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.