Evolution and Population Genetics: Study Notes for General Biology

Evolution

Principles of Evolution

Evolution is the process by which populations of organisms change over generations through variations in traits and differential reproductive success. The following are the main principles:

• Overproduction of Offspring: More offspring are produced than the environment can support, leading to competition for resources.

• Variation within a Population: Individuals in a population vary in their traits, some of which are heritable.

• Competition for Limited Resources: Organisms compete for food, water, mates, and territory. Only those best adapted to acquire these resources survive and reproduce.

• Differential Reproductive Success: Individuals with advantageous traits are more likely to survive and pass on their genes to the next generation.

Key Figures in Evolutionary Theory

• Charles Darwin:

  • Voyaged on the HMS Beagle for 5 years.

  • Wrote "On the Origin of Species" introducing the concept of natural selection.

  • Observed that species close together share traits (e.g., Galapagos tortoises and marine iguanas).

• Thomas Malthus:

  • Theorized that populations grow faster than food supply, leading to competition and suffering.

  • Influenced Darwin's thinking about the struggle for existence.

• Jean-Baptiste Lamarck:

  • Proposed inheritance of acquired characteristics (now discredited).

• Georges Cuvier:

  • Studied fossils and proposed catastrophism (major extinctions shape life).

• Alfred Wallace:

  • Independently conceived the theory of evolution by natural selection while working in Southeast Asia.

Genetics and Variation

Basic Genetic Concepts

• LUCA: Last Universal Common Ancestor; the most recent organism from which all organisms now living have a common descent.

• Chromosomes: Humans have 46 chromosomes (23 pairs, including X and Y sex chromosomes).

• Gene: A section of DNA coding for a protein (e.g., eye color).

• Allele: A form of a gene (e.g., blue or brown eye color).

Taste Genetics Example

• PAV (Proline Alanine Valine): Gene variant for tasting certain compounds.

  • PAV/PAV: Super tasters

  • PAV/AVI: Tasters

  • AVI/AVI: Non-tasters

  • About 70% of people are PAV/PAV or PAV/AVI; 30% are AVI/AVI.

Mechanisms of Evolution

Natural Selection

Natural selection is the process by which individuals with advantageous traits survive and reproduce more successfully than others. It is both adaptive and selective.

• Artificial Selection: Humans select for desired traits in organisms (e.g., dog breeding).

Genetic Drift

Genetic drift refers to random changes in allele frequencies in a population, especially in small populations. It is not selective or adaptive.

• Bottleneck Effect: A sharp reduction in population size due to environmental events, leading to reduced genetic diversity.

• Founder Effect: When a new population is started by a small number of individuals, leading to different allele frequencies compared to the original population.

Gene Flow

Gene flow is the transfer of alleles from one population to another, which can increase genetic diversity.

• Example: Western and Eastern Armadillo populations exchanging genes.

Sexual Selection

• Inter-Sexual Selection: Females choose males based on certain traits.

• Intra-Sexual Selection: Males compete with each other for access to females.

Modes of Selection

Types of Selection

• Disruptive Selection: Both extremes of a trait are favored (e.g., both tall and short individuals).

• Stabilizing Selection: Intermediate traits are favored (e.g., medium height).

• Directional Selection: One extreme of a trait is favored (e.g., only tall individuals).

Graphical Representation: Disruptive selection produces a bimodal distribution, stabilizing selection narrows the distribution, and directional selection shifts the mean trait value.

Evidence for Evolution

Fossil Record

• Carbon Dating: A form of isotopic dating using radioactive isotopes to determine the age of sedimentary rocks and fossils.

• Transitional Fossils: Fossils that show intermediate traits between ancestral and descendant groups (e.g., Tiktaalik, Archaeopteryx).

Anatomical Evidence

• Homologous Structures: Anatomical features that are similar due to shared ancestry (e.g., limb bones in humans, whales, and bats).

• Vestigial Structures: Structures that have lost their original function (e.g., human appendix, tailbone, wisdom teeth).

• Analogous Structures: Features with similar functions but different evolutionary origins, resulting from convergent evolution (e.g., wings of birds and insects).

Embryology

• Comparative embryology reveals similarities in early development among different species, supporting common ancestry.

Molecular Evidence

• Genetic Code: DNA sequences can be compared across species to determine evolutionary relationships.

• Example: Comparing the sequence CAGTAGACT in different species.

• Humans have 23 pairs of chromosomes; comparison with other primates (e.g., chimpanzees) reveals evolutionary relationships.

Summary Table: Mechanisms of Evolution

Mechanism	Description	Example
Natural Selection	Adaptive change due to differential survival and reproduction	Darwin's finches in the Galapagos
Genetic Drift	Random changes in allele frequencies	Bottleneck in cheetah populations
Gene Flow	Movement of alleles between populations	Migration between armadillo populations
Sexual Selection	Selection based on mating success	Peacock tail feathers

Key Terms and Concepts

• Hardy-Weinberg Equilibrium: A principle stating that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences. The equation is: $p^2 + 2pq + q^2 = 1$ where $p$ and $q$ are the frequencies of two alleles.

• Pesticide Resistance: An example of evolution in action, where populations of pests evolve resistance to chemicals due to selection pressure.

Need to Know for Exams

• Key People: Darwin, Wallace, Lamarck, Malthus

• Processes: Natural selection, artificial selection, genetic drift, gene flow, sexual selection

• Modes of Selection: Disruptive, stabilizing, directional

• Evidence: Homologous and analogous structures, fossil record, biogeography, vestigial structures, comparative embryology, molecular record, pesticide resistance

• Skills: Hardy-Weinberg equilibrium calculations