4 Biological Evolution

What is Evolutionary Theory?

Evolutionary theory provides an explanatory account for the origin of the immense diversity of life forms that occur on Earth, both those that currently exist, those that use to exist, and those that will exist in the future.  The evolutionary theory that become the predominant one was independently conceived by Charles Darwin and Alfred Russel Wallace in the mid-1800s.  It is known as evolution by natural selection.  Facets of this theory can actually be found in writings outside of western thought, such as those of several Islamic scholars dating back to at least the AD 800s.  At its core, evolution concerns four mechanisms that drive change in lifeforms: mutation, gene flow, genetic drift and natural selection.

When Charles Darwin was a boy, many biologists and indeed most educated people understood that the Earth was very old and that many features of extinct animals could still be seen today.  However, most scientists of Darwin’s time were very careful not to invoke the concept of evolution, which simply means change of form through time.  Those biologists that accepted the fact that species were not fixed but subject to change, often subverted their ideas – such as Charles Darwin’s grandfather Erasmus Darwin did in his book The Temple of Nature – or provided underdeveloped conceptualizations of them – as with Patrick Matthews.  When Charles Darwin and Alfred Russel Wallace finally introduced their ideas of species change over time, it divided opinion.  This was because nobody knew HOW the process took place or WHAT the mechanism(s) was(were) for change?

Previous to evolutionary theory, there was the speculative notion termed the Great Chain of Being.  In simple terms, this view held that God created all species as fixed entities, thus none could ever disappear, change form, or arise through time.  All living things were linked to one another in a great chain of being that was unchangeable: a fixity of species.  Species were not only linked but there was a hierarchical order of life with God at top and progressing downward to angels, humans, animals, plants, and even minerals.  This hierarchical notion was also applied to humans in society with rulers/priests on top extending extending down to peasants or slaves.

The notion that species were immutable with a given form created by God became more and more difficult to reconcile as explorations revealed more and more fossils of extinct dinosaurs and other life forms.  Some of the fossils came from animals that were profoundly different than anything still living, with some truly amazing forms.  As Europeans explored and colonized the world during the Age of Discovery, they brought back thousands of specimens of exotic plants and animals.  This vast biological diversity needed to be described, organized, and explained.

Carolus Linnaeus was the most famous of those who tackled this organization task.  He developed the 1st taxonomy to classify plants & animals.  He used a binomial naming system, which means each species has two names: a genus and a species. For example, we are Homo sapiens. The sapiens part distinguishes us from Homo erectus.  We are part of the same genus Homo, because we share a similar set of features but are different enough to be designated a separate species.

Linnaeus’s classification scheme was hierarchical.  Species formed the lowest level.  Multiple species get grouped together within a genus.  Multiple genera are grouped together within a family and on up to the highest level of kingdom.  Linnaeus put humans in the same order as apes and monkeys. He based this on observable traits that all could see.  He specified having collar bones & grasping fingers.  The latter is based on an opposable thumb, something that is unique in the animal kingdom.

His grouping of humans, apes and monkeys into the Primate Order suggests that all could have a common ancestor.  Linnaeus did not, however, think that there was an ancestral relationship.  He also didn’t mean to imply one.  Linnaeus believed in the fixity of species.  He did not view his system as tracking ancestry, even though it does, at least to some extent.  Linnaeus’s classification, which borrowed ideas from other taxonomists of the time such as John Ray, was based on morphology, physical attributes like size, shape, and body structure, and reproductive structures, which was especially important for plants.  This scheme continues to serve as the basis for all scientific classification of life on Earth, though adjustments have been made since its inception. Specifically in the 1950s with the DNA revolution, there were considerable rearrangements of species, genera, families and even higher orders, as DNA reveals a more detailed structure of ancestry (and species taxonomy) at a molecular level.  To learn more see this link.

Reconciliation of the concept of species fixity with evidence from extinct animals was attempted by various scientists through time. One of these was Georges Cuvier who speculated that extinct species were ones destroyed by catastrophic events such as fires, floods, volcanic eruptions, and the like.  And after each destructive event, new species that were already in existence elsewhere moved into the now empty environmental space.

Catastrophes certainly are important in evolutionary change, just not in the way that Cuvier thought: meteor impacts & super volcanos had especially significant effects in the distant past and changed the evolutionary course of life forms on Earth.  It turns out that extinction events of various magnitude are an ongoing facet of life on Earth.  These occur randomly after many millions of years, sometimes millions of millions of years.  In any mass extinction the slate was never wiped clean.  Some species survived and then multiplied and diversified to fill the open ecological niches (all factors that determine if a species can survive: food, competitors, predators, climate, etc.).  Geologists have identified five major and many minor mass extinctions over the past 600 million years. The last was the Cretaceous–Tertiary extinction event about 66 million years ago—when the dinosaur die off.  Dinosaur extinction was especially significant for human evolution because it gave mammals their chance to proliferate, with primates, our taxonomic order, appeared some 65 million years ago!

Also troubling in the 1700s and 1800s was how to explain the vast biological diversity of the world.

• Why so many types of plants & animals?
• Why do they appear so well suited to their environments?
• Why the apparent change through time?

Jean Lamarck proffered the Inheritance of Acquired characters, a theory to explain change in animals.  This can be termed the use-it-or-loose-it theory of organs and body parts.  The theory purports that if you use some body part a lot, such as stretching your neck higher and higher to reach leaves, your offspring will have longer necks.  Needs of parents leads to developing phenotypes that are inherited by offspring.  If you keep jumping high and then higher, your kids will be better at jumping and could try out for high jumping in track or basketball.  This sounds laughable to us now.

This is the nature of science: faulty ideas fall by the wayside whereas ideas with value persist. This is why Lamarck’s theory is dead while Darwin’s theory of evolution by natural selection endures, backed up by evidence for 160 years now.  Aspects that Darwin did not fully understand, such as heritability of characteristics, were eventually accommodated and incorporated into his theory, thereby strengthening its validity.

Setting the Stage for Darwin’s Explanation

Three things had a major impact on the debate about biological diversity and change:

• The 1st of these was Charles Lyell’s landmark work on the Principles of Geology (1837),
• The 2nd was Charles Darwin’s voyage on the HMS Beagle from 1831-1836,
• And the 3rd was Thomas Malthus’s “An Essay on the Principle of Population (1798).

Charles Lyell played an important role because his research irrefutably proved the theory of Uniformitarianism developed by James Hutton. This theory implied a vast time scale for the geologic history of the Earth.  Lyell demonstrated that the Earth’s features, such as high mountain ranges that consisted of limestone that formed within oceans, came about through slow changes that had to take millions and millions of years.  The Himalayas, for example, highest mountains in the world and getting higher daily at an incremental rate, are formed of sediment that originally filled deep ocean basins.  By studying rates of erosion and mountain building today, you can estimate how long it has taken to generate features such as Mount Everest or the Grand Canyon.  Once you realize that some mountains over 20,000 feet high are made of limestone that accumulated on the ocean bottom from millions of years of gradual accumulation, the time scales involved stagger the imagination.

Millions upon millions of years allow plenty of opportunity for evolutionary change.

Darwin’s career as a naturalist really kicks off with his five year ship voyage around the world.  We won’t belabor you with the details.  He collected abundant fossils and made extensive observations on modern animals and plants.  He was especially fortunate to spend time at the Galápagos Islands, where he observed some marked island to island differences in species such as finches and iguanas.  Finches of single islands varied according to habitat, especially beak shape, something obviously related to obtaining food (survival).  In addition, during this time Darwin starts to read seminal scientific works, such as Charles Lyell’s Principles of Geology, which convinces him that Earth is not a fixed system and change is possible.

Upon Darwin’s return to England in the fall of 1836, he befriends Lyell and other prominent members of the scientific community, and his work on the HMS Beagle gives him significant clout in his professional pursuits. Darwin’s major contribution to biology would be his eventual postulation of natural selection as the mechanism that leads to the evolution of adaptive traits.  Key in the development of this theory was his reading of the essay on populations by Malthus.

Malthus lived in a crowded and filthy industrialized London with slums full of impoverished people.  Such conditions clearly influenced his views about competition for food and resources.  Malthus wrote that “Population multiplies geometrically and food arithmetically.  Therefore, whenever the food supply increases, population will rapidly grow to eliminate the abundance.”  Unlike other species, humans have been ingenious at devising ways to increase our food supply.  Still, a Malthusian catastrophe is an ever-present specter on the horizon.

Malthus’ work convinced Darwin that there must be a struggle for survival and reproduction.  More offspring are born than can survive to age of reproduction.  Hence there is competition for food and resources and also for mates.  Though Malthus focused on human populations, Darwin recognized that his arguments are easily extended to other animals and also plants.  Darwin postulated that only those individuals best matched to their environment reproduced; thus, their characteristics were selectively retained.  Adaptive traits would be passed to the next generation, and, if this trend continues, eventually could become a defining characteristic of a new species.

Darwin did not publish his magnum opus until 1859, 23 years after he returned to England.  This was clearly no rushed hack job at scientific research!  It was well thought out, with careful reasoning and arguments.  Darwin presented his findings in exhaustive detail: the facts of living and fossil species as they pertain to evolution by natural selection.

Theory of Natural Selection, Major Principles:

Boiled down, Darwin’s argument consisted of 5 main principles:

1. High mortality; all species produce far more offspring than the food supply can support. This results in competition between individuals for resources.  Also a competition for mates.
2. Variation between individuals of a species; this is obvious to anyone paying attention.  Two monarch butterflies superficially look alike but there are subtle distinctions and some of these can make an important difference.
3. Differential survival & reproduction associated with the variability; those individuals that are better adapted, produce more offspring and pass adaptive traits to the next generation (they have higher fitness). Better adapted means traits that enable them to get food, to avoid being someone’s else’s lunch (survival), and to find a mate.  Eating well and avoiding predators does not matter if MATING does not occur—favorable traits will disappear if not passed on.
4. Heritability of characters; this just means that offspring inherit traits from their parents.  Again pretty straightforward, although Darwin did not understand the actual process.  If he had read the writing of his contemporary the friar Gregor Mendel, then he likely would have had a clear concept.  As it was, this was a major weakness in Darwin’s argument and remained the Achilles’ heal of his theory until genetics was incorporated into evolutionary theory in the 1940s.
5. Accumulation of small changes over long time periods leads to great change: new species, new genera, entire new families of animal life.

The two essential points that lead to evolution by natural selection:

• heritable variation;
• differential survival and reproduction associated with that variation.

A formal definition of natural selection might read like this:

A 2-step mechanistic explanation of how descent with modification takes place: (1) every generation, variant individuals are generated within a species because of genetic mutation, and (2) those variant individuals best suited to the current environment survive and produce more offspring than other variants.

Evolution is descent with modification and this takes place because of differential survival and reproduction of individuals.  Traits that enhance survival & reproductive success increase in frequency over time.  More favorable traits equals greater survival odds and greater reproductive success (RS).  Adaptive traits are ones that result in greater RS in a given environment.  Environmental context determines if traits are beneficial and it is important to keep this in mind.  The environment is constantly in flux, never static.  Traits that were highly adaptive at one time in a given environment might be less adaptive or even maladaptive or disadvantageous at another time after environmental change.

A commonly presented example is the peppered moth in England.  Industrialization in the 1800s resulted in pall of coal soot on tree trunks, turning white trunks dark.  Peppered moths, with two color variations, white and black, were significantly influenced by this environmental change. What was originally a great camouflage for the light-colored peppered moth, white against white trunks, became a liability as the trunks darkened.  As their hiding background changed, the moths became easy targets for insect eating birds.  In contrast, the dark-colored moths now had the camouflage advantage.  Because they died less frequently than the lighter colored variety, they proliferated in the new soot-covered environment.

Reproductive success is a measure of an individual’s genetic contribution to the next generation as compared to the contributions of other individuals.  Reproductive Success is measured by the offspring an individual of a species produces that survive to reproductive age; this is also known as fitness.  Do not confuse the evolutionary concept of fitness with its more informal meaning as the condition of being physically and mentally fit with good health.  An individual of a species could produce large numbers of offspring but if none survive to reproduce in turn, then the reproductive success (fitness) of that individual is nil.

Selective Pressures are forces in the environment that influence reproductive success in individuals by reducing their contribution to future generations.  These pressures can include predators, competitors both within and between species, resource availability, climate change, diseases, parasites, etc.  In the peppered moth example, birds applied the selective pressure with human activity causing a change in the environment.  Humans have now become a major selective force around the world for various animal and plant species.

A Bogus Common Criticism: One of the most common criticisms of the theory of evolution by natural selection is that “no one has actually observed speciation, so the process is just theoretical.”  There are hundreds of observed speciation events known in modern biology.  The two web sites listed here catalogue many and you can see for yourself:

http://www.talkorigins.org/faqs/speciation.html

http://darwinwasright.org/evidence-form-observed-speciation

Heritability of Characters

Darwin knew that traits were passed from parents to offspring, but he did not know the mechanism for how this occurred.  That did not prevent him from developing an explanatory theory for how evolution works, how life forms change through time.  Gregor Mendel’s work on heredity published in 1866 solved this quandary.  Unfortunately, Mendel obtained no recognition of his success in his lifetime and it wasn’t until the early 1900s that his contributions were fully appreciated.

Two of Mendel’s significant discoveries were:

• Genotype: The total compliment of inherited genes of an organism.
• Phenotype: The observable physical appearance of an organism, which may fully or partially reflect its genotype.

Remember, phenotype = morphology and physiology.  Genotype is the genetic code of the organism.  A gene is a section of DNA that codes for something in the organism. Each gene is comprised of two alleles, which are the various forms a gene can take (such as eye color, hair color, blood type, etc.).  Thus a genotype includes all possible alleles in an individual.  A phenotype may only show some of the genotype, as recessive alleles are masked (will not be displayed) by the presence of dominant alleles (will always be displayed) for the same traits.

Sources of Variability

There is only one source of new genetic variation: mutation!  This process is totally random.

There are two processes that influence how frequencies of genes are changed in populations:

• Gene flow—transfer of genetic material from one population to another; accomplished through migration and mating with an individual from a different population; gene flow tends to decrease differences between populations but increases variation within a population.
• Genetic drift—various random processes that change gene frequencies in small, relatively isolated populations by chance; includes bottlenecks and the founder effect).

Among humans, gene flow and isolation can occur for a variety of natural and cultural reasons: colonization, war, famine, massive destructive events such as asteroid strikes or smaller scale ones such as volcanic eruptions and typhoons, slave trade, taboos, social sanctions and other restrictions about mating those in other groups, etc.

An example of founder effect in humans is seen in Indigenous Americans.  The ancestral human groups that founded the peoples of North and South America were quite few in numbers and therefore had very few varieties of genes, including those for blood type.  Though type O blood is the most common in the world, most populations have an even distribution of types A and B blood as well.  Indigenous American populations are nearly exclusively type O blood. This is likely because those small founding populations that dispersed into North and South America were predominantly comprised of individuals with type O blood.

Epigenetics

Genetic inheritance plays an obvious important role in your life but not all of that inheritance in chiseled in stone.  Your behaviors and the environment that you live in also play a critical role, even at the molecular level.  What you eat, how physically active you are, whether or not you live surrounded by farm fields where pesticides are sprayed, whether or not you live in a dense city with thick air pollutants from combustion engines, coal-fired power plants, and the like, all can play a role.  This is what the field of epigenetics concerns.

Epigenetics is the study of how your behaviors and environment can cause changes that affect the way your genes work. While genetic changes alter the DNA sequence itself, epigenetic factors do not change the actual DNA. Instead they change the way your body reads a DNA sequence, turning genes “on” or “off” (whether or not they will be expressed) and altering the quantities of proteins produced (DNA contains instructions for how proteins are made and how often they are formed in an organism).  Epigenetic changes can be altered (think of moving to a new climate) but sometimes effects can be permanent.  This is especially true of epigenetic influences that occur in utero, before any of us had any agency in the world, the consequence of what your mother did or was exposed to from no fault of her own.  For example, the children of mothers pregnant during famines are more likely to develop heart disease, schizophrenia, and type 2 diabetes (e.g., this paper by Tessa J Roseboom and one by David St Clair and others).  And studies are now showing that dads play a role also–what they consume or are exposed to.

Microevolution vs. Macroevolution

Microevolution refers to small-scale changes occurring within species, such as changes in allele frequencies – recall that alleles are the different forms a gene can take.  Take for example a beetle species that has two possible colors, each color is represented by different sets of alleles. In the 1st generation, 75% of the beetles are the green color, 25% are brown. In the 2nd generation, 71% of beetles are green and 29% are brown. The frequencies of the alleles that code for green and brown have undergone a microevolutionary change.

Macroevolution refers to large-scale changes in allele frequencies in a population over a long time period; substantial change over many generations may culminate in the creation of new species.  Take for example the known evolutionary history of the modern horse as reflected in the fossil record.  Across 60 million years, the Eohippus—a small, dog-sized animal, with multitoed feet, that inhabited rainforests—evolved into the modern horse.  Among the large-scale changes were increases in overall body size and height and the loss of toes.  The single-toed hoof enable horses to run efficiently in the open grasslands they came to naturally inhabit.  Of interest is that much of horse evolution occurred in North America, but the species then went extinct there around 10,000 BC.  Fortunately for humans, the distant ancestors of horses crossed the Bearing land connection and spread into Asia where they persisted.  Ultimately the horse was reintroduced to North America again by the Spanish.

Speciation is the formation of new species by selective pressures that lead to changes in a subgroup of a population, such that they are recognizably different from the ancestral population.  In order for speciation to occur, a population needs to be reproductively isolated.  This is often accomplished by spatial or behavior isolation.  Over time, such microevolutionary change can become profound (macroevolution).  Species can develop then merge again or share genes if they have not been isolated for a sufficiently long time.  Growler Bears or Pizzleys provide an example of this.

Natural Selection & Culture

Some anthropologists are interested in links between natural selection and culture and potential biological aspects of our behavior.  Culture is a defining characteristic of our species and has helped us to become prolific and successful.  The link between culture and evolution is not clear and there is uncertainty what, if any, genetic factors are responsible for behavior.  Approaches exploring how culture has helped us evolve include Sociobiology, Behavioral Ecology, Evolutionary Psychology, and Dual-inheritance theory.

Sociobiology is largely the brainchild of E.O. Wilson (1978) who defined it as the systematic study of biological causes of behavior—the genetic component of behavior.  This approach is controversial in anthropology because of perceived genetic determinism.  Most human sociobiology unduly neglects culture but cultural scientists in turn unduly neglect the biological component behind human behavior.

Human Behavioral Ecology (HBE) tries to understand how biological adaptation and local ecology combine to produce behavioral patterns in humans; how ecological and social factors influence and shape our behavioral flexibility.  HBE explains variation in human behavior as adaptive solutions to the competing life-history demands of growth, development, reproduction, parental care, and mate acquisition.  Behavioral ecologists model economic tradeoffs (costs vs. benefits) because individuals have limited time & resources.

Evolutionary Psychology tries to identify which human psychological traits are evolved adaptations, the functional products of natural selection or sexual selection in human evolution.  Humans are not blank slates (the tabula rasa hypothesis) but evolutionary psychology sometimes overlooks the important role that culture plays.  Also, there was no single past environment for human psychological propensities.  Humans survived and thrived in diverse environments.

Dual-inheritance theory of Peter Richerson & Robert Boyd is concerned with cultural evolution, the processes by which culture is passed on and how this results in change. Learning culture by imitation or teaching is analogous to acquiring genes from parents and subject to a form of natural selection: cultural variants will decline in prevalence if they are possessed by people who have fewer offspring or recruits (& vice versa).   Cultural mutation occurs from random changes in cultural variants when learners do not reproduce them accurately or alter the final product.  Cultural drift occurs because the flow and transmission of cultural information cannot be completely uniform throughout a population; inevitably there are random differences in the cultural variants that groups are exposed to.  Groups within a culture whose members have little contact with one another will increasingly diverge.  Migration is one means by which new variants are introduced to a population and this results in decreased prevalence of the old ones.

Evolution of culture differs from genetic evolution most strikingly in being subject to “decision-making forces”.  Because cultural transmission is spread out over a significant fraction of our lives, the cultural variants we adopt are influenced by the behavioral choices of the individuals taking part in our social interactions.  Boyd and Richerson identify three learning biases that arise from the way individuals decide which cultural variants to adopt.

• Content-based bias is when individuals adopt a cultural variant based on considering the variant itself: is it functionally useful or superior to alternatives?  If an answer is not obvious, learners can weigh costs and benefits or determine how consistent a new variant is to a previous one.
• Model-based bias is when individuals adopt a cultural variant based on the social prominence of the presenter.  Rather than weighing whether a cultural variant is worth adopting, simply look at who else has adopted it and whether they are worthy of imitation.  Are they happy, confident and successful?  Much of the fashion industry works off this.  What cloths styles are Hollywood or music stars wearing? Advertisers also use this to get you to buy masses of other material, even stuff that can kill you, such as cigarettes.
• Frequency-based bias is when individuals adopt a cultural variant based on its popularity.  Common variants are likely to have been adopted and kept for good reason.  Individuals copying popular behavior will benefit from the experience of fellow group members.  Conforming also prevents a naïve individual inadvertently breaking some implicit rule of cooperation and risking punishment.  Even if the practical benefits of a variant are limited, there are social benefits associated with sharing group norms.