A lot of questions in ecology are to do with a species’ evolution, and evolution is distinctly concerned with the survival and reproduction of genes in their environment-be it an organism or a woodland. Although natural selection may refer to a gene’s and hence species’ survival, it goes hand in hand with sexual selection, which is concerned with its reproduction. There is no point for any gene to survive if it does not reproduce, a stark fact that is met by some male spiders in the genus Latrodectus, who are eaten by the female after copulation to give the offspring a better chance of survival. With this in mind, I feel it must be important to understand the very process that drives the great diversity of life we see today-inheritance. But first it might be useful to point out that all living organisms reproduce, but not all sexually reproduce. For now I shall explain inheritance in sexually reproducing species, particularly humans.
Consider a human that is made up from the building blocks of life-cells. Within each cell is a nucleus containing DNA (deoxyribonucleic acid), which carries the information needed to ‘function’ the human. This DNA is in the form of chromosomes, 46 altogether in each human somatic cell. Each chromosome is a long strand of DNA made of two chromatids joint together in the center by the centromere (see figure below). As humans are diploid (they inherit genes from both parents), there are two sets of chromosomes (one from the mother and one from the father), and these sets ‘hang out’ with each other’s matching chromosome to form homologous pairs (23 altogether). Each homologous pair shares the same size shape and function. It is quite confusing as the chromosomes themselves look like pairs, but a chromosome pair refers to two whole chromosomes that share similar characteristics (see figure below).
It is on the chromosomes, where the genes are situated. Each chromosome contains many genes, each of which contains information for different traits in an organism e.g. eye colour. Now here is the tricky part. An organism’s entire set of genes is called its genome, but not all genes in the genome are expressed in an organism. For every gene, there are several alleles, which are variations of a particular gene, and only one of these is expressed in an organism. Again, as humans are diploid, there are 2 alleles (one from each parent) per gene. The alleles for a particular gene are located at the same position or locus of a homologous pair (see figure above). An example of a gene is eye colour, and variations (alleles) of this gene could be green or brown. A person could have both these alleles but they will only express one of them. The collection of genes (or more rightly alleles) that are expressed is called an organism’s phenotype. So what are the other genes doing if they are not expressed? They are essentially getting a free ride on the back of the phenotype. To answer this we must understand what goes on during the process of inheritance.
So how do we inherit genes? Through reproduction of course, and this process begins with meiosis. Earlier I mentioned that humans have 42 chromosomes in every somatic cell. Somatic cells are the cells that make up the human body, but these exclude gametes, which are the other type of cells we find in humans and they are involved with reproduction. It is the gametes, which are responsible for passing genes on to the next generation. Each gamete-be it a spermatozoa or an ovum (male or female gamete), contains half the number of chromosomes than somatic cells, as humans only ever pass on half of their genes to the next generation. The other half comes from a sexual partner. This halving of the chromosome number to produce gametes is called meiosis.
Meiosis is a rather complicated process, which I will not describe in detail here but the main gist of it is that there are two divisions (meiosis I and II). The first division is the splitting of a homologous pair of chromosomes and the second is the division of each chromosome into four chromatids. The final result is four haploid nuclei. It is actually more complicated than that but the most crucial point is this. During these divisions, mixing occurs between the chromosomes so that the final four haploid nuclei contain a mixture of genes from each original homologous pair. During fertilization, a gamete from the opposite sex will fuse with the gamete, which restores the amount of chromosomes in the offspring. The offspring therefore can be said to have a mixture of genes from both parents, which is also a mixture of genes of each parent’s own genes. So why do we pass on only half of our genes and why are these genes not always the ones we express? Surely if they worked for the parent, they should work for the offspring too? Doesn’t this go against the whole selfish gene (see Dawkins, The Selfish Gene) concept? Well no because with the case of phenotypes, this is just another way of introducing variation and hence adaptability into individuals without having to lose genes. But we can’t ignore that at some point it in our evolution (by this I am not referring to human evolution but the evolution of life in general), it benefited one of our ancestors to sacrifice half of its genes and replace them with somebody else’s genes. Shouldn’t the selfish genes want a good chance of being passed on? Why did sex evolve?
This is a very complicated question, which there are many theories for, and I will not attempt to answer but just open up a way of thinking about it. Lets start with thinking about the alternative-asexual reproduction. This does exist in nature but to a lesser extent in the animal kingdom. With asexual reproduction, there is no reliance on another individual to reproduce and all of an individual’s genes may be passed on. If this is true then can there still be different species of asexual beings, given that species are defined as those that are reproductively isolated? This begs us to look deeper into the concept of a species. We may be able to distinguish a monkey from a seahorse because they look totally different. But why do they look different? How are new species formed?
Most new species are formed by an initial barrier to gene flow, such as a river separating a population of individuals. Whether these two populations were asexual or sexual, they would still eventually form different ‘species’ through natural selection. Although ‘species’ in this sense refers to groups of organisms that share certain characteristics, as opposed to those, which can only interbreed with each other. Sexual reproduction in this sense provides an extra barrier to gene flow-it helps form species! It concentrates evolution into one type of organism instead of working on individuals. Sexual reproduction forms gene pools instead of genomes, which are more adaptable, more changeable, more varied. Genes have formed organisms, which have in turn formed species. Genes have now become so hidden behind their ‘machines’; it is easy to forget they are there!
The benefits of sex are quite obvious for species or individuals but less obvious from a selfish gene point of view. Have genes now had to account for the organisms and species they have built? This aside, although sexual reproduction might be difficult to explain, it is undoubtedly shaped the evolution of life in so many ways. Without sex, the world would be a less colorful place, as sexual selection is the driver of so many fascinating traits, which the peacocks tail, is only one example of.
The following books were used to help make this article:
Mastering Biology (3rd edition), by OFG Kilgour and PD Riley.
Introducing Genetics, by Alison Thomas.