Genetics%20and%20Evolution

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Chapter 2

• Genetics and Evolution

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Chapter Preview

• What Is Evolution?
• What Is the Molecular Basis for Evolution?
• What Are the Forces Responsible for Evolution?

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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

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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.

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The Great Chain of Being An Example

• God
• Angels
• Humans
• Birds
• Terrestrial Animals
• Plants
• Rocks

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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)

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The Classification of Living Things after the
1700s

• Linnaeuss 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

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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

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The Classification of Living Things after the
1700s

• Modern Cladistics compares animals based on
• Analogies anatomical features with similar
  functions
• Homologies anatomical features evolved from a
  common ancestral form

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Visual Counterpoint Class
Discussion

• Using cladistics, are the wings of birds and
  butterflies analogies or homologies?

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Visual Counterpoint Class
Discussion

• Using cladistics, are the wings of bats and hands
  of humans analogies or homologies?

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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

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The Discovery of Evolution

• 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

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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)

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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

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The Discovery of Evolution

• An Example of Lamarcks 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.

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The Discovery of Evolution

• Sir Charles Lyell developed the idea of
  uniformitarianism
• Argued that the major features on earths 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.

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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

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The Discovery of Evolution

• Darwins Precursors
• Lamarcks theory of evolution recognized that
  species did change (but was wrong about how)
• Lyells 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

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The Discovery of Evolution

• Darwins 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.

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It was assumed that all offspring had a
mixture of parental traits.
DURING DARWIN'S TIME THE ORIGINS OF THE
INCIVIDUAL TRAITS UPON WHICH NATURAL SELECTION
ACTS WAS NOT KNOWN!!!
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Gregor Mendel and The Science of Heredity

• 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
• Mendels work gave rise to science of genetics

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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

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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

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Heredity and the Molecular Basis for Evolution
Genes (Mendels 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
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Humans have 23 pairs of Chromosomes
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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
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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)
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DNA cannot leave the cells nucleus.
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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.

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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).

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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).

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Protein Synthesis
The RNA travels to the ribosomes, the cellular
structure where translation of the directions
found in the codons into proteins occurs.
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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.

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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.

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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.

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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.

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Meiosis Sex Cell Production
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Meiosis Sex Cell Production

• Following Mendels 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.

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Phenotype and Genotype

• A persons phenotype are the traits that are
  visible or observable.
• A persons 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.

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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

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Mendel's Law of Dominance

• Best Example human blood groups
• 4 Phenotypes 6 Genotypes
• A AA, AO
• B BB, BO
• AB AB
• O OO
• Which alleles for human blood types are dominant,
  codominant, and recessive?

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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
  Mendels Laws of Segregation and Independent
  Assortment.
• How would it work for human blood types?

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Punnett Squares For Class Discussion

• Try using a simple punnett square for one
  generational cross between people with the
  following Mendelian traits
• 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)

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Polygenetic Traits

• Mendels 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

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Evolution, Population, and Individuals

• Evolution acts on individual traits but
  individuals do not evolve, only populations
  evolve (over an extended period of time)
• Populations that can still produce fertile
  offspring are still considered part of the same
  species
• Such populations have a distinctive gene pool or
  all of the genetic variation possessed by
  individuals in the population.

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Evolution, Population, and Individuals

• Over time, changes in the frequency of alleles
  among different populations lead to noticeable
  differences in the phenotypes of the members of
  these populations.
• 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

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The Hardy-Weinberg Principle

• Demonstrates that there will be no changes in a
  populations allele frequencies over time, if
• mating is entirely random
• the population is sufficiently large for
  statistical averages to express themselves
• no new variants are introduced into the
  populations gene pool
• 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!

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The Forces of Microevolution

• Mutation
• Genetic Drift
• Gene Flow
• Natural Selection

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Mutation

• Random genetic change occurring during mitosis or
  meiosis
• Can be beneficial, harmful, or not noticeable
• Mutagens or chemical in the environment can
  increase the chances of mutation
• Without the variation brought in through random
  mutations, populations cannot change over time in
  response to changing environments

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Genetic Drift

• 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

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Gene Flow

• Changes in the allele frequencies of a population
  due to the infusion of genetic material through
  interbreeding with another population
• Among humans, social factors like mating rules,
  intergroup conflict, and our ability to travel
  great distances can affect gene flow

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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 ones genes

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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
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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.
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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.
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Sickle Cell Anemia and Malarial Environments