EO

Evolution by Natural Selection

Evolution by Natural Selection

  • Introduction

    • Presented by Dr. David Booth at the University of Dundee

    • Key themes: struggle for existence, trophy hunting effects, individual variation, and mechanisms of evolution.

Key Terms

  • Allele: Alternative forms of a gene that may occur at a given locus.

  • Dominant Alleles: Suppress expression of recessive alleles in heterozygous conditions.

  • Gene: A sequence of DNA/RNA that controls the transmission and expression of traits.

  • Genotype: The genetic constitution of an individual/group.

  • Locus: The position in a chromosome of a particular gene or allele.

  • Diploid: Having two haploid sets of homologous chromosomes.

Struggle for Existence

  • Example: Atlantic Cod (Gadus callarias)

    • 1 female can produce approximately 2 million eggs per season.

    • High mortality rate: 99% of eggs consumed as plankton; 90% mortality before reaching 1 year.

Trophy Hunting and Inadvertent Evolution

  • Commercial Fishing for Cod:

    • Targets larger, older fish resulting in reduced maturity age from 6 years to 5 years.

  • Rock Shrimp:

    • Initially all born male; larger individuals become female.

    • Harvesting of largest individuals results in smaller, less productive females evolving.

  • African Elephants:

    • Poaching has decreased tusked elephants, leading to fewer males with tusks.

Mechanisms of Evolution

  • Natural Selection: Acts on phenotypes with genetic basis, altering allele frequencies.

  • Key Processes:

    • Mutation

    • Natural selection

    • Genetic drift

    • Gene flow

  • Phenotype: Observable characteristics determined by genotype.

Population Dynamics

  • Populations change size due to:

    • Birth, death, immigration, emigration.

    • Formula:
      N{t+1} = Nt + B - D + I - E

  • Geometric Growth:

    • Growth modeled by:
      N{t+1} = oldsymbol{λ} Nt

    • Where λ = geometric growth rate.

Types of Variation in Traits

  • Discontinuous Variation: Few discrete values (e.g., earlobe attachment).

  • Continuous Variation: A range of values (e.g., height, weight, milk yield).

Natural Selection Outcomes

  • Variations:

    • Stabilizing selection: Favors average phenotypes (e.g., Eurosta flies).

    • Directional selection: Favors one extreme (e.g., ground finches, larger beaks).

    • Disruptive selection: Favors extremes at both ends (e.g., African seedcrackers).

Adaptive Evolution

  • Influences of environmental/climatic conditions on species:

    • Changes in beak lengths in response to food availability among different geographic locations.

    • Notable cases with Drosophila exhibiting genetic clines correlated with latitude.

Further Reading

  • Suggested literature on population dynamics and natural selection:

    • Scott, Biological Science, Chapter 34.

    • Chiyo et al. (2015) on tusk size decline in African elephants: https://doi.org/10.1002/ece3.1769.

    • Saccheri & Hanski (2006) on natural selection and population dynamics: doi: 10.1016/j.tree.2006.03.018.