Season 1 Episode 9: Chapter IX - Hybridism

We dedicate this episode of the podcast to Chapter 9 - Hybridism in Darwin's Origin of Species (OoS). We all agree that this chapter is one of the most challenging chapters to read in OoS. In this chapter Darwin force marches the reader through example after example of hybrids while simultaneously admitting his ignorance on why these creatures should exist.

It is not inherently obvious to the reader why they are being subjected to this catalog of biological anomaly and minutiae concerning pollination of orchids and other flowers but Darwin is adamant to share his knowledge, like a proud new parent inflicting others to look at numerous photos of their newborn. Darwin , early in Chapter I of OoS, argued against the notion that new species are formed through hybridization of existing species and we expected him to make that point in this chapter, but he never really does.

It seems that by the end of the chapter we are exhausted from Darwin's recounting various animal and plant hybrids and admitting his ignorance of sterility in hybrids but that does not dissuade Darwin from concluding he was right all along - “…the facts given in this chapter do not seem to me opposed to the belief that species aboriginally [emphasis added] existed as varieties”. Darwin stays on point.

We started off our discussion identifying our favorite hybrids. Josh first suggested mermaids as his favorite hybrid but James required he offer up a "real" hybrid.

Mermaids James grew up with in Florida

Josh introduced us to the Liger, a hybrid between a male lion and female tiger both species have 38 chromosomes which also allows for reciprocal mating. Male tigers and female lions create tigons. The hybrid nomenclature is a portmanteau word derived from using the male species as the prefix and the female as the ending of the name hence a liger and tigon being separate types of hybrids.

900 lb Hercules

Sarah mentioned the Zedonk, a hybrid between a zebra and a donkey. This is another one of the hybrids that are created artificially in captivity. Few people realize that there are actually three different species of zebras - Grevy's, Plains and Mountain zebras - and each species has their own number of chromosomes. Grevy's zebras zebras have 46 chromosomes, Plains zebras have 44 and the Mountain zebra has 32 chromosomes whereas the donkey has 62.

Image from Carole Coleman

James suggested the Grolar, a hybrid between a grizzly and polar bear, was his favorite hybrid.

A grizzly bear with her grolar cubs

Unlike the liger and zedonk, the grolar occur in nature when the two species of bears interact. Historically grolar bears were less common since Grizzly bear distribution was further south than Polar bears but the warming of the climate has caused both bears to extend their range, polar bears southward and grolar bears northward, such they now more commonly overlap during the breeding season. Research suggests Polar bears "recently" diverged from brown bear ancestors with the speciation event occurring only 343,000-479,000 years ago. A mere blink of time in terms of the geological record.

We discussed the most famous of all hybrids, the mule and how it is formed from the male donkey breeding with a female horse. Pictured below is a horse (left) and a mule (right) showing how the mule is often much larger than either of its parents, an example of Hybrid Vigor.

picture by Merle

We noted that the mule hybrid can only be formed from male donkeys and female horses and rarely rarely formed from a male horse and female donkey. Sarah suggested that the reason the mule can only be formed from a male donkey and female horse has to do with the constraints of difference in gestation times between horses and donkeys. It turns out that horse gestation is 11-12 months whereas donkey gestation period is 11-14 months. There is a lot of overlap in gestation time between the two animals but it is an intriguing idea to consider it may influence non-reciprocal successful mating. 

We introduced the idea of prezygotic isolating mechanisms and how they would evolve to prevent hybrids from being formed. Behavioral isolation is frequently seen in the complex displays and calls given by a variety of birds species and James mentioned the beautiful flashing patterns exhibited by the different species of fireflies. The image below shows the species-specific color and flash pattern of lightning bugs in Florida.

Go here to learn more about fireflies

Josh introduced us to the sordid and dark world of duck mating behavior and male duck genital morphology. Here is just one example of the size and complexity of the male duck penis which has evolved in response to female duck vaginas. Female ducks have evolved elaborate shaped vaginas to avoid fertilization from other species of ducks.This interesting topic is covered nicely in this short article here.

Sarah spent some time explaining the intricacies of pollination and how pollen is actually greatly reduced multicellular structure that produces sperm - pollen is actually plant testicles. Plants evolve complex pollen grains, much like the extreme duck penis, to create reproductive barriers between species. The forms of inter-specific (between-species) barriers to reproduction would evolve because those individuals who are more discerning in their choice of mates would waste less time/energy fertilizing and producing those hybrid offspring which are often, at best, viable but infertile but more likely enviable. Often it is the female of the species that is the one who evolves the reproductive challenge for the male because she produces fewer, and often more metabolically expensive, gametes (eggs/ova) than her male (sperm) counterpart and therefore has more to lose if she errs on who she mates with than he. 

Public domain image (created by the Dartmouth Electron Microscope Facility)

Ultimately we conclude that hybrids exist because Darwin was correct in his model of speciation, that is new species derive from varieties of preexisting species (see Chapter 4 blog post) so they share many of the common genetic traits with their ancestral or closely related species. Surprisingly Darwin did not recognize that hybrids are his best argument against the special creation model of immutable species since one should not expect hybridism to occur between two immutably formed species that were specially created.



"That's a Wrap" & "Aces High" Kevin MacLeod (incompetech.com) Licensed under Creative Commons: By Attribution 3.0 http://creativecommons.org/licenses/by/3.0/

Season 1 Episode 8: Chapter VIII Instinct

MANY instincts are so wonderful that their development will probably appear to the reader a difficulty sufficient to overthrow my whole theory. [page 317]

In this episode of Discovering Darwin we covered Darwin's chapter on Instinct and how Chuck attempted to explain how animals exhibit complex behaviors that are not learned. More importantly Darwin was trying to outline how behaviors could evolve in the same way he explained the evolution of physical traits.

It is not too difficult to study the evolution of physical traits because we often have fossil evidence of their transformation. Last episode we discussed the evolution of whales and the plethora of fossil evidence that has allowed researchers to reconstruct the evolution of the terrestrial ancestor of whales to the streamlined marine mammals we see today. 

Behaviors are harder to imagine through the lens of natural selection because we can only see those behaviors that are exhibited by extant (living) organisms and behaviors can evolve much faster than physical traits. Cultural evolution can allow individuals within their lifetime to adopt a new behavior that they learn from others. One of the wonderful examples of this is dolphins using a sponge to protect their rostrum (beak) as they hunt for prey in coral structures. They can pass this idea on to other dolphins and you can track the rapid transmission of this behavior through a population.

Picture from http://www.livescience.com/21989-dolphin-sponge-tools-culture.html

Darwin was not interested in learned behaviors in this chapter but he was interested in behaviors that are known at birth or at specific developmental times in the organism's lifespan. Darwin called these behaviors instinct. In exploring this idea Darwin focused on three major examples of innate/instinctual behaviors:

1. Cuckoos and their behavior to dump their eggs in other birds nests.
2. "Slave making" ants species which capturing of other ant species to become sources of forced labor in their own colonies.
3. Honey bees and their complex, mathematically efficient, honeycomb making behavior.


Cuckoo
Cuckoos exhibit a behavior known as "brood parasitism" where they lay their eggs in the nest of a host species and leave the eggs to be incubated and hatched by the host. The host also raises the cuckoo baby as their own until it is strong enough to fledge from the nest. Below is a dramatic photo showing the poor host Reed Warbler dutifully feeding the ginormous common cuckoo (Cuculus canorus) nestling.

"Reed warbler cuckoo" by Per Harald Olsen - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Reed_warbler_cuckoo.jpg#/media/File:Reed_warbler_cuckoo.jpg

You may wonder why the Reed warbler does not recognize the cuckoo offspring is not their own offspring? What you are really asking is why has the Reed warbler not evolve the ability to recognize its own offspring from another species? If you were switched at birth in the hospital, do you think your mother would know? What ways do we know our offspring are actually ours? Only through hospital tagging or non-interrupted contact are we to "know" the offspring we have are the ones we gave birth to. 

Why have we not evolved an ability to recognize our own offspring? Probably because there has not been a selective advantage to recognize our offspring because over evolutionary time it has been rare for humans to be in a situation where we must recognize our newborn from other unrelated newborns. Since that ability is rarely useful, selection has not favored it in our species. In the same manner, birds that nest individually associate those eggs in their nest as being their own. Recognition has not evolved because there is little selective advantage for that ability. However there are birds that do exhibit an amazing ability to discern their own specific offspring among a throng of others. Colonial nesting birds like albatross and penguins have an unerring ability to discern their own chick from maddening crowds because selection has favored that ability.

baby penguins awaiting the return of their parents to feed them. Image from Mike Johnson http://www.dailymail.co.uk/travel/travel_news/article-2871252/Tourist-s-incredible-photographs-reveal-stunning-beauty-remote-Antarctic-island-teeming-thousands-penguins-seals.html

In nesting birds like the Reed warbler it would normally be rare for it to be stuck raising an unrelated offspring so they have not developed the ability to recognize that the over-sized baby is not really their own offspring. Instead the poor Reed warbler probably thinks she has the largest and healthiest reed warbler baby in the world. Feed it some more!

Because nesting birds are less discerning in recognizing their young, brood parasitism has the opportunity to evolve and based upon phylogenetic analysis it seems it has. Brood parasitism has evolved independently seven times in the evolution birds resulting in 75 species out of the 8600 known species of birds exhibiting forms of brood parasitism from occasional indiscretions to those species which never raise their own offspring, instead relying totally on other species to incubate and raise their young. 

This extreme form of brood parasitism intrigued Charles Darwin in that a cuckoo could be born in a Reed warbler nest, be raised by Reed warblers, fledged from the nest and go off to grow up and retain its identity as a cuckoo bird and not a reed warbler or whomever was its host species. The identity of the cuckoo was innate, instinctual and expressed itself in the adult females when they reached reproductive age.

Image from https://whyevolutionistrue.wordpress.com/2011/03/04/mimicry-the-nefarious-cuckoo/

Darwin predicted the cuckoo species we see this complex behavior in various stages of complexity, transitions if you will. There are some cuckoos which specialize in parasitizing a single species of birds while other cuckoo species are generalist and parasitize a wide variety of host species. The research suggests it is difficult to discern if the evolution of cuckoo behavior went from specialist (single host species used) to generalist (many potential host species used), or vice versa.  In addition, within the common cuckoo (Cuculus canorus there are families (gentes) who specialize on parasitizing a single host species and the cuckoo egg color has evolved to better mimic their hosts eggs. In the picture below see the cuckoo egg indicated by the arrow in a variety of host nests while showing the great variation in egg coloration within the single species of cuckoo.

By what steps the instinct of F. sanguinea originated I will not pretend to conjecture. But as ants, which are not slave-makers will, as I have seen, carry off the pupæ of other species, if scattered near their nests, it is possible that such pupæ originally stored as food might become developed; and the foreign ants thus unintentionally reared would then follow their proper instincts, and do what work they could. If their presence proved useful to the species which had seized them—if it were more advantageous to this species to capture workers than to procreate them—the habit of collecting pupæ, originally for food, might by natural selection be strengthened and rendered permanent for the very different purpose of raising slaves.[page 338]

Polyergus mexicanus workers return from a successful raid with captured pupae of the host species, Formica subsericea. Urbana, Illinois, USA Photo from http://www.alexanderwild.com/Ants

Sarah discussed the intriguing behavior of slave making ants and how Darwin thought it evolved and how modern science has supported, somewhat, his original view. The biggest hurdle for slave making to evolve is that each ant species relies on species-specific and colon-specific pheromones for individuals within a colony to recognize each other. Individuals from other colonies, and even more so, individuals from other species will smell distinctively different from the slave making ants so how do they suppress aggression towards their newly captured indentured pupae?  It was proposed that ants should raid closely related species so that their pheromones are more likely to be similar to reduce aggression between raiders and potential slaves. This is known as the Emery Rule.

Interestingly, scientists since Darwin have been studying the evolution of slave making behavior and much is still to be figured out. The closing remarks on a wonderful review paper about the evolution of social parasitism and slave making behavior in ants makes the following observation:

"Though slave-making ant species have been studied for more than 150 years, many problems are still open, the most prominent obviously being the evolution of slave  raiding itself. Recent investigations have given contradictory results concerning, for example, the mechanisms of  chemical integration of slave makers and their hosts in a  mixed society, the pattern of sex allocation in slave makers, and coevolution between social parasite and host….Considering that almost all  slave-making ant species are listed as threatened by the  World Conservation Union (IUCN), investigations on  their behavior, population structure, and genetic variability may help us learn more about how endangered they  really are and if and how they can better be protected." (D'Ettorre & Heinze 2001)

 
 Josh ended the program with a discussion of the perfection of honeycombs and how is that bees make such perfect hexagon shapes?

stockphoto from http://www.npr.org/sections/krulwich/2013/05/13/183704091/what-is-it-about-bees-and-hexagons

Josh explained that cell shapes within the colony can range from a circular shape to the beautifully distinct hexagon shape we associate with honey bees. Two major hypothesis, which are not mutually exclusive, have been proposed to explain the hexagon shape. The first is the efficiency hypothesis which argues that wax is expensive to produce so bees would evolve to be the most efficient in building their combs and hexagon shape requires the least amount of material and produces the least amount wasted space. 

PHOTOS BY KATHY KEATLEY GARVEY, UC, DAVIS

Notice this circular form above has many gaps between the cells in contrast the compact arrangement in the hexagon com.  

The second hypothesis Josh brought up was the idea that in the bee hive, the bees body heat melts the wax and the wax then forms a shape that requires less energy to maintain so the straight edges between the cells forms as an outcome of soft wax reaching a low energy resting state.  In this model the bees are not intentionally creating a hexagon shape but it emerges from their constant activity and body heat. 

It is interesting that the questions that Darwin outlined as interesting issues for evolution are still be investigated and we are beginning to understand these instinctual behaviors better because of the evolutionary framework that Darwin gave us over 150 years ago.







The opening theme to Discovering Darwin is "May" by Jared C. Balogh. http://freemusicarchive.org/music/Balogh/Revitalized_Eyes/MAY 
Interlude music is Rhapsody In Blue Part 1 by Paul Whiteman and George Gershwin Published 1924 https://archive.org/details/rhapblue11924

Human Instinct - Special Episode with Dr. Mark Jackson

In this episode we take a side trip with Dr. Mark Jackson, our colleague from the psychology department, to explore the historical view of instinct behaviors in humans. The next chapter of Darwin's Origin of Species deals specifically with instinct behavior in non-human animals and it stimulated us to question what the thought of the time was considering human instinct behavior. Dr. Jackson brings his sly wit to the conversation and tells us how the early field of psychology dealt with or explained innate behaviors in humans.

Mark introduced us to noted historical psychologist William James who is known as offering the first psychology class in the United States and considered the "father of American Psychology".

Dr. "Will.i.am" James

One of the interesting ideas that we discussed concerning instinctual behaviors in humans is how many of our emotions are frequently intertwined with physical aspects of our bodies. Mark recounted the example of you seeing a bear and how your body may initiate the flight response before your mind can consciously evaluate the situation and determine you are afraid.

Unknown man having his legs decide how to deal with the situation before his rationale thought process begins.

Season 1 Episode 7: Difficulties with the Theory

In this episode we decided to break from the standard form of the podcast and discuss two chapters at once instead of the traditional single chapter per episode. Episode 7 covers Chapter VI - Difficulties with the Theory and Chapter VII - Miscellaneous Objections to the Theory of Natural Selection. We note that Chapter VII is really more of a vanity press project where Darwin dedicates a whole chapter to personally rebuke the concerns and critiques of Mr. St. George Mivart, a fellow biologist who published criticisms against Origin of Species.

St. George Mivart, as Charles Darwin saw him

Transitions. 
"First, why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined?” pg 212

James used the imagery of a rainbow to argue that we often ignore the transitions or force transitional forms into the more distinct categories.
 

The biggest challenge of a transitional form is that it is a challenge that increases exponentially with each attempt to address the lack of transitions. We discussed that if you see Species A and Species C and believe they are closely related then the criticism of lack of transition can be leveled at the evolutionist. If the transitional form Species B is found in the fossil record then the challenge for transitional forms doubles because now you must find the transition between A & B and B & C. Each new transitional form increases the demands for more transitional forms. Sarah made the argument that the transitional challenge is often the case of moving the goal posts.


The other challenge of transitional forms is that speciation can occur in two major patterns - anagenesis and cladogenesis. This figure represents the two forms of speciation.

figure from http://krupp.wcc.hawaii.edu/

In anagenesis the species goes through transformation such that the original form is replaced by the new form and both are recognized as unique species. This shift in form can often occur quickly so that there is little opportunity for fossil evidence of the forms or the transitional forms are quickly replaced by the new forms.

Old photographs of common dog breeds show how quickly we have modified the breeds in just 100 years. A summary of this can be found at Science and Dogs website where we collected this intriguing comparison of how the bull terrier has changed over the past 100 years. Of course these are not true species but the premise is the same, shifting from one form to another can occur quickly and no transitional forms remain for comparison. This is where my rainbow metaphor comes to play.

The other form of speciation is called cladogenesis which involves a subset of the species that diverge from the ancestral form. Josh spoke about bat bugs and bed bugs and how they share an ancestry but it would be difficult to find the specific transitional individual between bat and bed bugs.

image from http://ohioline.osu.edu/hyg-fact/2000/2105a.html

Saltations
"It has been asked by the opponents of such views as I hold, how, for instance, could a land carnivorous animal have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted? [220]"

The evolution of whales, at one time a challenge to Darwin, have now become one of the best examples of evolution. The fossil record for whale evolution is robust because the ancestors of whales were semi-aquatic or aquatic mammals and their remains would often sink to the silty bottom of the ocean or bay where they lived. Buried in the mud the remains would quickly become entombed so their skeletons remained intact. Our current understanding of whale evolution is beautifully represented by this graphic from Berkeley website.

A nice video animation summarizes the proposed process of whale evolution and it can be watched here. As Darwin argues, the large differences in forms that we see today developed over long periods of time with successive accumulated changes in form. The scale involved in the process, millions of years with many millions of individuals is often difficult for humans to appreciate or comprehend.

James talked about the flying squirrel and how cute it is. Here is his photographic proof of its cuteness.

flying squirrel in Kentucky, cutest animal on earth?

Complex Structures 

The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder. - Charles Darwin in a letter to Asa Gray (February 1860).

The complexity of the eye with all of its intricate interconnecting parts seem, at first blush, to be too complex to have been formed by natural selection. This has historically been a difficult problem to solve because the eyeball does not fossilize so we are left to look for examples of eye evolution by looking at extant species. However, modern biology using molecular techniques and studying the genes involved in the production of the eye have beautifully reconstructed how a camera-like eye of vertebrates could have evolved from a simple light sensing structure seen in primitive chordates like the hagfish.

hagfish with slime - photo from ecouterre

There a nice TED talk with awesome graphics that summarizes our current state of knowledge  of how the eye evolved.


The opening and closing theme to Discovering Darwin is "May" by Jared C. Balogh. http://freemusicarchive.org/music/Balogh/Revitalized_Eyes/MAY 
Copyright: Attribution-NonCommercial-ShareAlike: 
http://creativecommons.org/licenses/by-nc-sa/3.0/

Season 1 Episode 6: Chapter V Laws of Variation

In this episode of Discovering Darwin we discussed the fifth chapter of Origin of Species titled Laws of Variation. Ironically there are no laws actually discussed in the chapter but Darwin does outline a variety of phenomenon and patterns of inheritance and variation that puzzled and intrigued him. When we consider the great variation in forms that we see in nature it is often difficult to tease apart the cause of that difference in form. For example, variation in form may be genetically determined as described by Mendel and his experiments with pea plants. Of course Darwin did not have a sense of genes and how multicellular plants (excluding mosses) and animals get two copies of each gene, one form their mother and one from their father, that codes for traits. Those two copies of the genes determine the expression of physical traits. Looking at the chart below of Mendel's pea plant traits you can see how variable the sweet pea plant could be.

Imagine Darwin's confusion if he saw pea plants with yellow or green seeds, purple or white flowers and variation on where the flowers are located on the plant. Even more intriguing is the notion that yellow seeded pea plants with purple flowers could be crossed and produce offspring with green colored pea seeds and white flowers! Today, with our knowledge of genetics, we understand the concept of recessive and dominant traits and how recessive traits can be hidden in the parents but expressed in the offspring when they inherit only the recessive versions of the gene.

We discussed if Darwin actually was aware of Mendel's work and Josh mentioned an interesting paper by Bizzo and El-Hani that argued Darwin was aware of Mendel's work. That paper can be found here.

Acclimation versus Adaptation
Sarah discussed how Darwin seemed to use the terms acclimation and adaptation interchangeably as he worked through his understanding of how organisms can exhibit variation in form. Today biologists see these two concepts as separate and unique characteristics.

Some of the best examples of acclimation can be seen in arctic animals, like this arctic fox, that can dramatically change their coats from winter to summer.

Acclimation is when the organism alters its form, physiology or behavior in response to local environmental conditions. Often these changes are induced by hormonal changes in the organism which themselves are triggered by environmental stimuli.  These changes are typically not permanent.

In contrast, if you look at a population of a species you may see variation in forms but these are adaptations to local conditions. An interesting example of local adaptations is seen in the north American rock pocket mouse (Chaetodipus intermedius) that lives in the southwest desert areas of US and Mexico. This small, bite sized rodent, lives in a variety of rocky habitats which differ in color depending upon the geological history. The brown sandy colored rocks are in contrast to the black balsaltic rock that formed from geologically recent lava flow events. The mice have strong local selection to avoid being seen by their natural predators so selection favors coat colors that blend with the background. The top two images are showing the local mouse in its native local condition, the bottom two panels are swapping mice from their native locations. All of these mice are the same species but their coat color variation is genetically determined by expression of Mc1r gene for black pigmentation and that gene has undergone local mutations and selection.

http://uanews.org/story/coats-different-color-desert-mice-offer-new-lessons-survival-fittest

  
We also discussed how creatures locally adapt to caves by losing the ability to see and often become pale colored or completely lose their coloration.  Josh read a wonderful quote by Darwin where he described blind cave crabs possessing components of the eye but it was incomplete in form.

Image from http://imgarcade.com/1/blind-cave-crab

We also discussed how Darwin recognized the pattern that certain traits correlated with individual sexes but did not understand how that was possible. Darwin spoke of calico cats and how he was puzzled that they are typically only female.  Of course we now understand that sex is determined by sex chromosomes and traits can be sex-linked when they are located on the sex chromosome, which in mammals are called X and Y chromosomes (females are XX, males are XY).A very nice summary of the genetics of calico cats can be found here.

image from Wikipedia

During the podcast we discussed Darwin's critique of the idea that every unique species was individually specially created by a Creator. Darwin mentioned how improbable it was that the creator would take the time to make different varieties of turnips. This discussion stimulated a flashback for Josh to remember when he was an undergraduate seeing outside a professors office a classic Farside cartoon concerning special creation. "Oh my, me...."

copyright Gary Larson

Modern View of Inheritance and Sources of Genetic Variation

In contrast to Darwin's semi-blending model of inheritance, biologist today rely on the particulate model of inheritance, which is based on the idea that individuals gain specific set of genetic particles (chromosomes) from both their parents. We now understand that the process of sexual reproduction creates great variation in offspring without relying solely on the process of genetic mutations to create new forms.Yes genetic mutations are important in creating new traits (see mouse story above) but much of the genetic variation we see in nature within a generation derives from shuffling and recombining preexisting traits when creating gametes (eggs and sperm) and combining those gametes (fertilization) into a new individual.

To illustrate how variation is created through sexual reproduction allow me to use an analogy where playing cards represent genetic information. Imagine you take a complete deck of cards sans the joker and partition them out such that you have all the heart and spade suits and your reproductive partner has all the diamonds and club suits. The red colored suits (diamonds and hearts) represent the original genetic material inherited from your mother's side and the black suits (clubs and spades) denote the original genetic material inherited from the father's side.  Each of these cards would signify a chromosome, a strand of DNA that has a variety of genes which code for traits. Since you would have two copies of each chromosome - a heart and spade set of ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, jack, queen and king - you are considered a diploid (di - two) individual. So is your reproductive partner who has the diamond and club suit of the same cards. In this example you would have a total chromosome count of only 26 where in reality humans have a total chromosome count of 46 but this simplified example will illustrate how inheritance and a majority of the variation occurs.

Your diploid genome - 1/2 the genetic material from your dad, 1/2 from your mom.

When you go to reproduce sexually you make gamete cells, either sperm or eggs depending upon your biological sex. To make those cells you randomly draw a single card from either the heart or spade suit to create a cell that has only one copy of each card - ace through king. This new cell would have a chromosome count of only 13, half of what you have, and is called a haploid cell. Your partner is doing the same thing with their cards, creating a haploid cell that contains ace through king cards randomly chosen from the diamond and club set they contain. When those two haploid cells fuse, a process called fertilization, you recreate a diploid individual who has two copies of each of the cards ace through king. So in sexual reproduction it is a reduction in total genetic material (in haploid gametes) and then a reconstitution of the total genetic material during fertilization.

The traits encoded in the genes on the chromosome, represented by the playing cards, get passed from parent to offspring as intake distinct "particles". Variation is also created in this process because when the gametes are formed the single chromosome donated to the cell came about from randomly drawing from the two possible sources. Imagine you are actually flipping a coin to decide which card to choose with the rule that heads picks the red card and tails picks the black card. You could randomly draw all the red cards to make your gamete, improbable but not impossible. It would require flipping 13 heads in a row which for a fair coin the probability of occurring is 1 out of 8192 times.  Improbable but not impossible. The inverse is true for drawing only black cards. Most likely the gamete would contain a mixture of red and black cards for the ace through king cards.  But even then, you could create a large number of possible combinations of just those thirteen cards - 8192 to be exact. You then repeat the process for the other gamete and it is easy to see how many different combinations of chromosome pairs the new fertilized gamete can contain. This process of creating genetic variation through random drawing of each chromosome is called independent assortment, that is each chromosome independently assorts into the gamete cell.

One of 8192 possible haploid gametes you could make

As you imagine all the probable outcomes of creating this gamete it becomes clear that you could create a great diversity of gametes - in humans with 23 pairs of chromosomes you can create 2²³=8,388,608 genetically different gametes! On top of this source of variation, there is also a phenomenon called crossing over where two sister chromosomes - in our example imagine two of the cards of the same value, e.g., the queen cards - break apart and recombine with each other. So take your queen of hearts and queen of spades, stack them together, rip them randomly in two and then tape the two opposite colored halves back together to create a queen card that now has top half red and bottom half black and the other card is the inverse, top half black and bottom half red. This would then allow traits on one chromosome to be combined with traits on a completely separate chromosome so they are now being inherited together. Assuming these traits were housed on the same chromosome, which is not true, imagine grandma had brown eyes and was farsighted, whereas grandpa had blue eyes but had normal vision and all of these traits were on the same chromosome. Now with crossing over, we have a chromosome with grandmas brown eyes and grandpas normal vision and grandpas blues eyes now combined with farsightedness.

A crude approximation of crossing over creating two new chromosomes.

As you can see with crossing over and independent assortment occurring you can appreciate the great array of genetic diversity that can be created in the offspring during sexual reproduction.

The opening and closing theme to Discovering Darwin is "May" by Jared C. Balogh. http://freemusicarchive.org/music/Balogh/Revitalized_Eyes/MAY 
Copyright: Attribution-NonCommercial-ShareAlike: 
http://creativecommons.org/licenses/by-nc-sa/3.0/  

Interlude music if Frenchy's String Band "Sunshine Special" recorded in Texas sometime around 1927. https://archive.org/details/FrenchysStringBand-SunshineSpecial 

Season 1 Episode 5: Chapter IV - Natural Selection; or Survival of the Fittest.

As we thought, this became the longest episode we have recorded yet but then again, we had a lot to cover. Chapter IV is the part of Origin of Species where Darwin outlines how he believes natural selection, over long periods of time, can generate new species. It is a rich and complex chapter and our wide-ranging conversation explored a large number of the issues Darwin brings forth in the chapter.

We discussed at great length the variety of ideas that Darwin encapsulates within the only figure in the book. James mentioned how Darwin first sketched the figure in his notebook B - Transmutation of Species with the understated "I think" title followed with "Case must be that one generation then should be as many living as now. To do this & to have many species in same genus (as is) requires extinction. Thus between A & B immense gap of relation. C & B the finest gradation, B & D rather greater distinction."

 

 Image from Original Notebook at Darwin Online.

 Ultimately this image became refined for Origin of Species to look like this -

In our discussion we noted that this image represents ideas like: a large number of extinction events, the lack of predictable direction in evolutionary change, no change (E & F), increase in species numbers, differential rates of evolution as indicated by the slope of the lines radiating out at a specified time era (e.g., era IV, z4), and that species can converge in character traits (e.g., how f lineage shifts to look like extinct d lineage). A larger version of the figure can be found here.

 Sarah noted how human evolution phylogenetic tree can show the same sort of pruned bushiness that Darwin represented in his figure.
This figure is from the Smithsonian and is an interactive figure at their site that is worth checking out.

Interestingly there has been recent researchers who reject the evolutionary tree model for human evolution. Instead of the classic tree structure they note that the mitochondrial DNA (mtDNA) analysis suggests all modern humans can be traced back to African ancestors who dispersed out of Africa only 100,000 years ago. However, those various subspecies of humans interbred and migrated back into Africa thereby creating a more reticulated, "trellis" evolutionary relationship than the classic branching independent lineages as represented in a tree. Follow this link to read a nice summary of the alternative view with a figure to illustrate the trellis view of human evolution.

Sarah mentioned how quickly other scientists adopted Darwin's tree model to represent relatedness and she noted how the embryologist Ernst Haeckel drew up a phenomenal evolutionary tree. To truly appreciate this tree  you should see it in a larger format.

 

We also discussed the rapid evolution of the Hawthorn maggot fly and its shift to feeding on introduced Apples and how that resulted in two populations with little genetic exchange between them. It is a beautiful house fly sized insect with painted wings which they flick to mimic the movement of jumping spiders. Photo Copyright © 2013 Harvey Schmidt

http://bugguide.net/node/view/817659

Do you see the jumping spider in its wings?

At the end of the podcast we attempted to read the epic opening sentence in Darwin's summary in a way that brings the words to life. Here it is for you to try.

If under changing conditions of life organic beings present individual differences in almost every part of their structure, and this cannot be disputed; if there be, owing to their geometrical rate of increase, a severe struggle for life at some age, season, or year, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of life, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, it would be a most extraordinary fact if no variations had ever occurred useful to each being's own welfare, in the same manner as so many variations have occurred useful to man.