In
this blog post, I thought I'd offer a brief explanation for how evolution came to
produce two sexes within the species of most organisms. The mixing of genetic
material has great advantages. Genetic tricks can be
shared between simple life forms to create better adapted life forms. Sex
overcomes many genetic problems because a good copy of the mutated gene can be
inherited from the parent without deleterious mutations. Mixing of genetic
information over populations is a mechanism that has been retained throughout
the majority of evolutionary history, because it is so beneficial in washing out
deleterious genes.
This mechanism is also part of the reason why distinct sexes evolved in the first place in multicellular organisms - they constitute an advantageous breeding strategy. Imagine that the genetic information is mixed together by the combination of two identical sex cells (these are called 'gametes'). Now, offspring with more nutrients available during development will have a greater change for survival than their more nutritionally impoverished peers, so sex cells would evolve to get bigger, in order to contain more nutrients (these are called 'eggs'). However, the bigger they get, the more cumbersome they are and the smaller the chances of two meeting and exchanging genetic material, so sex cells that are small and quick will have an evolutionary advantage as cells grow (this is one of the reasons why sperm cells are so small).
Consequently, identical members of the same multicellular species will almost invariably evolve into two forms. This asymmetry results in a fork of two reproductive strategies; a multicellular species will have a 'sperm'-strategy from which we call male, and an 'egg'-strategy form which we call female. Sperm combining with sperm do not have enough nutrients to support a developing offspring, and eggs are too bulky to meet one another. One sperm and one egg, however, will form a viable offspring, so that is why there has been an evolution of males and females over the years emerging from those tiny gradual changes in the genetics of the organisms. Male and female evolve concomitantly because two-sex sexual reproduction produces more viable offspring than both asexual reproduction and single-sex sexual reproduction.
Conversely, single-celled life has no need for distinct sexes, as there is no need for nutrient stores for development. All mono-cellular cells can reproduce, and one main motivation for doing so is to obtain good versions of damaged genes. In fact, single-celled organisms have machinery that can detect damaged genes, and that machinery starts a cascade of events which result in sex with the nearest available cell that can respond to those signals. Multi-cellular life reproduces with specific sex cells, and only those specific cells are used for reproduction. Because of the increased utility of the process of recombination of genes, it is unsurprising that over time a mechanism which suppresses the growth of non-sex cells in multi-celluar life would have taken precedence. Evolution works by small increments, and we would therefore expect that the signals for reproduction in single-celled organisms, which detect DNA damage, would be used for exactly the opposite function in multicellular organisms - to suppress their growth and division. This is exactly what we see. The same molecules (caspases, cytochromes and others) are involved in initiating sex in unicellular life and in initiating DNA repair and cellular suicide in multicellular life. Moreover, when this particular mechanism is damaged, single cells in multicellular life are likely to grow out of control, and this is what we see with cancer. These same molecules are very frequently in a mutated form in cancer tissue.
The upshot is, all organisms have a limited lifespan and all reproduce themselves to create offspring. The traits of an organism, called the phenotype, are determined by a heritable mechanism, called the genotype. The genotype is passed on to organisms' offspring through a process that creates randomly imperfect copies of the genotype. Therefore the offspring of an organism demonstrates variability in their phenotype as compared to each other and the offspring's parent. The varied phenotype of each offspring creates a varied probability of survival to reproduction age within that competitive environment. Those offspring that do survive to reproduction age pass on their more heritable phenotype to the next generation.
The laws of physics and chemistry completely determine the mutations and subsequent selection process, just like the laws of physics determine the outcome of a spinning roulette wheel, rolling dice, or where water runs when it lands on the ground. From the fact that all organisms are modified descendants of their parent(s), and that any two organisms, whether living or dead, have a common ancestor, we can infer the primary axiom discussed earlier in the series: that evolution produces a nested hierarchy. As we've seen, since each generation of offspring produce their own generations, organisms can diverge from each other in their succession of traits, especially when they are geographically isolated from each other. When plotted on a graph, this nested hierarchy forms a tree-like structure where the branches are the separate paths that come about as the traits diverge.
We have already covered the fact that, in evolutionary history, millions of traits and genetic differences have been evolving gradually slowly over the course of 4 billion years, so the tree like structure has a huge number of branches on it. Everything that has ever lived fits somewhere on that nested hierarchy, and where it fits can be evidenced by studying the configuration of its genotype to such a precise degree that evolution is as demonstrable a fact as any fact we know.
We also talked about how all life belongs in a nested hierarchy resembling a tree of life. One caveat though, the roots of the tree, the early formations of life, are not quite hierarchical in the same way - they are more like an intertwined network rather than a hierarchical tree, and it's for reasons I'll explain. Firstly, there are a few of examples when branches on the tree fuse - such as the mitochondrial merger and subsequent endosymbioses, which resulted in the evolution of eukaryotes; or the later merger which resulted in chloroplasts. Horizontal gene transfer is quite common in bacteria, but there is still a core set of genes for each species which can be used to define the species and place it on the tree of life. However, to place a branch on the tree requires a lineage from which it can branch. This is fine for all recent life - tracing back gives three clear lineages from which all modern life arose: prokaryotes (bacteria), archaea and eukaryotes. But the meeting point of these three lineages is where the problem arises.
Simplified, we can't say that archaea split from bacteria and eukaryotes then split from archaea, because there are some eukaryotic 'core' genes which are also present in bacteria, but not archaea. We can't say that eukaryotes split from bacteria then archaea split from eukaryotes because there are genes shared with bacteria and archaea but not eukaryotes. Eukaryotes and archaea couldn't have split from bacteria separately because they share core genes which aren't in bacteria. The only reconciliation is to assume that all three lineages (or at least two of them) were rapidly exchanging genetic material. Generally, the tree of life genes are transferred predominantly by inheritance, but horizontal gene transfer also played a major role in shaping these three lineages from which all life is derived.
