Variation and Inheritance: Mechanisms of Evolution and Mendelian Genetics

Learning Objectives and Historical Gaps in Evolutionary Theory

• By the end of this study guide, students should understand the fundamental mechanisms of evolution, variation, and inheritance.

• Core Objectives:

  • Explain the role of variation and inheritance in evolution using Darwin’s four postulates.

  • Differentiate between discrete and continuous variation with natural population examples.

  • Describe Mendelian mechanisms: Laws of dominance, segregation, and independent assortment.

  • Predict genotypic and phenotypic ratios using Punnett squares (monohybrid and dihybrid crosses).

  • Identify sources of variation: Mutation, recombination, and crossing-over.

  • Distinguish Mendelian inheritance from complex patterns: Co-dominance, incomplete dominance, pleiotropy, epistasis, and polygenic traits.

  • Evaluate environmental influences on phenotype, including maternal effects and genotype-environment interactions.

  • Define and calculate heritability using the formula $h^2 = \frac{V_g}{V_p}$.

  • Analyze experimental designs like common garden and cross-fostering studies.

  • Apply concepts to real-world scenarios such as antibiotic resistance, pesticide resistance, and human genetic disorders.

• Historical Gaps in Knowledge:

  • In early evolutionary theory, two major questions remained unanswered:

  1. By what specific mechanism does inheritance occur?

  2. How is variability generated within populations?

Darwin’s Four Postulates of Natural Selection

• Darwin proposed four key postulates as the framework for natural selection:

  1. Variation: Individuals within species are variable.

  2. Inheritance: Some of these variations are passed on to offspring.

  3. Differential Survival: In every generation, more offspring are produced than can survive.

  4. Extinction/Selection: The survival and reproduction of individuals is not random. Those who survive and reproduce possess the most favorable variations.

• Significance of Variation:

  • Variation is the essential "raw material" for evolution by natural selection.

  • Natural selection acts as a sorting process of differential survival and reproduction.

  • Without variation, evolution cannot occur.

Types and Causes of Variation

• Discrete Variation:

  • Involves multiple distinct forms within a species, often called polymorphism.

  • Examples include specific morphs or distinct traits (e.g., specific plumage color variations represented as $F01$ through $Fs4$ in data charts).

• Continuous Variation:

  • Traits show a range of values rather than distinct categories.

  • Examples:

  • Beak depth in Galapagos finches ($Geospiza \, fortis$); study size $N = 751$, with depths ranging from approximately $6\,mm$ to $14\,mm$.

  • Height of males in a biology class ($N = 30$), ranging from approx. $60$ to $80$ inches.

• Causes of Variation:

  • Genetic Factors: Inherited DNA sequences.

  • Environmental Factors: Conditions in which an organism is raised (e.g., different actors like Sean Connery or Roger Moore having different physical statures and traits).

Theories of Genetic Transmission

• Darwin's Model (Pangenesis):

  • Darwin lacked an accurate mechanism for heredity and favored pangenesis, which suggested all cell lines contribute to gametes. This implied that environmental influences on parents could be inherited.

• Weismann’s Germ-plasm Theory:

  • Proposed the distinction between germ cells (involved in reproduction) and somatic cells (body cells). Only germplasms contribute to the next generation, contradicting pangenesis.

Gregor Mendel and the Mechanics of Inheritance

• Gregor (né Johann) Mendel (1822-1884):

  • Educated in the Institute of Philosophy, Olomuc; joined the Augustinian Order in Brno.

  • Influenced by Darwin’s work on domestication, his objective was to deduceed the law according to which variations appear in successive generations.

• Mendel's Core Contributions:

  • Dominance: One trait can mask another.

  • Segregation: Alleles separate during gamete formation.

  • Independent Assortment: Traits are inherited independently of one another.

• Mendelian vs. Blending Inheritance:

  • Blending Theory: Proposed that offspring are a mix of parents, causing rare variants to be "blended out" and diluted from the population.

  • Mendelian Theory: Traits are particulate. Rare variants persist and can become established or "fixed" in a population. Rare allele "$A$" can survive through heterozygotes ($Aa$) and redistribute to homozygotes ($AA$).

Experimental Methodology: The Pea Plant

• Choice of Subject: Common Pea ($Pisum \, sativum$).

  • Readily available, self-pollinating, and true-breeding.

  • Mendel obtained $34$ cultivars displaying seven selected characteristics.

  • Characteristics Included:

  • Form of ripe seeds (wrinkled vs. round).

  • Form of seed albumin (green vs. yellow).

  • Position of flowers (terminal vs. axillary).

  • Length of stem (tall vs. dwarf).

• Design:

  • Used $22$ uniform cultivars crossed over generations.

  • Examined single isolated traits rather than complex hybrids.

  • Utilized large replicate numbers to minimize the influence of chance on mathematical power.

Laws of Inheritance

• Law of Dominance:

  • Characters transmitted entire or unchanged in the hybrid are "dominant"; those that become latent or hidden are "recessive."

  • Evidence: Crossing uniform cultivars resulted in offspring that resembled only one parent ($100\%$ dominance).

• Molecular Basis of Dominance (Examples):

  • Huntington’s Chorea (Dominant): Caused by a mutant gene with extra $CAG$ repeats leading to an abnormal protein with extra Glutamine. The toxic protein disrupts nerve function even if only one allele is present.

  • Tay-Sachs Disease (Recessive): Caused by a mutation of the $HEXA$ gene on chromosome $15$, leading to a defective enzyme (hexosaminidase A) that fails to degrade gangliosides. If one normal allele is present, enough enzyme is produced to prevent accumulation.

• Law of Segregation:

  • The two members of a gene pair segregate randomly and equally into gametes.

  • Results from Mendel's first-generation hybrids ($F1$ self-pollinated):

  • Seed Shape: $5474$ round to $1800$ wrinkled ($2.96:1$ ratio).

  • Seed Color: $6022$ yellow to $2001$ green ($3.01:1$ ratio).

  • Four Parts of Segregation:

  1. Inheritance is determined by "factors" (genes) passed unchanged.

  2. Individuals inherit one unit from each parent per trait.

  3. Segregation into gametes is random and equal.

  4. Traits may be latent but still passed to next generations.

• Law of Independent Assortment:

  • Characteristics are inherited independently of each other.

  • Tested via Dihybrid Crosses:

  • Parentals: $RRYY$ (round, yellow) x $rryy$ (wrinkled, green).

  • $F1$ Offspring: $RrYy$ (round, yellow).

  • $F2$ Generation: Yielded phenotypic ratio of $9:3:3:1$ ($315\,round/yellow$, $108\,round/green$, $101\,wrinkled/yellow$, $92\,wrinkled/green$).

Key Figures in Genetics

• Reginald C. Punnett (1875-1967):

  • "Father" of genetics as a scientific discipline.

  • Combined Mendel’s laws with statistics and created the Punnett Square method.

• William Bateson (1861-1926):

  • Translated Mendel’s work into English ($1901$).

  • Coined terms: "genetics," "allelomorphs" (alleles), "heterozygote," and "homozygote."

Sources of Genetic Variation

• Mutation:

  • The ultimate source of all heritable variation.

  • Point Mutations: Change in a single nucleotide.

  • Transition: Purine to Purine or Pyrimidine to Pyrimidine.

  • Transversion: Purine to Pyrimidine.

  • Sickle Cell Anemia: Substitution of Thymine ($T$) for Adenine ($A$) in the B-globin gene, changing glutamic acid to valine in the 6th amino acid position.

  • Unequal Crossing-over: Alters larger sections of DNA.

• Recombination: Shuffles existing variation into new phenotypic combinations.

• Nature of Mutation:

  • Mutations are random with respect to adaptation. Most are nonadaptive or maladaptive.

  • Mutation rates ($\mu$) are influenced by "sloppy copying" and DNA degradation.

• Bacterial and Pesticide Resistance:

  • Mutations for resistance occur randomly. Their spread through populations is not random but a predictable response to environmental selection (e.g., antibiotic application).

Non-Mendelian Inheritance and Complexity

• Co-dominance: Both alleles are expressed equally. Example: Blood types ($I^A I^B$ results in type $AB$).

• Incomplete Dominance: Alleles "blend" to create a third phenotype. Example: Red and white flowers producing pink offspring.

• Pleiotropy: A single gene affects two or more unrelated traits. Example: Holt-Oram syndrome (hand and heart malformations due to defective $TBX5$ allele).

• Epistasis: One gene interferes with the expression of another. Example: Albinism ($cc$) suppresses the phenotype of all other coloration genes because melanin production is disrupted.

• Polygenic Traits: Phenotype determined by interaction of multiple genes.

  • Example: Eye color can involve up to $16$ genes; accurate prediction is possible using six specific SNPs (Single-Nucleotide Polymorphisms).

Environmental and Maternal Influences

• Environmental Effects:

  • Determine the degree to which genetic "potential" is realized (e.g., maximum stature).

  • Sex Determination: In some species (e.g., turtles), nest temperature determines offspring sex.

• Maternal Effects:

  • Environmental features attributable to the mother: Womb/egg environment, yolk volume, and post-birth care.

• Evolutionary Note: Variation due strictly to environmental effects is not a factor in evolution.

Experimental Designs for Decomposing Variation

• Controlled Crosses: Validating Mendelian expectations.

• Common Garden Experiments: Raising different genotypes under identical conditions to isolate genetic effects.

• Cross-Fostering: Offspring are reared by non-biological parents.

  • Example: Testing hunting prowess in families. If prowess correlates with the Birth Family, it suggests genetic control. If it correlates with the Foster Family, it suggests learned behavior.

Quantitative Genetics and Heritability

• The Phenotype Equation:

  • $V_p = V_g + V_e$

  • Where $V_p$ = phenotypic variation, $V_g$ = genotypic variation, and $V_e$ = environmental variation.

• Heritability ($h^2$):

  • The proportion of phenotypic variation attributable to genotype: $h^2 = \frac{V_g}{V_p}$.

  • High heritability ($h^2$ near $1$) indicates high fidelity of transmission and a strong response to selection.

• Heritability and Fitness:

  • Traits tightly correlated with fitness (e.g., age at first birth) often have lower heritability ($h^2 \approx 0.08$ to $0.11$) because selection has already reduced genetic variation (mutations are purged).

  • Traits uncorrelated with fitness (e.g., height, age at menopause) have higher heritability ($h^2 \approx 0.56$ to $0.75$).

• Case Study: $Parus \, major$ (Great Tit) has an $h^2 = 0.3$ for "time of egg laying." Selection acts on the genetic component of this variation to favor female readiness.