Tag Archives: natural selection

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]

Sgt. Macaque and Dr. Shepherd, reporting for duty!

What’s that?  You’ve never heard of Sgt. Macaque or Dr. Shepherd?  Well that’s probably because I just made those names up.

Dr. Dog

What do you mean “questionable credentials”?

I did so, however, based on two very real headlines that I happened across in the past few weeks.  Both are fascinating examples of humans taking advantage of other species’ adaptations to help us meet our goals, and both got me thinking about the prevalence and significance of this kind of human/animal ‘partnership.’

Headline #1: “Meet the Monkeys Keeping Chinese Troops Safe” 

For those of you not inclined to click on the link, I will summarize.  The People’s Liberation Army of China (aka the Chinese military) has revealed that a small unit of trained macaque monkeys is being used at one of the country’s Air Force bases to prevent migrating birds from nesting in the area and potentially getting sucked into aircraft engines (an occurrence that is good for neither the bird nor the aircraft/pilot).  According to the PLA, the monkeys, which are trained to destroy birds’ nests in response to whistle commands, are proving a more effective deterrent than scarecrows, netting, firecrackers, or human soldiers.  Not surprising, given that monkeys have evolved to be more adept at arboreal maneuvering than men of either flesh or straw.

And in other news…..

Headline #2: “Dogs Sniff Out Prostate Cancer With 98 Percent Accuracy”

This one is pretty self-explanatory.  Researchers in Italy trained two German Shepherds that had previously worked as explosive-sniffing dogs to recognize the scent of volatile organic compounds — chemicals associated with cancerous tumors — in urine samples.  In a subsequent blind study of almost 700 men, the dogs correctly identified which urine samples came from men with prostate tumors 98% of the time.  This success echoes previous research with medical detection dogs, which has found evidence that dogs can detect lung and breast cancers by smelling a person’s breath, and that they can be trained to warn individuals with diabetes or epilepsy of low blood sugar or impending seizures.

Considered alongside the other tasks in which dogs aid humans (some, such as search-and-rescue efforts and drug-sniffing, are well-known; others, such as identifying diseased beehives, sniffing out bedbugs, and tracking orca poop, are not), two inescapable conclusions appear:

1. Dogs have an amazingly good sense of smell.  Their noses contain up to 300 million olfactory receptors (ours have a measly 6 million), and a substantial portion of their brain is devoted to analyzing the smells registered by those receptors.   This means that they can smell in parts per trillion.  Imagine being able to smell one drop of blood in 20 Olympic swimming pools worth of water (sharks, by contrast, smell in parts per million or billion), and you’ll start to get an idea of how natural selection has honed this adaptation in canines.

2. Humans are good at co-opting it.

Best picture ever? Dog in homemade bee suit. Image credit Josh Kennett.

Humans, in fact, have created quite a niche for ourselves by exploiting other animals’  abilities.  And we’ve been doing it for a long time.  Recent evidence suggests that humans have been co-evolving with dogs, the first domesticated animal, for 30,000 years.  The domestication of farm and labor animals was more recent but, as evolutionary biologist Jared Diamond has thoroughly explained, it has had an enormous impact on the development of human societies over the past 10,000 years.  The truth of the matter is that humans just wouldn’t be where we are today if we didn’t have these conscripts.

But here, I think, is where it is worth making a big distinction between the subjects of the two headlines above.  Macaques are not domesticated.  Nor are a variety of other species that humans have put into service more recently, such as military marine mammals.  And this raises some major ethical dilemmas.  Is it okay for humans to use animals in this way?  For decades, the US Navy has taken advantage of dolphins’ swimming and echolocation abilities to detect and clear mines (good for the human population), but military sonar has simultaneously contributed to making the ocean a more unhealthy environment (bad for marine wildlife).  How much of a qualitative difference is there between this and the efforts of an organization like Helping Hands, which trains capuchin monkeys to act as service animals to the severely disabled?  Under what circumstances, if any, does human need trump an animal’s (or species’) right to live undisturbed in its natural habitat?  Do the “rules” differ for domesticated vs. non-domesticated species?  As someone who has admitted to having a Grand Canyon-sized soft spot for animals, these are questions that I genuinely don’t have answers for, and I am curious to hear other people’s thoughts.

In the meantime, I will finish with one more current news story, this time about a handful of humans doing something to help out animals:

Headline #3: “Wolf Pups Rescued From Funny River Fire in Alaska’s Kenai National Wildlife Refuge”

Image credit Kenai National Wildlife Refuge, via livescience.com

Thanks, guys.


Medical Detection Dogs and the In Situ Foundation are but two of myriad organizations devoted to training dogs to use their noses in service of human health.  Read this essay, however, for an alternative perspective on the use of dogs in medical detection.

Hamadryas and Pangolins and Mastodons, oh my!

As mentioned in my very first blog post, one of the reasons I started with this endeavor is that I have a lifelong love of learning.  Education is an ongoing process, and knowledge, whether it is put to practical use or sought simply to satisfy personal curiosity, is a fantastic thing.  Imagine my excitement, therefore, when I learned of the 2014 Mammal March Madness competition run by Dr. Katie Hinde, an assistant professor of Human Evolutionary Biology at Harvard University.

Picture adapted from art by Tracy A. Heath, Matt Martyniuk, Sarah Werning, via Phylopic! As seen on the blog Mammals Suck…Milk!

This is my kind of pedagogy!

The competition first caught my attention, I must admit, simply because of its enthusiastic mention of mammals (note the exclamation point in the title of her blog post).  As I’ve said before, I have a rather large soft spot for animals, and am easily persuaded to investigate stories or headlines that promise some kind of faunal component.  The more I thought about it, though, the more I began to admire the ingenuity of this exercise.  Here’s why:

1. It brings science to the masses, and makes it fun.

The NCAA March Madness tournament is ubiquitous at this time of year, and the popularity of bracket competitions is continuing to increase.  ESPN even has its own “resident bracketologist,” which at first glance sounds like a job title as realistic as “space smuggler.”

Although it turns out space smugglers are not only real, but fairly common.

Taking advantage of the pervasiveness of this cultural phenomenon to educate people about science is brilliant.  It not only puts the lesson in a familiar context, but appeals to people’s natural competitive instincts by turning it into a game. And for those like me, who are already interested in science but not well versed in basketball, it provides a different kind of learning opportunity: I now understand the nature of “seeds” and know how to fill out a bracket.

2. It only looks simple.

You may be thinking, “If I want to play I just need to look at the bracket and pick some winners, right?”  Technically you’re correct, but as with the NCAA bracket your chances of winning are much better if you make educated picks.  And in this case, that means you need to know not only what an animal is (Hinde’s bracket, as seen below, has an entire division entitled “The Who in the What Now” that is populated by little-known species), but where it lives and how it lives.  

Hamadryas Baboon. Image from Arkive.org.

Pangolin. Image from Arkive.org.

Mastodon. Image from National Geographic.

A laundry list of questions quickly emerges:  How big is it?  How fast is it?  What kind of “weaponry” or defenses does it have?  Does it live by itself or in a group?  To make things even more exciting, the simulated battles in the early rounds of competition take place in the natural habitat of the higher-ranked seed — Hinde calls it the “home court advantage” — but battle locations are randomized at the level of the Elite Eight and beyond.  Could a pack of hyenas triumph over an orca?  Probably, if the battle were to take place on land!

Even with my more-extensive-than-average background in biology, I found myself diving into Wikipedia articles and professional journals to find the information necessary to assess each species and select winners.  Among other things, I’ve learned in the past week that Hamadryas baboons can amass in troops as large as 800 individuals, that the bowhead whale has a layer of blubber that can be 17-19 inches thick, and that, despite its name, the godzilla platypus (which lived 5-15 million years ago) is only twice as large as a modern-day platypus.

Like this.  But 3 feet long!  Image from National Geographic.

3.  It’s memorable.  

I mean this in a dual sense.  First, because it’s happening outside the classroom, the learning that occurs during the Mammal March Madness tournament is likely to be contextualized differently than information read from an assigned textbook.  And while it’s difficult to say whether any knowledge about animals that’s gained through participation in the MMM bracket game will be more likely to be retained due to its association with a cultural touchstone, it’s a form of education that I wholeheartedly endorse (see: my goal to someday teach a Primates & Pop Culture class).

Second, the game itself is memorable.  This may be an especially important factor for kids, or those who are kids at heart: you play, you learn, and then you will want to play again (and learn about a new set of mammals) next year.  It’s the circle of life learning!

_______________________________________________________

The Mammal March Madness tournament is now underway, and you can follow #2014MMM on Twitter to keep track of all the live action.  I have the Saber-toothed Cat going all the way, but anything can happen in this mammal-eat-mammal world!

The Saber-toothed Cat. 850 pounds. 12-inch canines. Good luck.

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.