Transcript 0495810843_246858

Chapter 2
Genetics and Evolution
Chapter Preview

What Is Evolution?

What Is the Molecular Basis for
Evolution?

What Are the Forces Responsible for
Evolution?
Creation vs. Evolution
All cultures have stories and myths
about the creation of the world and
human beings
 Evolution differs from these creation
stories by offering consistent and
testable explanation for the origins
and diversity of life

The Classification of Living
Things before the 1700s

Great Chain of Being – a classification
system developed by Aristotle that grouped
living and non-living things into groups
based on similarity.

Each group had a “primate” or best example
for the group.

The groups were organized in a hierarchy,
from inferior to superior.
The Great Chain of Being:
An Example
God
Angels
Humans
Birds
Terrestrial Animals
Plants
Rocks
The Classification of Living
Things after the 1700s

Linnaean Classification – a classification
system developed Carl Von Linné that
grouped living things into groups based on
similarity of form, function, and growth.

Placed humans among the primates (also
including apes, monkeys, and prosimians)
and mammals (animals having fur or hair
and who suckle their young)
The Classification of Living
Things after the 1700s
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Linnaeus’s system relied on a Binomial
Nomenclature which organized living
things into species (reproductively isolated
populations) and genera (singular –
genus), a more inclusive grouping of similar
organisms
Examples of genus/species:
humans = Homo sapiens
chimpanzees = Pan troglodytes
lowland gorillas = Gorilla gorilla
saber-toothed tigers = Smiladon fatalis
The Classification of Living
Things after the 1700s

Like Linnaeus, modern Taxonomy
uses body structure, body function
and patterns of growth but also
examines genetic material and protein
structures to make classifications
The Classification of Living
Things after the 1700s

1.
2.
Modern Cladistics compares
animals based on:
Analogies = anatomical features
with similar functions
Homologies = anatomical
features evolved from a common
ancestral form
Visual Counterpoint:
Class Discussion

Using cladistics, are the wings of birds and butterflies
analogies or homologies?
Visual Counterpoint:
Class Discussion

Using cladistics, are the wings of bats and hands of
humans analogies or homologies?
Visual Counterpoint:
Class Discussion

Are the anatomical features of the following
organisms analogies or homologies?
1. dolphin and shark morphology
2. bat and bird wings
3. primate opposable thumbs and panda
“thumbs”
4. seal flippers and human hands
The Discovery of
Evolution
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
1.
2.
At first, the fossilized remains of
animals found in Europe were
interpreted according to religious
doctrine.
These interpretations relied on
several assumptions:
Fixity of Species (species were
created only once and did not
change over time)
The Great Chain of Being
The Discovery of
Evolution
Early interpretations, relying on the
notion of fixity of species, argued that
fossil animals had become extinct.
 For example, George Cuvier invoked
catastropes like the Great Flood of the
Book of Genesis to explain the
existence of extinct animals such as
mammoths (catastrophism)

The Discovery of
Evolution
Jean-Baptiste Lamarck, was among
the first to suggest an “evolutionary”
mechanism to account for the
diversity of living creatures
 His theory of the inheritance of
acquired traits proposed that
intentional behavior on the part of
individuals brought about changes in
the form of entire species

The Discovery of
Evolution
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An Example of Lamarck’s Theory:
The first giraffe gained its long neck by
stretching to reach the leaves on the
highest tree top branches and in turn
passed this acquired long neck onto its
offspring.
The Discovery of
Evolution

Sir Charles Lyell developed the idea
of uniformitarianism

Argued that the major features on
earth’s surface (ex: mountains and
canyons) through the gradual
accumulation of minute changes,
brought about by the same natural
processes, such as erosion, that are
observable today.
The Discovery of
Evolution

A Major Consequence of
Uniformitarianism:
The time depth required for these changes
was not compatible with literal
interpretations of the Bible in which the
earth is said to be six thousand or so years
old
The Discovery of
Evolution
Darwin’s Precursors:
Lamarck’s theory of evolution recognized
that species did change (but was wrong
about how)
2.
Lyell’s uniformitarianism expanded the
age of the earth (allowing more time for
evolution to happen)
3. Malthus observed that animals produce
many offspring but not all of them live to
maturity
1.
The Discovery of
Evolution

1.
2.
3.
Darwin’s Theory of Evolution by Natural
Selection:
All species display a range of variation, and all have the
ability to expand beyond their means of subsistence.
In their “struggle for existence,” organisms with variations
that help them to survive in a particular environment will
reproduce with greater success than those without them.
As generation succeeds generation, nature selects the
most advantageous variations, and species evolve.
DURING DARWIN'S TIME THE ORIGINS OF THE
INCIVIDUAL TRAITS UPON WHICH NATURAL
SELECTION ACTS WAS NOT KNOWN!!!
It was assumed that all offspring had a mixture
of parental traits.
Gregor Mendel and The
Science of Heredity
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Experimented with plant pollination to
establish the laws of heredity
Mendel discovered that inheritance was
particulate, rather than blending as
Darwin thought
Ironically, Darwin had his 1866 paper but
did not read it
Mendel’s work gave rise to science of
genetics
Mendel's Law of Segregation
• During reproduction, the genes
governing the expression of a trait
will be separated and keep their
individuality
• They will be passed on to the
next generation, unaltered
• Today, we know this is due to meiosis
Mendel's Law of Independent
Assortment
•During reproduction, each parent donates
segregated genes
• In the offspring, these segregated genes
recombine in a random manner and
independently from one another
•Thus, individual traits are inherited
independently
Heredity and the Molecular
Basis for Evolution
Genes = (Mendel’s particles) a section of DNA which
codes for the production of a specific protein
DNA = Deoxyribonucleic Acid (limited to the nucleus
of a cell)
Chromosomes = compacted and coiled DNA
(usually occurs when a cell divides)
Alleles = variants of a gene that occur in the same
location on a chromosome or DNA molecule
Humans have 23 pairs of
Chromosomes
Humans have 23 pairs of
Chromosomes
Humans have 23 pairs of chromosomes
or a total of 46.
Cells with all 46 (23 pairs) are known as diploid
Cells with only 23 (no pairs) are known as
haploid
The DNA Molecule
DNA looks like two strands of a
rope twisted around each other
with ladderlike steps between the
two strands
Alternating sugar and phosphate
molecules form the backbone of
these strands connected to each
other by four base pairs: adenine
(A), thymine (T), guanine (G), and
cytosine (C).
Connections occur between
complementary pairs of bases
(A to T; G to C)
DNA cannot leave the cell’s
nucleus.
Not All DNA Occurs In the
Nucleus
• Mitochondrial DNA (mtDNA) found in the
mitochondria of animal cells (does not code for
any physical traits but can be used to examine
genetic relationships to others in a population).
• Retroviruses do not have DNA but consist of
RNA molecules.
Protein Synthesis
• Groupings of three base pairs (codons) code
for particular amino acids.
• A gene is nothing more than a series of codons
which tell cells which amino acids to make in
order to produce a protein (this process is known
as protein synthesis).
Protein Synthesis
• Because DNA cannot leave the nucleus of a
cell, the directions for a specific protein are first
converted into ribonucleic acid or RNA in a
process called transcription.
• RNA differs from DNA in the structure of its
sugar phosphate backbone and in the presence
of the base uracil (U) rather than thymine (T).
Protein Synthesis
The RNA travels to
the ribosomes, the
cellular structure
where translation of
the directions found
in the codons into
proteins occurs.
Mitosis - Cell Division
• In order to grow, maintain good health, and
heal, the body cells of an organism must
divide and produce new cells.
• Cell division is initiated when the
chromosomes form a second pair that
duplicates the original pair of chromosomes
in the nucleus.
Mitosis - Cell Division
• The DNA “unzips” between the base pairs
(adenine from thymine and guanine from cytosine)
• Afterwards, each base on each now-single
strand attracts its complementary base,
reconstituting the second half of the double helix.
• Each new pair is then surrounded by a
membrane and becomes the nucleus that directs
the activities of a new cell.
Meiosis – Sex Cell
Production
• Sexual reproduction actually increases genetic
diversity in a species.
• However, if two regular body cells, each
containing 23 pairs of chromosomes, were to
merge, the result would be a new individual with 46
pairs of chromosomes, followed by individuals with
up to 92 pairs of chromosomes in the next
generation and so on. These individuals would not
live.
Meiosis – Sex Cell
Production
• To solve this problem, meiosis begins like mitosis,
with the replication and doubling of the original
genes in chromosomes through the formation of
sister chromatids, but it proceeds to divide that
number into four new cells rather than two.
• Each resulting sex cell (sperm and ova) has only
half the number of chromosomes compared to the
parent cell.
Meiosis – Sex Cell
Production
Meiosis – Sex Cell
Production
• Following Mendel’s law of segregation, the alleles
from the parent chromatid are separated.
•In homozygous individuals with identical alleles,
the sex cells have the same alleles.
•In heterozygous individuals with different alleles,
half the sex cells will one allele, the other half will
have a different allele for the same trait.
Phenotype and Genotype
• A person’s phenotype are the traits that are
visible or observable.
• A person’s genotype or genetic composition can
never be fully predicted because of the segregation
and independent assortment of genes and alleles.
• In addition, during meiosis corresponding portions
of one chromosome may “cross over” to the other
one, somewhat scrambling the genetic material
compared to the original chromosomes.
Mendel's Law of Dominance
• Not all of the genes/alleles present in an organism
(the genotype) will be expressed physically (the
phenotype).
• Some genes/alleles are recessive and will not be
expressed in the presence of dominant genes or
alleles
• In some cases, genes/alleles may be codominant
with others and both will be expressed
Mendel's Law of Dominance
• Best Example = human blood groups
4 Phenotypes:
6 Genotypes:
A
B
AB
O
AA, AO
BB, BO
AB
OO
Which alleles for human blood types are dominant,
codominant, and recessive?
Punnett Squares
• A method for measuring
the probability of a certain
genotype appearing
based on the crossing of
two organisms with
known genotypes. Based
on Mendel’s Laws of
Segregation and
Independent Assortment.
• How would it work for
human blood types?
Punnett Squares: For Class
Discussion
•
Try using a simple punnett square for one
generational cross between people with the
following Mendelian traits:
1. Tongue rollers – homozygous for the dominant
trait (TT) or heterozygous (Tt); non-tongue rollers
are homozygous for the recessive trait (tt)
2. Dwarfism – homozygous for the dominant trait
(DD); non-dwarfs are homozygous for the
recessive trait (dd) or are heterozygous (Dd)
Polygenetic Traits
• Mendel’s laws work best for Mendelian traits –
physical traits coded by one gene (with multiple
alleles)
• Most human traits (like height and skin color) are
polygenetic and are coded on several genes
Evolution, Population, and
Individuals

Evolution acts on individual traits but individuals do not
evolve, only populations evolve (over an extended period of
time)
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Populations that can still produce fertile offspring are still
considered part of the same species
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Such populations have a distinctive gene pool or all of the
genetic variation possessed by individuals in the
population.
Evolution, Population, and
Individuals
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Over time, changes in the frequency of alleles among
different populations lead to noticeable differences in the
phenotypes of the members of these populations.
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Eventually, the genetic and phenotypic differences will
result in reproductive isolation – the populations will no
longer be able to interbreed.

So how do populations in a species accumulate enough
genetic and phenotypic differences to be considered
different species? ANS - Micorevolution
The Hardy-Weinberg Principle
Demonstrates that there will be no changes in a population’s allele
frequencies over time, if:
1.
mating is entirely random
2.
the population is sufficiently large for statistical averages to
express themselves
3.
no new variants are introduced into the population’s gene
pool
4.
all individuals are equally successful at surviving and
reproducing
However, none of these conditions ever applies to populations of
living things so allele frequencies do change over time!
The Forces of
Microevolution
Mutation
Genetic
Drift
Gene Flow
Natural
Selection
Mutation

Random genetic change occurring during
mitosis or meiosis
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Can be beneficial, harmful, or not noticeable
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Mutagens or chemical in the environment
can increase the chances of mutation
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Without the variation brought in through
random mutations, populations cannot
change over time in response to changing
environments
Genetic Drift
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Chance changes in the allele
frequencies of a population due to
accidents or other events

Result in greater changes when
populations are small or isolated = the
founder effect
Gene Flow

Changes in the allele frequencies of a
population due to the infusion of
genetic material through interbreeding
with another population
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Among humans, social factors like
mating rules, intergroup conflict, and
our ability to travel great distances
can affect gene flow
Natural Selection

Natural selection refers to the evolutionary
process through which genetic variation at
the population level is shaped to fit local
environmental conditions.

Natural selection ≠ survival of the fittest in
the sense meant by Herbert Spencer

Best measured through reproductive success mating and production of viable offspring
who will in turn carry on one’s genes
Adaptation and Physical
Variation
Evolution often involves balancing the
beneficial and harmful effects of a
specific allele.
Such is the case with sickle-cell anemia
Sickle Cell Anemia
A painful disease in which the oxygen-carrying
red blood cells change shape (sickle) and clog
the finest parts of the circulatory system
Originated as a mutation.
Sickle Cell Anemia
Individuals who are
homozygous for the sickle cell
trait frequently die at a young
age.
Individuals who are
homozygous for normal red
blood cells are more susceptible
to malaria.
Individuals who are
heterozygous for the sickle cell
trait enjoy some protection from
malaria but risk creating a
homozygous offspring.
Sickle Cell Anemia and
Malarial Environments