Evolution has always been about the complete package. Traits, like pea shape or hummingbird tongues, do not evolve – genomes do. It is true that evolution is not real unless the list of instructions that makes up a genome is expressed in the real world but our genetic material is the scorecard of what succeeds and what fails. The ability to sequence whole genomes has given biologists a brand new view of evolution; one that shows what is going on behind the curtain of the greatest show on earth.
People are often surprised about the fact that humans share 98% of the genetic material of chimpanzees. We tend to forget that genomes are instruction manuals for building an organism. All organisms are composed of cells with very similar structures and processes and all multicellular organisms develop (in similar ways) from a single fertilized egg. Most of the genes in our genome influence the day to day business in cells or the once-in-a-lifetime process of self-assembly. So it is not surprising that plants and animals have a lot of genetic material in common.
One of the most fascinating examples of this is the Hox gene complex in animals. These genes are found in the same sequence in the genomes of virtually all animals -a sequence that relates to their function; which is to serve as a locator for cells as they develop in an embryo. The eight main Hox genes create different amounts of proteins in each cell depending on where they end up between the head and the tail of the animal. This information, in turn, triggers other genes to switch on to develop tissues appropriate to different parts of the body. The fact that most animals use the same system points to our common ancestry as well as to how easy (and fatal) it is to screw up the process of building an organism. The theme that crops up in evolutionary genomics again and again is that copies of old genes get re-purposed for new functions. In a lecture to the Linnean Society, Peter Holland gives examples of repurposed Hox genes leading to the evolution of terrestrial larvae in moths as well as placentas in mammals. In each case, a small shift in DNA results in a gene that launches a new branch of organisms with a world of new habitats to which they can adapt.
Another example of re-purposing genes happens when the sperm and egg cells for sexual reproduction fail to form properly and double the genetic content in the newly fertilized egg. This is called polyploidy and it is an excellent example of reproductive isolation – the driving force behind evolution. It rarely occurs in mammals – not, as I initially supposed, because of complications due to an unusual number of sex chromosomes but because polyploid fetuses do not develop to term. Among plants, it is quite common – as much as one in ten speciation events are the result of polyploidy. Figure 1 shows likely doubling and tripling events in the evolution of plants. Notable are the rapid sequence of events in the evolution of rape seed (Brassica napus)
and cotton (Gossypium). The major branching events in the plant kindom, like the origin of the monocots or the split between rose-like and aster-like flowers , are accompanied by the extra genetic material and reproductive isolation of a doubling or tripling event. A couple of things to know about polyploidy is that :
– asexual reproduction is needed so that these new “hopeful monsters” have someone to breed with;
– extra genetic material can be pared away over the next several generations – effectively returning the organism to the normal amount of genetic material;
– sometimes polyploidy comes about from a hybrid of different species.
Genomes are not a better representation of what an organism is but they do give us some pretty persuasive clues about how species came to evolve. Retaining the gene sequences that produce useful proteins just makes sense. Stepping out cautiously into the new genetic variations that make life better for reproductively isolated populations is an act of desperation that has yielded the beautiful diversity we see around us.