NATURAL SELECTION
Ch 14
p. 296-324
THE MODERN THEORY
• Key discoveries:
• Invention of more powerful microscopes & precision tools
• Use of short lived organisms for breeding experiments
• Rigorous application of the scientific method
BEYOND NATURAL SELECTION
• We now understand that natural selection
is just ONE of a number of processes that
can lead to evolution. This knowledge has
resulted in the development of a more
complex understanding of genetic change
= THE SYNTHETIC THEORY OF
EVOLUTION = a combo of Charles
Darwin’s Natural Selection & Gregory
Mendel’s Genetic Inheritance along with
population genetics and molecular biology
of the 20th century
NATURAL SELECTION
• Genes are the carriers of inheritable
characteristics
• They are also the source of random variation
upon which natural selection operates
• Mutation causes some variation
• Meiosis is when most variation occurs during the
copying, shuffling and dealing of genes to the gametes
and crossing over
• Remember, genetic variation is random, it is not
controlled or directed toward a goal
How Natural Selection Works
• In the struggle for
survival, entire
organisms, not genes,
either survive &
reproduce, or do not
• Natural Selection
operates on the
phenotypic variation
between individuals
• Phenotypic = physical &
behavioral characteristics
produced by the
interaction of genotype &
environment
• More “fit” individuals
will have more success
mating and will live
longer therefore
producing more
offspring in the next
generation
• In essence, this is selection
because individuals who are
phenotypically fit will
produce more offspring, thus
causing a change in the
genetics of the next
generation
EVOLUTION AS GENETIC CHANGE
• POPULATIONS: individuals of the same species in an
area who breed with one another
• GENE POOL: all of the alleles of genes within a
population
• RELATIVE FREQUENCY: number of times an allele
appears in a population compared to the other alleles of
the same gene
• EVOLUTIONARY CHANGE INVOLVES A CHANGE IN
THE RELATIVE FREQUENCIES OF ALLELES IN THE
GENE POOL OF A POPULATION
POPULATION GENETICS
• Study of genetic traits in individuals
• Genotype – actual allele inherited from each parent that the
individual has (blue eyes, brown eyes)
• Phenotype – resulting observable characteristic from the
interaction of the individuals genotype (brown eyes)
• Study of genetic traits in populations
• Gene Pool: combined genetic material of all the members of a
population
• Alleles: name for the 2 or more different forms of a gene. (gene =
eye color, alleles = blue, brown, green )
• Allele Frequency: is a fraction that represents the frequency of a
particular allele within a population
HARDY-WEINBERG PRINCIPLE
If a population is not evolving,
allele frequencies are in
Genetic Equilibrium
1.
•
•
Under specific conditions allele
frq. Remain constant
If those conditions are not met the
population may EVOLVE
Conditions required to maintain
G.E. are:
2.
•
•
•
•
•
3.
No natural selection
Random mating
No mogration
No mutation
Very large population size
4.
Even when HardyWeinberg
conditions are
met, equilibrium
can be affected by
RANDOM
CHANGES in
allele frequencies
A random change
in allele
frequencies in a
popultion due to
chance events is
called GENETIC
DRIFT
DISTRUBUTION & SELECTION
• The normal distribution of variations in a population can
change by natural selection
• Evolution results from disruptions in genetic equilibrium
STABALIZING SELECTION
• These curves are
called normal
distributions because
the patterns is so
common in nature
DIRECTIONAL SELECTION
• This type of selection
occurs when a
change in the
environment makes it
infavorable to have
an extreme
phenotype
DISRUPTIVE SLELECTION
• This type of selection
occurs when an
environmental
change makes it
unfavorable to have
the most common
phenotype
CONCEPT OF A SPECIES
• Speciation and extinction were extremely important in
determining the history of life on earth
• Before we can know whether speciation has occurred
in a group of organisms, we have to know what a
species is!
• A species is a group of interbreeding individuals
or populations that are reproductively isolated
from other such groups
SPECIATION
• Speciation involves environmental processes that lead to
the splitting of a gene pool into two or more separate
pools
• MAJOR CAUSES:
• Reproductive isolating mechanisms
• Geographic isolation
• Courtship behaviours
SPECIATION - REPODUCTIVE ISOLATING MECHANISMS
• The key event in
• PRE-ZYGOTIC =
• Prevent fertilization
• Seasonal/habitat barriers to
mating, behavioral isolation,
physical isolation
• POST-ZYGOTIC =
• Come into play despite
fertilization
• Gamete mortality, hybrid nonviability, hybrid sterility
speciation is
Reproductive
Isolation between
two diverging
populations
• Isolating mechanisms
are all genetically
based and can be
either pre-zygotic or
post-zygotic
SPECIATION - GEOGRAPHIC
ISOLATION
• Simplest mechanism of isolation
• Continents collide, mountains are uplifted & suddenly a
population finds that it is splintered into a large population as
well a number of geographically isolated smaller population
• Most of these small populations eventually go extinct
SPECIATION - COURTSHIP
BEHAVIORS
• Often the behaviors a male must engage in to convince
the female of his proper species-hood are quite elaborate
and cannot be reproduced by member of a different
species
• Various behavioral cues allow males & females to
recognize their own species
PUNCTUATED EQUILIBRIUM VS
GRADUALISM
Darwin saw evolution as a slow, continuous process,
without sudden jumps (GRADUALSIM) however, if you
study the fossil record, you will see long intervals in
which nothing changed (EQUILLIBRIUM) punctuated
by short, revolutionary transitions in which species
became extinct and replaced by new forms
(PUNCTUATED EQUILIBRIUM)
Instead of a slow progression, evolution of life seems
more like the life of a fire-fighter: long periods of
boredom interrupted by rare moments of terror.
TYPES OF EVOLUTION
• The basic evolutionary mechanisms-mutation, migration, genetic
drift, and natural selection- can produce major evolutionary can
change if given enough time
• THERE ARE 3 MAJOR TYPES OF EVOLUTION:
• Divergent
• Convergent
• Co-evolution
DIVERGENT EVOLUTION
• = THE PROCESS OF 2+ RELATED SPECIES
BECOMING MORE DISSIMILAR
• Ex. Red Fox (forests, red) and Kit Fox (prairies, tan)
Similarities in structure show that they have a common
ancestor, but as they adapted to different environments, they
diverged
CONVERGENT EVOLUTION
• = WHEN ORGANISMS THAT ARE NOT CLOSLY
RELATED EVOLVE SIMILAR TRAITS
• Ex. Antifreeze proteins in both Antarctic and Arctic fish
• Ex. Wings of a bat and bird
COEVOLUTION
• = JOINT CHANGE IN 2+ SPECIES IN CLOSE
INTERACTION
• Often occurs between
• Predators and prey
• Parasites and hosts
Ex. Plants and the animals that pollinate them
EXTINCTION
• 99% of the species that have
ever lived on earth have gone
extinct!
• Present day biodiversity is a
result of an accumulation of
changes due to extinction
events over millions of years
• EXTINCTION
CONTAINS 3 KEY
ELEMENTS:
• For geographically
widespread species
extinction is rare
• Large mass extinctions
produce major restructuring
of the biosphere and
allowing previously minor
groups of expand and
diversify
• It is generally impossible to
predict which species will
be victims of an extinction
event
SUMMARY
• EVOLUTION CAN
• EVOLUTIONARY
RESULT FROM:
PROCESSES
ALLOWS US TO
UNDERSTAND:
• Mutation
• Natural selection
• Genetic recombination
• Crossing over
• Non-random mating
• Mutation
• Genetic drift
• Extinction
• Gene flow
• Convergent evolution
• Extinction
• Divergent evolution
• Co-evolution