Why Evolution Is True (41 page)

51
Actually, it has been tried at least once. In 1927, Ilya Ivanovich Ivanov, an eccentric Russian biologist whose forte was making animal hybrids through artificial insemination, used that technique to try to create human/chimpanzee hybrids (dubbed “humanzees” or “chumans”). At a field station in French Guinea, he inseminated three female chimps with human sperm. Fortunately, there were no pregnancies, and his later plans to do the reverse experiment were thwarted.

52
Biologists have identified at least two genes responsible for much of the difference in skin pigmentation between European and African populations. Curiously, they were both discovered because they affect the pigmentation of fish.

53
A similar case was recently described for
amylase-1,
the salivary enzyme that breaks down starch into simple sugars. Human populations with a lot of starch in their diets, such as Japanese and Europeans, have more copies of the gene than populations who subsist on low-starch diets, such as fishermen or rain-forest hunter-gatherers. In contrast to the lactase enzyme, natural selection increased the expression of
amylase-1
by favoring the duplication of genes that produce it.

54
Remember that no food has an inherent flavor—how it “tastes” to individuals depends on their evolved interactions between taste receptors and neurons stimulated in the brain. It’s almost certain that natural selection shaped our brains and taste buds so that we’d find the flavors of sweet and fatty foods appealing, prompting us to seek them out. Rotten meat is probably as delicious to a hyena as an ice cream sundae is to us.

55
Most evolutionary psychologists feel that the EEA was a reality—that over the millions of years of human evolution, the environment, both physical and social, was relatively constant. But of course we know no such thing. After all, during seven million years of evolution our ancestors lived in different climates, interacted with diverse species (including other hominins), interacted in various types of societies, and spread out over the whole planet. The very idea that there was some “ancestral environment” that we can invoke to explain modern human behavior is an intellectual conceit, an assumption made because, in the end, it is all we can do.

Glossary

 

adaptation:
A feature of an organism that evolved by natural selection because it performed a certain function better than its antecedents. The flowers of plants, for example, are adaptations to attract pollinators.

adaptive radiation:
The production of several or many new species from a common ancestor, usually when the ancestor invades a new and empty habitat such as an archipelago. The radiation is “adaptive” because the genetic barriers between species arise as by-products of natural selection adapting populations to their environments. An example is the profuse speciation of honeycreepers in Hawaii.

allele:
A particular form of a given gene produced by mutation. For example, there are three alleles at the protein-coding gene that produces our blood type: the A, B, and O alleles. All are mutant forms of a single gene that differ only slightly in their DNA sequence.

allopolyploid speciation:
The origin of a new species of plant beginning with the hybridization of two different species, followed by a doubling of the chromosome number of that hybrid.

atavism:
The occasional expression in a living species of a trait that was once present in an ancestral species but has since disappeared. An example is the sporadic appearance of a tail in human infants.

autopolyploid speciation:
The origin of a new species of plant that occurs when the entire set of chromosomes of an ancestral species is doubled.

biogeography:
The study of the distribution of plants and animals on the surface of the earth.

continental islands:
Islands, like Great Britain and Madagascar, that were once part of continents but became separated from them by continental drift or rising sea levels.

ecological niche:
The set of physical and biological conditions, including climate, food, predators, prey, etc., encountered by a particular species in nature.

endemic:
An adjective referring to a species confined to a particular region and found nowhere else, such as the endemic finches of the Galapagos Islands. The word can also be used as a noun.

evolution:
Genetic change in populations, often producing changes in observable traits of organisms over time.

fitness:
In evolutionary biology, a technical term that denotes the relative number of offspring produced by carriers of one allele versus another. The more offspring, the higher the fitness. But “fitness” can also be used more casually, referring to how well an organism is adapted to its environment and way of life.

gametes:
Reproductive cells, including the sperm and eggs of animals, and the pollen and eggs of plants.

gene:
A segment of DNA that produces a protein or an RNA product.

genetic drift:
Evolutionary change that occurs by random sampling of different alleles from one generation to the next. This causes nonadaptive evolutionary change.

genome:
The entire genetic complement of an organism, comprising all of its genes and DNA.

geographic speciation:
Speciation that begins with the geographic isolation of two or more populations, which subsequently develop genetically based reproductive isolating barriers.

heritability:
The proportion of observable variation in a trait that is explained by variation among the genes of individuals. Varying from zero (all variation due to the environment) to one (all variation due to genes), heritability gives an idea of how readily a trait will respond to natural or artificial selection. The heritability of human height, for example, ranges from 0.6 to 0.85, depending on the population tested.

hominin:
All species, living or extinct, on the “human” side of the evolutionary tree after our common ancestor with chimpanzees divided into the two lineages that would produce modern humans and modern chimpanzees.

homologs:
A pair of chromosomes that contain the same genes, though they may have different
forms
of those genes.

lek:
An area where males of a species gather to perform courtship displays.

macroevolution:
“Major” evolutionary change, usually thought of as large changes in body form or the evolution of one type of plant or animal from another type. The change from our primate ancestor to modern humans, or from early reptiles to birds, would be considered macroevolution.

microevolution:
“Minor” evolutionary change, such as the change in size or color of a species. One example is the evolution of different skin colors or hair types among human populations; another is the evolution of antibiotic resistance in bacteria.

mutation:
A small change in the DNA, usually changing only a single nucleotide base in the sequence of bases that forms an organism’s genetic code. Mutations often arise as errors during the copying of DNA molecules that accompanies cell division.

natural selection:
The nonrandom, differential reproduction of alleles from one generation to the next. This usually results from the carriers of some alleles being better able to survive or reproduce in their environments than the carriers of alternative alleles.

oceanic island:
An island that was never connected to a continent, but, like the islands of Hawaii and the Galapagos, was formed by volcanoes or other forces producing new land from beneath the sea.

parthenogenesis:
A form of asexual reproduction in which individuals form eggs that develop into adults without fertilization.

polyandry:
A mating system in which females mate with more than one male.

polygyny:
A mating system in which males mate with more than one female.

polyploidy:
A form of speciation involving hybridization in which the new species has an increased number of chromosomes. This can involve either autopolyploidy or allopolyploidy (see above).

pseudogene:
An inactive gene that does not produce a protein product.

race:
A geographically distinct population of a species that differs from other populations in one or more traits. Biologists sometimes call races “ecotypes” or “subspecies.”

reproductive isolating barriers:
Genetically based features of a species that prevent it from forming fertile hybrids with another species—for example, differences in courtship rituals that prevent cross-mating.

sexual dimorphism:
A trait that differs between males and females of a species, such as size or presence of body hair in humans.

sexual selection:
The nonrandom, differential reproduction of alleles that gives their carriers different success at obtaining mates. This is one form of natural selection.

sister species:
Two species that are each other’s closest relatives; that is, those that are more closely related to each other than to any other species. Humans and chimps are one such pair.

speciation:
The evolution of new populations that are reproductively isolated from other populations.

species:
A group of interbreeding natural populations that are reproductively isolated from other such groups. This is the definition of “species” preferred by most biologists, and is also called the “biological species concept.”

stabilizing selection:
Natural selection that favors “average” individuals in a population over those at the extremes. One example is the higher survival of human babies having average birth weight than those born either heavier or lighter.

sympatric speciation:
Speciation that takes place without the existence of any geographic barriers that physically isolate populations from one another.

systematics:
The branch of evolutionary biology involved in discerning the evolutionary relationships between species and in constructing evolutionary trees that portray those relationships.

tetrapod:
A vertebrate animal with four limbs.

vestigial trait:
A trait that is the evolutionary remnant of a feature once useful in an ancestral species but that is no longer useful in the same way. Vestigial traits can be either nonfunctional (the wings of the kiwi) or co-opted for new uses (the wings of the ostrich).

Suggestions for Further Reading

Browne, J. 1996.
Charles Darwin: Voyaging.
2002.
Charles Darwin: The Power of Place.
Knopf, New York. (Issued in 2003 as a set by Princeton University Press.) Janet Browne’s two-volume biography of Darwin is a magisterial and beautifully written treatment of the man, his milieu, and his ideas. By far the best of the many Darwin biographies.

Carroll, S. B. 2005.
Endless Forms Most Beautiful.
W. W. Norton, New York. A lively discussion of the interface between evolution and developmental biology by one of the foremost practitioners of “evo devo.”

Chiappe, L. M. 2007.
Glorified Dinosaurs: The Origin and Early Evolution of Birds.
Wiley, Hoboken, NJ. A clearly written and up-to-date account of the origin of birds from feathered dinosaurs.

Cronin, H. 1992.
The Ant and the Peacock: Sexual Selection from Darwin to Today.
Cambridge University Press, Cambridge, UK. An introduction to sexual selection for the general reader.

Darwin, C. 1859.
On the Origin of Species.
Murray, London. The book that started it all; a world classic. The best popular science book of all time (it was, after all, written for the English public), and the science book anyone
must
have read to be truly educated. Although the Victorian prose puts off some people, there are beautiful stretches, and the arguments trump everything.

Dawkins, R. 1982.
The Extended Phenotype: The Long Reach of the Gene.
Oxford University Press, Oxford, UK. One of Dawkins’s best—a discussion of how selection on one species can produce a diversity of traits, including alterations in the environment and the behavior of other species.

———. 1996.
The Blind Watchmaker: Why the Evidence of Evolution Reveals a Uni
verse Without Design. W. W. Norton, New York. Dawkins’s paean to the power and beauty of natural selection. An engrossing read by our best science writer.

———. 2004. The
Ancestor’s Tale:
A
Pilgrimage to the Dawn of Evolution,
Weidenfeld & Nicolson. New York. A large, lavishly illustrated account of evolution, starting with humans and working its way back to our common ancestors with all other species.

———. 2006. The
Selfish Gene: 30th Anniversary Edition.
(First published 1976). Oxford University Press, Oxford, UK. Another classic—perhaps the best book written about modern evolutionary theory, essential for anyone who wants to understand natural selection.

Dunbar, R., L. Barrett, and J. Lycett. 2005.
Evolutionary Psychology: A Beginner’s Guide.
Oneworld, Oxford, UK. A short but valuable guide to this growing field.

Futuyma, D. J. 2005.
Evolution.
Sinauer Associates, Sunderland, MA. The best academic textbook on evolutionary biology. Unless you’re studying biology, this may be too technical for a straight read-through, but is well worth consulting as a reference.