Image from The Malay Archipelago by Alfred Russel Wallace
This is the first entry in what will hopefully be a series. I'll start off with a brief description (more details at the end) saying that my goal is an edu-technical yet entertaining series about biodiversity and its role in our lives. This is the international year for biodiversity and biodiversity is facing a crisis that is unprecedented within the existence of humanity. So it seems timely. I'll try and publish every Sunday evening or at least as many Sundays as I can.
The title is part of a quote by Alfred Russel Wallace. He was comparing the tropical environment to the 19th century European landscape that his readers would have known. I think it is an accurate term. We live in a world that has been transformed by technological change and human manipulation of the world's ecosystems. But the world we know coexists and is dependent upon a more ancient world, a world shaped by some 3.5 billion years of organic evolution. To the lineage of this moon jellyfish, itself a newcomer compared to bacteria, the human world has appeared in the blink of an eye (that is if jellyfish had eyes).
Image by the author. St Andrews State Park, Panama City, Florida
Our first topic is the evolution of sex. I choose this topic in honor of George C. Williamswho died this past week at the age of 84. Williams was among the most influential evolutionary biologists from the 1960s through the 1980s. His book 'Adaptation and Natural Selection' was frequently pointed out to me as a graduate student as a good source for clear thinking and avoiding misconceptions about adaptation.
One of the topics Williams tackled was the evolution of sex, probably the greatest 'unsolved mystery' in evolutionary biology. It is an appropriate topic for this diary as sex is ancient in origin, extraordinarily diverse in practice, and a major driver of much of the world's biological diversity.
What do we mean by the evolution of sex? This may seem straightforward but I can think of at least four possible interpretations.
- The evolution of the physical act of copulation (as shown by these lovebugs).
Image by the author. Car windshield, Tallahassee, Florida
- The evolution of sexual reproduction. Meaning the production of eggs and sperm through meiosis and their fusion (fertilization) to form a new individual.
Image in the public domain, source unknown as per Wikipedia
- More generally, the evolution of methods of generating new genetic combinations due to the mixing of DNA from different individuals.
- The evolution of two distinct sexes.
You might think that these are all part and parcel of the same process. However there are many cases in the natural world that involve some of these phenomena but not all of them.
For example. Bacteria have a process called conjugation which involves the physical linkage between two bacterial individuals and the movement of DNA from one to the other. This may sound very similar to sex as we understand it but there are two important differences. First conjugation is not related to reproduction which in bacteria is entirely asexual. Second there are no sexes in bacteria.
Image from Wikimedia commons
In this diary I'm going to focus primarily on the evolution of sexual reproduction (#2 on the list) as that is the greatest mystery. I will touch a bit on item #4 - the evolution of different sexes. So those of you hoping for a discussion of #1, I'm sorry but perhaps you should check out the 'documentary' below. At least I didn't put the word 'boobies' in my title.
So why is sexual reproduction a mystery? Simply put because it is evolutionarily costly to individuals who use it instead of asexual reproduction. The diagram below illustrates this nicely. If females can have two offspring then every asexual female will produce two copies of all of her genes in her offspring (each offspring is genetically identical to the mother). A sexual female will produce only one copy (on average) of each of her genes as each of her offspring will have half of her genes with the other half coming from her mate.
Image from Wikimedia commons
Left - sexual reproduction, Right asexual reproduction (all female).
Sexual reproduction is also very costly in other ways.
Image from "The Malay Archipelago" by Alfred Russel Wallace.
This male bird of paradise has used a lot of energy to grow fancy ornaments and perform elaborate displays in order to attract mates. This attributes also increase his probability of being taken by a predator. An all female species would have no need for these attributes.
Image by the author. St Marks NWR, Florida
These plants have invested considerable energy in growing brightly colored, elaborate flowers filled with nectar that serves no other purpose than to attract animals to pollinate their flowers. Without sexual reproduction there would be no need for showy expensive flowers.
Despite these costs, sexual reproduction is very widespread and appears to have evolved very early in history of eukaryotes (organisms with complex cells). Asexual reproduction (cloning) is also very common but has rarely spread to become the only mode reproduction for entire groups.
Let's look a few examples.
Images by the author. Left image from Chiricahua Mountains, Arizona. Right image from Bald Point State Park, Florida
The lizard on the left belongs to an all female species, the closely related species on the right belongs to a closely related sexual species. This group of lizards is widespread in the western US and points south (the right hand picture is of the sole species that occurs east of the Mississippi). There are quite a few all female asexual species. They produce eggs which develop into lizards without fertilization.
In this system it is known that the asexual species arise due to matings between individuals from two different sexual species (hybridization).
So if an asexual species can breed at twice the rate of a sexual species why do any sexual species still exist?
It is true that sexual reproduction greatly increases the genetic variation among individuals and that sexual populations can evolve much faster than asexual ones. But that is an evolutionary advantage to the entire group (i.e. group selection) whereas asexual individuals have a 2 fold reproductive advantage (i.e. individual selection). The general consensus among evolutionary biologists (although there are exceptions) is that group selection is a very weak evolutionary force compared to individual selection.
But the diversity of life indicates that there must be an advantage to sex. Here are some examples.
Aphids and water fleas.
Image all over the web, original source unknown
Aphids and small, common aquatic animals known to biologists as Daphnia and to others as water fleas have life cycles in which they have several asexually produced generations per year. In the fall they produce eggs that hatch out into males and females. They mate and produce eggs that will overwinter. It appears that sexual reproduction allows the offspring and advantage in surviving the most environmentally stressful time of the year.
Image by the author. Tallahassee, Florida
This insect is actually a preying mantis and not a walking stick (note the forelimbs). It is also an asexual all female species according to someone who had encountered them elsewhere locally (we see these fairly often on our porch). Most mantids have 'normal' sexual reproduction.
Left image from Wikimedia Commons, public domain. Right image by the author. Sydney Aquarium, Australia
More and more examples of asexual reproduction are coming to light in reptiles (none are known in either birds or mammals) The file snake on the left is an example of a species where isolated females have given birth in a zoo with no opportunity to mate. The same is true of the Komodo Dragon. It is not known in such species if this happens frequently in the wild. The tiny snake on the right is known as a Flowerpot Snake. It is a burrowing species and has been transported around the world in flower pot soil. It is also an asexual all female species which has undoubtedly made migration easier as only a single snake can establish a new population.
Image from the Bishop Museum website.
This is the Indo-Pacific gecko. It is a member of a group of geckos (genus Hemidactylus)that frequently live in close association with humans and are transported by us across great distances, usually inadvertently. Most species are sexual but this species is asexual and all female which may have been an advantage in colonizing Pacific islands.
Image from Wikimedia commons.
This is a bdelloid rotifer. They are microscopic aquatic animals. The bdelloids are famous among biologists as being the largest group of animals known (a few hundred species) that is exclusively asexual. Otherwise asexual animals are widespread but there are only a few species in any given group. It never appears that asexuals are out-competing sexuals and evolving into entire clades (groups of species with a common ancestor) in which sexual reproduction does not exist.
So far we have largely looked at animals. Does the same pattern hold true for other non-bacterial organisms? Well, sort of. Asexual reproduction is more common in plants and fungi than it is animals but sexual reproduction is still widespread and there is little evidence of large groups of exclusively asexual organisms.
Probably the most common situation is illustrated by this clump of Aloes.
Image by the author. Fairchild Botanical Gardens, Miami, Florida
Like many plants, aloes can reproduce asexually by sending out runners or simply budding off new shoots. Almost anyone who has gardened or kept house plants will have encountered this. This clump of aloes probably represents a clone, the genetically identical offspring of a single individual, produced asexually.
However the aloes are also flowering, attempting to reproduce sexually. This is a common pattern among many groups of organisms, reproducing both sexually and asexually. Exclusively asexual plants and fungi are certainly known but I believe the same basic pattern is true that you see in animals. You don't see large groups of closely related exclusively asexual species. Exclusive asexuality seems to be (mostly) an evolutionary dead end. Asexual plants tend to be 'weedy' colonizer species, similar to many of the animals I've mentioned.
Image by the author. Aravaipa Canyon, Arizona.
Or they are species for whom sexual reproduction is impractical. Ignore the Gila Monster and look at the two clumps of green near its tail. Those are desert ferns. Ferns, unlike seed plants, have sperm that swim through water to fertilize eggs. Hence the abundance of ferns in wet habitats. Desert ferns have largely if not wholly adopted asexual reproduction because the ecological conditions for sexual reproduction do not exist for them.
So why is sex so common? There are quite a few hypotheses. I'm not going to try and cover them all but just give you a feel for some of the major ones.
- The Red Queen hypothesis. The idea here is that the world around you (parasites, predators, competitors) is constantly evolving and to keep up 'you' need to be constantly evolving as well. The hypothesis name comes from 'Through the Looking Glass' where the Red Queen needed to keep running just to stay in place. The hypothesis was framed in the context of parasites which have a very close association with their hosts.
- The Tangled Bank hypothesis. The term comes from the last paragraph of the 'Origin of Species' and refers to the variation in time and space of the natural world. The idea here is that producing more genetically variable offspring has an immediate advantage because it increases the chance that some of your offspring will end up in environments suited to their particular genetic composition.
- Evolution of advantageous genetic combinations. Sex allows 'good' mutations that have occurred in different individuals to combine and occur in the same individuals in the future. So by reproducing sexually you may be bringing in novel beneficial genes for your offspring.
- Removal of mutations. Based on a couple of different mechanisms it may be that sex helps get rid of 'bad' mutations and prevents favorable genetic combinations from being lost due to chance in small populations.
- DNA repair. This hypothesis states that the main function of sexual reproduction is repair of DNA damage. Normally we would think of the DNA repair as a beneficial side function of sex but this hypothesis turns the whole concept upside down. The generation of genetic variation is a side effect of a process whose primary function is repairing DNA. Interestingly. the bdelloid rotifers, mentioned above have a mechanism for DNA repair that does not involve sex.
The problem with options 1-4 is that they can be shown to work mathematically but it seems unlikely that the range of conditions in which they can work is restricted enough that they are unlikely to be a general explanation. Hypothesis 5 is plausible but it is difficult to know how to test it.
My last topic is a bit of a side note. You may have noticed that the cost of sex in terms of reproduction could be called the cost of males. Or why do males exist? A more accurate way of phrasing this is to call it the cost of anisogamy. What is anisogamy? The term refers to having gametes (reproductive cells) of unequal sizes, in other words sperm and egg. In a biological sense, male and female are defined by gamete size. Individuals that produce relatively few large gametes are female and individuals that produce many small gametes are male. Outside of the fungi most sexually reproducing organisms have two sexes that can easily be classed as male and female. I think part of the answer of the mystery of sex may lie here but how I don't really know.
There are organisms that are isogamous. Most fungi fall into this category, as do a number of algae such as the Spirogyra below.
Image from Wikimedia Commons
Spirogyra looks like a mat of fine green threads in a pond. Up close it looks like this. The connections between cells in different filaments are matings. This species, and most fungi and many other algae have sexual reproduction but don't have males and females. All gametes are the same size. In this situation the cost of sex might not exist or might be much smaller because, presumably, an asexual form would have to make gametes that were twice as large to make up for not fusing with a gamete of equal size.
It is sobering to think that something so central to our existence remains wreathed in mystery. Even those of us, like myself, who have no children, were produced through this process. To quote John Maynard Smith, another famous evolutionary biologist who wrote a book on the topic: it is hard not to feel "that something essential is being overlooked."
Why should we care? Is this just an arcane topic for academics? While the evolutionary origins and maintenance of sexual reproduction may be mysterious, the consequences are obvious. Sex greatly increases genetic variation, the raw material of evolution. The great variety of plant, animal, fungal, and other life on earth is, in part, driven by sex.
Human changes to the planet probably, on average, favor weedy asexual species. We fragment habitats, simplify ecosystems, and leave large disturbed areas to be colonized. Our new world favors species that can move around and breed rapidly. We certainly aren't going to eliminate sex but we might take it down a peg.
Well that's it for this time. I realize that I didn't cover other topics related to evolution and sexuality such as the evolution of sexual behaviors, variation in matings systems, and so on. If there is interest I can discuss these in a future diary.
Next time I'll talk a bit about the ecology of one or more gulf of mexico habitats and some possible effects of the oil spill. I attended a workshop over the last couple of days to plan research on ecological effects of the oil and I learned some fascinating stuff (a lot was basic ecology of different gulf habitats and also some stuff on oil chemistry). I intend to have a pretty varied menu of topics as long as they have some link to biodiversity. I am interested in exploring possible evolutionary responses to climate change as well. And some fun diaries with cool natural history.