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T. Dobzhansky (geneticist)
“Nothing in biology makes sense
except in the light of evolution”
Adaptation
A genetically determined characteristic
that influences an organism's ability to
survive and reproduce in a particular
environment.
Characteristics can be morphological,
behavioral, or physiological.
Evolution
• Is a genetic change in a population (not an
individual) over time
• Ecologists look at phenotypic (physical
changes), in most cases, because that is
how we recognize populations.
• It is, in fact, changes in the genotype, or
more specifically, the gene pool.
Allele Frequencies
The frequency of occurrence of alleles in a
population.
If we use the simple one dominant and one
recessive allele model, this can be
demonstrated by:
p = frequency of the dominant allele
q = frequency of the recessive allele
Example
AA - 30 individuals
Aa - 20 individuals
aa - 50 individuals
p = 2(# individuals AA) + # individuals Aa
2(Total # individuals in population)
p + q = 1; therefore q = 1 - p
Example
p = 2(30) + 20
200
= 0.4
p + q = 1; therefore q = 1 - 0.4 = 0.6
With these values, we can calculate the
probability of what genotypes would be
present in the next generation if this
population were to mate randomly
Genotypic Frequencies
p2 = probability of AA
q2 = probability of aa
2pq = probability of Aa
p2 + 2pq + q2 = 1
Mechanisms for Evolutionary
Change
 Mutation
 Genetic Drift (small population size)
 Gene Flow (immigration and emigration)
Non-Random Mating
Natural Selection
Basic Tenet of Natural Selection
The most fit organisms (survivors)
will reproduce and pass their genes
on to the next generation.
Hardy-Weinberg Equilibrium
In diploid, sexually reproducing
organisms, phenotypes, genotypes and
genes all tend to come to equilibrium in
populations in certain conditions are met
Hardy-Weinberg Equilibrium
 No Mutation
 Large Population Size
 No immigration or emigration
 Random Mating
 No Selection for Traits
Genetic Drift
• Random or chance events lead to a
change in the genetic makeup of a
population
• Limited to small populations
• “Bottleneck”
p = 0.7
p = 0.33
Random event leads to loss of individuals
Population will come to reflect surviving population
Effective Population Size (Ne)
The number of individuals actually
participating in random mating.
This number is always smaller than the
actual population size
- number of old
- number too young
- small number of one sex
Gene Flow
• Change in gene pool of a population from
immigration or emigration.
• Founder Effect
Identification of Human Migration from
Mitochondrial DNA
Steps for Natural Selection
• Variation occurs in every group of living
organisms. Individuals are not identical in any
population.
• Every population produces an excess of
offspring.
• Competition will occur among these offspring
for the resources they need to live.
Steps for Natural Selection
• The most fit offspring will survive.
• If the characteristics of the most fit
organisms are inherited, these favored
traits will be passed on to the next
generation.
Common Types of Individual Selection
• Stabilizing selection
• Direction selection
• Disruptive selection
figure 21-12.jpg
Figure 21.12
Figure 21.12
Figure
21.13
Figure 21.13
Figure 21.14
Similarities in Adaptive Strategies
• Convergent Evolution – similar responses
to similar environmental situations BUT
not related evolutionarily
• Example – Fig. 2.9
!!!!!!!!!!VARIATION!!!!!!!!!!!
Properties of Fitness
• Fitness is a property of a genotype, not an
individual or population.
• Fitness is specific to a particular environment.
As the environment changes, so does the
fitness of genotypes.
• Fitness is measured over one generation or
more.
Outcomes of Selection
• Changes in Genetic Make-up of a
Population
• Rise of new species – IF certain
conditions met
Allopatric Speciation
Distribution of a Species
Geographic Barrier Splits Distribution
No longer interbreeding; therefore, no exchange
of genes and could be undergoing different selection
pressures
Over time, the gene pool of each group
can become quit different
If two groups are brought back together and
do not interbreed, they are now two separate species
Parapatric Speciation
Distribution of a Species
Individuals move into a new habitat
If no interbreeding occurs between individuals
in new habitat and those in the old, reproductive
isolating mechanisms can develop.
Sympatric Speciation
Isolating mechanism develops within the existing
distribution of a species
Sympatric Speciation
Isolating mechanism develops within the existing
distribution of a species
Reproductive Isolating Mechanisms
• Prezygotic mechanisms prevent fertilization or
zygote formation.
 Temporal shifts - do not become reproductively
active at the same time.
 Behavioral shifts - do not recognize courtship
behaviors (female bird doesn't recognize the dance
of a male).
 Mechanical shift - change in reproductive structure
making it physically impossible to mate.
 Habitat shifts – populations live in the same regions
but occupy different habitats/microhabitats.
Reproductive Isolating Mechanisms
• Postzygotic mechanisms zygote forms but
 Does not complete development
 Develops into a weak, unhealthy individual
 Is sterile in either the F1 or F2 generation
Evolution of Interactions Among Species
Mimicry
• Batesian mimicry - a benign species
resembles a noxious or dangerous species.
• Müllerian mimicry - both the mimics and
the model are noxious or dangerous.
Coevolution
• Evolutionary change in one species
results in a reciprocal response of another
species
• Many examples – excellent one is
diversification of flowering plants and
insect pollinators
• Parasitism
• Predator-Prey Interactions (including
herbivory)
• Competition
• Other topics of note
– Sexual selection
– Kin Selection