evolution 101

What follows is my quick overview of evolutionary theory, adapted from course materials that I have developed over the last few years.  Some understanding of genetics is required-- at the least, look up the terms "gene", "allele", and "mutation", and you should be good to go!  All of the text content is mine, but obviously images are borrowed, and attributed when possible. For now this just covers evolutionary theory.  Eventually I'll add my own primer on HUMAN evolution, but that's a whole big thing.

Natural Selection


Darwin, the 1800's naturalist who developed ideas about evolution after sailing around the world, knew that species had diverse traits, and that the specific traits an individual has will help or inhibit its survival and reproduction.  He didn’t know about genetics, so he couldn’t know that the source material for this variation was genetic mutation.  Random mutations in genes produce new alleles (allele= any alternative form of a gene, coding for a slightly different protein).  Of course, some alleles will be beneficial, while others are harmful in some way.  Organisms that are better suited to their environment– thanks largely to their unique combination of alleles– will out-produce other individuals, leaving more offspring.  These organisms are said to have greater fitness. In this way, advantageous alleles become more common in a species, and disadvantageous alleles become less common.  This is evolution by natural selection. Given a novel environment and isolation, populations can even evolve into new species, like the marine iguana above. (note: Evolution is an observation.  We can see that populations, even species, change over time.  Natural selection is a theory explaining how this occurs.)


wikipedia

Darwin’s ideas about evolution were not widely accepted during his time (the 1800’s), due partly to prevailing views about the fixed and created nature of life, but also because he lacked a mechanism to explain natural selection.  It was only in the early/mid 1900’s, once DNA had been discovered, that evolution by natural selection fully made sense.  This unifying of genetics and evolutionary theory, called the “modern synthesis”, forms the basis of our current understanding of biological evolution.  The graphic above illustrates how natural selection works.  Notice how none of it makes much sense without the genetics piece– mutation.  Mutation is random.  Natural selection will work to increase or decrease the frequency of a mutation IF AND ONLY IF that mutation is present in a population.  Mutation is random; natural selection is quite the opposite. 

A basic definition of evolution is “biological change over time within a population”.  So let’s define population: A group of organisms living in an area that can interbreed with each other.  All of the genes in this group of organisms is called a gene pool.  How the frequency of these genes—or rather, the particular alleles of each gene—changes over time is what evolution is all about.  Let’s also get a definition of species, using the most common (but not the only!) species concept, the “biological species concept”: any group of organisms who can interbreed and produce fertile offspring.  So, different populations of humans are still considered one species because we can interbreed. 

mansfieldct.org
A classic example of natural selection: in 1800’s Britian, as the industrial revolution covered the landscape around London in soot, white moths of a particular species were suddenly at a disadvantage.  Birds could easily detect them on the once-white trunks of birch trees.  Alleles for mottled black color—already present in low frequencies in the species– became more common. 


Natural selection can take several patterns.  Stabilizing selection occurs when both extremes of a trait—say, large or small body size– are detrimental and become less common.  Directional selection favors one extreme over the other.  Disruptive selection favors both extremes and makes the middle phenotype less common.  The distribution of human infant birth weight is a great example of stabilizing selection.  For most of human history (and still today, in many parts of the world), babies born small had a decreased chance of survival, and large babies increased the risk of death for both the mother and infant during childbirth.  Genes influencing birth weight have thus been strongly selected for, and today the distribution of infant birth weight forms a bell curve, with “medium” weight babies being most common. 
The graphic above is misleading, however: The grey shaded area shows the “population before selection”, implying that early humans had higher proportions of small and large babies.  This is unlikely; stabilizing selection has operated to produce a bell curve probably for several million years, at least since hominin brains began increasing in size, posing a problem for childbirth.

Let’s look at another example of natural selection acting on our species.  Probably in every human population there has always been a low frequency of a particular allele of the hemoglobin gene—let’s call the allele “S”– that, instead of producing normal red blood cells, produces sickle-shaped ones.  This mutation has certainly arisen randomly in many populations many times, as a result of several possible single-nucleotide copying errors that all produce the same trait (sickle-shaped red blood cells).  This gene has a codominant expression pattern: SS individuals are sick and until recently died before reproducing; AS individuals (with one normal allele) are mostly healthy; and AA individuals have totally normal hemoglobin.  However, the S allele persists in amazingly high frequencies in Africa.  Why has it not been reduced to the allele frequencies seen elsewhere? 

It has to do with another disease- not a genetic one, but one transmitted by mosquitoes: Malaria.  It so happens that this parasite cannot live in sickle-shaped red blood cells.  Individuals carrying one copy of the S allele– AS individuals– are immune to malaria.  This survival advantage keeps the S allele at high frequencies in places where malaria has been and continues to be prevalent.  Evolution by natural selection has favored AS individuals, in spite of the deadly effects of having an SS genotype.  AS individuals on average will have excellent reproductive success, keeping the S allele frequent; if they have offspring with another AS individual, there's a 25% chance that their offspring will be SS, fully affected by sickle cell anemia.

Below, the top map shows the allele frequency for allele S.  The map on the bottom shows the prevalence of malaria.  Notice how similar these maps are for Africa:


Clearly the same process hasn't happened in malaria-endemic South America and Western Asia.  Perhaps the S allele was never present in a high enough frequency, or natural selection hasn't yet had time to produce the same effect. Surely somebody has studied this.  Look it up!

Can natural selection explain altriusm?

Humans, like many other animals, will risk their lives for each other.  Such risky behavior without immediate reward is called altruism.  Research first done with squirrels in the 1970’s demonstrated that it’s not just an animal’s direct fitness (its own ability to survive and reproduce) that matters… natural selection will also select for behaviors that help an organism’s inclusive fitness, which includes the reproductive success of genetic relatives.  A squirrel sounding an alarm call is more likely to be eaten, but in saving her genetic relatives (who comprise her group), they can pass on more of her genes than she can! (Example- 3 of her siblings, each of whom has 50% of their genes in common with her, can, if they have offspring, pass on their mutual genes 1.5x more than she can.)  In this way, genes that influence altruistic behaviors are passed on by the beneficiaries of altruistic acts.  Human behaviors too have surely been shaped by kin selection– our love of family, especially offspring, is at a biological level resultant from the fact that your relatives carry your genes (and your offspring carry your genes and your enormous time/energy investment).  But highly social animals, like primates and humans, have an additional biological altruistic urge towards non genetic relatives.  In short: close-knit societies depend upon trust.  Cheaters—those who will take or accept help without reciprocating in the future- stand to gain in the short term, but because humans have evolved instincts for detecting and then punishing cheaters (highly reinforced by cultural norms), genetic factors that influence selfish and antisocial behavior are selected against.  At least in complex animal societies like those humans form.  A game called the Prisoner’s Dilemma, first developed for economics, illustrates how a strategy of “reciprocal altruism”– I’ll help you now because I’m expected to, but if you cheat me, I’ll stop helping you– can evolve and have genetic roots in a species. 

Mutation and natural selection are not the only processes that cause evolution.  Let’s look at 3 other forces, or mechanisms, that explain how populations and species change over time.

Sexual Selection: producing traits enhancing reproductive success

Darwin himself lamented that some traits appear to be counterproductive.  Especially troubling to him was the plumage of the peacock.  These bright, heavy feathers would be a burden, and would expose the bird to predators.  How could they persist?  The answer, which dawned upon him years later, is called sexual selection, a variation of natural selection.  (video clip)  Some elaborate traits are an honest indicator of genetic quality.  A peacock with gorgeous feathers is very fit– strong, healthy, free of parasites, and capable of evading predators even with the handicap of an enormous tail.  The mating behavior of females (peahens) also evolves–they have evolved to select males with the best feathers, because these males have the best genes, and together they produce more successful offspring.  And so such traits become “fixed” in a species.  The mane of a male lion is another example.  In fact, all sexual dimorphism (physical differences between males and females) has evolved due to sexual selection.  Remember that evolution has no “end goal” in mind– it is simply a long series of competitions between alternative genes and traits.  The results aren't always what you'd design if you were an engineer of species. 

As we said, traits that are honest indicators of good genes will become more common in a species, because the sex doing the “choosing” will evolve to be attracted to individuals with these traits.  This sounds great in theory, but has it been experimentally tested?  Yes, and experiments with peafowl have demonstrated that males with “better” tails get more mates, and their offspring have higher survival rates.  Having an extravagant tail really does correlate with having high genetic fitness– a strong immune system, and perhaps a strong physique for carrying around such a tail without getting eaten. 
Why does sexual selection occur?  Because, in many animal species, males and females have very different levels of investment in reproduction.  Males of most species invest very little: sperm are cheap to produce; they don’t gestate young (if mammals) or lay eggs; they seldom raise young (at least not to the extent that females do).  

Sexual selection can take 4 distinct forms, and sometimes several will be at play in a given species.
1)Female choice/male indirect competition.  Simply put, females are highly selective in choosing males.  This results in males evolving ornate traits or behaviors to indirectly compete with each other for female interest.  
2)Male direct competition.  In many species, males develop traits useful only for physical combat or display.  In these species, the victorious male gains access to most of the females, while most other males won’t mate at all.  This evolutionary process can lead to traits that are really disadvantageous for survival, such as large antlers.  It also explains why male animals are often larger than females: larger males gain access to mates more often, passing on their “large” genes. Once, on a trail run, I encountered a group of female deer who had formed a circle.  In the center were two males actively fighting for the right to be the dominant male.  So important was this contest, for all involved, that they didn't notice when I stopped and watched. It was like a middle school fight only the stakes were real.
3)Sperm competition.  In species where females mate with multiple males in a mating season, there is an evolutionary pressure for increased sperm number and vigor.  Even if a male isn’t the first to mate with a given female, if he has more sperm than other males, his sperm may displace previous sperm or outcompete them in the race for the egg.  This explains the relatively enormous testicles of animals like sheep.  (Full disclosure: I noticed this once at a fair and I was stunned, but quickly realized the evolutionary explanation.)  Damselflies even have tiny barbs on the end of their reproductive organs for removing sperm from a previous male!
4)Infanticide.  Male lions will sometimes kill a female’s cubs.  Without cubs to rear, she will soon become fertile and responsive to mating again, and the new male is likely to sire her next offspring.  Some researchers have even suggested that this phenomenon can be seen in humans: men are much more likely to commit violence against their stepchildren than biological children.  This could also be due to the stronger biological instinct we have towards our genetic progeny, versus nongenetic children that we rear…either way, part of the explanation has roots in our evolutionary history.
….whether and how sexual selection has played out during human evolution is a fun question to ask at a dinner party.  Try it sometime.  What human traits are likely due to our evolved need to procreate?...This will get lively.

Genetic Drift: random evolutionary change
Natural Selection and Sexual Selection are NOT random- they are nonrandom, and they are adaptive, meaning they increase the frequency of beneficial genetic traits.  But some evolutionary change is random.  Genetic drift is a random force of evolution– it changes the frequency of alleles (and the traits they produce) randomly, and in ways that may or may not benefit survival and reproduction.  It takes 3 forms. 
1)In small populations, any particular allele may increase or decrease in frequency quite drastically, simply because of sampling error (top picture below).  If there are, say, only 100 individuals in a population, and the allele frequency for a particular allele is 50%, odds are high that it won’t get passed on exactly as often as you’d predict.  It’s like flipping a penny 10 times: you expect 5 heads and 5 tails, right? You probably won’t get those odds.  But if you flip the penny 1000 times, you’ll get much closer to 50% heads and 50% tails.  Assuming an allele isn’t highly advantageous or disadvantageous (which would make it the target of natural selection!), genetic drift can drive an allele to high frequencies, or make it disappear altogether, in small populations.



2) An event killing off most of a population can have the same effect.  This is called a bottleneck event (bottom picture, above).  Genetic variation in the surviving population will likely not have the same allele frequencies as it did before the event.  Bottleneck events reduce genetic diversity and can drive the frequencies of particular alleles to zero or near 100%. 
phys.org

See the chart at right depicting groups of early humans.  Notice that just before our species (Homo sapiens) expanded, our population became quite small, due to some disaster, or drought, etc.  While survival of individuals certainly had something to do with fitness (some being better adapted), many people perished without passing on their genes due simply to random factors– living in an area that experienced drought, for example.  The resulting population was quite small, and had lost many alleles.  Other alleles were then present at an unusually high frequency. This small population rebounded and gave rise to all current humans.  As a result, we are a very genetically homogenous species

3) The Founder Effect.  Humans are very numerous (there are about 7 billion of us), but when our ancestors existed in small populations, genetic drift had a big effect.  Consider a group of an early Homo sapiens subspecies, the Neanderthals, who lived in Europe until about 30,000 years ago.  They were founded by a small population of travelers who settled Europe. and Western Asia.  Whatever genetic traits these founders carried, they were copied forward into subsequent generations.  Some genetic traits were left behind with the source populations, in the Middle East, and before that, Africa.  This phenomenon is called a founder effect: the founders of a new population disproportionately carry some genetic traits with them, and leave others behind, simply as a consequence of being a small population that does not represent all of the genetic diversity found in the entire original population.

Gene Flow: more random change; works to keep populations similar

Gene flow is the final force of evolution that we will discuss. Gene flow is also known as admixture. The basic idea here is that genes can flow across population boundaries, introducing new genetic variants into the gene pool. 
wikipedia
 As one might expect, the closer two populations are to one another, the more likely it is that genes can flow. Populations separated by great distances, or by great geographic barriers, have less opportunity for gene flow. An example of gene flow in human populations is the distribution of blood type B across Eurasia (see map below). Notice that in India and China, blood type B has a frequency of nearly 30%, but it drops to nearly 0% in far western Europe and parts of the Americas. The drop is not precipitous, but quite gradual, suggesting that nearby populations have exchanged genes and that the blood type B allele has slowly flowed east to west across Eurasia over the millennia.  Gene flow changes allele frequencies, so it is by definition a force of evolution; but it is non-adaptive, in that it does not produce traits “for” a purpose.  Note also that gene flow prevents two populations from becoming too different, as genetic changes are shared through mating
blogs.sas.com

Speciation: forming new species

We've run through all of the forces of evolution.  But a question remains: how do we get new species?  It is impossible to pinpoint a moment in time when this occurs, much less to observe this moment, as the process of speciation is gradual and very slow.  According to the biological species concept, two populations have become different species when they can longer interbreed and produce fertile offspring.  This occurs when just a handful of genes have become incompatible.  An example of speciation in process is shown below:

evolution.berkely.edu, a great site

  A species of salamander has migrated down the West coast, splitting into two different populations as they moved down two mountain ranges, on each side of the central valley in California.  When they had spread further south and reconnected in Southern California, the two groups of salamanders had become quite different, and do not interbreed.  (However, they probably could).  A few more genetic changes and these could be considered different species.  Note that speciation requires isolation between populations.  Under these conditions, mutation, natural selection, and genetic drift will drive the two populations to become different from each other.  Gene flow would prevent this. 

nature.com

As I just implied, “species” is not a concept that nature recognizes.  Not really.  It’s a useful concept for biologists.  When two populations are diverging into what we will eventually consider separate species, the process occurs on a continuum.  If we sampled the DNA from these two populations over the course of their speciation (say, thousands of years), we’d see gradual, nonlinear change.  It would be very difficult to pinpoint a moment in time and say “now we have 2 species!”.   And let’s not even get started on the various species concepts that exist.  We will revisit them a bit later, but for now, let’s go with the concept biologists use, the “biological species concept”, mentioned earlier.  It considers species to be reproductively-isolated groups of organisms; that is, a group of individuals who can and do reproduce with each other. 

Because there are multiple ways to define a species, and because species hypotheses are difficult to test on extinct populations, anthropologists disagree about how many species of hominin (upright ape) existed before we came on the scene.  The above chart shows one interpretation of human evolution– one depicting many species.  Other anthropologists, like your textbook author, don’t recognize some of these populations as distinct species, and instead consider them as geographically-separated populations of the same species, using terms like “Archaic Homo sapiens” in place of Homo neanderthalensis and Homo heidelbergensis.  And with that, we're into human evolution, beyond the scope of this primer.  I hope you've enjoyed this! Take your evolutionary knowledge with you and use it as a lens for seeing the biological world!


Links and resources- primers on human evolution


Becoming Human-- website by the Institute of Human Origins.
Becoming Human part 1 - PBS Nova video series on human evolution.
Becoming Human part 2
Becoming Human part 3

The latest (Nov 2016) on the first known hominins outside of Africa:
http://www.sciencemag.org/news/2016/11/meet-frail-small-brained-people-who-first-trekked-out-africa

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