Download 7th Evolution Population Genetics.key

Evolution
Genes and Variation
Review of Genetics
• What are genes?
– Specific genetic makeup of an individual
• What is an allele?
– Different forms of a gene
– Remember BB, Bb, bb?
• What is a mutation?
– A change in the DNA sequence that affects genetic
information
Genes & Variation
• What is a population?
– all the organisms of the same group or species
who live in the same geographical area and are
capable of interbreeding
• What is a gene pool?
– The total number of alleles in a population
• What is variation?
– The differences among members of a population
Natural Selection
• Natural selection acts on populations
• Environmental pressures cause
individuals to survive or die
Modes of Selection
Directional Selection
Low Mortality
Old average
High Mortality
New
average
• In a given population, the most fit individuals
are now at one extreme or the other
• Causes the entire curve to shift
Stabilizing Selection
• Organisms at the
center of the graph
(the average) are the
most fit
• Often occurs after a
natural disaster
• Nature tries to
maintain the ‘status
quo’
• Same average
Low Mortality
High Mortality
Original
Population
# of
birds
Beak size
• Curve stays at same position, but narrows
New
Population
population
Disruptive Selection New
– 2 phenotypes!
Original
Population – 1
phenotype
Low Mortality
High Mortality
# of
birds
Beak size
# of
birds
Beak size
• Individuals at both extremes have a higher
fitness than individuals in the center
• Leads to two distinct phenotypes!
Genetic Drift
• Over time, a series of chance occurrences
can make one allele more common in the
population.
• A chance event wipes out members of the
population and leaves only a select few à
doesn’t represent original population
• Small populations more easily affected
• Random chance – no way to plan
Genetic Drift
No grey!
Survivor
Group #1
Survivor
Group #2
Original population
3 phenotypes – black,
white, grey
No black!
Types of Genetic Drift-
Founder Effect
• Members of a
population migrate to
a new area
• New population
established
• May be very different
gene pool than original
population
• Starts as smaller gene
pool & fewer
individuals
The “Blue People” of Kentucky
Types of Genetic Drift-
Bottleneck Effect
• Severe reduction in population size due to
a random event
• What could these random events be?
– Predation
– Disaster
– Hunting
• New population may look nothing like
original
A bottleneck effect is genetic drift in which a
severe reduction in population size results from
natural disaster, predation, or habitat reduction.
Cheetah:
A Real Life
Bottleneck
•
Every one of today's 20,000
Cheetahs is genetically almost
identical. They descend from
survivors of a near-extinction
catastrophe that resulted in
generations of close inbreeding
10,000 years ago.
Does evolution ALWAYS take place?
• Nope.
• Populations can sometimes be stable.
• Genetic equilibrium
Genetic Equilibrium
• 1. Random mating & all members of a population have equal
opportunity to mate
• 2. Large population
– Genetic drift has less of an affect on big groups
• 3. No net movement in or out of a population
– Migration & Emigration bring alleles to and from a population
• 4. No mutations –
– no new alleles/traits introduced
• 5. No natural selection
– No phenotype has an advantage
Types of Reproduction
• Asexual Reproduction
– No mate needed
– Quickly reproduce
– Every individual can
reproduce
– Traits can easily be
passed on
– Mutation/disease
affects everyone
– No variation
– No evolution
• Sexual Reproduction
– Variations (driving force
behind evolution!)
– New combinations of
traits could be
beneficial
– Bad traits could
disappear or be hidden
– Fewer offspring
– Competition for mates
Sexual Dimorphism
Distinction in appearance due to secondary sex characteristics
Males of a species are distinctly different from Females
Intrasexual
Selection
- Fighting for mates
- Males usually fight for
females
Intersexual Selection
- Also called Mate Choice
- Usually the choosing is done by the females.
- Males with the most impressive masculine
features are most attractive to the ladies.
What makes
a species?
Speciation
What makes a species?
Species = latin = kind à
Means many types!
• Morphology = similar physical form
• Similarities in…
• Body Function
• Biochemistry
• Behavior
• Genetic Make-up
Biological Species concept: Ernest Mayr, 1942
Defines a species as a population or group of
populations whose members have the potential to
produce viable, fertile offspring but who cannot
produce viable, fertile offspring with members of
other species.
Barriers for Speciation
• In order to have a new
species, they must be
reproductively
isolated
• Two ways:
– Prezygotic: before
sperm and egg meet
– Postzygotic: after
fertilization
Prezygotic (Pre-Fertilization) Barriers
to Speciation
• Habitat isolation
– Don’t live in same area
• Behavioral isolation
– Mating rituals : song, dance
• Temporal isolation
– Mate in different seasons, times of day, etc.
• Mechanical isolation
– Things don’t fit where they should.
• Gametic isolation
– Egg and sperm don’t attract each other
Postzygotic (After Fertilization) Barriers
to Speciation
• Reduced hybrid viability
– Zygote (fertilized egg) dies or offspring never
reach sexual maturity
• Reduced hybrid fertility
– Offspring are sterile
• Hybrid breakdown
– Offspring can produce only 1 new generation
Behavioral Isolation: Specific Courtship Displays
Temporal Isolation: Different frog species mate
at different times of the year.
Colors indicate same habitat range
When species cross…
Man-made & Sterile
Mule: offspring of a male donkey
and a female horse
Cama : offspring of a camel and a
llama
Produced by artificial insemination
Hinny: offspring of a male horse
and a female donkey
A horse has 64 chromosomes resulting in 32
in the gamete. A donkey has 62, resulting
in 31 the gametes. How many does a mule
have in its cells? 63
+
=
Male and female mules are typically sterile because
the horse and donkey chromosomes differ in number
and they are not homologous. Therefore, the horse and
donkey chromosome doublets fail to properly pair up
with each other during synapsis of meiosis I.
AGAIN - These
Usually Do Not occur
in nature!
Liger : Male lion,
female tiger
Tigon:
Male tiger,
Female lion
Zebroid: zebra & another
equine animal
Zebroid hybrids are STERILE.
What is that called?
Reduced Hybrid Fertility