Evolution And Natural Selection
Chapter 13
Evolution
• Evolution is a change in the frequency of
genetically determined characteristics within a
population over time.
Evolution???
Evolution
• Microevolution occurs when there are minor
differences in allele frequency between
populations of the same species. This results
in genetic differences between subspecies.
• Macroevolution occurs when there are major
differences that have occurred over long
periods of time that have resulted in so much
genetic change that new kinds of species are
produced.
Microevolution
Macroevolution
History of Evolutionary Thought
• French naturalist Georges-Louis Buffon
wondered if animals underwent change
(evolved) over time. He observed changes of
animals in the fossil record.
• In 1809, Buffon’s student, Jean-Baptiste de
LaMarck, proposed that acquired
characteristics were transmitted to offspring.
Georges-Louis Buffon
Jean-Baptiste de LaMarck
History of Evolutionary Thought
• Acquired characteristics are traits gained
during an organism’s life and not determined
genetically.
• Lamarck’s theory has since been proven
wrong because we know that acquired
characteristics are not inherited; however, it
stimulated evolutionary thought.
Theory of Natural Selection
• In 1858, Charles Darwin and Alfred Wallace
suggested the theory of natural selection as a
mechanism for evolution.
Charles Darwin
Alfred Wallace
Theory of Natural Selection
• The theory of natural selection is the idea
that some individuals whose genetic
combinations favor life in their surroundings
are more likely to survive, reproduce, and pass
on their genes to the next generation than are
individuals who have unfavorable genetic
combinations.
Natural Selection
Assumptions Necessary for the Theory of
Natural Selection
• 1. All organisms produce more offspring than
can survive.
• 2. No two organisms are exactly alike.
• 3. Among organisms, there is a constant
struggle for survival.
Assumptions Necessary for the Theory of
Natural Selection
• 4. Individuals that possess favorable
characteristics for their environment have a
higher rate of survival and produce more
offspring.
• 5. Favorable characteristics become more
common in the species, and unfavorable
characteristics are lost.
Applying Gregor Mendel’s Genetics
Concepts To These Assumptions
• 1. An organisms ability to overproduce results
in surplus organisms.
• 2. Mutation occurs and new genetically
determined traits enter the gene pool. Sexual
reproduction allows for genetically unique
offspring. Meiosis and fertilization allow for
new genetic combinations in every
generation. The genetic information is
expressed in the phenotype of the organism.
Applying Gregor Mendel’s Genetics
Concepts To These Assumptions
• 3. Resources such as food, nutrients, water,
mates, and nest materials are in short supply,
so some individuals do without.
Environmental factors such as disease
organisms, predators, and helpful partnerships
with other species affect survival. All the
environmental factors that affect survival are
called selecting agents.
Applying Gregor Mendel’s Genetics
Concepts To These Assumptions
• 4. Selecting agents favor the individuals that
are most likely to survive and reproduce,
thereby passing their genes on to the next
generation. An organism is selected against if
it has fewer offspring than other individuals
with a more favorable combination of alleles.
The organism does not have to die to be
selected against.
Applying Gregor Mendel’s Genetics
Concepts To These Assumptions
• 5. Alleles or allele combinations that produce
characteristics favorable to survival become
more common in the population and, on the
average, the members of the species will be
better adapted to the environment.
Gregor Mendel
Evolution
• Evolution results when there are changes in
allele frequency in a population.
• Individual organisms cannot evolve, only
populations can.
• The mechanisms that bring evolution about
operate at the level of the individual.
Natural Selection in Evolution
• Natural selection brings about evolution by
selecting which organisms will survive,
reproduce, and pass their genes on to the next
generation.
• These processes affect the phenotype
displayed and not the genes directly.
Factors That Affect How A Species
Changes Over Time
• Environmental Factors
• Sexual reproduction
• The amount of genetic diversity
Environment
• The reproductive success of an individual
within a population is determined by how well
an individual’s characteristics match the
demands of the environment in which it lives.
Fitness
• Fitness is the success of an organism in
passing on its genes to the next generation,
compared with other members of its
population.
Genetic Diversity
• A large gene pool with great genetic diversity
is more likely to contain genetic combinations
to allow some individuals to adapt to a
changing environment.
• Structural, behavioral, biochemical, and
metabolic characteristics all determine how
well an organism can adapt to its
environment.
Common Misconceptions About
The Theory Of Evolution
• Evolution happened only in the past and is not
occurring today.
• Evolution has a predetermined goal, or “it was
meant to be.”
• Changes in the environment cause the
mutations that are needed to survive under
the new environmental conditions.
Common Misconceptions About
The Theory Of Evolution
• Individual organisms evolve.
• Many of the current species can be shown to
be derived from other present-day species
(e.g. apes gave rise to humans).
• Alleles that are valuable to an organisms
survival become dominant.
Common Misunderstandings
About Natural Selection
• 1. “Survival of the fittest.” While individual
survival is important for reproduction, the
more important factor is the number of
descendants an organism leaves.
• 2. “Struggle for life”. This struggle does not
necessarily refer to open conflict and fighting.
Common Misunderstandings
About Natural Selection
• 3. Phenotypic characteristics that are
acquired during life, but not genetically
determined. Acquired characteristics may be
important to an individual’s success; however,
they are not genetically determined and
therefore cannot be passed on to future
generations.
Common Misunderstandings
About Natural Selection
• 4. Relationship between the mechanism of
natural selection (death, reproductive success,
mate choice) and the outcomes of the
selection process. While the effects of natural
selection appear at the population level, the
actual selecting events take place at the level
of the individual organism.
Factors That Influence Natural
Selection
• Genetic Diversity within a species.
• The degree of genetic expression.
• The ability of most species to produce excess
offspring.
Mechanisms That Affect Genetic
Diversity
• Mutation and Migration
• Sexual Reproduction and Genetic
Recombination
Genetic Diversity in Humans
Genetic Diversity in Maize
Spontaneous Mutations
• Spontaneous Mutations are changes in DNA
that cannot be tied to a particular factor.
• Mutations may alter specific genes.
• Cosmic radiation or naturally occurring
mutagenic chemicals might be the cause of
these mutations.
Spontaneous Mutations
• Mutation rates in nature are low (1 in
100,000).
• Most mutations are harmful; however, over
millions of individuals with thousand of genes,
over thousands of generations, beneficial
mutations can occur.
• For mutations to be important, they must be
in cells that give rise to the gametes.
Migration
• Migration is another way in which new
genetic material can enter a population.
• When individuals migrate from one
population to another, they bring alleles that
may be rare or absent.
• When they leave, they can remove alleles
from the population.
Migration
Migration
Sexual Reproduction
• Sexual reproduction creates new genetic
combinations in individuals.
• Sexual reproduction does not generate new
genetic information, but it does allow for the
mixing of genes into new combinations.
Genetic Recombination
• Genetic recombination is the mixing of genes
that occurs when the genes from the male are
intermingled with those from the female.
• Genetic recombination can sometimes occur
between organisms of a different species
(hybridization). This often results in sterile
offspring.
Cama Hybrid
Liger Hybrid
Zeedonk Hybrid
The Role of Gene Expression
• Degrees of expression.
• Why some genes may avoid natural selection.
• Natural selection works on the total
phenotype.
Degrees of Gene Expression
• For genes to be selected for or against, they
must be expressed in the phenotype of the
individual expressing them.
• Penetrance is a term used to describe how
often an allele expresses itself.
• Expressivity is a term used to describe
situations in which the allele is penetrant, but
not expressed equally in an individuals.
Polydactyly
Why Some Genes May Avoid
Natural Selection.
• Some genetic characteristics can be expressed
only during specific periods in the life of the
organism.
• The organism may die before the
characteristic is expressed. Consequently, the
characteristic does not contribute to the
overall fitness of an organism.
Why Some Genes May Avoid
Natural Selection.
• Some genes require environmental triggers
before being expressed.
• Recessive alleles must be present in the
homozygous condition before they can be
expressed.
Natural Selection Works On The
Total Phenotype.
• “Good” genes can be passed on in
combination with “bad” genes.
The Importance of Excess
Reproduction
• Successful organisms reproduce at rates in
excess of that necessary just to replace the
parents when they die.
• High death rates tend to offset high birth rates
and the population size remains relatively
stable.
The Processes That Drive Selection
• Differential Survival
• Differential Reproductive Rates
• Differential Mate Choice – Sexual Selection
Differential Survival
• If a population consists of a large number of
individuals that are genetically and
phenotypically different, some of them will
possess characteristics that make their
survival difficult.
• Some individuals will have characteristics that
make their survival more likely in a given
environment.
Differential Reproductive Rates
• Two organisms may have similar survival rates
and very different reproductive rates.
• The organism with the greater reproductive
rate will be selected for and more of its genes
will be in the subsequent population.
Differential Mate Choice – Sexual
Selection
• Sexual selection occurs within animal
populations when some individuals are
chosen as mates more frequently than others.
Differential Mate Choice – Sexual
Selection
• Those that are more frequently chosen have
more opportunities to pass on more copies of
their genetic information.
• The chosen characteristics could include body
size, aggressiveness, or specific conspicuous
characteristics attractive to the opposite sex.
Sexual Selection – Elephant Seals
Fighting
Sexual Selection
Sexual Selection
Patterns of Selection
• Stabilizing Selection
• Directional Selection
• Disruptive Selection
Stabilizing Selection
• Stabilizing selection occurs when individuals
at the extremes of the range of characteristic
are consistently selected against.
Directional Selection
• Directional selection occurs when individuals
at one extreme of the range of characteristics
are consistently selected for.
• A consistent change in the environment can
call for this kind of selection.
• Changes in allele frequency result from this.
Disruptive Selection
• Disruptive selection occurs when both
extremes of a range for a characteristic are
selected for and the intermediate condition is
selected against.
• This kind of selection occurs when there are
sharp differences in the nature of the
environment where the organisms live.
The Hardy-Weinberg Concept
• English mathematician G.H. Hardy and
German physician Wilhelm Weinberg
developed and applied a simple mathematical
relationship to study gene frequencies.
The Hardy-Weinberg Concept
• Constant gene frequencies over several
generations indicated that evolution is not
taking place at that time.
• Changing gene frequencies indicate that
evolutionary changes were happening.
Necessary Conditions For Gene
Frequencies To Remain Constant
• 1. Mating must be completely random.
• 2. Mutations must not occur.
• 3. The migration of individual organisms into
and out of the population must not occur.
• 4. The population must be very large.
• 5. All genes must have an equal chance of
being passed on to the next generation.
(Natural selection is not occurring.)
Why Hardy-Weinberg Conditions
Rarely Exist
• Random mating does not occur.
• Spontaneous mutations occur.
• Immigration and emigration of individual
organisms are common.
• Populations are not infinitely large.
• Genes are not all equally likely to be passed
on to the next generation.
The Next Phase???