Genes and Evolution

All cells, including bacterial calls, are under consistent competition for nutrients and energy resources. Bacteria that are best fit to survive in intensely aggressive conditions usually outlast cells that have handicaps, for example larger DNA sequences. We will find that small, simple cells are more selective and tend to survive better than large, complex bacteria cells.

For a typical E. Coli cell, its takes about forty minutes for it to replicate its DNA. It takes even less time for the cell to actually divide into two. Bacteria cells that can replicate faster than others tend to dominate their populations. So, cells that replicate their DNA faster reproduce faster. But, even though cells can make several copies of DNA at a time, other factors such as the length of the DNA strand can influence whether or not a certain strand will dominate the population. For example, a DNA sequence with ten genes will reproduce twice as fast as a DNA sequence with twenty genes. Therefore bacteria try to reduce their number of genes in order to be more “physically fit” for the environment. But, this isn’t always the case as sometimes bacteria with larger DNA tend to dominate populations.

In areas where nutrients for energy are scarce, larger genomes tend to thrive. But how can this be? Where nutrients are scare, competition for those resources are high. So, a bacteria cell that can more readily create ATP from those nutrients is more fit for the environment. Under such conditions, selection for speed of replication is a much smaller factor than ability to produce ATP. In this case bacteria have large, much more complex genomes which allow the cell to produce ATP more quickly.

Humans inherit genes “vertically,” from their parents. There is variation in humans because our genes are a combinations of two different genes. But, bacteria’s variation is accounted for in lateral transfer of genes. Lateral transfer is the ability of bacteria cells to pick up genes from other bacteria or dead cells in order to make up for any gene loss due to selection. Although bacteria makes clones of themselves during reproduction, much of their DNA is changed throughout its lifetime because of lateral transfer. This accounts for about 90 percent of variation in bacteria cells.

Other factors such as surface area to volume ration have also lead to the simplicity of bacterial cells. For example, a 1x1x1 cell has a volume of 1 and a surface area of 6 and a 2x2x2 cell has a volume of 8 and a surface area of 24. Their surface area to volume ratios are 6:1 and 24:8 or 3:1, respectively. The smaller cell has a higher surface area to volume ratio which means it has a greater surface area per volume of molecules to absorb nutrients in order to survive. Selection tends to favor smaller cells because are more efficient in absorbing resources for producing ATP.

Selection tends to favor cells that are more simple, that have less genomes, and are relatively small. Bacteria with more genes take longer to replicate and larger cells have lower surface area to volume ratios which reduce their chances of soaking up nutrients and surviving.