustaceans, insects & spiders) # of legs decreased & segments fused
4. coevolution—evolutionary changes in one species cause evolutionary adjustments in
another
a. flowering plants & pollinators
b. predator-prey interactions
c. parasites & their hosts
G. Extinction: The Loss of Species
1. approx 99% of all that existed are extinct
2. rapid or gradual
3. bad genes or bad luck
4. mass extinctions—multitude of species eliminated
5. humans—natural habitat destructors—have increased extinction rate from long term
average of about one species per 1,000 yrs to hundreds or perhaps thousands per one year.
a. Additional examples of how humans impact variation in other species:
- artificial selection
- loss of genetic diversity within a crop species
- overuse of antibiotics
H. Pace of Evolution
1. gradualism—steady uninterrupted process (Darwinism)
a. new species evolve gradually in small adaptive steps
b. possible problem: fossil evidence had not shown a gradual succession of forms;
intermediate forms missing. More recent finds supplying more evidence
2. punctuated equilibrium—long periods of ‘stasis’ interrupted by short rapid periods of
change
a. allopatric speciation—can be rapid
b. speciation in small populations also rapid
c. both a and b together (allopatric speciation of small populations) –spurts of speciation
can occur
d. change in environment—provides opportunity for change
I Determining Evolutionary Relationships
1. Homologous Features
a. related species share characteristics resulting from common ancestors
b. forelimbs of mammals (horse, cat, whale, bat & human) now used for different
purposes, but were present & used in common ancestor for walking.
2. Analogous features or homoplasy
a. unrelated species evolve similar features independently as adaptations to their
environments
b. convergent evolution
c. paddlelike front limbs & streamlined bodies of many aquatic animals are analogous
3. Phylogenetic trees: dynamic models that show common ancestry
J. Evidence of Evolution
1. Fossil Records: link past and present organisms
a. fossil—any trace of life from the past
b. fossil record—entire collection of remains;
paleontologists attempt to reconstruct the history, etc
c. fossil dating
(1) stratigraphy—layers top to bottom (oldest layer)
(2) radioactive isotopes (radioisotopes)
(a) half-life—time required for ½ to decay into stable form
(b) 14C –half life of 5730 years
(c) ratio of one isotope to another, ex 14C to 12C during life about the same as ratio
of 14C & 12C in atmosphere
(d) decay rate is constant, so age can be determined
(e) 40K half life of 1.3 billion years
d. example of fossil evidence—Archaeopteryx
(a) pigeon-sized
(b) both avian & reptilian features
2. Comparative Anatomy of Living Organisms
a. homology
b. vestigial structures—structures of marginal, if any, importance to an organism, but are
remnants of structures that served important functions in the organisms’ ancestors
(1) leg bones & pelvis in snakes
(2) human appendix—degenerated cecum.
3. Comparative Embryology
(1) comparisons of early stages of development
(2) example: vertebrate embryos all similar, appearance & features (notocord, paired gill
slits, dorsal hollow nerve cord)
4. Biochemistry & molecular biology
(1) DNA sequencing—shows relationships, ex. man & chimpanzees more closely
related than chimps & gorillas
(2) similar nucleotide sequences in master control genes that regulate other gene groups
during development
5. Biogeography: the geographical distribution of organisms-links past and present organisms
a. features of plants & animals in different areas that suggest common ancestry
b. plants & animals in similar environments on different continents tend to be quite
different
c. species that live closer to one another tend to be more closely related
6. Final thoughts
a. The environment is always changing
b. There is no “perfect” genome
c. A diverse gene pool is important for the long-term survival of a species
d. Human directed processes, such as genetic engineering, can also result in new genes and
combinations of alleles that confer new phenotypes
e. Populations of organisms continue to evolve. Evidence:
o Chemical resistance (mutations for resistance to antibiotics, pesticides,
herbicides, or chemotherapy drugs occur in the absence of the chemical)
o Emergent diseases
o Observed directional phenotypic change in a population (Grants’ observations of
Darwin’s finches in the Galapagos)