Download brown hair

The National Academy of Sciences
In everyday usage: "theory" often refers to a
hunch or a speculation. The word, “theory” is
often used when drawing a conclusion based
on inconclusive evidence.
The formal scientific definition of theory:
It refers to a comprehensive explanation of some
aspect of nature that is supported by a vast
body of evidence.
National Academy of Sciences
“Scientific theories are so well-established that no new evidence is
likely to alter them substantially.”
• Example: No New Evidence will Demonstrate:
–
–
–
–
that the Earth does not orbit around the sun (heliocentric theory)
that living things are not made of cells (cell theory)
that matter is not composed of atoms (atomic theory)
that the surface of the Earth is not divided into solid plates that have
moved over geological timescales (the theory of plate tectonics)
• How does this relate to the theory of evolution?
– It is supported by so many observations and confirming
experiments that the basic components of the theory will not
be overturned by new evidence.
– However, like all scientific theories, the theory of evolution is
subject to continuing refinement as new areas of science
emerge or as new technologies enable observations and
experiments that were not possible previously.
Micro-Evolution
Chapter 17
• Individual organisms DO NOT evolve.
• Populations of organisms do.
• Micro-Evolution is one way that
evolution occurs.
• Micro-Evolution simply refers to a
change in allelic frequency within a
population of organisms
• Population – a group of individuals belonging to the
same species, occupying the same given area
• They exhibit variation among the individual members,
but they also hold certain morphological (structural),
physiological (functional), and behavioral traits in
common.
– Alleles – each gene exists in 2 or more slightly
different molecular forms. Offspring inherit the
parent’s alleles and express them in their
phenotypes.
B
b
Bb
Chromosome # 1
Chromosome # 1
Inherited from mom
Inherited from dad
Gene for
Hair Texture
Mom’s allele codes for
Straight hair
Gene for
Hair Texture
Dad’s allele codes for
Curly hair
Picture all of the alleles of a population as being together
in a large pool called a gene pool.
The percentage of any specific
allele in the gene pool is called
the allelic frequency.
Look around the room….
What is the gene pool for hair
color???
What is the allelic frequency
of people with blonde hair?
5 Factors that affect the mix of alleles
during gamete formation and
fertilization
• Gene mutations create new alleles
• Crossing over and genetic recombination are
normal results of meiosis (sex cell creation).
• Independent assortment of chromosomes occurs
in meiosis (when sex cells are created).
• Fertilization between genetically varied gametes
produces “new” combinations of genes
• Abnormal changes in chromosome structure or
number can occur.
• Mutations – heritable changes in DNA that can
alter gene expression.
• Mutations are random and the phenotypic
outcome may be neutral, beneficial, harmful, or
lethal to the individual depending on other
interactions
The Effect of Mutations
• Lethal Mutation an
expression of a gene
that results in death
• Neutral Mutation
have no effect on
survival and
reproduction
regardless of whether
or not they are
expressed in a
phenotype
• Beneficial Mutations
provide some survival
advantage
Mutation: Remix
• Mutations are the ONLY source of NEW alleles.
• This is the genetic foundation for biological
diversity.
Recombination & Crossing Over
Chromosomal crossover - recombination between the
paired chromosomes inherited from each of one's
parents, generally occurring during meiosis.
Homologous sites on two chromatids can mesh with
one another, and may exchange genetic information
Crossing-Over
Crossing over can occur at any location on a chromosome, and
it can occur at several locations at the same time.
This causes the traits found in each gamete varied. This is
why siblings of the same parents can look so different from
each other.
FROM
MOM
This
chromosome
inherited by
mom could code
for straight hair,
brown hair, and
green eyes…
FROM
DAD
After crossing-over
This
chromosome
inherited by dad
could code for
curly hair,
black hair, and
brown eyes…
Now one chromosome
passed on to offspring
could have traits from
both homologous… curly
hair, brown hair, and
green eyes…
The Law of Independent Assortment
genes for different traits—(for example, eye
shape and eye color)—
are inherited independently of each other.
bb
Bb
rr
RR
Bb
Rr
Fertilization
Zygote
Chromosomal Abnormalities
Micro-Evolution
• Those factors lead to variation and change in
alleles before birth.
• Change in allele frequencies within a
population can also be brought about by
non-genetic factors: genetic drift, gene flow,
and natural selection.
Genetic Drift
• Genetic Drift – the random fluctuation in
allele frequencies over time, due to chance
occurrences alone
• It is when few individuals rebuild a population
• It increases the chance of any given allele
becoming more or less prevalent
• Bottleneck Effect – When a disturbance
greatly reduces the size of a population leaving
a few individuals to reestablish the population
•Disease/Pollution
• Habitat loss
• Predation/Hunting
Deforestation in the Amazon
• Founder Effect – a few individuals leave the
original population to establish a new one. They
may or may not carry phenotypes that are
common to other members of the original
population
Speciation may occur rapidly due to chance allelic frequencies of the
“founder” organisms that differ from the original population.
Inbreeding: Possible bad side to genetic drift
• Mating among closely related individuals
• It tends to increase deleterious
homozygous recessive conditions, thus
leading to lower viability and survival rates.
Reduced fertility both in litter size
and sperm viability
Increased genetic disorders
Fluctuating facial asymmetry
Higher infant mortality
Slower growth rate
Smaller adult size
Loss of immune system function
The problem that all endangered or
threatened species encounter is this
tendency to be forced to “inbreed”.
When there are only 100 of a species left,
and they are forced to inbreed…then the
tendency is for genetic diversity to
diminish even further.
So what??? Why is genetic diversity
important?
Another consequence of inbreeding in
small populations is that recessive and
possibly fatal combinations of alleles
becomes greater.
Ethiopian Wolf
Genetic diversity gives
species the chance to
evolve. It provides
enough variation for
possible positive
evolutionary outcomes..
The smaller the
population, the less likely
that is to occur.
Gene Flow
• Genes move with individuals when they
move (emigrate or immigrate) into and out of
a population…and it changes the gene pool
Essence of Darwin’s Ideas
(1) Variation exists in natural populations
(2) Many more offspring are born each season
than can possibly survive to maturity
(3) As a result, there is a struggle for existence
- competition
(4) Characteristics beneficial in the struggle
for existence will tend to become more
common in the population, changing the
average characteristics of the population
over time.
Natural Selection
• Accounts for more changes in allele
frequencies than any other micro-evolutionary
process
• When organisms with the most beneficial traits
in a population are the ones that survive and
reproduce, and pass those traits on…