9.IV: The Sexual Evolution

The original “tree of life” model of speciation from Charles Darwin’s notebook, 1837.

The science of life’s early history is much like the study of the early universe.  Abiogenesis itself, the initial emergence of life from ocean chemistry, was a singular event that may never be fully understood. From that point forward, though, science has a deep understanding of how life continued and developed over the last few billion years.  Evolution is the study of life’s variation over time and place.  This section will summarize the basic principles of evolution, a subject that is widely misunderstood or misrepresented.

A.  How Life Evolves

B.  Studying and Understanding Evolution

C.  Citations

A.  How Life Evolves

1. Natural selection

2. Speciation and phylogeny

3. Survival of the fittest

1. Natural selection

Evolution occurs in bacteria and even some non-living things such as viruses and pre-biotic chemistry.   I have deferred this discussion to chapter 9 because, when eukaryotes reproduce sexually, it changes the nature of their evolution in fundamental ways.  Some of the examples in this discussion will make use of commonly known modern-day animals, though of course life was much more primitive a billion years ago.


Life requires metabolism (“eating”) and reproduction.  In order to survive, an organism must
metabolize to get matter and energy from its environment.  For life to continue, some organisms must reproduce.  Those who are most successful at eating and reproducing will
outlast those who starve, get eaten, or otherwise die before reproducing. 

Many traits that make good survival skills are determined by genes and are thus hereditary; they get passed to the next generation.  If a gene confers survival advantages and is hereditary, then it is stable and will become more common within its species.  This anthropic principle of biology is one of the most important facts of life, and one of the central tenets of this book:  The only “reason” that any heritable trait exists today is that it survived well in the past.  We wouldn’t be here if our ancestors weren’t good at surviving and reproducing by virtue of qualities that could be passed from one generation to the next.  That’s how we have come to be “programmed” to obsess over food, sex, and child-raising.  Some things never change! The only “reason” that any heritable trait exists today is that it survived well in the past.

The traits that are survivable depend on the environment.  Imagine a colony of rabbits, some black and some white.  One winter, some of them take shelter among black rocks, while others forage for food out on the snowy plain.  Birds of prey will easily spot the white rabbits in the black rocks and the black rabbits in the snow.  Due to this natural selection, the surviving rock rabbits will be predominantly black and will pass down their black genes.  The snow rabbits will become increasingly white.  If an explorer came along later, he would get the impression that each colony of rabbits had been “designed” for its environment pre-emptively when in fact the cause and effect was quite the opposite.

The “currency” of evolution is diversity.  If all members of a species were identical, they would all live or perish together.  But individuals naturally exhibit a degree of diversity, which is enhanced by sex.  Initially, that diversity is random.  There is no “reason” that some of those rabbits were born white and others black.  Fur color is simply determined by genes, molecules over which the rabbits have no control.  When the environment favors one trait more than others, it creates an evolutionary pressure toward that trait.  In our example, the caves, snakes, snow, and bears were evolutionary pressures separating the black and white rabbits into different environments.   

Sometimes, percentages of genes drift gradually from one generation to the next.  At other times, genes copy themselves incorrectly and actually change.  That is called a mutation.  Ultimately, mutations are the source of all genetic diversity. 

If the beaks within a particular family of finches grow shorter and stouter over the generations, those finches might find it harder to pick worms out of holes, and easier to crack hard seeds instead.  If they live in a forest without many seeds, they will have to move. 1  Then, with fewer worm-eating finches in the forest, worms may become more plentiful, performing more soil fertilization, helping more plants to grow, providing more shade for lizards who eat spiders who eat flies … .  Things change, and they are all inter-connected.

2. Speciation and phylogeny

Let’s follow the finches with short, stout beaks who got frustrated by the offerings of their habitat.  Suppose they left the lush forest full of worms and relocated to a sparser meadow with a few shrubs and fruit trees.  They have become an isolated population of finches.  They have brought part of the gene pool with them, including their genes specialized for the new environment.  As they continue to interbreed, the shape of their beak will get reinforced or even further exaggerated.  After all, those birds that do best at eating fruit seeds will thrive there and have lots of hatchlings with similar genes. 

After thousands or millions of years of isolation, a strange thing will happen.  If one of those seed-eating finches makes a return visit to its old ancestral forest, it may find that it is unable to mate with the finches that were left behind.  This may be because the meadow population has become physically or genetically different from its forest cousins, or because the forest birds are not attracted to the funny-looking meadow birds.  There may have been a mutation in one population or the other, making their DNA incompatible.  Whatever the reason, if they can no longer mate with each other, then these populations have become two separate species.  They will continue to look alike for quite some time, but the slow-and-steady change from one generation to the next will occur independently in the two species.  Each species will settle into its own habitat and its own role within that habitat, a combination that ecologists call a niche.  Each niche will influence which genes survive or die out.  As time goes by, the two species will become more and more different.  This process of isolation to the point of mating incompatibility is called speciation.

Over millions of years, a population may diversify into several related species called a genus.  For example, house dogs / wolves, coyotes, and jackals are several species in the genus Canis.  Then, one of the species may go through such a noticeable change that biologists classify it into an entirely new genus.  Vulpes (fox) is a genus closely related to Canis.  House dogs, wolves, and coyotes had a common ancestor relatively recently, while the common ancestor of wolves and foxes lived earlier.  In turn, two closely related genera are connected by a common ancestor in the same biological familyCanis and Lupus are both in the family CanidaeCanidae and Felidae (cats) are two families in the same order of carnivores.

This biohistory can be graphed like a family tree called a phylogeny.  A phylogenic chart can be difficult to understand, especially because most of its members are extinct.  Some evolution skeptics raise objections like, “Dogs are dogs and cats are cats.  Dogs can’t ‘turn into’ cats!”  The following chart shows how the relationship between cats and dogs actually evolved. Dogs did not evolve into cats, or vice versa.  Instead, they both descended from a common ancestor, a carnivore.  This carnivore’s lines of descent evolved into dogs in one environment and cats in a different environment.  Likewise, each family of cats / dogs developed many specialized forms.  I made this chart easy to visualize by showing images of the common ancestors (many of which are extinct) at each node of the tree. 

The term basal refers to an organism at the early base of the lineage.  The basal carnivore was a strange cat / dog creature called a miacis.  The opposite of basal is derived.  House dogs are more derived than jackals, because house dogs have changed more than jackals from their common ancestors.

phylogeny evolution cats dogs canine feline
Example of phylogeny. Dogs and coyotes had a common ancestor that was neither dog nor coyote, but belonged to their common genus of Canis. Cats and dogs descended from a common ancestor in the order Carnivora, which was neither cat nor dog at the time.
Diagram by Scot Fagerland, 2015, with image credits here.

3. Survival of the fittest

Evolution is often described as a competition, a “survival of the fittest” 3 in a particular environment.  Who exactly are the competitors in the evolutionary game?  There are many different levels.  Species compete against each other for domination of a niche.  Individuals within a species compete against each other for mates.  The fundamental unit of biological competition is the gene.  In fact, different varieties (alleles) of the same gene compete against one another.  Sperm genes compete against egg genes, and even parent / child genes compete within the womb.   The game of life unfolds according to which alleles survive best.  The “fortresses” that they use for survival are the bodies they construct.  You are the result of a four-billion-year-long allele survival tournament.

B.  Studying and Understanding Evolution

Phylogenic categories continue up a hierarchy called the Linnaean system of taxa, originated by 18th century Swedish biologist Carl Linnaeus and familiar from high school:

Domain > Kingdom > Phylum > Class > Order > Family > Genus > Species

Domains are the oldest taxa and species are the most recent derivations; some new species have evolved within your lifetime. 4 Most Linnaean taxa are man-made and arbitrary.  For example, it’s a matter of biologists’ preferences whether large cats and small cats should be divided into two genera, and whether the lynx should be classified as a Panthera or a Felis.

There are two classifications that are truly rooted in biological nature.  The second-most clear-cut evolutionary unit (as elaborated below) is the species, the set of individuals that could potentially mate with each other.  Even more objective and complete is the monophyletic clade, the full set of descendants of a given common ancestor.  Small clades are easy to visualize.  The monophyletic clade descending from you includes you, your spouse, and all your kids, grandchildren, and great-grandchildren who have ever been born.  Your siblings are not part of that clade, but they would be included in the larger clade that descends from your parents.  The larger the clade, the more difficult it is to visualize.  You have to pretend to be an omniscient genealogist who can trace back everyone’s family tree to any number of generations.  The clade going back to one set of your great-great-grandparents would consist of thousands of relatives living and dead.  One mega-clade would include all human beings alive today (and just as many dead ones) as long as we traced back everyone’s family tree far enough to discover an ancestor that we all had in common.  Even larger clades include all humans who have ever lived or all mammals who have ever lived.  The set of all dolphins and all lizards is not a monophyletic clade.  Dolphins and lizards had a common ancestor about 300 MYA.  That ancestor’s descendants would include all reptiles, mammals, and birds who descended from her – and that would be the monophyletic clade of interest.

Evolution can be hard to understand because people learn best visually, and evolution cannot be easily visualized.  It is not just the scale of time or the structure of the trees that challenge us.  Posing more subtle problems, the “species” as a biological unit is usually identified visually.  This is especially true of fossils.  If two fossilized seashells look alike, a paleontologist is likely to conclude that they belonged to the same species.  But recall that species is defined by mating compatibility.  What if the sea creatures lived a million years apart?  Is it really accurate to say that they could have mated with each other?  A species according to the classic definition must exist only in a slice of time.  And if that seashell pattern vanishes from the fossil record, it is impossible to determine whether the species went “extinct” (its descendants all died) or simply changed its appearance.  This is an issue that even professional biologists rarely acknowledge, so it is no surprise that it can be confusing for laymen.

Because the Linnaean system dates to the 18th century, before genetics or evolution were understood, it is not a strict mapping of monophyletic clades.  Nowadays, fossil evidence is supplemented by DNA analysis to help piece together the “phylogenetic” tree of life.  Biological language is shifting from the classic-but-fuzzy Linnaean system based on “looks” to the more objective system of clades and parent-child descendancy.  The Linnaean system is acceptable for classifying living species.  Cladistic trees are a much more meaningful model for long-term evolutionary descent, 5 but they are derived from and unavoidably influenced by the Linnaean trees handed down by classic biology.  

Back to Section 9.III:  Sexual Reproduction


Continue to Section 9.V:  From Amoebas to Amniotes

C.  Citations

  1. You probably recognize this discussion of the finches as one of Darwin’s major breakthroughs.  By studying Galapagos finches, he provided compelling evidence that evolution and even speciation are caused by natural selection.  Darwin’s original writings on these finches were published in Narrative of the Surveying Voyages … (1839), Journal of Researches into the Natural History … (1845) and On the Origin of the Species (1859).  These books are all available in full at John van Wyhe (ed.), The Complete Work of Charles Darwin Online (since 2002), http://darwin-online.org.uk (accessed 8/04/19).
  2. Carnivora drawing:  “Miacis” by “Mr. Fink”, GFDL CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0), https://commons.wikimedia.org/wiki/File%3AMiacis.jpg .  Felidae drawing: “Proailurus: Common cat ancestor”, public domain, http://www.wpclipart.com/animals/extinct/Proailurus__common_cat_ancestor.png.html .   Canidae drawing: “Life restoration of Hesperocyon (formerly Cynodictis)” by Robert Horsfall, illustration for book A History of Land Mammals in the Western Hemisphere, W.B. Scott, MacMillan, 1912 (public domain).  Tiger photo: Karen Arnold, public domain, http://www.publicdomainpictures.net/view-image.php?image=33775 .   Lynx photo: Petr Kratochvil, public domain, http://www.publicdomainpictures.net/view-image.php?image=40903&picture=lynx .  Fox photo = “Fox red”, public domain, http://www.wpclipart.com/animals/F/fox/fox_2/fox_red.jpg.html .  Jackal photo = “Black backed jackal”, public domain, http://www.wpclipart.com/animals/J/jackal/Black-backed_Jackal.jpg.html .  Cat photo = “Vesta”, by Scot Fagerland, 2015.  Jungle cat photo = “Jungle cat photo”, public domain, http://www.wpclipart.com/animals/wild_cats/jungle_cat/Jungle_cat_photo.jpg.html .  Dog photo = “Golden retriever dog”, Karen Arnold, public domain, http://www.publicdomainpictures.net/view-image.php?image=35696 .  Coyote photo = “Coyote howling”, public domain, http://www.wpclipart.com/animals/C/coyote/Coyote_howling.png.html
  3. Herbert Spencer, Principles of Biology, 1864.
  4. Peter R. Grant and B. Rosemary Grant, “The secondary contact phase of allopatric speciation in Darwin’s finches”, PNAS (11/16/2009), https://www.pnas.org/content/early/2009/11/12/0911761106 (accessed and saved 8/04/19).
  5. M. Alan Kazlev and T. Mike Keesey, “Taxonomy: Cladistic and Linnaean Systems – Incompatible or Complementary?” Palaeos (1998 – 2002), http://palaeos.com/systematics/taxonomy/incompatible.html (accessed and saved 8/04/19).
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