Plant Biology: Evolution and Plant Names

Evolution

Outline:

  • Intro to evolution
    • Agriculture
    • Species extinction and invasion
  • Early development of evolutionary concepts
    • First, Second, and Third Revolutions
  • Charles Darwin
  • Evidence for Evolution
  • Microevolution
    • Natural Selection
    • Mutations
    • Migration
    • Genetic drift
    • Rates of evolution
  • Macroevolution
    • geographic isolation
    • ecological isolation
    • mechanical isolation
  • The Role of Polyploidy in Evolution

Introduction to Evolution

Natural selection - tendency of organisms with favorable adaptations to their environment to survive and produce new generations

In agriculture

humans have used artificial selection to change agricultural and domestic plants and animals

Artificial selection - used in plant and animal breeding, evolution directed by humans

ex: wild tomatoes have small fruits, but were bred to have larger ones for consumption

agricultural systems influence evolution of native species

ex: insects evolving resistance to insecticides

Biotechnology and Evolution

Transgenes - foreign genes introduced into plants through genetic engineering

Transgenes can affect native populations through the potential of gene exchange

Development of Evolutionary Concepts

Aristotle - Arranged organisms from simplest to most complex, called the scale of nature, implying that organisms are static and don’t evolve

Leonardo da Vinci - observed that fossils are parts of previously existing organisms that have gone extinct

Jean Baptiste Lamarck - believed that characteristics acquired during life were passed on and become cumulative

ex: a giraffe’s neck stretches through life as it stretches for leaves on high branches, and then passes the long neck on to babies

Epigenetics - the study of heritable changes in gene expression not due to mutation

Epigenetics is accomplished by blocking gene expression by adding methyl groups to DNA, which can be reversed when the genes are needed

Occurs during development and is typically reset during sexual reproduction

Can carry across generations

May be a short term adaptation that allows an organism to switch between phenotypes and respond to a changing environment

Epigenetics - behaviors and environment change how genes work and are expressed, turning them on and off

Revolutions in Thought

First Revolution - brought about by Charles Darwin in 1859. Darwin proposed natural selection as a mechanism for how evolution occurs

Second Revolution - occurred in the 1930s, when Darwinian natural selection and Mendelian genetic provided a better understanding of how evolution occurs

Third Revolution - currently happening and characterized by an increase in interest in and understanding of evolution of development

Charles Darwin (1809-1882)

HMS Beagle - the boat on which Darwin served as an assistant naturalist and voyaged around the world, collecting plants and animals, especially in the Galapagos Islands

Darwin was influenced by Charles Lyell’s geology book that suggested the Earth is much older than previously believed as well as Malthus’s ideas on population growth relative to food supplies and other limiting factors

Evidence for Evolution

Homology - a characteristic shared by different organisms with common ancestry

convergent evolution - similarities not due to common ancestry

Plants that come from different ancestry adapt in similar ways to common environmental conditions

Common use of ATP, common DNA structures and relationships with proteins, the fossil record, and geographical distribution of species all support evolution

Microevolution and Macroevolution

Darwin observed a number of evolutionary processes

  • artificial selection - changes in populations of domestic animals by retaining animals with desirable traits
  • a limited supply of resources, such as food, prevent some individuals from reproducing
  • the ability of any individual to compete varies within a population

Seeing this, he reasoned that individuals that are best adapted to use available resources will increase in number in the following generations

Natural selection - descent with modification

Four Principles of Natural Selection

  1. Overproduction of offspring

  2. Struggle for existence

    1. Competition for resources

  3. Inheritance and accumulation of favorable variations

  4. Survival and reproduction of the fittest

Darwin was criticized in his time because this does not explain how variations occur

Microevolution - evolution within a species

How individual variations occur and how a species as a whole changes

Mutation - change in a gene or chromosome

Deletion - part of a chromosome breaks off

Translocation - part of a chromosome breaks off and attaches to another chromosome

Inversion - part of a chromosome breaks off and reattaches in an inverted position, resulting in one or more nucleotide pair changes

Most mutations are harmful, but some are silent or produce characteristics that help an organism survive change

Migration - gene flow between populations when individuals or gametes migrate from one population to another

Effects of migration depend on the size of the population and extent of the isolation

Genetic drift - changes in the genetic make-up of a population due to random events

Macroevolution - how species evolve

How does one species become two?

Geographic isolation - occurs when two populations are separated, preventing gene flow

Random mutations only spread through the population in which they arise. Eventually, genetic changes are so great that gene flow between the two populations can no longer occur

Ecological Isolation - Factors such as climate or soils causing the separation of two populations

Results in species that occupy the same range of territories but do not exchange genes

Mechanical isolation - populations separated by a lack of physical ability to breed, for example

Polyploidy and Evolution

Polyploidy - occurrence of double the normal chromosome number in an individual

Result of a failure to halve the number of chromosomes during meiosis

Hybrids - offspring produce by parents that differ in one or more characteristics

Hybridization is common in plants, but not animals. Hybrids may have gene combinations that are better suited for new environments

Hybrids are often sterile due to chromosomes not pairing properly during meiosis, but if a polyploid is formed in a hybrid, chromosomes can pair and overcome sterility

Introgression - intercrossing between hybrids and parents

Many domesticated crops are polyploid

Allopolyploid - a polyploid generated from a cross between two species

Autopolyploid - a polyploid generated from a single species

Apomixis - production of seeds without fertilization

Provides a way for sterile hybrids to reproduce asexually

Plant Names and Classification

Outline

  • Introduction to Plant Classification
  • Development of the Binomial System of Nomenclature
    • Linnaeus
    • International Code of Botanical Nomenclature
  • Development of the Kingdom Concept
  • Classification of Major Groups
  • Species Concepts
    • Morphological
    • Interbreeding
    • Ecological
    • Phylogenetic
    • Eclectic
    • Nominalistic
  • The Future of Plant Classification

Introduction

All living organisms are given a two-word, Latin scientific name (the species name)

Each species only have one correct scientific name, as opposed to the possibility of many common names for one species or the same common name for many species

The first attempt to organize and classify plants occurred in the 4th century BC when Theophrastus tried to classify plants by leaf characteristics

The 13th century saw the distinction between monocots and dicots made, and at the beginning of the 18th century the details of fruit and flower structure as well as form and habitat was used in classification.

Latin phrase names were given to plants and animals with the first word of the phrase referring to the genus.

Linnaeus

Carolus Linnaeus - established the Binomial System of Nomenclature

Linnaeus published Species Plantarum in 1753 and changed Latin phrases to reflect relationships between species. He also placed one too many species in each genus and appreciated the names to two parts.

Binomial Nomenclature

All species are named according to this system

This system standardizes plant identification across languages and countries

The International Code of Botanical Nomenclature is a book that standardizes the rules that govern the naming and classification of plants. It has English, French, and German translations and details two steps to officially recognize a new plant species

  1. the DNA sequence has to be deposited in a public database such as GenBank

    1. GenBank - A public database in which DNA sequence data can be deposited

  2. the author has to designate a type specimen that is deposited in a herbarium

The International Code of Nomenclature for Cultivated Plants

Cultivated plants have origins or selection that is mostly due to intentional human activity, whether deliberate or accidental and may be from a wild population, making the plants unsuitable for the ICBN, requiring an independent catalogue.

Phylocode

New data changes out knowledge of evolutionary relationships

  • many taxonomic groups are being reorganized
  • if a taxon is reclassified, its name and ranks in the hierarchy may all change
  • phylocode - used by some taxonomists to govern phylogenetic nomenclature and to name clades
    • ranked categories such as genus and family are not mandatory

Development of the Kingdom Concept

When classification schemes first developed, organisms were placed in either the Plant Kingdom or the Animal Kingdom

  • This was great for complex animals, but not for simpler organisms
    • E.g.: Slime molds
  • Hogg and Haeckel proposed a third kingdom in the 1860s
    • All organisms that did not develop complex tissues were placed in Kingdom Protoctista
  • In 1938, Copeland assigned single-celled, prokaryotic organisms to Kingdom Monera, leaving algae, fungi, and single-celled eukaryotic organisms in Protoctista
  • In 1969, Whittaker developed the five-kingdom system
    • this split fungi from Protoctista
  • In the 1980s, Woese argued that Monera should be split into Archaea and Bacteria, giving us six kingdoms
    • Archaea, Bacteria, Protista, Fungi, Plantae, Animalia
  • This is still a work in progress (slime molds are still weird)

Classification of Major Groups

three domains (bacteria, archaea, and eukarya) group the kingdoms, and so on.

Domain → kingdom → phylum → class → order → family → genus → species

Did King Philip Come Over For Good Soup?

Categories in between (subphylum, suborder, subspecies) are also used

Taxonomists specify in identifying, naming, and classifying organisms using this system. There is frequent disagreement.

Systematists incorporate evolutionary processes to sort out natural relationships

Dichotomous keys help identify organisms, choosing features from paired statements that most closely apply to an organism

Classification of Species

There are at least 22 different ways to classify species, which can be grouped into six species concepts

Morphological Species Concepts

A species is defined by its growth form

This concept relies heavily on the examination of specimens in herbariums, and it is subjective and may not lead to clear distinctions. Recently computers have been able to aid analysis of similar features.

Interbreeding Species Concepts

A species is a population capable of interbreeding and is reproductively isolated from other groups

This is used by many biology textbooks, but crosses can be made in a lab that could not occur in nature

Ecological Species Concepts

A species is a group of related individuals that occupy a unique ecological niche

Assumes that the evolution of species is governed by ecological differences and species identities are maintained by ecological niches

Phylogenetic Species Concepts

A species is determined by phylogenetic history

Individuals with common evolutionary backgrounds are considered to be a species. Cladistic methods are used to determine evolutionary history.

The Future of Plant Classification

The organization of species is ongoing

Today, DNA sequence analysis is aiding the reorganization of species. Most dicots are now called eudicots and flowering plants (Phylum Magnoliophyta) have been separated into two large classes (monocots and dicots).