Download Ch. 15.3 Notes

Ch. 15.3
Shaping Evolutionary Theory
Mr. Gaede | Biology
I.
POPULATION GENETICS
Q: 1900s – Why don’t dominant alleles take over recessive
alleles in a population?
A: 1908, scientists Hardy & Weinberg–
Discovered evolution will not occur in a population unless allele
frequencies are acted upon by forces that cause change
Hardy-Weinberg Principle:
When allele frequency 𝑣 stays constant  pop. is in genetic
equilibrium (EQ)  NOT EVOLVING
I.
POPULATION GENETICS
Conditions of Hardy-Weinberg EQ:
For allele 𝑣 to stay the same:
1. No mutations
2. No migration (gene flow)
3. Large population size
4. No natural selection
5. Random mating
 Evolution is always occurring because these conditions can never be met
II. GENETIC DRIFT
 Genetic drift— change in the allele
𝑣 in a population due to chance
(crossing over of DNA during Meiosis)
 Occurs in small populations 
 Recessive individual breeds more than
normal & allele 𝑣 increases quickly
 Reduces genetic variation
II. GENETIC DRIFT
1. Founder Effect
 Occurs when a small sample of a
population settles in a location
separate from the rest of the pop.
2. Bottleneck Effect
 Occurs when a population declines to
very low number + then recovers 
problem: loss of genetic variability
Northern Elephant Seals
• Bottleneck Event: Human Hunting (1890s)
• Population decreased to 20 seals
• NOW  ~30,000 seals w/ reduced variation
III. GENE FLOW
 Gene flow – (AKA: migration)
 organisms move from one
population to another
 Increases genetic variation
IV. NONRANDOM MATING
Nonrandom mating –
Organisms choose their mates
based on proximity
Genetic variation decreases
V. NATURAL SELECTION
Natural Selection –
Organisms with traits well suited
to their environment survive and
reproduce at a greater rate than
un-fit organisms in the same
environment
V. NATURAL SELECTION
 Stabilizing selection
 Eliminates extremes of
trait when middle
phenotype is better for
the environment
V. NATURAL SELECTION
 Directional selection
 Selects for one extreme
 Population average
moves in one direction
V. NATURAL SELECTION
 Disruptive selection
 splits a population into
two groups
 often the cause of
speciation
 selects against the middle
Individuals w/ extreme variation of a trait have greater fitness than
individuals w/ the average form of the trait
VI. SEXUAL SELECTION
 Organisms choose a mate based
on a behavior or appearance
 Male competition & female choice
 Ex: peacocks and peahens
VII. REPRODUCTIVE ISOLATION
 Prezygotic (before zygote)
 Fertilization is unlikely
 Different mating behaviors
 Ex: E. vs W. meadowlark song
 Postzygotic (after zygote)
 Can physically reproduce but
offspring are sterile
 Ex: Liger or Mule
E. Meadowlark & W. Meadowlark
Liger
VII. REPRODUCTIVE ISOLATION
 Geographic Isolation (allopatric speciation)
 Population separated from
another population by the
land barrier (geography)
 Genetic differences over time
prevent mating
Antelope Squirrels vs. Grand Canyon
VIII.PATTERNS OF EVOLUTION
 Adaptive Radiation (divergent evolution)
 Evolution of an organism into a wide
variety of different types, each fill different
ecological niches
 Niche – role of an organism in its
environment
 Occurs after large extinctions
Cichlid Fish
300+ different species
VIII.PATTERNS OF EVOLUTION
 Coevolution
 The relationship between 2
species is so close that the
evolution of one species affects
the evolution of the other
 Mutualism – benefits both
Hummingbirds & Flowers
Beaks have evolved to fit shape
VIII.PATTERNS OF EVOLUTION
 Convergent Evolution
 Unrelated species evolve
similar traits even though they
live in different parts of the
world
 Occurs in environments w/
similar ecology
VIII.PATTERNS OF EVOLUTION
 Rate of Speciation Theories
1. Gradualism
 Evolution occurs in small
gradual steps over time
2. Punctuated Equilibrium
 Rapid spurts of genetic change
cause species to diverge quickly
Q: What are the healthiest populations?
 Large populations with lots of genetic diversity! They are least
likely to change.
 Large populations with the most diversity are most likely to
survive a disease outbreak.
 Small populations where many mutations occur are most likely to
evolve.
 https://www.youtube.com/watch?v=cW45C9BY1aU (Bird Beak Lab Video)