Biology II Lecture Notes - Evolution Part I and II

BIOLOGY II

LECTURE I

SYLLABUS

Learn about Evolution Part I

EVOLUTION PART I

Chapter 20 - Biology: How Life Works, Morris et al. (Sections 20.1 - 20.3)

Objectives

  • Adaptation, Unity, Diversity
  • Genetic Variation
  • Allele Frequencies
  • Evolution and the change in allele frequencies

Three Key Observations About Life

  • Organisms are suited to their environments.
  • Shared characteristics indicate unity among species.
  • There exists a rich diversity among species.

Adaptation, Unity, and Diversity

  • Onymacris unguicularis (Headstander beetle) from Southwest Africa illustrates adaptation.
  • Camouflage in grasshoppers is a dramatic example of adaptation.

Genetic Variation

  • Since Darwin's time, species have been recognized as not conforming to a single type, exhibiting a range of variants.
  • Natural selection depends on the differential success in survival and reproduction among these variants.
  • In humans, there exists a high degree of phenotypic variation; however, we rank low in overall genetic variation compared to other species.
  • Variation can be phenotypic (observable physical traits) or genotypic (genetic makeup).

Comparison of Genetic Variation

  • Adelie penguins: Exhibit low phenotypic variation but are 2-3 times more genetically variable than humans.
  • Fruit flies: Exhibit about 10 times more variability than humans in terms of DNA base differences.

Population Genetics

  • Population genetics studies patterns of genetic variation in natural populations.
  • Important Terminology:
    • Species: A group of individuals that can exchange genetic material through interbreeding.
    • Gene Pool: All alleles present in a species.
    • Populations: Interbreeding groups of organisms of the same species in the same geographical area.
    • Individuals represent different combinations of alleles from the species’ gene pool.

Mechanisms of Genetic Variation

  • Mutation generates new variation:

    • Somatic mutations: Affect non-reproductive cells.
    • Germ-line mutations: Inherited from reproductive cells.
    • Deleterious mutations: Harmful mutations that may lead to genetic disorders.
    • Neutral mutations: Cause no change in fitness.
    • Advantageous mutations: Lead to adaptations.

  • Recombination: Shuffles mutations creating new combinations, leading to new alleles.

Measuring Genetic Variation

Allele Frequencies

  • Determining genetic variation in populations requires knowledge of allele occurrence rates.
  • Frequency of an allele = Number of that allele in the population / Total number of alleles.

Example 1: Pea Plant Genotypes

  • Genotypes for pea color:
    • AA = Yellow pea plant
    • Aa = Yellow pea plant
    • aa = Green pea plant
  • If every plant is green, can infer population fixed for allele ‘a’.

Example 2: Pea Plant Genotype Frequencies

  • Population of 100 pea plants, genotypes:
    • 50% aa
    • 25% Aa
    • 25% AA
  • Allele frequency calculation:
    • Total alleles = 100 plants x 2 = 200 alleles in total.
    • Calculation method:
    • Homozygotes (aa, 50 plants): 50 x 2 = 100 copies of allele a.
    • Heterozygotes (Aa, 25 plants): 25 x 1 = 25 copies of allele a.
    • Total copies of allele a = 100 + 25 = 125
    • Frequency of allele a = \frac{125}{200} = 0.625 or 62.5%.

Further Calculations and Examples

  • Frequency for allele A in another example: Population of 200 plants has 33% aa, 40% Aa, 27% AA.
  • Complete frequency calculation similar to previous examples shown in lecture.

Hard-Weinberg Equilibrium

  • The Hardy-Weinberg equilibrium specifies the relationship between allele frequencies (p, q for two alleles).
  • If key conditions are met, the frequency relation is preserved: p + q = 1 and genotype frequencies p^2 + 2pq + q^2 = 1.
  • Conditions required for Hardy-Weinberg:
    • No selection (differences in survival/reproduction).
    • No migration into or out of the population.
    • No mutations occurring.
    • Population sufficiently large to prevent sampling errors.
    • Random mating must occur (non-random mating affects genotype but not allele frequencies).

Definitions and Observations on Evolution

  • Evolution is defined as a change in allele or genotype frequency over time and varies with the environment.
  • Darwin's Theory: Proposed descent with modification as an explanation for species divergence and evolution via natural selection.
    • Major points of his theory in “On the Origin of Species” include:
    • Current species descend from ancestral species, differing from them.
    • Natural selection functions as a mechanism for evolutionary change.

Historical Views on Evolution

  • In the early 19th century, it was widely accepted that the Earth was only about 6,000 years old and species had remained unchanged since their creation.
  • Notable historical figures influencing Darwin's ideas include:
    • Aristotle: Suggested species are fixed and unchanging —a view supported by the Old Testament.
    • Carolus Linnaeus: Created a binomial naming system and classified species in hierarchical order, which reflected creation rather than evolutionary relatedness.

Influences on Darwin’s Thinking

  • Paleontology explained fossil records; Cuvier’s catastrophism versus gradual evolutionary changes were significant in understanding transitions.
  • Geologists Hutton and Lyell provided ideas on slow, continuous changes in Earth's surface, supporting the theory that mechanisms of change are constant over time.
  • Malthus' Essay: Argued populations have the potential for geometric increase, leading to competition for limited resources, adapting the best individuals for survival.

Lamarck's Hypothesis of Evolution

  • Proposed evolution through use and disuse and inheritance of acquired traits, ideas now dismissed by modern genetics.

Modern Synthesis

  • Darwin's ideas incorporated Mendelian genetics to explain descent with modification and continuous variation.

Natural Selection

  • Defined as changes in allele frequencies over time based on survival and reproduction of those with advantageous traits.
  • Key points:
    1. Individuals do not evolve; populations do.
    2. Natural selection acts on heritable traits.
    3. Environmental factors determine traits selected for or against.
    4. Natural selection modifies existing traits rather than creating new ones.

Patterns of Natural Selection

  • Can have positive, negative, or balancing selection effects.
  • Illustrates types of selection: stabilizing, directional, and disruptive.

Migration and Genetic Drift

  • Migration involves gene flow between populations leading to homogenization.
  • Genetic drift is change in allele frequencies due to random events, significant in small populations and can result in bottlenecks and founder effects.

Molecular Evolution

  • Explains how sequence differences arise due to mutation and geographical barriers.
  • The Molecular Clock: Correlates time of separation of species with genetic divergence, varying across genes.

Species and Speciation Part I

Definition of Species

  • A dynamic unit capable of changing over time through evolution, presenting the “species problem.”

Distinguishing Species

  • Key is reproductive isolation and the capacity to produce fertile offspring, redefining species through concepts like Morphospecies and Biological Species Concept (BSC).

Limitations of Species Concepts

  • Both the Morphospecies and Biological Species concepts have limitations tied to reproductive compatibility and overlapping characteristics in species definitions.
    • Addressed with concepts like Ecological Species Concept (ESC) and Phylogenetic Species Concept (PSC) to refine definitions, especially in asexual organisms.

Elements of Reproductive Isolation

  • Categories include pre-zygotic (before fertilization; e.g., behavioral, temporal, mechanical) and post-zygotic (after fertilization; e.g., hybrid inviability) factors that create barriers leading to new species development.