Variation in Chromosome Number and Arrangement Timothy G

The Modern Synthesis � Charles Darwin recognized that variation existed in populations and suggested natural selection as a mechanism for choosing some variants over others resulting in survival of the fittest and gradual changes in populations of organisms. � Without a mechanism for generation of new variation, populations would be selected into a corner where only one variation would survive and new species could never arise. � The Modern Synthesis combines the mechanism of mutation in DNA to generate variation with natural selection to produce new species. © 2000 Timothy G. Standish

Mutation � Mutation = Change � Biologists use the term “mutation” when talking about any change in the genetic material. Not all result in a change in phenotype. � There are two major types of mutations: � Macromutations - Also called macrolesions and chromosomal aberrations. Involve changes in large amounts of DNA. � Micromutations - Commonly called point mutations and microlesions. © 2000 Timothy G. Standish

Macromutations Six major types of Macromutations in two categories are recognized: A Change within or between chromosomes � 1 2 3 Deletions - Loss of chromosome sections Duplications - Duplication of chromosome sections Inversions - Flipping of parts of chromosomes 4 Translocations - Movement of one part of a chromosome to another part B Changes in Chromosome number 5 Polyploidy - Addition of whole sets of chromosomes 6 Anuploidy - Addition or subtraction of chromosomes, but not whole sets © 2000 Timothy G. Standish

1 Deletion Chromosome Centromere Genes A B C D E F A B C

Deletions Can Be Seen During Pairing of Homologous Giant Chromosomes A B C D

Deletions Can Be Seen During Pairing of Homologous Giant Chromosomes Compensation loop E A

Production of Deletions Two types of deletions occur: 1. Terminal deletions � A B

� Large Impact of Deletions deletions are generally lethal � In a very general way, the larger the deletion, the larger the impact � Deletion of genes essential for survival is always lethal � Pseudodominance, in which typically recessive alleles behave in a dominant fashion may result from deletions due to short stretches of chromosomes which become hemizygous due to a deletion in one homolog � The Notch mutation, deletion of a short stretch of the Drosophila X chromosome, provides an example of this � When the notch deletion is on one X chromosome, white, facet and split alleles on the other X chromosome behave in a dominant fashion © 2000 Timothy G. Standish

Cri-du-Chat Syndrome The cry of the cat � Cri-du-chat syndrome results from deletion of approximately 1/2 the human chromosome 5 p arm � It is one of very few human deletions known to survive to live birth � The larger the deletion the more severe the symptoms which include: – Gastrointestinal and cardiac malformation – Mental retardation – Abnormal development of the glottis and larynx resulting in a cry resembling that of a mewing cat � Incidence of about 1/50, 000 live births © 2000 Timothy G. Standish

2 Duplication Chromosome Centromere Genes A B C D E F G H Duplication

Duplication Also Produces Compensation Loops A B C D E F G H ©

Duplication Also Produces Compensation Loops A A B B C C D D E

Production of Duplications � Duplications are thought to be the result of unequal crossing

Impact of Duplications �More copies of a gene results in more of the gene product of the gene �This can result in dosage problems �Position effects may also have an influence on the way gene duplication impacts organisms © 2000 Timothy G. Standish

3 Inversion Chromosome Centromere Genes A B C D E F A B C

4 Translocation Chromosome Centromere A B C A B E Genes D F E

Does Speciation Occur? �Yes �There a number of well documented examples of speciation �A

Normal Chromosome Numbers In Selected Species Organism Chromosomes Homo sapiens Cat Rat 46 38

American Planetree or Sycamore Platanus occidentalis © 2000 Timothy G. Standish

The London Planetree Oxford Botanical Gardens c 1670 North America Platanus occidentalis l l Platanus acerifolia Europe >5, 000 km 50, 000 Years Atlantic Platanus orientalis Africa Speciation in the production of P. acerifolia was the result of hybridization, not mutation and selection If they can interbreed, are P. occidentalis and P. orientalis really separate species? © 2000 Timothy G. Standish

In His Own Words "I may, of course, be egregiously wrong; but I cannot persuade myself that a theory which explains several large classes of facts can be wholly wrong. . " Charles Darwin, November 13, 1859, in letter to L. Jenyns © 2000 Timothy G. Standish

Has Natural Selection Been Shown To Produce New Species? � No � Darwin emphasized the slow gradual change that is necessary if natural selection is to work on changing a population gradually from one species to another � “Many large groups of facts are intelligible only on the principle that species have been evolved by very small steps. ” – The Origin of Species Chapter VII under “Reasons for disbelieving in great and abrupt modifications” © 2000 Timothy G. Standish