Evolutionary theory

• Evolution is not just change over time. All individuals change over time.

• Evolution is descent with modification.

• Evolution is, by definition... the change in allele frequency, in a population, over time.

  • Allele: one of many alternative forms of a gene; found at the same place on a chromosome (e.g. winkled and smooth pods, yellow and green seeds)

    • An allele is a gene that codes for the same function (e.g. flower color), but with different variants (e.g. red vs white flowers)

  • Frequency: the ratio of a particular allele, to the total of all other alleles of the same gene in a given population

  • Population: a group of organisms in the same species in an area

For example, a population of flowering plants will all have genes to make flower color, but some individuals may have different variants of the gene (alleles) such as red petals vs pink petals.

• • In the population, red may be preferred by pollinators, with fewer pollinators choosing the pink variants.

  • Over time, pollinators may start to prefer the pink color over the red, allowing those pink-flowered plants have more successful pollination interactions, and thus making more seeds.

  • The plants with the pink petals will increase in the population, and therefore the frequency of those "pink alleles" will change in relation to the "red alleles". 

  • To put a fine point on this: The allele frequency, in this plant population, changes over time. That IS the definition of evolution 

  • Over a extended period of time, the red-flowered plants might not receive pollinators anymore, and the "red group" goes extinct. The pink-flowered plants continue as the dominant species.

  • Another possibility is that the pink and red flower variants may become discrete populations, with their own pollinators. The gene pools of these two groups become different enough that they can no longer interbreed. New species are formed.

Convergent Evolution 

• Species from uncommon ancestors, evolve a similar form or function

• There are many examples of convergence in the plant kingdom

  • Plant form in arid environments: cacti (Cactaceae) and spurges (Euphorbiaceae) look alike, but evolved from different ancestors. 

  • Carnivorous "pitcher plants" (i.e. Sarracenia, Cephalotus, Nepenthes) have a similar form for catching insects, but originate from different orders

  • Conifers and the She-oaks, a flowering plant in the Casuarinaceae 

  • Ancient horsetails (Calamites) and modern day bamboo (e.g. Bambusa)

  • Tree-like gymnosperm cycads (Cycadaceae) and modern day palms (Arecaeae)

  • Gnetophytes and angiosperms share several features: netted veins in leaves, vessels in wood, flower-like cones, double fertilization, angiosperm-like seed

    • There is a debate whether these features are convergence or homologous

  • Grass-like morphology, or linear leaves from the ground, can be found in the quillworts (Isoetales), the curlygrass ferns (Schizaeaceae), as well as true grasses (Poales), 

Cladistic Methodology

• Method to determine natural relationships of organisms based on the history or evolution of groups

• Computer programs (algorithms) calculate the evolutionary relationships of groups based on the coded features for each taxa in the study

  • Coded means that the feature is either "present" (1), "absent" (0), or even "unknown" (?) for this taxa

    • For example, plants could be coded for vascular tissue, in which mosses would be "absent" for this trait, but ferns, conifers, and flowering plants would be "present" for this trait.

  • The result is a cladogram or phylogenetic tree, which, based on the data, is a hypothesis for the evolution of these plant groups in the study

Ancestral vs. Derived Features

• Researchers also need to determine which characters or features are “ancestral” (appeared long ago), and which are “derived” (recently appeared in evolutionary history)

  • Apomorphy: A derived feature, called an apomorphy, is a uniquely evolved feature to a group and is passed on to its descendants, and thus defines a group or a clade (e.g. the carpal for flowering plants)

    • When comparing groups, researchers may refer to a synapomorphy, or shared derived trait. This is a feature that is shared among plant groups, arising from a common ancestor.

  • Pleisomorphy: An ancestral feature, called a pleisomorphy, is one that is shared by many taxa, and does not help define the origin of a group

    • e.g. a nucleus is not informative for resolving the relationships between angiosperm families, although a nucleus would be a useful trait for defining eukaryotes versus prokaryotes

    • The nucleus would be pleisomorphic for resolving angiosperm phylogenetics, but it would be apomorphic for resolving domain-level phylogenetics (e.g. bacteris, archaea, eukaryotes)

• Note: taxa that exhibit pleisomorphic features, are not called "primitive"; taxa with apomorphic features are not called "advanced". 

  • The terms "primitive" and "advanced" denote inferiority and superiority, respectively, but species are adapted to their niche in the environment.

  • Therefore, comparing groups, such as mosses to pines, is meaningless. One isn't more advanced or primitive to one another. There are tradeoffs and adaptations that each has to their niche in the environment.

• We use "ancestral" or "derived" to refer to features, and "sisters" to refer to taxa 

  • e.g. mosses are sister to other land plants, and exhibit some ancestral features (pleisomorphies) such as spore-bearing, but also derived features (apomorphies) such as leaves in the gametophyte

Clades and Grades

• The goal of cladistics is to find true evolutionary groups

  • True groupings come from a single / common ancestor, which passed along 1 or more features to their descendants during evolution. This feature(s) is an apomorphy that defines the group.

  • We call this grouping a clade or a monophyletic group

  • Monophyletic groupings include a common ancestor and all its descendants

    • e.g. "Angiosperms" come from a single common ancestor, and include all flowering plant descendants

• There can be groups of organisms that we have traditionally recognized in science, but evidence shows that they are not true evolutionary groupings according to cladistics.

  • Paraphyletic groupings include an ancestor and some descendants, but not all; one evolutionary branch is removed

    • e.g. "Gymnosperms" are seed plants that come from a common ancestor, but the branch of angiosperms would need to be removed to make them a clade

    • Referred to as grades

  • Polyphyletic groupings include an ancestor and a few descendants scattered on the tree, but more than one evolutionary branch needs to be removed

    • e.g. "Algae" are aquatic plant-like organisms, but this term can refer to many unrelated groups like green algae, brown algae, and blue-green algae