Post on 02-Jan-2016
transcript
Gene Pool • The combine genetic information of
a particular population• Contains 2 or more Alleles for each
inheritable trait
Relative Frequency • Is the number of times an allele occurs
in a gene pool • Compared with the number of times
other alleles occur
• Example
• Relative Frequency:
• 70% Allele B
• 30% Allele b
Sources of Genetic Variation
• 2 main sources of Genetic Variations are Mutations and Genetic Shuffling that result from sexual reproduction
Mutations: any change in DNA sequence
♦ Can occur because of: ♦mistakes in replication♦ environmental chemicals
♦ May or may not affect an organism’s phenotype
Genetic Shuffling • Occurs during the formation of gametes• Chromosomes move independently during…
23 chromosomes• Can produce 8.4 million different combo’s of
genes• Crossing over can also occur during meiosis
further increasing the possible number of gene variation
Despite gene shuffling, the frequency of alleles does not
change in a population. Explain why this is true.
Similar to a deck of cards – no matter how many times you shuffle, same cards (alleles) are always there.
Gene Traits
• The number of phenotypes produced for a given trait depends on how many genes control that trait
Single Gene Trait • Controlled by a single gene (2alleles)• Examples: widow’s peak, hitchhiker’s
thumb, tongue rolling
Polygenic Trait• Controlled by 2 or more genes can
produce many genotypes and thus many phenotypes
♦Examples: height, hair color, skin color, eye color
Most human traits are polygenic.
Do the following graphs show the distribution of phenotypes for single-gene or polygenic traits? Explain.
• type: single gene
• why? Only two phenotypes possible
• Example: tongue roller or non-tongue roller
• type: polygenic
• why? Multiple (many) phenotypes possible
• Example: height range 4feet to 9 feet all
Natural Selection
• On single gene traits can lead to changes in allele frequency and thus to evolution
• Can effect the distribution of phenotypes in any of 3 ways: directional selection, stabilizing selection, or disruptive selection
Directional Selection• Evolution causes an increase in the
number of individuals with a certain trait at one end of the bell curve
♦ Individuals with highest fitness: those at one end of the curve
♦ Example: Galapagos finches – beak size
Stabilizing Selection• Evolution causes individuals at the center of the
curve to have a higher fitness than those at either end of the curve thus narrowing the curve
– Individuals with highest fitness: near the center of the curve (average phenotype)
– Example: human birth weight
Key
Per
cen
tag
e o
f P
op
ula
tio
n
Birth Weight
Selection against both extremes
keep curve narrow and in same place.
Low mortality, high fitness
High mortality, low fitness
Stabilizing Selection
Disruptive Selection• individuals at both ends of the curve survive better
than the middle of the curve
• Example: birds where seeds are either large or small
Disruptive Selection
Largest and smallest seeds become more common.
Nu
mb
er o
f B
ird
sin
Po
pu
lati
on
Beak Size
Population splits into two subgroups specializing in different seeds.
Beak Size
Nu
mb
er o
f B
ird
sin
Po
pu
lati
onKey
Low mortality, high fitness
High mortality, low fitness
Genetic Drift • Random change in allele frequency that
occurs in small populations• Usually in smaller populations where the laws
of probability are less likely to be predicted • In small populations, individuals that carry a
particular allele may leave more descendants than others by chance. Over time, this type of chance occurrence can cause an allele to be more common in a population
a. Founder effect: allele frequencies change due to migration of a small subgroup of a population
b. Example: fruit flies on Hawaiian islands
Two phenomena that result in small populations and cause genetic drift
1.Founder Effect2.Bottleneck Effect
Founder Effect: : Fruit Flies on Hawaiian islands
• Sample of • Original Population
• Founding Population A
• Founding Population B
• Descendants
Bottleneck Effect
major change in allele frequencies when population decreases dramatically due to catastrophe
♦ Example: northern elephant sealsdecreased to 20 individuals in 1800’s, now
30,000no genetic variation in 24 genes
Bottleneck Effect: Northern Elephant Seal Population
♦ Hunted to near extintion♦ Population decreased to♦ 20 individuals in 1800’s,♦ those 20 repopulated so ♦ today’s population is ~30,000♦ No genetic variation in 24 genes
Evolution vs Genetic Equilibrium
• Hardy-Weinberg Principle– States that allele frequencies in a
population will remain constant unless 1 or more factors cause the frequencies to change
• This situation is called Genetic Equilibrium
• If allele freq.’s do not change… the population will not evolve
5 Conditions are required
to maintain genetic equilibrium form
generation to generation:• Random mating• Large Population• No movement into or out of the
population• No Mutation• No Natural Selection• If these conditions are not met then
equilibrium is lost and the population will evolve
The Process of Speciation
I. The formation of new biological species,
II. Caused by natural selection and chance events that cause a change in relative allele frequencies
III. As new species evolve, they become reproductively isolated from one another
Reproductive Isolation
• Is when 2 species evolve so that they cannot interbreed
• Populations now have separate gene pools
• Develop by Behavioral Isolation, Geographic Isolation, and Temporal Isolation
Behavioral Isolation
• Two populations are capable of interbreeding but do not interbreed because they have different ‘courtship rituals’ or other lifestyle habits that differ.
Example: Eastern and Western Meadowlark
• Eastern and Western Meadowlark
populations overlap in the middle of the US
Geographic Isolation
• Two populations separated by a geographic barrier; river, lake, canyon, mountain range.
Temporal Isolation
• Populations reproduce at different times
January
1 2 3 4 5 6
7 8 9 10 11 12 13