• Evolution is not a property of individuals,
but of populations…in other words, you
have to look at a large number of
individuals to see evolutionary change, not
just a small number of them.
• Species:a group of organisms that is distinct from
other species in appearance and behavior AND
that regularly breed together producing FERTILE
• Population: a group that includes all the members
of a species IN A GIVEN AREA that are
somewhat independent of other populations and
are substantially more likely to mate with each
other than with members of other populations
within the same species.
Definitions* within populations
• Gene pool: sum of all genes in a population
(meaning all of the alleles of all the genes in
all of the individuals of a population.
• Allele frequency: sum of all copies of
EACH allele in each individual in a
• Gene Flow: movement of alleles into or out
of a population.
• As we already know, “mutations” in DNA
result in different alleles (genes)
• The result of these mutations is different
proteins that determine an individual’s traits
• So how does evolution happen?
HOW EVOLUTION HAPPENS*
• 1.) Mutation
• 2.) Gene flow
• 3.) Small population size
• 4.) Nonrandom mating
• 5.) Natural selection
• Occurs during DNA replication (cell makes copy
of its DNA)
• Most mutations fixed by DNA repair proteins,
BUT if a mutation slips by the repair proteins and
if it occurs in a cell THAT WILL BECOME AN
EGG OR SPERM, the mutation can be passed to
offspring and ENTER THE GENE POOL of a
How common is that?
• A mutation in the DNA of a sperm cell, in an organism
that creates billions of sperm over its lifetime, can only
possibly spread to very few individuals (first that sperm
has to fertilize an egg) and then that organism has to mate
and pass on the mutation to its offspring
• Human example: we have 20,000-25,000 genes, so 40,000-
50,000 alleles (2 alleles for each gene). Each allele has 1 in
100,000 chance of mutation. Each newborn has 1-2
mutated genes on average.
• These NEW ALLELES are the material for evolution.
Result of mutation
• The protein that the mutated allele codes for will be
altered, but perhaps not so much that the protein
malfunctions….perhaps it will make the protein work
• If the mutation does result in a malfuction, or any other
change in phenotype that renders that organism LESS able
to reproduce, natural selection will likely wipe out that
organism and therefore that allele. Conversely, a better
more efficient organism will survive longer, thus spreading
that mutated allele to its offspring and moving it through
the gene pool.
2.) Gene Flow*
• When organisms leave their home population and travel to
another population, they take their alleles to a new gene
• Alleles can move through gene pools even if the organisms
are incapable of movement. Ex: plants release seeds and
pollen that can travel on the wind or on organisms that
• The MAIN EFFECT OF GENE FLOW is to INCREASE
the genetic SIMILARITY between different populations
within a species. The destination population is more
similar to the home (source) population
3.) Small population size affects
• Allele frequencies (how many of each allele
are in each population) can change due to
events OTHER THAN mutation.
• Genetic Drift: process by which chance
events change allele frequencies. Two
common events that cause genetic drift:
– Population Bottleneck
– Founder effect
– Population drastically reduced as the result of
natural catastrophe or over hunting
– BY CHANCE, allele frequencies are changed
and there can be reduced genetic variability.
• Isolated colonies (populations) are founded
by a small number of organisms
– Small flock of birds becomes separated from
source population in a storm or by wind and
starts its own new population in a new place
– BY CHANCE, the new population has allele
frequencies that are different from its source
Drift could result in LOSS of
• Genetic Drift occurs more rapidly in a small
population than a large one. (less alleles to
begin with, change will be less “visible” in
a large population)
• Sudden reduction of individuals in a small
population COULD remove an allele from
that population COMPLETELY AND
4.) Non-random mating
• Non random mating: organisms seldom mate randomly.
• They usually mate with others that are in their nearby area.
• Organisms MAY mate with their own relatives…called
• This is harmful because relatives are genetically similar,
and inbreeding tends to increase the number of individuals
who inherit the same alleles from both parents. This can
increase homozygosity for each gene.
5.) Natural Selection*
• Covered in previous lecture, Natural Selection will favor
traits that increase survival to the extent that this
increased survival leads to improved reproduction.
– Traits that may increase likelihood of living long
enough to reproduce
– Traits that may lengthen the life of the organism,
therefore giving it more TIME to reproduce.
– These traits, given to the organism by it’s genes/alleles,
are then passed on to more organisms.
– (Remember that the result of these alleles is the
phenotype of the organism)
Part of the success is the
• All of these alleles/traits that make an
organism live longer must also depend on
– Availability of water
– Presence of predators/competitors
• Buffalo grass in Wyoming
– Roots have evolved to be efficient at taking up water
and nutrients from soil (it can be dry there)
– Roots have evolved to be long and go deep in the soil,
thereby making it “better” at taking up water than other
grasses growing nearby
– Grass is tough (silica compounds exist in the grass) so
that animals are discouraged from grazing
• When one species evolves to adapt to the
environment or competition, other species in the
area also change…COEVOLUTION.
• Example: wolves & deer
• Slow deer are killed by wolves….fast deer are left
which pass on their genes.
• Faster deer means that slow wolves won’t catch
them…slow wolves die of hunger, leaving faster
wolves to pass on their genes.
Species & Reproductive Isolation
• Recall that a species is a group of organisms
that regularly reproduces and produces
• Members of a species are also
reproductively isolated….meaning that
individuals outside the species cannot
reproduce with members of the species.
• How do we insure reproductive isolation?
Premating isolating mechanisms*
• Geographical isolation: organisms LIVE IN
PHYSICALLY SEPARATED locations
– This also helps the formation of new species
• Ecological isolation: organisms OCCUPY DIFFERENT
• Temporal isolation: organisms BREED AT DIFFERENT
TIMES OF THE YEAR
• Behavioral isolation: organisms HAVE DIFFERENT
COURTSHIP AND MATING RITUALS
• Mechanical Incompatibility: SEX ORGANS OF
ORGANISMS DO NOT FIT (Internal fertilization)
Postmating isolating mechanisms*
• Gametic incompatibility (especially important in
marine invertebrates and wind-pollinated plants):
GAMETES HAVE CHEMICAL OR
MECHANICAL STRUCTURES THAT ONLY
ALLOW OTHER GAMETES OF THEIR OWN
SPECIES TO FERTILIZE
• Hybrid Inviability:HYBRID OFFSPRING FAIL
TO SURVIVE TO MATURITY
• Hybrid infertility: HYBRIDS DO SURVIVE BUT
CANNOT REPRODUCE (Ex: Mules)
How do new species form?*
• Speciation: process by which new species
• Depends on two factors:
– Isolation of populations: there must be no gene
flow between 2 populations
– Genetic divergence: genetic drift or natural
selection causes organisms in one population to
become different (new alleles spread through
2 pathways to speciation*
• Allopatric speciation: two populations of a species become
separated by a geographical barrier (islands, reefs). Natural
disasters can cause this separation: earthquakes, volcanoes,
• Sympatric speciation: two populations of a species become
separated, but not geographically. The two populations
occupy same area but different habitats.
• Eventually, after enough time, the two populations become
so genetically different that they could not mate with
members of other population: therefore they are now two
What causes extinction?
• Extinction: death of all members of a
• 99.9% of all species that ever existed are
– Environmental change
– Interaction with competing species
• Habitat destruction due to natural disasters
or destruction by man
• If a species is limited to a specific area,
more chance of extinction if that specific
area is damaged
• Some species become so specialized (can
only eat certain food, can only live in
certain place/temp) that destruction of their
area can cause extinction.
Interaction with competing
• Species unable to compete with other
species in same area can go extinct.