Chapter 16: Population
Genetics and Speciation
• Darwin died in 1882
• general acceptance of natural selection and
evolution
• Within the next century we gained knowledge
of genetics and were able to strongly link
genetics with theories of evolution and
natural selection
Variation of Traits Within a Population
• Population genetics = the study of evolution from
a genetic point of view
• Microevolution = evolution at the genetic level, or
a change in the collective genetic material of a
population
• Population = a group of individuals of the same
species that routinely interbreed; smallest unit in
which evolution can occur
Bell curve
• In nature, many quantitative traits in a
population (i.e. height and weight) tend to
show variation that follows a bell curve
pattern
What causes Variation?
• Environmental factors
– Food available
• Heredity
– Range of phenotypic possibilities (i.e. body size)
– Set of specific phenotypes (i.e. possible flower
colors)
– Phenotypes within families (resemble family
members but not identical to; different
combinations of genotypes and therefore
phenotypes)
Populations have Gene Pools
• gene pools are used to describe the total genetic
information available in a population
– If you could know all the alleles present in this imaginary
pool you could use probability to predict expected
genotypes & their frequencies for the next generation
Why do genes vary?
(Reasons we know right now)
• 1) Mutation – random change in a gene that is passed on
to offspring
• 2) Recombination – reshuffling of genes in a diploid
individual (occurs during meiosis by independent
assortment and crossing-over)
Why do genes vary?
(Reasons we know right now)
3) random pairing of gametes – because each
organism produces large numbers of gametes (union
by chance)
• expression of some genes depend upon the presence
or absence of other genes or factors in the
environment
Natural Selection can act on species variations in
a number of ways
Stabilizing Selection
- individuals with average form of a trait have the highest
fitness
• average
represents
optimum for
most traits
•think about
bird beak lab
Directional Selection
- individuals that display more extreme form of a trait
have greater fitness than individuals with an average
form of the trait
• Peppered
moth
simulation
Disruptive Selection
- individuals with either extreme variation of a trait
have greater fitness than individuals with average form
of the trait
• extremes
are better
than blends
Genetic Equilibrium: a theoretical state
• Based on assumptions about an ideal
hypothetical population that is not evolving
– No net mutations occur; that is, the alleles remain
the same
– Individuals neither enter nor leave the population
– Population is large (ideally, indefinitely large)
– Individuals mate randomly
– Selection does not occur
Equilibrium Condition #1: no net mutations occur;
that is, the alleles remain the same
Disruption: MUTATIONS
o spontaneous mutation occur constantly at very low
rates under normal conditions
o exposure to mutagens (mutation-causing agents)
o chemical
o radiation
Mutation
• provide new alleles for a trait
• many of them are harmful
• although some have no effect
– Natural selection operates only on genes that are
expressed so it takes a very long time to eliminate
harmful recessive mutations
– In long run: beneficial mutations are a vital part of
evolution
Equilibrium Condition #2: individuals neither enter nor
leave the population (population remains constant)
Disruption: GENE FLOW
o size of population remains constant
o remember if an individual moves, genes move with
them!
Gene Flow
Immigration
- the movement of individuals into a population
Gene Flow
Emigration
- the movement of individuals out of a population
Gene Flow
Gene Flow
-the process of genes moving from one population
to another
- migration of individuals
-dispersal of seeds or spores
Example
• common baboons live on
eastern African savannas
in social/breeding groups
called troops (10-200
members)
– A few adult males
dominate the troop; young
and less dominant males
leave their birth, eventually
joining another troop
Equilibrium Condition #3: the population is large
(ideally, infinitely large)
Disruption: GENETIC DRIFT
o allele frequencies in a population change as a result of
random events, or chance
o in small populations the failure of even a single organism
to reproduce can significantly disrupt the allele frequency
of the population
o if there is no variation, then extinction results due to
natural selection
Equilibrium Condition #4: individuals mate
randomly
Disruption: NONRANDOM MATING
o many species do not mate randomly
o Geographic proximity can result in some degree of
kinship
o Amplify certain traits
o Offspring with disorders caused by recessive genes
o Individuals may select a mate that has traits similar
to their own traits (assortative mating)
Nonrandom Mating continued…
Sexual Selection
- tendency of females to males to mate with based
on certain traits
- EXAMPLE: male peacock feathers
Reminder: natural selection favors an increase in the
genes of successful REPRODUCERS, rather than
merely those of successful SURVIVORS.
Equilibrium Condition #5: selection does not
occur
Disruption: NATURAL SELECTION OCCURS!
o some members of population are more likely to survive and reproduce
over others and thus are able to contribute their genes to the next
generation
o traits such as body size or color
o trait distribution in a population may change when natural selection
is at work over time
o shift away from bell curve
Formation of Species
Speciation
- process of species formation;
results in closely related species
Morpholgical Concept of Species
• Morphology - internal and external structure and
appearance of an organism
– Used to be the chief criterion for classifying it as a species
• Has limitations though: e.g. phenotypic differences among individuals
in a single population
• Some individuals that appear different enough to belong to different
species interbreed in the wild and produce fertile offspring
The Biological Species Concept
• Ernst Mayr (1904-2005) – (German born, American
Biologist) proposed the biological species concept which
states that a species is a population of organisms that can
successfully interbreed but cannot breed with other groups
– We cannot test this concept with extinct organisms of the
past; some organisms’ reproductive capacity compatibility
cannot be tested; does not work for organisms that do not
reproduce sexually
Blend of ideas = current definition
• Species = a single kind of organism whose
members are morphologically similar and
can interbreed to produce fully fertile
offspring.
Speciation begins with:
GEOGRAPHIC ISOLATION
• Geographic isolation = the physical separation of
members of a population
– Habitat divided (e.g. deep canyon develops; river changes
course; drying climate in a valley)
– Natural selection and genetic drift cause the two populations
to diverge to the eventual point of being incompatible for
mating
– Organism: can you move around? e.g. birds can fly across a
canyon
Allopatric Speciation
• Allopatric speciation happens when species arise as a
result of geographic isolation
–
–
–
–
Means “different homelands”
Geographic barrier = no more gene flow between them
More likely in small population (smaller gene pool)
If the geographic barrier is removed, could the groups
interbreed and produce fertile offspring?
http://evolution.berkeley.edu/evolibrary/article//evo_45
Sympatric Speciation
- occurs when 2
subpopulations
become
reproductively
isolated within the
same geographic
area
- population dies or
lives in one spot,
some individuals
move; eventually
groups will not breed
Reproductive Isolation
- results from barriers to successful breeding
between population groups in the same area
Reproductive isolation and the new species formed
from it may result through disruptive selection (two
extremes are selected for and begin to diverge)
o Once successful mating is prevented, it’s like
the pattern of geographic isolation
THERE ARE TWO TYPES OF REPRODUCTIVE ISOLATION:
PREZYGOTIC ISOLATION
POSTZYGOTIC ISOLATION
Prezygotic Isolation
 pre-mating isolation
 occurs before fertilization
 can reduce the chance of hybrid
formation
 e.g. a mating call that is not
recognized as such by a
potential mate
 e.g. different peak mating
times
Postzygotic Isolation
 post-mating isolation
 occurs after fertilization
 e.g. different breeding times
 offspring of interbreeding species may not develop
completely and may die early; or live but not be fertile
Rates of Speciation
Gradualism
- idea that speciation occurs at a regular, gradual rate
- some scientist disagree: they think it happens in
“bursts” relative to the geologic time scale
Rates of Speciation
Punctuated Equilibrium
- change occurred in a few thousands years rather than
a few million
- periods of little change
- sudden, rapid changes
- e.g. see periods of no change with some cases of great
diversity of new forms evolved rapidly in fossil record