Rapid Evolution of Genome Size

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Different genome size in two “related” species of cress (genus Arabidopsis) is called the result of a rapid evolutionary process.

Thale and rock cress plants supposedly shared a common ancestor 10 million years ago.

Since thale cress has five chromosomes and rock cress has eight, researchers are trying to figure out how they evolved from the same ancestor. Thale cress has 125 million base pairs with 27,025 genes. Rock cress has 207 million base pairs with 32,670 genes. About 80% of the gene sequences match up, but thale cress has 17% fewer genes, reports the source article.1

The researchers conclude that the ancestral plant had a large genome similar to that of rock cress. They consider thale cress to be a more highly evolved, streamlined version of the plant, stripped of superfluous information.

Many of the sequences “missing” in the smaller genome are intergenic sequences and sections of transposable DNA. Intergenic sequences, sometimes called “junk DNA,” are not useless vestigial leftovers from evolution. They often regulate the genes that surround them. Transposable DNA sections are often found near mutation-prone hotspots which produce many variations in the organism. Reshuffling or eliminating these bits of DNA from a genome does not result in the evolution of a new kind of organism but just more variations within the organism’s “kind.”

Molecular clock calculations, also reported in the source article, assign thale cress an age of 3.1 million years and rock cress, 1.1 million years. Rock cress plants are said to have experienced “less natural selection because they are on average younger.” Molecular clock calculations assume constant mutation rates, compare two genomes, postulate the existence of a common ancestor, and back-calculate how long it would take the two modern genomes to have diverged from the hypothetical one. These calculations are very problematic because they assume that conditions have never caused the mutation rate to change and also use the presumed existence of a common ancestor to prove the age of the common ancestor.

Natural selection does not produce new kinds of organisms. Natural selection allows the organisms with characteristics better adapted to their environment to survive and reproduce.

Note that rock cress plants are thought to have a larger genome because they have experienced “less natural selection.” Natural selection does not produce new kinds of organisms. Natural selection allows the organisms with characteristics better adapted to their environment to survive and reproduce. Thus, if the thale and rock cress are in any way “related” to one another, they have not evolved; they merely represent different variations of the same biblical kind.

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  1. Tina T Hu et al., “The Arabidopsis lyrata Genome Sequence and the Basis of Rapid Genome Size Change,” Nature Genetics 43 (April 10, 2011): 476–481, doi:10.1038/ng.807.


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