Tag Archives: evolution

Ask the Biological Anthropologist: Issue #2

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.

wolverine

Such as incredible healing powers that allow for the implantation of adamantium claws.

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.

Romance is optional.

Romance is optional.

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?

It turns out this isn't a great answer in evolutionary biology.

It turns out this isn’t a great answer in evolutionary biology.

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”:

Illustration by John Tenniel, courtesy of Wikipedia

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!

Natural Selection: Helping stack the deck in your favor for over 1 billion years.

Stay tuned next week for Issue #3 of Ask the Biological Anthropologist!


References Cited:
1. Burt A, Bell G (1987) Mammalian chiasma frequencies as a test of two theories of recombination.  Nature, 326, 803-805.

Further Reading:
1. Fabian D, Flatt T (2012) Life History Evolution.  Nature Education Knowledge.  [http://www.nature.com/scitable/knowledge/library/life-history-evolution-68245673]

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Evolution: this time it’s personal

Usually if I’m thinking about evolution, it’s in a broad academic sense, wondering how the pressures of natural selection have shaped certain characteristics in humans or other species over thousands or millions of years.  Rarely do I stop to think about my own personal relationship with the evolutionary process, or about how my membership in the taxonomic group Homo sapiens predestines me to particular biological challenges.

But now that I am — SPOILER ALERT — in the midst of my first pregnancy, the realities of my evolutionary history are hitting me fast and hard.  I’m suddenly distinctly aware of the physical and physiological difficulties of maximizing my reproductive fitness.  Though it means I am successfully playing the evolutionary game, it turns out that in practice, passing on my genetic material  to future generations is much more of an ordeal than I had anticipated!

Of course, this is not an entirely new revelation for me.  As a student of anthropology I have long been taught that the evolution of bipedalism combined with increases in brain size substantially increased the difficulty of human birth by necessitating a larger head to pass through a narrower pelvic outlet than is seen in chimpanzees and other primates.  In order to enter the world, a human baby must engage in a series of gymnastic maneuvers through the birth canal, and women everywhere — regardless of their knowledge of evolutionary history — know that it’s no exaggeration to say that the process of labor and childbirth is a difficult one.

Image copyright Karen Carr, Smithsonian Institution

Left: chimpanzee infant head size relative to pelvic opening; Right: human infant head size relative to pelvic opening

But the seemingly endless weeks of overwhelming fatigue, constant nausea, headaches, and light-headedness that characterized my first trimester and have now continued into my second have got me thinking about the challenges of reproduction beyond the context of childbirth (still months away), and for the first time I find myself envious of the seemingly easier reproductive pathways that have been favored in other lineages.

For example, my hormone-drenched brain has recently been wondering what’s so great about internal gestation.  After all, it’s not the only viable reproductive strategy.  And with all the discomforts I’m experiencing as a result of the baby growing inside me, I can’t help but fantasize about how much more pleasant things would be if I could emulate a bird and bring forth life simply by sitting on an egg.  Seems ridiculous, right?  Perhaps, but when it comes down to it, some of my favorite activities — reading, crossword puzzles, jigsaw puzzles — are best accomplished while seated, and I’ve gotten  quite good at it over the years.  Moreover, external gestation would go a long way toward evening the sexual division of labor associated with reproduction.  My husband and I, equally capable of egg-sitting, could share incubation duties (as in most species of penguin), and neither of us would need to be subjected to back pain, heartburn, or any of my other aforementioned symptoms.

In a similar vein, I find I have a newly disdainful attitude toward the functionally useless nipples of men.  I am perfectly aware that males have nipples only because of the default human developmental trajectory; selection has had no reason to eliminate them.  Yet I recently found my train of thought wandering along rather critical tracks:
“Come on selection, can’t we step things up a notch?  The equipment is more or less in place — let’s follow the example of the Dayak Fruit Bat* and put it to use!”

And what about sex determination mechanisms?  In humans, sex is dictated by the chromosomal combination that results when sperm randomly meets egg: females have an XX genotype while males are XY.  But in some species, primarily in the reptile class, sex determination is temperature-dependent.  Alligator eggs incubated at or below 86°F produce females, while those incubated at or above 93°F yield males.  In still other cases, both chromosomes and temperature are involved, and “certain incubation temperatures can “reverse” the genotypic sex of an embryo.”

Image copyright Laura Dewan, University of Hawaii

Now, I’m not saying that I want to choose the sex of my baby.  In fact, my husband and I have decided not to find this out pre-delivery.  But I’m sure that some couples, given the choice, would gladly renounce their evolutionary heritage in order to gain this ability.

As for me, I have no doubt that I am oversimplifying.  The seeming perks of other animals’ reproductive pathways must surely be counterbalanced by their own set of drawbacks.  And, as my mother reminded me, I should keep certain broad facts in mind:
1. Memories of my discomfort during pregnancy will fade over time
2. It could always be worse — the road to reproductive success for a female elephant begins with a 22-month gestation!

Kudos to you, Madame Elephant.  Kudos to you.

newborn-elephant

*The Dayak Fruit Bat example may not have been part of the original train of thought recounted here.

The Full Spectrum

Let’s talk for a moment about the special status that humans have in the natural world — or rather, the special status that we often accord ourselves.   Though most people no longer think of the world in terms of Aristotle’s scala naturae, we still have a tendency to consider ourselves unique, separate from and far more advanced than the rest of the animal kingdom.  And it’s true that humans do lots of things that other animals don’t: we  build cities, we fly airplanes, we write books (and blogs) and create works of representational art.  As a species, our accomplishments are substantial.

But if there’s one thing that modern ethological research has demonstrated, it’s that many of our abilities and behaviors can be found at least to some extent in other species, especially in the primate family.  Since Jane Goodall’s early observations of tool use among wild chimpanzees at Gombe Stream National Park led her academic adviser, paleoanthropologist Louis Leakey, to comment that “now we must redefine tool, redefine man, or accept chimpanzees as human,” several once-vaunted hallmarks of humanity have been eliminated from the list of defining characteristics that separate Homo sapiens from other species.  Primatologists have provided evidence that chimps hunt, participate in campaigns of organized aggression toward neighbors, maneuver for status in Machiavellian ways, and reconcile following conflict.  Research on language use by great apes similarly suggests that in many cases the difference between humans and our closest relatives is one of degree rather than kind.

The existence of this behavioral spectrum  has been illustrated yet again in the new issue of the journal Current Biology, in which anthropologists Richard Wrangham and Sonya Kahlenberg report that young chimpanzees in Kibale National Park in Uganda carry and play with sticks in a manner similar to children with dolls.

chimpanzee-stick-doll

A female chimpanzee holds a “stick doll” between her leg and body. (Sonya Kahlenberg)

Examples of play among juvenile animals are not new.  But it is intriguing that these researchers identified a sex-based bias in their data: as in humans, female chimpanzees were more likely to carry stick dolls.  Does this mean that young females are simply practicing for the future by imitating the mothering behaviors they observe within a group?  Or might it suggest innate psychological differences between young male and female chimpanzees?  In either case, how do these data pertain to studies of sex/gender differences in humans? Nature/Nurture debaters, start your engines!!

Whatever the long-term implications of this new study, it serves to illustrate that the more we learn about the behavior of other animals, especially apes, the less reason we have to consider ourselves wholly separate from them.  Of course, this isn’t to say that you should expect to see orangutans in Indonesia building treehouse cities anytime soon.  But try not to be too taken aback the next time you read a story about a wild animal doing something “surprisingly” human-like.
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To read more about play-related sex differences in non-human primates, click here.  And don’t forget to subscribe to The Tinkering Primate if you want to get emails about new posts.  Just click the “Yes Please” button underneath the chimpanzee on the site banner.

The Basics

If I had to choose three adjectives to describe Darwin’s theory of evolution by natural selection, they would be:

1. Elegant
2. Powerful
3. Simple

Yes, simple.  Although evolutionary biologists now know that forces other than natural selection (e.g. genetic drift and gene flow) can cause populations to evolve, Darwin’s theory still provides the basic framework for understanding the history of life, and explains much of the variation we see on our planet.  And it does it with just three premises (hence my simplicity claim).  Any student who passes through my Introductory Biological Anthropology course can recite them for you:

1. Individuals in a population vary
2. Variation is heritable
3. Because of this variation, certain individuals are able to survive and reproduce more successfully in a given environment than others.

That’s it.  Those three premises are all that is required to understand the idea of evolution by natural selection.  So why is there so much confusion and misunderstanding surrounding Darwin’s ideas?  Why, during last year’s celebration of the 200th anniversary of Darwin’s birth and the 150th anniversary of the publication of On the Origin of Species, did a poll reveal that only 4 out of 10 Americans believe in evolution?

The ongoing evolution vs. creationism/intelligent design battle aptly demonstrates that religious background plays a significant role.  But in some cases, people may not believe in evolution by natural selection because they don’t understand the process.  Many misconceptions abound simply because people haven’t been properly introduced to the Darwinian basics.

So in the spirit of defending evolutionary thought, I want to share a few  clarifications that I have found useful when introducing students to Darwin’s theory of evolution by natural selection.

  • The third premise — that because traits vary, certain individuals are able to survive and reproduce more successfully in a given environment than others — is often abbreviated to “Survival of the Fittest.”  But this shorthand only works if people understand that the term “fit” is relative.  Individuals that are highly fit (i.e. able to produce many offspring) under one set of environmental circumstances are not necessarily the “fittest” under different conditions.  In other words, changes in an organism’s habitat can turn underdogs into winners and vice versa.  This is key.  Remember it.
  • Natural selection and Evolution are not terms that can be interchanged at will.  As I tell my students, natural selection is a process that affects individuals, but it is populations that evolve as fitter individuals reproduce more successfully and pass on beneficial trait variations.
    A quick example: Among a population of tree frogs, bright red individuals who are more visible against the green backdrop may be subject to higher predation and die before they reproduce.  As a result, fewer “red color” genes will be passed on to subsequent generations and over time the population will have higher frequencies of the beneficial-for-camouflage “green color” genes.
  • Humans (aka Homo sapiens) are not the pinnacle of evolution.  In fact, there is no pinnacle of evolution, because natural selection is not a unidirectional process in which organisms become “better” in an absolute sense (see the above point about underdogs).  Humans are simply the most recently evolved members of one particular primate lineage.  And, despite what many of my students think, most experts agree that we are still evolving.

That’s the end of today’s lesson.  Now go and explain these premises to everyone you know and maybe, just maybe, we can top 50% in the next Gallup poll.