Anthro

Heredity and Biological Evolution Notes

Overview

  • Course: Anthropology 2200

  • Big Question: Where does biological variation come from?

Early Theories of Inheritance

  • Preformation Hypothesis: Early concept suggesting that organisms develop from miniature versions of themselves.

  • Blending of Fluids: Proposed that offspring are a blend of parental traits.

    • Problems:

    • Not enough variation for selection to act on.

    • No mechanism for introducing new variation; thus, questioning how new species could evolve.

Mechanisms of Inheritance

  • Gregor Mendel (1822-1884):

    • Conducted foundational work in genetics.

    • Laws of Inheritance:

    • Opposed blending inheritance.

    • Identified genes as discrete units of inheritance.

    • Introduced alleles as different versions of a gene.

    • Defined traits as dominant or recessive.

Examples of Traits in Peas

  1. Seed color: Yellow (dominant) vs. green (recessive).

  2. Seed shape: Round (dominant) vs. wrinkled (recessive).

  3. Pod shape: Inflated (dominant) vs. constricted (recessive).

  4. Flower color: Purple (dominant) vs. white (recessive).

  5. Plant height: Tall (dominant) vs. short (recessive).

Generational Outcomes

  • F₁ Generation: All offspring exhibit the dominant yellow seed trait.

  • F₂ Generation: Ratio observed is 3 yellow: 1 green.

Genetics Terminology

  • Blending vs. Particulate Inheritance:

    • Blending: Traits mix, leading to uniformity.

    • Particulate: Traits are transmitted as distinct units.

DNA - The Molecular Basis of Evolution

  • DNA (Deoxyribonucleic Acid):

    • Traits coded by genes located on chromosomes.

    • Discovered structure in 1953 by Watson, Crick, and Franklin.

Structure of DNA

  • Components:

    • Sugar,

    • Phosphate,

    • Nucleotide Bases:

    • Adenine (A), Thymine (T), Guanine (G), Cytosine (C).

    • Base pairing: A with T, C with G.

DNA Properties

  1. Stability: Preserves genetic message.

  2. Replicability: Ensures inheritance.

Steps of DNA Replication

  • Unzipping of DNA: Enzymes break hydrogen bonds between nitrogen bases to form parent template strands.

  • Complementary Strands: New strands are synthesized using complementary base pairing with free nucleotides.

  • Completion: Results in two DNA molecules—each with one parent strand and one new strand.

Function of DNA

  • Role in Coding for Proteins:

    • Proteins serve as the building blocks of biological structures.

    • Involvement in cellular processes via enzymatic proteins.

  • Flow of Genetic Information:
    \text{DNA} \rightarrow \text{mRNA} \rightarrow \text{tRNA} \rightarrow \text{amino acid} \rightarrow \text{protein/polypeptide}

Genetics Overview

  • Humans' Genome:

    • Contains approximately 3 billion base pairs.

    • About 1%-1.5% consist of genes, totaling around 20,000-25,000 genes.

  • Gene: A sequence of DNA coding for a protein.

  • Locus: Specific location of a gene on a chromosome.

  • Allele: Variations of a gene leading to different trait expressions.

  • Phenotype: Observable characteristics.

  • Genotype: Genetic makeup of an individual.

Chromosomal Structure and Cell Division

  • Chromosome Count: Humans have 46 chromosomes (23 pairs).

  • Cell Division Modes:

    • Mitosis: Produces two daughter cells.

    • Meiosis: Leads to gametes with half the chromosome number (23 single-stranded).

Mendel's Laws of Inheritance

  • Law of Segregation: During gamete formation, alleles for a trait segregate from each other so that offspring acquire one trait from each parent.

  • Law of Independent Assortment: Genes for different traits are inherited independently of each other, provided the genes are on different chromosomes or far apart on the same chromosome.

Evolutionary forces are mechanisms that drive changes in allele frequencies in a population over time, contributing to the process of evolution. Key forces include:

  • Natural Selection: Individuals with traits that provide a survival or reproductive advantage are more likely to pass on their genes, leading to adaptation within populations.

  • Genetic Drift: Random changes in allele frequencies can occur, particularly in small populations, leading to loss of genetic variation.

  • Gene Flow: The movement of alleles between populations through migration can introduce new genetic material, increasing genetic diversity.

  • Mutation: Spontaneous changes in DNA sequences can create new alleles and contribute to genetic variation, serving as the raw material for evolution.

These forces interact and can influence the evolutionary trajectory of populations, shaping their genetic diversity and adaptation to changing environments.

Natural selection is a key mechanism of evolution, based on several core principles:

  • Variation: Individuals within a population exhibit variations in traits, which can be genetic and may affect their survival and reproduction.

  • Overproduction: Most species tend to produce more offspring than can survive due to limited resources such as food, space, and mates.

  • Struggle for Existence: Because of overpopulation and competition for resources, individuals must compete for survival, leading to a struggle for existence.

  • Survival of the Fittest: Individuals with traits that provide a survival or reproductive advantage are more likely to survive and reproduce, passing on their advantageous traits to the next generation.

  • Adaptation: Over time, natural selection leads to adaptations, where the population becomes better suited to its environment as advantageous traits become more common.

These principles illustrate how natural selection acts on variation within a population, driving the process of evolution.

  • Gene Flow: The movement of alleles between populations through migration. It can introduce new genetic material into a population, increasing genetic diversity. This process occurs when individuals from one population mate with individuals from another population, transferring alleles and thereby affecting allele frequencies.

  • Genetic Drift: Random changes in allele frequencies within a population, which can lead to the loss of genetic variation, particularly in small populations. It occurs due to chance events (such as a natural disaster or random mating) that disproportionately affect which alleles are passed on to the next generation. Over time, genetic drift can result in the fixation of certain alleles and the loss of others, independent of their adaptive value.

  • Founder Effect: Occurs when a new population is established by a small number of individuals from a larger population. The new population may have reduced genetic variation and may be genetically distinct due to the limited gene pool from its founders.

  • Bottleneck Effect: This occurs when a population's size is significantly reduced for at least one generation. Events like natural disasters can sharply decrease the population size, leading to a loss of genetic diversity. The remaining population may not represent the genetic diversity of the original population, causing potential long-term effects on the population's evolution and adaptability.