Last year I introduced a Q&A feature to the blog, inviting any and all questions related to primates/evolution/anthropology. Today I’m pleased to present Issue #2 of Ask the Biological Anthropologist!
Q: Since evolution relies on the mixing of genes to create new offspring, with the chance that a random mutation can result in something “interesting,” do organisms with longer replication intervals fare poorer at the evolution game than those with very short replication intervals (like viruses)? –Matt, Arlington
A: This is a great question. But I’m going to pause to clarify some terminology and basic reproductive biology before I give an answer that will complicate the dichotomy Matt has proposed.
A Note on Terminology: As I explained way back when I first started this blog, it’s important to distinguish between ‘evolution’ and ‘natural selection.’ Evolution describes the gradual changes that occur in populations as new genetic/physical/behavioral traits emerge. These changes, however, are driven by natural selection, which is a process that acts on individuals. Essentially, individuals must compete for the resources necessary to survive and reproduce, and those individuals who have more advantageous traits (i.e. those that are the most ‘fit’ in a particular environment), are able to reproduce more and pass on these traits. I emphasize this because while, as Matt’s question suggests, mutation is the ultimate engine of evolutionary change, beneficial mutations don’t just produce something “interesting.” They produce adaptations that improve an individual’s odds for survival/reproduction, leading those advantageous traits to become more common in subsequent generations.
A Note on Reproductive Biology: Mixing genes from two parents is not the only way to create offspring! Asexual reproduction, in which a single organism reproduces by passing on its full complement of genes (producing, in theory at least, a genetically identical individual), occurs in species such as bacteria, sea stars, and many fungi and algae. Viral replication is a more complex process, but still different from sexual reproduction, in which gametes from two parents (i.e. egg and sperm) combine to produce a genetically novel offspring. As for mutations — changes to a DNA sequence due, for example, to copying errors during cell division — they can occur in any of these reproductive scenarios. More often than not mutations have deleterious or neutral effects but, as stated above, natural selection will favor a mutation in those cases in which it produces a trait that confers an advantage to survival/reproduction.
So how do mutations and a species’ method of reproduction relate to replication intervals — what I will refer to as generation times — and species-level competition? Let’s start by thinking about those species that have short generation times. In populations of such species, mutations arise more frequently simply because new individuals are being produced more often: individual members of these species reproduce at frequent intervals, and may produce many offspring with each reproductive event. On the one hand, this means that beneficial mutations can appear in these species more often, allowing populations to evolve more quickly as natural selection favors the rapid spread of new adaptive characteristics. In the case of asexually reproducing species, however, there are two major downsides. First, mutations are the only way to introduce new genetic variation into the population. Second, deleterious mutations are also able to accumulate and, with no mechanism for ‘correction,’ may raise the likelihood of extinction (see: Muller’s Ratchet).
Species with long(er) generation times provide a notable contrast in both life history pattern and reproductive strategy. “Life history” is a term used in biology to refer to the pacing or scheduling of events in an organism’s life, and encompasses factors such as rate of maturation, age and size at first reproduction, frequency of reproduction, size and number of offspring, and total lifespan. Life history theory posits that for any species, all of these factors have been shaped by natural selection to maximize individual reproductive success in the context of specific ecological challenges (e.g. predation pressures). The result is that species with a ‘slow’ life history reproduce more infrequently and produce relatively few offspring with each reproductive event. Of particular interest to us in the present context, they also tend to reproduce sexually.
But why?? The truth is that sex has long been considered a bit of a conundrum from an evolutionary perspective. It is costly, both in terms of the time and energy that individuals must use to find, access, and potentially keep a mate, and in terms of the fact that a sexually reproducing individual is able to pass on only 50% of his/her genetic material to each offspring. From a fitness standpoint, this means that a sexually reproducing individual must produce twice as many offspring as an asexually reproducing individual in order to pass on its genes as successfully. But as the differences in life history patterns mentioned above illustrate, this is highly unlikely. So why? Why rely on such a complex and costly system?
This brings us back to species-level competition. The Red Queen hypothesis, proposed by WD Hamilton, states that sexual reproduction is widespread, especially among species with long generation times, precisely because one of the perks of sex is that it produces offspring with increased genetic variability. This is a necessary consequence of the mechanics of sexual reproduction — the process of creating haploid gametes and having them fuse to combine the DNA/chromosomes of two individuals creates opportunities for what is known as genetic recombination — and is, according to Hamilton, a key tactic in the ‘arms race’ between parasites and host species. Put another way, the reproductive mode of species with long generation times is likely an adaptation that compensates for the faster rates of reproduction and evolutionary change characteristic of parasitic organisms.
This is an important point, so I’m going to hammer home the logic:
- Because they have short generation times, parasites undergo rapid evolutionary change that allows them to adapt to the most common host genotype.
- This means that genetic diversity and new combinations of resistant genes — exactly the outcomes produced by sexual reproduction and genetic recombination — are important ‘counterstrategies’ in host species.
- Sexual reproduction allows species with long generation times (such as humans) to ‘keep up with’ viruses and other potentially threatening organisms that have faster life histories.
Just as the Red Queen describes in a passage in Lewis Carroll’s “Through the Looking Glass”:
“Well, in our country,” said Alice, still panting a little, “you’d generally get to somewhere else — if you run very fast for a long time, as we’ve been doing.” “A slow sort of country!” said the Queen. “Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!”
This may seem like a very long-winded answer to a seemingly straightforward question about how reproductive rate affects success in “the evolution game.” But hopefully I have made it clear that it’s not quite as simple as ‘fast’ vs. ‘slow’ reproduction, and that while natural selection may favor different life history patterns in different environments, it also produces adaptations that even the playing field in other ways. The Red Queen hypothesis, for example, has been supported by data demonstrating that animals with longer generation times have higher levels of genetic recombination (Burt & Bell 1987). So cool!
Stay tuned next week for Issue #3 of Ask the Biological Anthropologist!
1. Burt A, Bell G (1987) Mammalian chiasma frequencies as a test of two theories of recombination. Nature, 326, 803-805.
1. Fabian D, Flatt T (2012) Life History Evolution. Nature Education Knowledge. [http://www.nature.com/scitable/knowledge/library/life-history-evolution-68245673]