How Populations Evolve
Chapter
14
Populations Evolve
Biological
evolution does not change individuals
It
changes a population
Traits
in a population vary among individuals
Evolution
is change in frequency of traits
The Gene Pool
All of the genes
in the population
Genetic resource
that is shared (in theory) by all members of population
Variation in Phenotype
Each
kind of gene in gene pool may have two or more alleles
Individuals
inherit different allele combinations
This
leads to variation in phenotype
What Determines Alleles in New Individual?
Mutation
Crossing
over at meiosis I
Independent
assortment
Fertilization
Change
in chromosome number or structure
Hardy-Weinberg Equilibrium
A
hypothetical population in which allele frequencies at a locus are not changing
Population
is not evolving (evolution is the change in allele frequencies in a
population)
Five Conditions
No
mutation
No
gene flow
Large
population
Random
mating
No
natural selection
Hardy-Weinberg Rule
At genetic equilibrium, proportions
of genotypes at a locus with two alleles are given by the equation:
p2 AA
+ 2pq Aa + q2 aa = 1
Frequency of allele A = p
Frequency of allele a = q
Punnett Square
Frequencies in Gametes
No Change Through Generations
Agents of Evolution
Mutation
Small
populations
Non-random
mating
Natural
Selection
These
agents plus lack of gene flow between populations can lead to speciation
Gene Mutations
Infrequent but
inevitable
The source of new
alleles
Not goal directed
Small Populations: Size Matters
Chance events
drive a small population away from genetic equilibrium
Genetic Drift
Genetic Drift
Bottleneck
A
severe reduction in population size
Causes
pronounced drift
Example
Elephant
seal population hunted down to just 20 individuals
Population
rebounded to 30,000
Electrophoresis
revealed there is now no allele variation at 24 genes
Founder Effect
Effect of drift
when a small number of individuals start a new population
By chance, allele
frequencies of founders may not be same as those in original population
Effect is
pronounced on isolated islands
Inbreeding
Nonrandom
mating between related individuals
Leads
to increased homozygosity
Can
lower fitness when deleterious recessive alleles are expressed
Amish,
cheetahs
Natural Selection
A
difference in the survival and reproductive success of different phenotypes
Acts
directly on phenotypes and indirectly on genotypes
Reproductive Capacity
& Competition
All
populations have the capacity to increase in numbers
No
population can increase indefinitely
Eventually,
the individuals of a population will end up competing for resources
Variation in Populations
All
individuals have the same genes that specify the same assortment of traits
Most
genes occur in different forms (alleles) that produce different phenotypes
Some
phenotypes compete better than others
Change Over Time
Over
time, the alleles that produce the most successful phenotypes will increase in
the population
Less
successful alleles will become less common
Change
leads to increased fitness
Increased
adaptation to environment
Results of Natural Selection
Three
possible outcomes:
A
shift in the range of values for a given trait in some direction
Stabilization
of an existing range of values
Disruption
of an existing range of values
Directional Selection
Allele
frequencies shift in one direction
Stabilizing Selection
Intermediate
forms are favored and extremes are eliminated
Disruptive Selection
Forms
at both ends of the range of variation are favored
Intermediate
forms are selected against
Pesticide Resistance
Pesticides
kill susceptible insects
Resistant
insects survive and reproduce
If
resistance has heritable basis, it becomes more common with each
generation
Sexual Selection
Selection favors
certain secondary sexual characteristics
Through nonrandom
mating, alleles for preferred traits increase
Leads to
increased sexual dimorphism (differences between males and females)
Gene Flow
Physical
flow of alleles into a population
Tends
to keep the gene pools of populations similar
Counters
the differences that result from mutation, natural selection, and genetic drift
Barriers to Gene Flow
Whether or not a physical barrier deters gene flow
depends upon:
Organisms
mode of dispersal or locomotion
Duration
of time organism can move
Genetic Drift in
Snail Populations
Robert
Selander studied Helix aspersa
Collected
snails from a two-block area
Analyzed the allele frequencies for five genes
Genetic Drift in
Snail Populations
Snail Speciation?
Will
the time come when the snails from opposite sides of the street are so
different that they can no longer interbreed?
If
so, then they will have become two distinct species
Speciation & Natural Selection
Natural
selection can lead to speciation
Speciation
can also occur as a result of other microevolutionary processes
Genetic
drift
Mutation
Appearance & Species
Appearance may not be useful in distinguishing
species
Members
of same species may appear different because of environmental conditions
Appearance
can vary with age and sex
Different
species can appear identical
Same species, but they look different
Biological Species Concept
Species are groups of interbreeding
natural populations that are reproductively isolated from other
such groups.
Ernst
Mayr
Reproductive Isolation
Cornerstone
of the biological species concept
Speciation
is the attainment of reproductive isolation
Reproductive
isolation arises as a
by-product of genetic change
Genetic Divergence
Gradual
accumulation of differences in the gene pools of populations
Natural
selection, genetic drift, and mutation can contribute to divergence
Gene
flow counters divergence
Genetic Divergence
Reproductive Isolating Mechanisms
Premating
isolation
Mating is prevented
Postmating
isolation
Prevents formation of vigorous, fertile offspring
Zygotes may die early, be weak, or be sterile
Premating Isolation
Geographic isolation
Ecological Isolation
Temporal Isolation
Behavioral Isolation
Mechanical Isolation
Postzygotic Mechanisms
Gametic
incompatibility
Hybrid
inviability
Hybrid
infertility
Speciation
Speciation
in geographically isolated populations is probably most common mechanism
Some
sort of barrier arises and prevents gene flow
Effectiveness of barrier varies with species
Speciation
in Wrasses
Isthmus of Panama
arose and separated wrasses in Atlantic and Pacific
Since separation,
genes for certain enzymes have diverged in structure
Divergence may be
evidence of speciation in progress
Speciation by Polyploidy
Change
in chromosome number
(3n, 4n, etc.)
Offspring
with altered chromosome number cannot
breed with parent population
Common
mechanism of speciation in flowering plants
Possible Evolution of Wheat
Extinction
Irrevocable
loss of a species
Mass
extinctions have played a major role in evolutionary history
Fossil
record shows 20 or more large-scale extinctions
Reduced
diversity is followed by adaptive radiation
Extinction
Today:
Introduced
species
Habitat
loss
Climate change
Over-harvesting
Pollution
Who Survives?
Species
survival is to some extent random
Asteroids
have repeatedly struck Earth destroying many lineages
Changes
in global temperature favor lineages that are widely distributed
Back to schedules:
MWF
TTH
Biology Home
Merced College Home