Chapter 8 Chapter 7 How to Identify Almonds
If you're a hiker who's tired of eating farm-grown fruits and vegetables, it's fun to try a little wild food.You know, some wild plants, like the wild strawberries and the blue berries of the blackberry, are not only delicious, but safe to eat.Their appearance is quite similar to familiar farm varieties, so we can easily recognize these wild berries, although they are much smaller than those we grow.Adventurous hikers should be careful when eating mushrooms, as they know that many species of mushrooms can be fatal.But even the most dried fruit lovers won't eat wild almonds, because dozens of them contain deadly cyanide (the kind used in the Nazi gas chambers).Many other plants that are considered inedible are found throughout the forest.
However, all crops come from wild plants.How did certain wild plants become crops?This is a particularly perplexing question because the ancestors of many crops (such as almonds) were either deadly or unpalatable, while others (such as maize) resembled their wild ancestors in appearance. The difference is too big.What kind of people are those men and women who live in caves to come up with the idea of "domesticating" plants?And how?
Plant domestication can be defined as the cultivation of a plant and thereby, intentionally or unintentionally, undergoing genetic changes from its wild ancestors that are more conducive to human consumption.Cultivation of crops today is a conscious, highly specialized job done by professional scientists.They have learned about hundreds of existing crops and set out to develop new ones.To achieve this goal, they plant many seeds or roots, select the best offspring, plant their seeds again, and apply their knowledge of genetics to breed elite varieties capable of breeding purebreds, perhaps even using the latest Genetic engineering techniques to transfer certain useful genes.UC Davis has a department (Department of Pomology) devoted entirely to apples and a department (Department of Viticulture and Enology) devoted to grapes and wine.
However, the history of plant domestication dates back more than 10,000 years.The earliest farmers certainly would not have used molecular genetic techniques to achieve this result.These earliest farmers didn't even have any existing crops to serve as samples to inspire them to breed new varieties of crops.Therefore, it is impossible for them to know that no matter what they do, they will finally have a good meal.
So how did the first farmers unknowingly domesticate plants?Like how did they turn poisonous almonds into non-toxic almonds without knowing what they were doing?Besides making certain wild plants bigger or less poisonous, what changes did they actually make in them? Even for some important crops, the timing of domestication varied widely: the timing of domestication of peas, for example No later than 8000 BC, olives around 4000 BC, strawberries until the Middle Ages, and pecans as late as 1846.Many important wild plants for food production are valued by millions of people. For example, oak trees are sought for their edible acorns in many parts of the world, but oak trees remain undomesticated even today.What makes some plants easier to domesticate than others, or more attractive to domesticate?Why did the olive tree succumb to the Stone Age peasant, while the oak tree baffles our brightest agronomists?
Let us first look at the problem of domestication from the plant's point of view.As far as plants are concerned, we are just one of thousands of animals that have inadvertently "domesticated" plants.
Like all other animals (including humans), plants must disperse their offspring into areas where they can thrive and pass on the genes of their parents.Small animals disperse by walking or flying, but plants have no such freedom of choice, so they must somehow hitchhike along the way.Several plants produce seeds that are easily blown away by the wind or float on water, while many others coax animals to take the seeds away by enclosing the seeds in tasty fruit and announcing the ripeness of the fruit by color and smell.The hungry animal rips off the fruit and eats it, or walks away, or flies away, and spits out or defecates the seed somewhere far from the parent tree.Seeds can be carried thousands of miles in this way.
It might surprise you to learn that the seeds of plants can't even be digested in your stomach, and can still sprout out of your waste.But any adventurous readers who are not prone to vomiting may wish to do this experiment and verify it for themselves.The seeds of many wild plants actually have to pass through the guts of animals before they can germinate.For example, a melon in Africa is so easily eaten by a hyena-like animal called a coyote that most of the melons grow where the coyote excretes.
For an idea of how plants that want to hitchhike along the way can attract animals, consider the example of the wild strawberry.When strawberry seeds are immature and cannot be sown immediately, the surrounding fruits are green, sour and hard.When the seeds finally ripen, the fruit becomes red, sweet and tender.The change in fruit color becomes a signal that attracts birds such as thrushes to peck the fruit and fly away, finally spitting or excreting the seeds with feces.
Of course, strawberries aren't meant to attract birds when the seeds are ready to disperse.The thrush did not intend to domesticate strawberries either.Instead, strawberries evolved through natural selection.The greener the color and the more sour the taste of the less ripe strawberries, the fewer birds will eat the strawberries before the seeds are ripe and damage the seeds; more birds.
Countless other plants have fruits adapted to be eaten and dispersed by certain kinds of animals.As strawberries are to birds, so are acorns to squirrels, mangoes to bats, and certain sedges to ants.This fits part of our definition of plant domestication as a genetic change in the ancestral plant that made it more favorable for human consumption.But no one would seriously call this evolutionary process domestication, because birds, bats, and other animals don't fit the other part of that definition: They didn't grow plants on purpose.Likewise, the early unconscious stages of the evolution of crops from wild plants included ways in which plants attracted humans to eat them and disperse their fruit, but did not yet grow them on purpose.The human excretory, like that of the coyote, may have been a proving ground for the first mindless crop breeders.
Our excretory site is just one of the many places where we accidentally sow the seeds of the plants we eat.When we collect edible wild plants and bring them home, some are scattered on the road or at home.Some fruit rots while the seed is still perfectly intact, so it's thrown into the trash and never eaten.Strawberry seeds are part of the fruit that we actually put in our mouths, but because the seeds are so small, they are swallowed and passed out with feces.But there are also fruits whose seeds are so large that they are spit out.Thus our latrines, together with our spittoons and rubbish-heaps, make the best agricultural research laboratories.
No matter which "laboratory" these seeds end up in, they all come from some kind of edible plant -- that is, a plant that we like to eat for some reason.From the days when you were picking berries, you've known to pick certain berries or berry bushes.Finally, when the first farmers began to sow consciously, they sowed the seeds of those plants which they deliberately gathered, though they did not understand that the seeds of large berries were expected to grow into bushes bearing more large berries. 1. The principle of inheritance.
So when you're slogging your way into a mosquito-infested thorny bush on a hot, humid day, you're not doing it just for any strawberry bush.Even unconsciously, you decide which clump of strawberries looks the most promising and whether it's worth the trip.What is the standard of judgment in your subconscious mind?
One criterion, of course, is size.You like big berries because you don't deserve the sun roast and mosquito bites for a few unsightly little berries.This is part of the reason why the fruits of many crops are much larger than those of their wild ancestors.We are all too familiar with the fact that supermarket strawberries and the blue berries of blackberry trees look huge and fat compared to their wild counterparts; the difference has only emerged in the last few hundred years.
This size difference among other plants dates back to the early days of agriculture, when cultivated peas were selected and evolved to weigh more than 10 times their wild counterparts.It was these little wild peas that hunter-gatherers gathered for thousands of years, just as we gather the blue berries of the blackberry tree today.They then selectively harvested and planted the most attractive and largest wild peas—this is what we call farming—and it started automatically causing the average pea size to increase each generation.Likewise, supermarket apples are typically around 3 inches in diameter, while wild apples are only 1 inch in diameter.The earliest cobs were barely more than half an inch long, but by 1500 A.D. Mexican Indian farmers had grown cobs as long as six inches, and modern cobs can be as long as — “a foot and a half.
Another stark difference between the seeds of our own plants and those of many of their wild ancestors is bitterness.The seeds of many wild plants have evolved to be bitter and unpalatable, or even poisonous, to prevent animals from eating them.Natural selection, therefore, acts oppositely on seeds and fruits.Plants with tasty fruit let animals scatter their seeds, but the seeds in the fruit must be unpalatable.Otherwise, the animals will chew up the seeds and the seeds will not germinate.
Almonds provide a striking example of bitter seeds and how they have changed during domestication.Most almond seeds contain an extremely bitter chemical called amygdalin, which (mentioned earlier) produces toxic cyanide when it breaks down.If anyone was foolish enough to eat wild almonds despite the warnings of their bitter taste, he would die from it.Since the first stage of unconscious domestication was the collection of edible seeds, how did the domestication of wild almonds reach this first stage?
The explanation for this is that by chance a few almond trees have a mutation in a gene that prevents them from synthesizing the bitter-tasting amygdalin.These trees became extinct in the wilderness without leaving any descendants, because the birds found their seeds and ate them up.But the children of the original farmers, out of curiosity or hunger, got a little bit of every kind of wild plants around them, and finally tasted and discovered these almond trees without bitterness. (Similarly, European farmers today would still be thankful if they stumbled upon a few oak trees with sweet acorns but not bitter ones.) The seeds of these non-bitter almonds are the seeds that ancient farmers might have planted, beginning by inadvertently It was left to grow on the rubbish dump, and later it was intentionally planted in its own orchard.
Wild almonds, dated no later than 8000 BC, have been found in excavated Greek archaeological sites.By 3000 BC, wild almonds were being domesticated in lands east of the Mediterranean Sea.After the king of Egypt died around 1325 BC, almonds were among the posthumous foods placed in his famous tomb.Among the many other familiar crops are lima beans, watermelons, potatoes, eggplants and cabbages.The wild ancestors of these crops, some bitter, some poisonous, and occasionally a few luscious varieties, must have grown from the excrement of ancient travelers.
If size and taste were the most obvious criteria for hunter-gatherers to select wild plants, other criteria included fleshy or seedless fruit, oily seeds, and long fibers.Wild pumpkin seeds have little or no pulp on the outside, but early farmers preferred to choose pumpkins with much more pulp than seeds.The long-ago selection of bananas for all-meat, seedless varieties has inspired modern agricultural scientists to breed seedless clementines, seedless grapes, and seedless watermelons.Seedless fruit is a good example of how human selection has completely reversed the original evolutionary role of wild fruit, which was really just a means of dispersing seeds.
In ancient times, many plants were similarly selected for their oily fruits and seeds.The earliest domesticated fruit tree in the Mediterranean region was olive, which was cultivated after about 4000 BC for its oil.Cultivated olives are not only larger than wild olives, but also have higher oil content.Ancient farmers chose sesame, mustard, poppy and flax for their oily seeds, while modern botanists choose sunflowers, safflower and cotton for the same purpose.
The cultivation of cotton for oil is a modern matter.Before that, of course, cotton was chosen for the fiber used in textiles.Cotton fibers, or lint, are the hairs on the cotton seeds, and early farmers in the Americas and the Old World independently selected different varieties of cotton for their long lint.Two other plants cultivated for ancient textiles are flax and hemp, which get their fibers from stems, so they were selected for their long, straight stems.While we think of most crops as being grown to eat, flax was also one of our earliest crops (domesticated no later than 7000 BC).It is the raw material for linen, which remained the main textile raw material in Europe until it was replaced by cotton and synthetic fibers after the Industrial Revolution.
The changes we've described so far in the evolution of wild plants into crops are directly related to traits that early farmers might have actually noticed—such as fruit size, bitterness, fleshiness, oiliness, and fiber length.By harvesting these wild plants with particularly desirable qualities, ancient peoples unwittingly spread them, putting them on the path to domestication.
Beyond this, however, there were at least four other major changes that failed to elicit compelling choices from berry pickers.In these cases, if the berry pickers did cause any change, it was either by harvesting available plants because other plants remained unavailable for some unknown reason, or by changing the selection conditions that acted on the plants .
The first such alteration affected the wild structure of seed dispersal.Many plants have specialized mechanisms for dispersing their seeds (thus making them impossible for humans to gather efficiently).Only seeds produced by mutations lacking this structure were harvested and became progenitors of the crop.
An obvious example is peas.The seeds of peas (the peas we eat) are enclosed in pods.For vetches to germinate, they must break out of their pods.To do this, the pea has evolved a gene that causes the pod to burst open, catapulting the pea to the ground.The pods of the accidentally mutated peas do not burst.In the wild, the mutant peas died encased in pods on the parent plant, and only the bursting pods passed their genes on to offspring.But in contrast, the only pods that humans can harvest are probably the ones left on the plant that don't burst.So, as soon as humans started taking vetches home to eat, there was immediate selection for this single-gene mutation.Examples of selection for mutations that also do not burst are lentils, flax, and poppy.
Wild wheat and barley are not enclosed in pods that can burst, but grow at the top of stalks that shed themselves, allowing the seeds to fall to the ground where they can germinate.A single-gene mutation keeps wheat stalks from falling off.In the wild, this mutation can be devastating to a plant because the seeds cannot germinate and take root if they cannot fall to the ground.But the mutated seeds may have been left on the stalk with ease, waiting for humans to harvest them and bring them home.When humans then planted these harvested mutated seeds, the farmer could again get all the mutated seeds from the progeny of these seeds, harvest them, sow them, and the non-mutated seeds of the progeny fell to the ground, and could not get out.In this way, the farmer reversed the direction of natural selection by 180 degrees: the previously successful gene suddenly became destructive, and the destructive mutation became successful. This involuntary selection for non-shedding wheat and barley stalks, more than 10,000 years ago, is apparently the first major human "improvement" of plants.This change marked the beginning of Fertile Crescent agriculture.
The second change was even more imperceptible to ancient travelers.For annuals grown in regions with erratic climates, it could be devastating if all the seeds germinated rapidly and simultaneously.If this had happened, it only took one drought or frost to kill off the seedlings, leaving no seeds for future generations.Consequently, many annuals have evolved to reduce losses by means of germination inhibitors, which cause seeds to initially go dormant and then germinate in batches over several years.That way, even if most of the seedlings die off due to one bad weather event, some of the remaining seeds will germinate later.
A common adaptation that wild plants rely on to reduce losses is to enclose their seeds in a thick husk, or protective covering.Among the many plants that have produced this adaptation are wheat, barley, flax or sunflower.While there is still a chance that these post-germinated seeds will germinate in the wild, consider what is bound to happen as agriculture grows.Early farmers may have discovered through trial and error that they could get higher yields by loosening the soil, watering it, and then planting the seeds.If so, the seeds germinate immediately and grow into plants, and their seeds can be harvested and planted the following year.But the seeds of many wild plants don't germinate right away, so they don't yield anything.
Occasionally mutated individuals in wild plants lack thick seed coats and other inhibitors of germination.All of these mutants germinated rapidly and eventually produced mutant seeds.Early farmers may not have noticed the difference, they just knew to notice and selectively harvest the large berries.But the sow-grow-harvest-sow cycle immediately and unconsciously selects for those mutants.As with the changes in seed dispersal, these changes in germination inhibition are also true for the monoecious plants of wheat, barley, and pea.But most wild plants are not reproduced in this way.They are either monoecious, but cannot self-fertilize, and must cross breeds with other monoecious individuals (my stamen fertilizes your pistil, your stamen fertilizes my pistil), or they are like all normal Like other mammals, they are dioecious.The former plant is called self-incompatibility monoecious and the latter plant is called dioecious.Neither scenario is a relief for ancient farmers, who could lose all mutants in their favour.
The solution involves another imperceptible change.Many plant mutations affect its own reproductive system.Some mutant individuals did not even need to be pollinated to produce fruit, the result of which we have seedless bananas, grapes, oranges and pineapples.Some mutant androgynous plants lose their self-incompatibility and become capable of self-fertilization—a process exemplified by many fruit trees such as plums, peaches, apples, apricots, and cherries.Some mutated grapes are usually dioecious, but also become self-fertilizing monoecious.Ancient farmers, who did not yet understand the biology of plant reproduction, still used all these methods to end up with useful crops capable of breeding purebreds and thus worth replanting, rather than promising mutants, which His descendants are lost to oblivion because of their worthlessness.
The farmer thus selects from among a number of characteristic plants, and he selects on the basis of not only visible qualities such as size and taste, but also such invisible qualities as seed dispersal mechanisms, germination inhibition, and reproductive biology. specialty.As a result, different plants have been selected for quite different or even opposite characteristics.Some plants, such as sunflowers, are selected for having much larger seeds, while others, such as bananas, are selected for having small or no seeds.Choose lettuce for its lush leaves and forgo its seeds or fruit; choose wheat and sunflower for its seeds and forgo its leaves; choose pumpkin for its fruit and forgo its leaves.It is particularly interesting that different selections can be made for one wild plant for different purposes, resulting in crops with very different appearances.Beets have been grown since Babylonian times, but then for their leaves (like the modern beet variety called chard), later for their edible roots, and finally (in the 18th century) for their For their sugar content (sugar beets).The earliest cabbages were probably originally grown for their oilseeds, and after even greater divergence there was selection for them; some selected for the leaves (modern cabbages and kale), Some opt for stems (kohlrabi), buds (robes kale), and flower buds (cauliflower and broccoli).
So far we have discussed the transformation of wild plants into crops through conscious or unconscious selection by farmers.That is to say, the farmers initially selected the seeds of certain individual plants, brought them back and planted them in their own gardens, and then selected the seeds of some offspring every year, and planted them in the gardens next year.But a large part of this variation was also affected by the plants' self-selection.Darwin's "natural selection" refers to the fact that some individuals of a species can survive and/or reproduce more successfully under natural conditions than competing individuals of the same species.In reality, natural processes that differentiate survival and reproduction make this selection.If conditions change, individuals of the same species are more likely to survive or reproduce better, and are thus "selected for by nature," with the result that the population undergoes evolutionary change.A classic example is the development of the phenomenon of melanization of the moth industry in England: in the 19th century, as environments became dirtier, darker moths were more common than lighter moths, as they inhabited dark and dirty places. Dark-colored moths on trees are, by contrast, more likely than light-colored moths to escape the attention of predators.
Much in the same way that the Industrial Revolution changed the environment for moths, farming has changed the environment for plants.A loosened, fertilized, watered, and weeded garden plot provides a completely different growing environment than a dry, unfertilized garden plot on a hillside.Many of the changes in plants produced in domestication have resulted from such changes in conditions, and consequently in those plant varieties which were favored.For example, if a farmer sows his garden closely together, there will be intense competition among the seeds.Large seeds, which can take advantage of favorable conditions to grow rapidly, are now in a better position than small seeds, which formerly grew on dry, unfertilized hillsides, where seeds are scarcer and there is less competition. Less aggressive, so only growing in that kind of place is good for them.This constant competition among the plants themselves played an important role in the acquisition of larger seeds and many other changes that evolved as wild plants became ancient crops.
As far as the domestication of plants is concerned, some plants were domesticated a long time ago, some were not until the Middle Ages, and there are still some wild plants that remain largely unaffected by all our activities.What is the reason for the huge difference in this regard?We can infer many answers by examining the fixed sequence in which different crops developed in the Southwest Asian Fertile Crescent.
Here's how it turns out: The earliest Fertile Crescent crops, such as wheat, barley and peas, were domesticated about 10,000 years ago, and they all descended from wild ancestors that presented many advantages.They are already edible and produce high yields in the wild.They are easy to grow, just sow or plant.They grow quickly and can be harvested within a few months of sowing.This was a huge plus for early farmers who were still somewhere between wandering hunters and settled villagers.They stored easily, unlike many later crops such as strawberries and lettuce.They are mostly self-pollinating: that is, the various crops pollinate themselves, passing on their own desirable genes unchanged without interbreeding with other less useful varieties.Finally, their wild ancestors required little genetic change in order to be converted into crops—in the case of wheat, for example, mutations that allowed kernels not to shed and germinated quickly and evenly.
The next stage of crop development includes the earliest fruit and nut trees domesticated around 4000 BC, among them olives, figs, dates, pomegranates and grapevines.Compared with cereals and legumes, their disadvantage is that it takes at least 3 years after planting to start to set fruit, and it takes 10 years to reach the peak period.Therefore, the cultivation of these crops was only possible for those who had lived a fully settled rural life.However, these early fruit and nut trees are still the easiest crops to grow.Unlike later domesticated trees, they can be planted directly by cuttings or even by seeding.Cuttings also have an added benefit: Once ancient farmers discovered or bred a productive tree, they could be sure that all descendants of that tree would grow exactly like it.
The third stage involved some fruit trees that were much harder to cultivate, including apples, pears, plums and cherries.These trees cannot be grown from cuttings.It is useless to grow from seed, for the progeny of even the best of them are very changeable, and the fruit they bear is mostly worthless.The trees are grown by the difficult technique of grafting, which was developed long after Chinese agriculture began.Even if you understand how grafting works, grafting is still a difficult job.Not only that, but the principle itself can only be discovered through conscious experimentation.Inventing such a thing as grafting is probably no longer a wanderer who takes advantage of a certain excrement, and then returns to find that there has grown a tree bearing sweet fruit.
Many fruit trees that developed at a later stage present another problem because their wild ancestors were not self-pollinating at all.They must be cross-pollinated by another plant of the same variety but with genetic variation.So early farmers either had to find fruit trees that didn't require cross-pollination, or they had to consciously plant different genetic varieties or nearby male and female individuals in the same orchard.All these problems delayed the domestication of apples, pears, plums, and cherries until around the classical period.At about the same time, however, a more recent crop of domesticated plants, which were wild as weeds in fields of intentionally cultivated crops, came without much trouble.Crops that were originally weeds included rye, oats, turnips, radishes, beets, leeks and lettuce.
While the set of crops I have just detailed applies to the Fertile Crescent, a partially similar set of crops also occurs elsewhere in the world.In particular, the Fertile Crescent wheat and barley are representative of the class of crops known as cereals (Poaceae), while the Fertile Crescent peas and lentils are legumes (of the Fabaceae family, which includes soybeans) representative.Cereal crops have the advantage of being fast growing, high in carbohydrates and producing 1 ton of food per hectare of arable land.Thus, grains today account for more than half of all calories consumed by humans and include five of the modern world's 12 major crops (wheat, corn, rice, barley, and sorghum).Many cereal crops are low in protein, but this deficiency can be made up for by pulses, which typically contain 25% protein (38% for soybeans).Thus, together, grains and legumes provide many essential ingredients for a balanced diet.
As outlined in Table 7.1, the domestication of local cereal and legume combinations marked the beginning of food production in many regions.The best-known examples are the combination of wheat and barley with peas and lentils in the Fertile Crescent, maize with several legumes in Central America, and rice and millet with soybeans and other legumes in China.Less well known are the combinations of African sorghum, African rice and barnyard barnyardgrass with cowpea and wild beans, and the non-cereal quinoa of the Andes with several legumes.
Table 7.1 also shows that flax was domesticated early for fiber in the Fertile Crescent and similarly elsewhere.Hemp, 4 types of cotton, yucca, and agave provided fibers for rope and cloth at various times in China, Central America, India, Ethiopia, sub-Saharan Africa, and South America. In some places, the fluff of domesticated animals is also used as a supplement.Of these early centers of food production, only the eastern United States and New Guinea remained without fiber crops.
Against these similarities, there are some important differences in food production systems around the world.One of the differences: In many parts of the world, agriculture began with issues of seed dispersal and monocropping of fields and eventually plowing with livestock.That is to say, the seeds are scattered one by one by hand, so that the whole field can only grow one kind of crop.Once cattle, horses, and other large mammals were domesticated, they were harnessed to plows, and farmland was worked by animal power.However, no animal that could be harnessed to a plowshare had been domesticated in the New World.On the contrary, the land there is always plowed with a stick or a hoe, and the seeds are planted one by one by hand instead of spreading them in handfuls.As a result, most gardens in the New World are mixed crops rather than monocultures.
There are 5 types of crops in this table, which come from early agricultural sites in different regions of the world.Crops that were first domesticated elsewhere are in square brackets.Crops that were imported or became important only later were omitted, such as bananas in Africa, corn and bean crops in the eastern United States, and sweet potatoes in New Guinea.Cotton is 4 species in the genus Cotton, each native to a particular region of the world; squash is 5 species in the genus Cucurbita.PLEASE NOTE: Cereals, pulses and fiber crops mark the beginning of agriculture in most areas, but root crops, tubers and tubers are only beginning to be important in some areas.
Another major difference in farming systems concerns the primary source of calories and carbohydrates.We have seen that in many areas the main source of this direction is grain.Xiao Guo, in other areas, this task of grains was taken over or shared by roots and tubers, which apparently were irrelevant in the ancient Fertile Crescent and China.The staple food is cassava and sweet potato in equatorial South America, potato and sorrel tubers in the Andes, African yam in Africa, and Indo-Pacific yam and taro in Southeast Asia and New Guinea.Tree crops, principally bananas and breadfruit, are also carbohydrate-rich staples in the Southeast and New Guinea.
Thus, by the time of the Romans, almost all of today's crops had been domesticated somewhere in the world.As we will also see with domestic animals (Chapter 9), ancient hunter-gatherers were well acquainted with local wild plants, and ancient farmers apparently discovered and domesticated nearly every animal worth domesticating.Of course, medieval monks did start cultivating strawberries and raspberries, and modern plant breeders are still improving ancient crops and have added some new secondary crops, mainly berries (such as the blueberry of the black rice tree). berries, bilberries, and kiwi) and nuts (macadamia, pecans, and cashews).但和古代的一些主食如小麦、玉米和稻米相比，这些新添的现代作物始终只具有不太大的重要性。
不过，在我们所列举的关于驯化成功的例子中仍然缺乏许多野生植物。尽管它们具有食用价值，但我们却不曾成功地驯化它们。在我们驯化失败的这些例子中，引人注目的是橡树。橡实不但是欧洲农民在荒年作物歉收时的应急食物，而且也是加利福尼亚和美国东部的印第安人的主食。橡实具有营养价值，含有丰富的淀粉和油。和许多在其他方面可以食用的野生食物一样，大多数橡实含有味苦的丹宁酸，但爱吃橡实的人学会了用处理杏仁和其他野生植物中味苦的化学物质的同样办法来处理丹宁酸：或者用研磨和过滤来去掉丹宁酸，或者只从丹宁酸含量低的偶然产生突变的橡树上收获橡实。
为什么我们不能驯化像橡实这样宝贵的粮食来源呢？为什么我们花了那么长的时间去驯化草莓和树莓7对那些植物的驯化，即使是掌握了像嫁接这样困难的技术的古代农民也会束手无策，这又是怎么一回事呢？
事情原来是这样的：橡树有3个不利因素。首先，它们生长缓慢，可能使大多数农民失去耐心。小麦种下去不消几个月就可得到收成；杏仁种下去3、4年后就可长成能够结果实的树；但种下—颗橡实可能在10年或更长的时间里不会有什么收益。其次，橡树所结的坚果无论是大小还是味道都适于松鼠，而我们都见到过松鼠埋藏、挖掘和吃橡实的情景。如果偶尔有一颗橡实松鼠忘记把它挖出，那么这颗核实就可长出橡树来。有数以10亿计的松鼠，每一只松鼠每一年把数以百计的橡实传播到几乎任何一个适于橡树生长的地方。这样，我们人类就不可能为我们所需要的橡实去选择橡树。橡树生长缓慢和松鼠行动迅速这些问题大概也说明了为什么山毛榉和山核桃树同样未能驯化的原因，虽然欧洲人和美洲土著分别对这两种树种大量地加以利用以获得它们的坚果。
最后，杏仁和橡实的最重要差异也许是：杏仁的苦味由单一的优势基因所控制，而橡实的苦味似乎由许多基因所控制。如果古代农民栽种了偶然产生不苦的突变的杏仁或橡实，那么根据遗传规律，如果是巴旦杏树，结果，长成的树上的杏仁有一半可能也是不苦的，而如果是橡树，则几乎所有橡实可能仍然是苦的。仅仅这一点就足以使任何想要种橡实的农民的热情荡然无存，尽管他们已经赶走了松鼠并且保持耐心。
至于草莓和树莓，我们在与鸫和其他吃浆果的鸟儿的竞争中遇到了同样的困难。是的，罗马人的确在他们的园子里照料过野草莓。但是，由于千百万只欧洲鸫把野草莓的种子排泄在每一个可能的地方（包括罗马人的园子里），所以草莓始终是鸫想要吃的小浆果，而不是人想要吃的大浆果。由于近来保护网和温室的发展，我们才终于能够把鸫打败，并根据我们自己的标推来重新设计草莓和树莓。
因此，我们已经看到，超市上的大草莓和野生的小草莓之间的差异只是一个例子，用来说明把人工培育的植物与其野生祖先区别开来的许多特征。这些差异首先来自野生植物本身之间的自然变异。有些变异，如浆果的大小和坚果的苦味的变异，可能很快就被古代的农民注意到了。其他变异，如种子传播机制或种子休眠方面的变异，在现代植物学兴起之前，可能并未被人类认出来。但是，不管古代旅行者对可食用的野生植物的选择是否依赖于自觉的或不自觉的选择标准，由此而产生的野生植物向作物的演化起先总是一种无意识的过程。这是我们对野生植物个体进行选择的必然结果，是园子里各植物个体之间竞争的结果，而这种竞争所偏爱的个体和在野外得天独厚的个体是不同的。
这就是为什么达尔文在他的伟大著作（物种起源）中并不是一开始就解释自然选择问题的原因。他的第一章反而详细说明了我们的驯化动植物是如何通过人类的人为选择而出现的。达尔文不是讨论我们通常认为和他联系在一起的的鸟类，而是一上来就讨论——农民是怎样培育出不同品种的醋栗的！他写道，“我已经看到园艺学著作中对园丁们在用这样差的材料取得这样了不起的成果方面的令人叹为观止的技术所表现出来的巨大的惊奇；但这种技术是简单的，就其最后结果来说，对这一技术的采用也几乎是无意识的。它在于总是去培育最出名的品种，播下它的种子，然后当碰巧出现了一个稍好一点的品种时，再去选择它，就这样地进行下去。”通过人为选择来培育作物的这些原则仍然可以成为我们的关于物种起源通过自然选择的最可理解的模式。
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