Why Did Sexual Reproduction Evolve?
Asexual reproduction seems to be an extraordinarily successful way to propagate. It accounts for the pervasive presence of prokaryotic single-celled organisms in a vast diversity of habitats on Earth. Yet sexual reproduction persists, and it dominates the reproductive mode of multicellular life. This is a mystery because sex has a twofold cost. What the “twofold cost” means is that because only females of some species can bear young, a 50:50 female:male split of a population of 100 sexually reproducing organisms can only produce 50 offspring in the first generation if each female produces one offspring. However, a population of 100 asexually reproducing organisms dividing once can produce 100 offspring. So why would sex evolve given these costs? Clearly, however, once it did evolve, sex was successful, and it has been selected for since.
We do not know exactly why sex evolved, but there are a number of hypotheses that need not be mutually exclusive. First, we should note that the twofold cost is not strictly true for all species. Isogamous species are species in which males and females are not distinguished, and all members of a species can produce offspring. This counter-argument does not apply to all species, however, so other explanations are required.
Many ideas to explain sex lie in the genetic cross-over process that occurs in meiosis. One potential benefit is that it allows for radical genetic reorganization, with its potential to pass on many genes with combined beneficial effects. Asexually reproducing organisms must wait until different mutations collect over time that together can produce improvements. In sexual reproduction, because an entire segment of chromosome can be passed on from both parents, new large-scale genetic combinations can be tested. Cross-over may further allow genes to escape their surrounding genes, which may be deleterious, and recombine in new beneficial combinations.
Another theory is that sex may have arisen to protect against parasites. Organisms are constantly in a race to catch up with the deleterious effects of parasites. Even without changing physical environments, organisms must adapt constantly to deal with this on-going evolutionary pressure. This has been termed the Red Queen Hypothesis, named after the Red Queen in Lewis Carroll's book Alice in Wonderland, who had to constantly keep running just to stay in one place. Sexual reproduction allows for multiple genes that might be needed in new combinations to resist new parasites to be passed on to offspring and for new arrangements of these genes to be fabricated quickly in evolutionary time.
Yet another concept is that cross-over is a type of DNA repair process. As large segments of DNA are crossed over to form the chromosomes of the progeny, they can be used to patch-up segments of damaged DNA. Related to this concept is the idea that sex is a way of reducing mutational load. Asexually reproducing organisms continue to build up mutations sequentially in their DNA with each round of replication, ratcheting up, generation after generation. This “Muller's ratchet” (first discussed by geneticist Hermann Muller) eventually loads an organism with many potentially lethal mutations. Sexual combinations of genes may provide a mechanism to reduce this load by generating new genetic assortments from mixing of chromosomes from different lineages.
The mystery of sex is interesting because, to return to a time-honored question we have discussed already multiple times, would we presume such a process to be universal? Could we imagine a planet covered in asexually reproducing organisms in which stable genetic systems and populations could persist without sex? Is sex an idiosyncratic system that was “discovered” by evolution and because it provides advantages in certain situations, it has spread and persisted, or is it a system that is somehow essential for the development and emergence of multicellular life? These questions are difficult to answer without a definitive understanding of how sex evolved and what advantages it provides, but they are nevertheless profound. They strongly influence our view of how living things can reproduce and to what different extent systems of reproduction are an ineluctable part of biological evolution or chance events in our own particular evolutionary experiment.
5.10 The Growth of Populations of Cells
The modes of reproduction that we have just examined allow populations of cells to grow. In multicellular eukaryotes, this process is controlled by a network of genes that lead to cell differentiation, whereby initially unspecialized cells change into the whole panoply of cell types in a given organism, such as skin, liver, and heart cells. These cells are under the control of genes that produce hormones influencing cell differentiation and development. These pathways encompass an entire field of science that deals with cell developmental biology. We won't delve into this field any more here, but you are encouraged to find out more. Cell developmental biology provides many insights into the origin and evolution of plants and animals, and particularly the evolution of cell differentiation and multicellularity. However, in Chapter 15 we consider the rise of multicellularity again, why this happened on Earth, and whether it is inevitable.
Perhaps the best characterized growth patterns are to be found in the prokaryotic cells (Figure 5.20). Generally prokaryotes do not differentiate, although some fungi and slime molds do exhibit primitive differentiation. Some bacteria have specialized cell structures such as the heterocysts in cyanobacteria, which are specialized cells for fixing atmospheric nitrogen gas from the atmosphere into biologically available nitrogen compounds.
Figure 5.20 The major phases of growth in a population of prokaryotes.
The life cycle of a population of prokaryotic cells, for example, a culture of bacteria studied in the laboratory, is quite simple and follows some well-established patterns illustrated in Figure 5.20. In the first stage, the lag phase, the cells grow slowly. This represents the phase during which the cells are beginning to reproduce in the presence of newly available energy and nutrients. Following this stage, when the cells have adjusted to the new environment, they enter the logarithmic phase or exponential phase when growth is rapid. The organisms are not always growing according to an exact mathematical logarithmic function. The true growth rate depends on the energy available. In the next stage, the cells run out of some essential nutrient or energy supply, and they enter the stationary phase. Following the stationary phase, the cells begin to die as they enter the death phase.
5.11 Moving and Communicating
One often sees prokaryotes described in the literature as “primitive.” In the sense that they are simpler than eukaryotic cells, this description is not wrong. However, prokaryotic cells have remarkable features including the ability to move and communicate. We now investigate these characteristics. They are of relevance to understanding the evolution of life on Earth for two reasons. First, they show that the prokaryotes that inhabit the planet today are likely much more complex than the first cells that emerged on Earth. We should therefore be careful when we think of these organisms as “primitive” and recall that there has been at least 3.5 billion years of evolution since life emerged. At the same time, many features across cell types are conserved and seem ancient, such as the genetic code, whose commonality across life suggests that certain cellular structures reflect early events in cellular evolution. Second, learning about these cellular features is an important part of grasping that prokaryotes are not just simple bags of fluid that take in nutrients and excrete waste; single-celled structures can evolve remarkably complex behaviors. This tells us that there is potentiality for great complexity in even the simplest cellular structures.
5.11.1 Movement in Prokaryotes
Cells are not necessarily sessile. For many microorganisms, moving around is essential. By changing location, they can get access to new energy resources and nutrients or move away from toxins (chemotaxis).
Movement