Biology 1406 - Notes for exam 5 - Population genetics Ch 13, 14, 15
Species - group of individuals that are capable of interbreeding and producing fertile
offspring; genetically similar
13.7, 14.2
Population - group of individuals of a species that live in the same place at the same
time; “local species” ; high probability of actually interbreeding
Gene pool - “genotype of population” - all of the alleles of all of the genes in the
population
Population gene pool can change over time - this is called organic evolution
(microevolution)
Change in gene pool measured as change in allele frequencies
13.9
Allele frequency is proportion of chromosomes with each allele of a gene Fig 13.9b
Ex.
Aa
AA
Aa
aa
aa
5 Diploid individuals, 10 “a” genes in population
A = 4/10 so allele frequency = .40
a = 6/10 so allele frequency = .60
Since the proportion of sex cells produced is the same as the allele frequency, we can
predict the next generation
Fig. 13.9C
This gives reference point to measure change
Hardy - Weinberg Equilibrium:
13.9
- sexual reproduction (meiosis and fertilization) will not change allele frequencies
- a population that is in equilibrium (allele frequencies stable) must have five
conditions:
- the population must be large
- the population must be isolated, no migration into or out of population
- no mutations occur to change allele frequency
- mating is random
- all individuals reproduce equally
Any of these conditions that are not true can cause evolution (change in allele frequency)
Mutations :
13.8
- produce new alleles, genes, chromosomes
- only inheritable mutations (in gametes) important
- important as ultimate source of variation
- too rare ( 1 in 100,000 genes / generation) to change allele frequency within
short time
Population not isolated:
13.11
- migration into or out of population, called “gene flow”
- gene flow between populations can shift allele frequencies; increases genetic
variation because different populations may have different allele frequencies
- gene flow between populations maintains genetic similarity of populations
- isolation (no gene flow) important in speciation
Population small :
13.11
- large population size reduces the effects of chance, small populations effected more by
chance events
- “genetic drift” is random change in allele frequencies due to random events (matings,
deaths, etc)
- allele frequencies can change erratically to higher or lower frequency by chance
- alleles can be eliminated (0%) or “fixed” (100%)
- tends to reduce genetic variation within population
Ex. a population with random deaths ----and matings <---->
AA1
AA
AA
Aa
AA
AA
aa
AA
Aa
Aa
AA
Aa
aa
First generation
allele frequency A = 11/16
A1 = 1/16
a = 4/16
Aa
Aa
Next generation
A = 8/16
A1 = 0
a = 8/16
aa
Several conditions can cause very small population size:
Founder effect - a few individuals form a new population (colonizers or introduced)
AA
AA
aa
Aa
AA
Aa aa
population
AA2
-------------------------------->
AA2
new population
Bottleneck effect - a few individuals survive a catastrophe (severe unfavorable condition)
- surviving individuals form population
Genetic drift, founder effect and bottleneck effect can seriously reduce genetic variation
in population and threaten long term survival - can not adapt
- endangered species, zoo populations
Mating is not random: “sexual selection”
13.14
- individuals (usually females) select mates not by chance
- favors phenotypes that mates choose
Ex. large showy tails in peacocks
- leads to species specific traits, sexual dimorphism, reduces genetic variation
Individuals do not reproduce equally: “natural selection”
13.12-13.17
- proposed by Charles Darwin as major cause of adaptation and change in living
organisms - “descent with modification”
13. 2
- one allele gives individual a reproductive advantage over other individuals these leave more offspring, produces adaptation; “relative fitness”
- “survival of the fittest” - what does “fittest” mean?
- alleles that allow the greatest reproductive success increase in frequency, other
alleles decrease
How much “better” or more “fit” does an allele have to be to be selected ?
Ex. with a selection coefficient of .01 (individuals with mutant allele leave 101 offspring
: individuals with normal allele leave 100 offspring)
mutant allele will be fixed in population within about 2,000 generations
What determines if an allele is better than another allele?
How do these alleles become more common in population?
Natural selection is the outcome of set of conditions:
- individual phenotypes are determined by genetic material and are heritable
(13.2)
- variation exist in genetic material among individuals (different phenotypes)
- some variations give reproductive advantage (includes surviving, gaining
energy, etc.)
- many more offspring produced than can survive
Natural selection is powerful shaping force in living organisms:
- explains more of observed details in biology than any other idea - unifying
concept
- acts on phenotypes (individuals) but selects alleles
Does this affect recessive alleles and neutral variation?
13.16
- editing process, not creating process(structures are modified, not created) 13.2
Natural selection leads to accumulation of alleles that allow individuals to survive and
reproduce the best in their particular environment - adaptive evolution
Natural selection does not
13.17
- lead to perfection, only the best of available variation in current environment;
often compromise among competing variables
- plan ahead; selected in current environment (recessive alleles and neutral
variation not selected against)
- necessarily benefit the species; works on individual reproductive success,
species may indirectly benefit
- not goal directed, much variation in evolutionary trends
15.13
- limited by historical constraints:
- can only act on existing variation due to random mutations;
- chance events can drastically alter genetic structure of life
Natural selection can alter variation in population in three ways
13.13
- stabilizing selection favors intermediate phenotypes; typical in stable
environment
- directional selection favors one phenotype extreme over others; typical in
changing environment ; “tracking the environment”
- disruptive selection favors phenotypes at extremes of variation; “balanced
polymorphism”
13.8
Five things can change allele frequency in gene pool and cause evolution:
mutations
gene flow
genetic drift
increase genetic variation
“ “
no consistent result
decrease genetic variation
“ “
sexual selection
natural selection
random chance,
“
species specific traits
“
adaptation
Can natural selection produce complex organs?
- random mutations produce variation = chance
-natural selection accumulates best variations through better reproductive success
in current environment = not chance
- small variations accumulate, build on earlier structure
ex. the eye
15.12
Speciation - the origin of species
ch 14
- species is a reproductive isolated group
14.1-14.2
- usually results from geographic isolation; “allopatric speciation”
14.4
- no gene flow between populations
- mutations and natural selection make populations different
- mutations produce reproductive barriers that prevent reproduction
between groups
How many mutations are necessary to produce a new species?
- homeotic “control” genes very important, especially during embryonic
development
15.11
- natural selection (and speciation by natural selection in different populations)
modify existing structures
- genetic variations allow different reproductive success
What is exaptation?
15.12
Reproductive barriers prevent interbreeding
Fig 14.3 A
prezygotic barriers (before fertilization):
- habitat isolation - live or breed in different places
- temporal isolation - mating at different times
- behavioral isolation - courtship rituals, odors, colors in sexual recognition and
attraction
- mechanical isolation - reproductive structures are not compatible
- gametic isolation - sex cells do not survive or can not unite
postzygotic barriers (after fertilization):
- hybrid inviability - zygote does not develop properly
- hybrid sterility - hybrids can not produce functional sex cells
Speciation can occur without geographical isolation “sympatric speciation” 14.6
- increased number of chromosome due to errors in meiosis - “polyploidy”
4N, 6N, 8N
- most common in plants which self fertilize
14.7
- new species produced in one generation
Speciation is process of isolation and genetic change - if populations come back together
before reproductive barriers completely develop, are they species?
Ex. Ensatina salamanders, deer mouse
14.10
Adaptation and speciation (genetic change) tract the environment
14.11
- in rapidly changing environment change is rapid
- punctuated equilibrium is rapid change (500,000 to 1,000,000 years)
followed by long period with little change
- in stable environment change is slow, stabilizing selection leads to slow change
over long periods of time
- most rapid change usually occurs in small isolated populations; smaller gene
pool, more easily isolated, more environmental fluctuations
New environments lead to rapid speciation - “adaptive radiation”
- new islands formed
- reduced competition after mass extinction
Evidence of change : fossil record
13.4-13.6
biogeography
comparative anatomy and embryology
evolutionary development
15.11
molecular biology (changes in DNA and proteins)
14.8
History of the study of evolutionary change:
Carolus Linnaeus - 1760’s, introduced method of classifying living organisms
- assumed fixed and unchanging species
biologists of late 1700’s - concept of changing species
Jean Baptiste Lamarck - 1800, first scientific hypothesis of evolution
- based on inheritance of acquired characteristics
Charles Darwin and Alfred Wallace - 1850’s, natural selection as most important force
causing genetic change - “modification through decent”
geneticists of late 1800’s and early 1900’s - genetics explain how traits are inherited
Godfrey Hardy and Wilhelm Weinberg and other population geneticists of early 1900’s
- mechanisms of change within populations
modern synthesis – 1940’s – combined ideas of modification through descent, genetics,
population studies, paleontology (fossils) into modern view of organic evolution
molecular biologists - 1950’s to present – compared genetic material (DNA) and proteins
to show how species are related
embryologists of 1800’s to present – described how embryos develop from egg
evolutionary development – 1990’s to present – how genes produce body design, in
individuals and in the evolution of animals
Ability of life to change and role of natural selection (adaptation) is the central unifying
concept in biology