Last week’s post discussed the science behind DNA and how DNA works to generate the proteins needed for an organism to grow, develop, and function, but how do different species, and even different individuals belonging to the same species, do these things while still having distinctly unique characteristics? This is explained by natural recombination and mutations, which influence species populations over the short term, but also play a role in species evolution and ecosystem interactions.
First, let’s talk about reproduction. When two individuals of a species reproduce, they each contribute a gamete, which is the equivalent of one-half of their genome. What does this mean on the scale of DNA? Reproductive cells (i.e. sperm and egg or ovule and pollen, etc.) form from the division of a cell which contains a complete set of the organism’s DNA. Through this process, called meiosis, two gamete cells are formed from this original cell, meaning that the DNA of the original cell is split in half between these two gametes. Each time gamete cells are formed, the DNA is divided in different ways, generating gametes with distinctly unique sets of DNA sequences, and therefore unique combinations of genes. Reproduction occurs when a male gamete cell fuses with a female gamete cell and the two half-sets of DNA are combined into a complete set, creating a new individual with a unique DNA sequence. When thought of in human context, this explains why siblings from the same two parents have similar characteristics but are still unique in their combination of these traits.
The formation of gametes through meiosis occurs every time an organism reproduces sexually, therefore with the creation of each new individual, a shuffling of DNA has occurred. This shuffling of parental genes generates a population of unique individuals who then reproduce to shuffle the genes even further. On a population scale, you might think that over time, the genes of each individual have been equally mixed throughout the population, but this is where the ideas of gene flow, natural selection, and mutations come into play. Gene flow is the movement of genes between populations, for example, if two mice colonies overlap and begin to reproduce with one another, the genes from these two colonies can then begin to mix and new combinations of traits created. Over time, individuals with certain combinations of traits will be more successful in surviving and reproducing than others. This is called survival of the fittest, meaning that individuals who have traits that make them more suited to their environment will be more likely to survive and reproduce, therefore passing their genes onto the next generation. Over several generations with this environmental selection pressure, populations will shift toward having more of the genes that make an individual more suited to survival. (If you remember from the ancient agriculture post, this process also occurs during domestication, where the selection of individual plants or animals by humans, acts as an artificial selection pressure for the maintenance of certain genes within a population. This is also how antibiotic and pesticide resistances form in bacteria or pest populations, respectively, but with the antibiotics/pesticides/etc. acting as the selection pressure.)
Another important aspect of this process is that of natural mutations. In nature, spontaneous changes can occur in an individual’s DNA nucleotide sequence, caused by external factors or just by chance. These spontaneous changes can be manifest as deletions or additions of nucleotides, or the inversion of nucleotide sequences (so that they are in reverse order), among other changes. If these mutations occur in genes essential for an organism to grow, develop, and function, they may prevent the gametes from fusing or be fatal to the developing organism. If a mutation occurs in a gene where it does not alter function, it may be less advantageous (and lost from the population gene pool with the death of that organism) or it may prove advantageous and make that organism better suited to its environment, in which case it will be more likely to reproduce and pass the mutation on to its offspring. Over the billions of years that life has existed on earth, this process of natural recombination, gene flow, mutations, and natural selection has contributed to the evolution of all living species that have ever lived, and these forces continue to work today, both through natural and artificial (human-promoted) selection.
So why is this relevant to today’s technologies like transgenics? Oftentimes we hear transgenic or genetically engineered organisms referred to as ‘genetically modified’, however, this is a misnomer as the DNA and genes of organisms have been ‘modified’ since the beginning of life on earth through natural recombination, mutations, and the other forces discussed above. Without this ‘modification’, humans as well as all other species, would not exist as they do today because evolution cannot occur without the presence of gene variability. The next post will discuss some of the science behind the development of transgenic organisms and compare the effects of those techniques with natural genetic recombination as discussed here.
- Reproduction occurs through the combination of a male and female gamete which each represent one-half of the parent individual’s DNA sequence. Each gamete contains a unique combination of the parent’s genes, and will therefore result in offspring who have similar traits, but in different combinations.
- Within a population, individuals have different combinations of genes which determine their level of ‘fitness’ for survival in their particular environment. The individuals with more suitable traits will survive and reproduce, passing the genes for these traits on to their offspring.
- New traits can arise through new gene combinations or through natural mutations. Over time, these new traits will encourage the evolution of existing species, or the divergence of some individuals to form a new, but related, species.
- DNA sequences and genes have been naturally ‘modified’ since the beginning of life on earth to generate the genetic variability essential for evolution to occur. Without changes in DNA, there would be no species diversity or adaptation to new and changing environments.