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Five different forces have influenced human evolution- natural selection, random genetic drift, mutation, population mating structure, and culture. All evolutionary biologists agree on the first three of these forces, although there have been disputes at times about the relative importance of each force.
The fourth and fifth forces are new in the sense that they are not explicated in more traditional texts. This is not an attempt to develop a “new” theory of human evolution. Instead, the forces of population mating structure and culture are arbitrary categorizations used to organize several different phenomena of human evolution.
Scientists agree on the phenomena themselves, although they do not always organize them the same way. Each of the five forces will be explained in turn. This is a risky approach because it can lead to the false impression that the five operate quite distinctly and differently from each other.
Force # 1. Natural Selection:
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Natural selection is defined as the differential reproduction of organisms as a function of heritable traits that influence adaptation to the environment.
There are three essential components to this definition:
(i) Differential reproduction,
(ii) Heritable traits, and
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(iii) Adaptation to the environment.
Darwin noted that most species reproduce at a rate that, if unchecked, would lead to exponential population growth. However, such growth is seldom realized in nature because many organisms fail to reproduce. Darwin reasoned that if this differential reproduction was associated with adaptation to an environmental niche and if the adaptive traits were transmitted to a subsequent generation, then the physical and behavioural traits of a species will change over time in the direction of better adaptation.
Genetic variation fuels natural selection and genetic inheritance transmits adaptive traits from one generation to the next. If all the members of a species were genetically identical, then there would be no genetic variation and hence no natural selection. The organisms in this species could still differentially reproduce as a function of their adaptation, but they would transmit the same genes as those who failed to reproduce.
Biologists index natural selection by reproductive fitness, often abbreviated as just fitness. Reproductive fitness can be measured in one of two ways. Absolute reproductive fitness may be defined as the raw number of gene copies or raw number of offspring transmitted to the subsequent generation.
It may be expressed in terms of individuals (e.g., George has three children), phenotypes (e.g., on average the red coloured birds produce 3.2 fledglings), or genotypes (e.g., on average genotype Aa has 2.4 offspring). For sexually reproducing diploid species like us humans, a convenient way to calculate absolute fitness is to count the number of children and divide by 2. For example, someone with 2 children would have an absolute fitness of 1.0, indicating that the person has left one copy of each allele to the next generation.
The second way of measuring reproductive fitness is relative reproductive fitness. Relative fitness is simply the absolute fitness of an individual, phenotype, or genotype divided by the absolute fitness of a reference individual, phenotype, or genotype. For example, suppose that the absolute finesses of genotypes aa, Aa, and AA are respectively 1.8, 2.4, and 2.5.
If AA is the reference genotype, then the relative fitness of aa is 1.8/2.5 = .72, the relative fitness of Aa is 2.4/2.5 = .96, and the relative fitness of AA is 2.5/2.5 = 1.0. It is customary, but not necessary; to express the relative fitness of genotypes in terms of the most fit genotype.
It is crucial to distinguish reproductive fitness from desirability. The fastest, the most agile, the longest-lived, and the most intelligent do not need to be the “fittest” in a reproductive sense. Fitness is defined solely and exclusively in terms of gene copies left to the subsequent generations. There is no mention of social values in this definition. A genotype that promotes longevity is more fit than one leading to a shorter lifespan only if it leaves more copies of itself.
Similarly, fitness is correlated with survival but it is not synonymous with survival. Unfortunately, popular culture has equated natural selection with the term “survival of the fittest,” implying a tooth and claw struggle in the jungle. Natural selection often involves subtle mechanisms, some of which may actually end in the organism’s death! After a perilous journey from salt water to the headwaters of a stream, salmon reproduce and then die. The male preying mantis is literally devoured by the female while in the very act of copulation.
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An important part of fitness and natural selection is competition with other conspecifics (other members of the same species). The environment for an organism is much more than physical surroundings. It also includes the behaviour of conspecifics. Hence, reproductive fitness for many organisms is defined less in terms of their physical capacity to reproduce and more in terms of being able to out-reproduce other conspecifics.
A male gorilla, for example, can survive, be healthy, and be physiologically capable of producing many offspring. His main problem with reproductive fitness lies with other males. Unless he entices fertile females away from an established male, his reproductive fitness will be low.
Modes of Natural Selection:
For continuous traits, there are three modes of natural selection—directional, stabilizing, and disruptive. In directional selection, fitness increases with trait value. Most human evolutionists suspect that human brain size underwent directional selection.
About 4 million years ago (mya), the brain size of our probable ancestors, the Australopithecines, was around 450 cc (cubic centimeters), only slightly larger than that of a contemporary chimpanzee. Around 2 mya, brain size almost doubled with the emergence of Homo Habilis and later Homo Erectus. Brain size increased so that modern humans average between 1300 and 1400 cc.
The second mode of natural selection is stabilizing selection. Here, trait values that are close to average have the highest fitness and fitness decreases as one move away from the mean. In the popular mind, natural selection is almost always equated with directional selection.
Yet most biologists suspect that stabilizing selection is the most frequent mode of natural selection. Most species are well adapted to their ecological niches—otherwise, they would have gone extinct many eons ago—so being somewhere around the average is more likely to be beneficial than having an extreme phenotype. Stabilizing selection will not change the mean of a distribution but it may reduce the genetic variance over time.
Human birth weight is a classic example of stabilizing selection. Before modern medical interventions, low birth weight neonates had high mortality. Similarly, neonates much larger than the average posed serious problems for their mothers and themselves. In terms of infant survival, it was preferable to be near the mean rather than at the extremes.
The third mode of natural selection is disruptive selection. Here phenotypes close to the average have reduced fitness compared to phenotypes at the extremes. Disruptive selection appears to be the rarest form of natural selection and, indeed, there are few well-documented cases of it.
There does not appear to be a good example of disruptive selection in human evolution. Despite its rarity, however, disruptive selection may be very important for the emergence of species. After suitable time, disruptive selection can lead to bimodal distributions that might eventually lead to different species.
Effect of Natural Selection:
The ultimate effect of natural selection is to change allele frequencies. It operates only on what is already present in the genome of a species and, makes some alleles more frequent and other less frequent. Nevertheless, the appearance, anatomy, and physiology of a species may change over time simply because some alleles become rare after lengthy natural selection.
Programs in artificial selection where humans control the selection process are the best illustrations of the tremendous genetic variability hidden in a species’ genome. All contemporary strains of dogs had their origin in the wolf. The fact that dogs come in all sizes, colour patterns and temperaments is due to – deliberate selection of rare allelic combinations in the wolf genome. Yet despite these differences, dogs can still reproduce with wolves.
Force # 2. Genetic Drift:
Genetic drift is defined as the change in allele frequencies over time due to chance and chance alone. To illustrate drift, imagine the change over time in allele A in a small isolated population of 10 individuals. Suppose that the frequency of A is .50 and the frequency of the other allele, a, is also .50.
Hence, with 10 individuals, there will be 20 alleles—10 A alleles and 10 a alleles. In transmitting alleles to the next generation, the probability of transmitting A is the same probability as flipping a fair coin 20 times and getting 10 heads and 10 tails. This is the most likely of all possible outcomes, but the probability of this outcome is only .17; the probability of an outcome other than an even 50/50 split is 1 – .17 = .83.
Suppose that we actually flipped the fair coin and ended up with 12 heads (or A alleles) and 8 tails (or a alleles). The frequency of A is now .60. The probability of transmitting allele A to the next generation is equal to flipping a biased coin that has a 60% chance of heads and a 40% chance of tails. In 20 flips of this biased coin, the most likely outcome is 12 heads and 8 tails, but once again the probability of this single event is only .18. Again, we are more likely to experience an outcome other than a 60/40 split.
Suppose that we flipped this biased coin and ended up with 13 heads and 7 tails. In this generation, the frequency of A is .65. In the next generation, the probability of transmitting A is equal to the flip of yet another biased coin, but one that has a probability of heads being .65.
You can see how chance changes in allele frequencies in one generation alter the probability of transmitting the allele to the next generation. The process of genetic drift is equivalent to tossing biased coins in each generation. The degree of bias is determined by the allele frequency in that generation. As a result, a plot of the frequency of allele A by generation should show that A usually changes in frequency from one generation to the next.
Force # 3. Mutation:
Mutation is defined as an error in copying the DNA. Although there is a bewildering array of terms that geneticist use to classify mutations, we consider only two different classifications, the first depending upon the type of cell affected by mutation and the second on the amount of DNA that is mutated.
In terms of the type of human cells affected by mutation, geneticists distinguish somatic mutations from germinal mutations. Somatic mutations influence somatic cells—i.e., all cells of the body other than those that directly produce the gametes (sperm and egg). Germinal mutations affect the cells that directly turn into the gametes.
Because there are many more somatic than germinal cells in us humans, the overwhelming majority of detectable mutations are somatic. Somatic mutations may have no discernible effect on an organism when, for example, they take place in an unused section of DNA, or they can influence the physiology of the cell when they occur in a coding region or a regulatory region of DNA.
In some cases, somatic mutations result in abnormal cell growth, ranging from benign moles to malignant carcinomas. Although somatic mutations can affect the reproductive fitness of the organism experiencing them, they cannot be passed to offspring.
Germinal mutations, on the other hand, are the life force behind evolution. The ultimate effect of germinal mutations is to introduce new genetic material. Without germinal mutation, there would be no genetic variation, no natural selection, no genetic drift, and hence, no evolution.
According to contemporary evolutionary theory and modern reproductive biology, germinal mutations are the only method of introducing new alleles and new arrangements of DNA into a species. All of the other forces of evolution change allele and/or genotypic frequencies; they do not introduce new genetic material.
The effect of a mutation depends on where the mutation occurs in the genome. If the mutation occurs in a section of DNA that does not contain code for a peptide chain, does not regulate the production of a peptide chain, does not influence subsequent replication of the DNA molecule etc., then it may have no influence on the organism or the organism’s progeny.
Some mutations that actually occur in coding regions may also have no effect. For example, a point mutation that changes the DNA codon from AAA to AAG will still result in the amino acid phenylalanine being placed in the peptide chain. Mutations that do not influence the ultimate reproductive fitness of an organism are called neutral mutations and give rise to what are called neutral genes and neutral alleles.
Although neutral alleles may not be important for the evolutionary change, they are of extreme important to geneticists tracing the evolution of populations and species. For example, if two human populations diverged recently, the frequency of the neutral alleles should be similar in the two groups.
But if they separated a long time ago, then the allelic frequencies of neutral alleles should differ. Similarly, older human populations should have accumulated more neutral mutations than populations that have more recently fissioned from one another. Hence, genetic similarity as well as genetic variation on neutral alleles can assist in reconstructing human evolutionary trees.
The most likely effect of a mutation that actually has an effect on a phenotype is to reduce fitness. Proteins and enzymes have been honed and shaped by generations of natural selection to make sure that they work appropriately for the organism. An abrupt, random change to a protein or enzyme is akin to tossing an extra gear into a finely tuned motor or capriciously rearranging a circuit on a computer chip.
Most such random acts harm rather than help functioning. If the affected allele is recessive, the loss of functioning is not critical. In all likelihood, the other allele will produce a functioning protein or enzyme. Consequently, deleterious mutations can build up for recessive alleles. This is probably the reason why several hundred different deleterious alleles have been identified for any single recessive disorder.
Occasionally, however, mutations can be beneficial and increase in fitness. On the primate X chromosome, the gene for green retinal cone pigment is located quite close to the locus for red cone pigment. It is suspected that at one time very long ago in mammalian evolution there was only one gene, but a gross mutation resulted in its duplication.
Further mutations altered the gene product in the duplicated locus (or perhaps the original one) so that it responded to light of a different wavelength. Natural selection favoured the resulting increase in colour discrimination and ultimately gave us the colour vision that we primates have today.
Mutations are both rare and common depending on the type of mutation. We have seen that gross chromosomal mutations are quite common in human fertilizations, but the majority of embryos die in utero. Mutation rates for a single allele are very difficult to quantify.
Those that occur in coding regions for dominant alleles that influence an organism’s prenatal development may suffer much the same fate as chromosomal anomalies and hence be undercounted. Most geneticists, however, agree that mutation rate for an allele is rare and is on the order of one mutation for several thousand or several tens of thousands of gametes.
Force # 4. Population Mating Structure:
Although the concept of population mating structure is implied in all texts on evolution, the actual term population mating structure is seldom encountered. Here, population mating structure is defined as all those factors—physical, temporal, anatomical/physiological, and behavioural—that result in nonrandom mating among members of a species.
To understand this concept, we must first understand the meaning of a population. A population is a group of individuals who belong to the same species, have a characteristic set of allele frequencies, usually reside in the same geographic area, and have a history of mating among themselves. Some examples may help to clarify a population and how population structure influences evolution.
Marmots are a genus of rodent and several species are found only in alpine (i.e., areas far above sea level) ecologies. Imagine two populations of marmots in the Rocky Mountains, one group inhabiting the alpine region of Long’s Peak in Rocky Mountain National Park, the other group residing on neighbouring Meeker’s Peak.
In order for a marmot born on Long’s Peak to mate with a marmot living on Meeker’s Peak, the first marmot must leave the alpine region of Long’s Peak, traverse a valley, and then climb into the alpine region of Meeker’s Peak. Although this may actually occur, it happens infrequently.
Most Long’s Peak marmots are born on Long’s Peak, live their whole lives on Long’s Peak, and mate with marmots who have been born and raised on Long’s Peak. The same occurs with marmots on Meeker’s Peak. In short, the marmots on Long’s Peak are one population while the marmots on Meeker’s Peak are another population. Hence, geographical separation of populations is a major factor influencing the population structure of a species.
In some cases, different populations may actually reside in the same geographical area. Mayflies spend two years living as nymphs in the bottom of lakes and streams before they metamorphose into winged insects, reproduce, and die. Imagine mayfly nymph Elmer who has just met the love of his life, mayfly nymph Esmeralda.
Elmer could be the persistent suitor who wines and dines Esmeralda every night for a year. But if Elmer is scheduled to metamorphose in an odd year while Esmeralda is programmed to change in an even year, the two will never be able to mate. Consequently, even-year mayflies are one population while odd-year mayflies are another population, even though the two may physically reside next to each other.
Physical and temporal separation permits different populations to evolve in different ways. Imagine that an unusually large avalanche on Long’s Peak decimates the local marmot population. With lowered population size, genetic drift may be accentuated for a few generations and alter allele frequencies. Similarly, a drought in one year may deplete the number of hatching mayflies, again intensifying natural selection and accentuating the effects of drift.
Another factor in population structure is the founder effect that occurs when only a few members of a species colonize a new territory. The South American finches that originally colonized the Galapagos Islands were probably few in number. Genetic drift, the effects of natural selection in adapting to a new environment, and their geographical isolation contributed to their evolution.
The amount of immigration and emigration among populations also influences allele and genotypic frequencies—large amounts of immigration/emigration reduce the differences between local populations while small amounts of immigration/emigration permit the populations to diverge.
The evolution of human populations has been dramatically influenced by physical population structure. Even today, the physical separation of human populations maintains genetic diversity that would otherwise be absent.
For example, people born and raised in the tropical rain forests of the Amazon basin are more likely to mate with other people born and raised in the same geographical area than they are with, say, North American Eskimos. Even within national boundaries, there are local populations. Someone living in Nebraska is more likely to mate with a fellow cornhusker than with a Yankee from Maine.
Force # 5. Culture:
Culture is not unique to humans. Species of monkeys and apes—and quite possibly other mammals and some birds — can transmit information and behaviour from one generation to the next. Examples include termite fishing in chimps, potato washing among macaques, and even swimming!
But culture has influenced human evolution to a degree unprecedented in any other species. Medicine is a clear example. Instead, clever people developed new insights into the causes of infection and then transmitted this information horizontally to their colleagues and vertically to the next generation. And the result in all likelihood has been a reduction in the pressure from natural selection.
There are many other examples of culture’s effect on human evolution. The social and religious attitudes that are a part of culture influence allele frequencies. Feelings and beliefs on population growth, birth control, and abortion clearly influence reproductive fitness, and social and religious attitudes, like virtually all behaviour, have a moderate heritability.
Similarly, social attitudes about whom to marry and not to marry influence mating structure. Military culture obviously has influenced the reproductive fitness of individuals continually since recorded history began. Travel technology has made it possible for people in different parts of the world to meet and mate, diminishing the reproductive isolation of human populations.
Domestication of the horse and camel intensified population migrations, and developments in oceanic transportation removed the reproductive isolation between Native Americans and Europeans. Even our new information age effects evolution. People on different continents can now meet over the Internet.
The prospect of genetic engineering is a developing cultural event that may have profound consequences for human evolution. It is still much too early to predict the long-term outcome of genetic engineering. For some traits, genetic engineering, even if it is technologically feasible, may not be the option of choice.
For example, suppose that you had a child with a growth hormone deficiency. Would you pay $100,000 for genetic engineering or pay $2,000 to give your child injections of growth hormone at important stages of development?
Other aspects of genetic engineering inspire awe and dread. In the past, the overwhelming effect of human culture on humans has been to alter allele and genotypic frequencies. Genetic engineering could open what may be Pandora’s box by allowing science to actually create new alleles, thus changing mutation from a random phenomenon into a deliberate, scientifically guided enterprise.
Suppose for example that altering the regulatory region of a few genes could allow them to operate for a longer time during fetal development and increase the number of neurons in the brain and human cranial capacity. We would have the potential for creating humans with new and novel genotypes that do not currently exist in the human genome.
Prognosticating on the long-term future is best left to science fiction writers, astrologers, and crystal ball gazers, so there is no immediate urgency to act. If history is any judge, then some parts of contemporary science fiction will turn into science while others remain fiction. In the case of genetic engineering, it is impossible at the present time to distinguish the two.
Integration of the Forces
The five forces of evolution do not operate in five individual vacuums with each force doing its own thing independently of the other four. Instead the five forces have dynamic interactions, making it very difficult for us humans to conceptualize the evolutionary process. The most elaborate attempt to combine the forces is called shifting balance theory developed by the famous geneticist Sewall Wright.
This three dimensional terrain represents the adaptive landscape of a species—the pits are regions of strong adaptation to the environment while the ridges are areas of poor adaptation. A species is represented as a blob—not a rigid and firm structure like a pinball, but a heavily viscous blob like a dollop of heavy grease with all types of dust and dirt particles in it. Natural selection is the force of gravity.
When a species is well adapted to its environmental niche, the species resides in a pit. As the environment for the species changes, the landscape itself alters. When the environmental changes are small, the pit changes only slightly, rising a bit here or sinking a bit there.
Some movement is imparted to the blob so that it appears to rock back and forth a bit, but the overall change in terrain is too modest to expel the blob from its pit. This is a situation known as stasis in which a species remains the same for a long period of geological time.
However, the environment can change in a big way, sometimes physically (e.g., an ice age), sometimes physiologically (e.g., development of a new pathogen), but often from competition from other species. When this happens, there is uplift in the adaptive pit so that the terrain changes from a deep pit to a shallow bowl to, eventually, a ridge.
In such circumstances, natural selection influences fecundity so population size decreases. The decrease in population size permits random genetic drift to come into play. In this scenario, drift is equal to shaking the whole landscape in an unpredictable way, and population size equals a change in the size of the blob.
As the uplift changes the pit into a ridge, the blob— much smaller and lighter than it used to be—begins rolling downhill. The effect of genetic drift, however, in shaking the landscape is unpredictable. If the population is on a ridge, drift may bounce it to the left or to the right.
If the population is in a shallow depression where gravity would otherwise keep it, drift might be large enough to eject it from this potential adaptive pit. Eventually the blob will be rolled and jostled into deeper and deeper pits. The species is adapting to the environmental changes. It will grow in size, reducing the effect of drift. As the shaking subsides and as the environment stabilizes, the blob will remain in the pit and another period of stasis occurs.
The blob, however, is no longer the same. During the period of intense change, the blob will have lost some particles of dirt and dust while other particles of dirt and dust have replicated themselves. This represents allele loss and the increase in frequency of rare advantageous mutations. Effectively, the blob has changed perhaps even enough to become a new species. In some cases, the blob may have split into two different parts, each eventually settling into its own adaptive pit.
The effects of population structure and culture may be incorporated into Wright’s model by imagining that we place the adaptive pit under a powerful microscope. Magnification shows that what appeared to be a single blob is in fact a large series of “bloblettes” that have settled into the tiny crevices among the boulders in the deep pit.
Small bloblettes are connected to their neighbours through small crevices while others are isolated from a partner by a pile of rubble. This denotes the spatial and temporal aspects of population structure. Mate preferences may be represented as different forms of magnetism that attract certain dust particles to others.
When the landscape changes, the appearance from far away is that of a single, large blob moving when in fact it is the movement of a large number of these bloblettes, all responding to the gravity.
Culture changes the bloblettes from inanimate matter into something that can move and act on their own accord. The can now dig downwards by themselves, becoming better adapted and allowing the population size to increase. As population size increases, bloblettes grow larger and merge together.
Like all analogies, this account of shifting balance is not perfect. But it does demonstrate the important interactions of the various forces of evolution. When selection pressure is intense, population size decreases, allowing a greater role for drift. Viewed from a distance, the blob may appear to split apart, but under the microscope, the fission is really an isolated bloblette going its own way.
The ultimate fate for most isolated bloblettes is extinction—they get trapped in small local pits where the decrease in population size is too rapid to permit them to slide into deeper adaptive areas. But occasionally, one bloblette makes it into one adaptive pit while another end in a second pit where both increase in size until another upheaval takes place. In this case, speciation has occurred.