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

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 sapienschimpanzees = Pan troglodyteslowland gorillas = Gorilla gorillasaber-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

Modern Cladistics compares animals based on:

1. Analogies = anatomical features with similar functions

2. 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 At first, the fossilized remains of

animals found in Europe were interpreted according to religious doctrine.

These interpretations relied on several assumptions:

1. Fixity of Species (species were created only once and did not change over time)

2. 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 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:

1. 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

The Discovery of Evolution

Darwin’s Theory of Evolution by Natural Selection:

1. All species display a range of variation, and all have the ability to expand beyond their means of subsistence.

2. 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.

3. As generation succeeds generation, nature selects the most advantageous variations, and species evolve.

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!!!

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

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 chromosomesor 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 AA, AOB BB, BOAB ABO 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)

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.

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

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 DriftGene Flow Natural Selection

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

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

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

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