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Darwin (Charles Robert Darwin,1809-1882) is an omnipotent naturalist who has made great achievements in all fields of natural history.

However, generally speaking little is known about his important contributions to botany, although he cited a lot of botanical evidence in the Origin of species and spent the rest of his life mainly engaged in botanical research. and published six botanical works. Three of them are about the botany or reproductive biology of flowers, with a particularly far-reaching impact: fertilization of Orchidaceae plants (1862), effects of cross-flower and self-fertilization in plants (1876), and different flower patterns of the same plant (1877). Like many of Darwin's paradoxes and paradoxes, a deep study of these strange phenomena is both useful and interesting to understand Darwin's great contribution to evolutionary botany and plant ecology.

Many people may have been under the illusion that young Darwin took the Beagle around the world, passed through the Galapagos Islands and saw ground finches with different beak shapes on different islands very close to each other. Inspired by this observation, he came up with the idea of a variety of species, and when he returned to England, he wrote the immortal classic "the Origin of species" that shocked the world.

The truth is far from that simple! Darwin's skepticism about species fixation began when he discovered fossils of lean-toothed mammals similar to the living sloth in South America. In the Galapagos Islands, he was far more impressed by the localization of plants on each island than by the localization of ground finches. As Cohen and others pointed out in 2005 in the journal Nature, when Darwin was a student at Cambridge University, he was most influenced by his mentor and professor of botany, Mr. Henslow 1796-1861, who laid a good foundation in botany.

When he saw similar but not exactly the same plant species on the small islands of the Galapagos Islands, he immediately realized that this was probably what Professor Henslow called a different "variety" of the same plant in class (Darwin later called it a "prototype" in the Origin of species). As a result, Darwin carefully collected these plant specimens and recorded in detail the exact origin and date of collection.

More than a decade later, these plant specimens were studied by his good friend and famous botanist Mr. Hooker Hook (J.D. Hooker 1817-1911), which confirmed Darwin's initial conjecture. Darwin, by contrast, did not mark ground finches in the same detail when collecting specimens from various islands, so much so that the ornithologist Mr. Gould (J. Gould 1804-1881) was puzzled by the lack of exact information about the origin of the birds. As a result, Cohen and others wrote: "indeed, when Darwin first landed on the Galapagos Islands, he obviously thought plants were more interesting than birds, so he did not mark the exact origin of birds as carefully."

Henslow (John Stevens Henslow,1796-1861)

Joseph Hook (Joseph Dalton Hooker,1817-1911)

Gould (John Gould,1804-1881) goes back to the paradox: why did Darwin spend so many years on botanical research? Why did he contribute a lot to botany and have a far-reaching influence, but he was not known as an outstanding botanist before his death?

In a sense, Darwin's interest in botany is innate. His grandfather Erasmus (E. Darwin 1731-1802) was a famous British botanist and translated the works of Linnay (C.von Linne,1707-1778).

Darwin has been fond of plants and plants since he was a child. when he was eight or nine years old, he took a picture with his sister Catherine with a pot of flowers in his hand. when he was a teenager, he helped his father take care of the peonies in the back garden. When he studied medicine at the University of Edinburgh School of Medicine at the age of 15, although he was not interested and dropped out, he was quite interested in the course "Medicinal plants". At Cambridge, his favorite professor was Mr. Henslow, a professor of botany, and became "inseparable" with him.

In Professor Henslow's botany course, he not only learned the classification and anatomy of plants, but also had the opportunity to familiarize himself with many plant specimens in Professor Henslow's herbarium. It is worth pointing out that not only did he collect a large number of plant specimens for himself and Professor Henslow during the Global Scientific Expedition, but he also collected samples of sandy violets for Professor Henslow when he went to North Wales with Professor Sedgwick (A. SedgwickPhiladelphia 1785-1873)-the earliest recorded specimen collected by Darwin for the herbarium. In addition to his above experience, there are at least three factors that made Darwin interested in botanical research.

Erasmus (E. DarwinMedi 1731-1802)

Linnai (C.von Linne,1707-1778)

First of all, like all Darwin's studies, his botanical research aims to provide evidence for his great theory of species variability and the mechanism of natural selection. Darwin studied invertebrates from barnacles and earthworms to bees and beetles, domestic pigeons, dogs, horses, cattle, sheep and other domestic animals that people turned a blind eye to, and many rare and interesting plants, but all these studies were originally intended to serve the "big book" on species he planned to write.

For well-known reasons, a Wallace (A.R. Wallaced1823-1913) was killed halfway, causing his original plan to "miscarry". Darwin apologized to readers countless times in the Origin of species. Due to the limited space, he was unable to elaborate on the large amount of evidence supporting his species theory, and expected readers to "give some trust in the accuracy of his argument". In a sense, all Darwin's works after the Origin of species are written to "compensate" the reader's trust. For example, "the Origin of Human beings and sexual selection" is a supplement to the famous saying that "the pipa half-covers the shy face" at the end of the book "the Origin of species" ("the origin and history of mankind") and the section "sexual selection" in Chapter 4. The theme of "the effects of cross-flower and self-fertilization in the plant kingdom" is only lightly mentioned in the Origin of species: "however, I suspect that no creature can reproduce on its own permanently." What's more, Darwin's last book, the formation of humus and the role of earthworms, made its debut at the end of the Origin of species with only nine words: "worms crawling over wet land." Therefore, I have always had a private opinion: if you want to really understand the Origin of species, you have to read all of Darwin's works.

Second, Darwin is a born naturalist who has a penchant for collecting from an early age and enjoys it all his life. Everything from minerals and fossils to beetles, jellyfish, barnacles, etc., will naturally not let go of beautiful flowers and plants.

As a matter of fact, his private house party, Downe House, not only has a famous English garden, but also has greenhouse gardens. Darwin and his wife Emma (Emma Darwin,1808-1896), both flower lovers, often went to see the Orchid Bank on the southern outskirts of Downe, where Darwin first encountered the insectivorous plant Castanopsis thunbergii.

In addition, "birds of a feather flock together"; Darwin's circle of friends includes many botanists, horticulturists, breeders, plant collectors and collectors, including Professor Henslow, a mentor at Cambridge University, Hook, director of the Royal Botanical Gardens (Kew Gardens), Harvard University botanist Asa Gray,1810-1888, and so on.

Natural history research belongs to interest-driven research, and Darwin is a typical leisure class gentleman scientist. He found bosom friends in this botanical "ticket circle" and continued to increase knowledge and expand interest in his contacts and discussions with them. to be able to drill deeper and further in botanical research. Therefore, Darwin's study of botany came from true love, and he confessed more than once: "Orchidaceae plants have given me unparalleled pleasure in my life." in my whole scientific career, I have not found anything to satisfy me. Comparable to the understanding of heteromorphic style structure.

The last (and perhaps most important) portrait of Mr. and Mrs. Darwin in their old age, botanical research not only provides a great deal of evidence for Darwin's theory of biological evolution, but also compared with other biological groups, it is more suitable for his work habits, research methods and experimental methods. In addition to the chapter on instinct and the chapter on geology, the botanical evidence mentioned in the Origin of species can be found everywhere.

Darwin has always been praised for his ability of observation. He can always see what people have not seen, and he will observe many things that people turn a blind eye to. He will observe them carefully over and over again, and can quickly capture their importance. He was also an extremely smart and creative experimenter. In the 19th century, when the experimental equipment was poor (the optical microscope just came out), he began to design many experiments that seemed very simple but very effective. Quite reliable experimental results were obtained.

Darwin contracted a strange disease on his way around the world, which afflicted him for the rest of his life after he returned home, and when it was serious, it was often worse than death, which greatly limited the possibility of his field work. The research and writing of the following decades were basically done in Danghaoshi. Plants are relatively easy to cultivate and immobile, so they are well suited to Darwin's work habit of staying at home. For all these reasons, more and more of his research interests turn to plants.

Therefore, understanding Darwin's botanical research not only helps to deepen the understanding of his theory of biological evolution, but also helps to further understand his keen observation and dexterous experimental skills. But why have biologists and historians of science not regarded Darwin as a botanist for a long time? It is not so much that Darwin's botanical research is overshadowed by his reputation for biological evolution, but rather that his botanical research is too advanced. If G.J.Mendel (1822-1884) was decades ahead of his genetic studies, Darwin's botanical studies were at least 100 years ahead!

During the Darwinian era of G.J.Mendel (1822-1884) and the following decades, botanical research mainly focused on taxonomy and anatomy. It was in the Alpha stage of biological taxonomy, the so-called "alpha stage" of biological taxonomy, which refers to the primary stage of biological system taxonomy, usually only the morphological description of genera and species and the comparison of common characters and general classification, so as to facilitate identification and retrieval. Lack of in-depth research and discussion of evolutionary system.

His friends Hook and Gray were both famous plant taxonomists at that time. Darwin's botanical works, mainly from the perspective of biological evolution, examine how plants evolve various mechanisms to adapt to the environment under the pressure of natural selection (embodied in plant structures and organs). Such as "climbing plant movement and habits", "plant movement skills", "insectivorous plants". Or to explore the process and mechanism of species formation, from varieties (or prototypes) to the evolution of new species, that is, speciation, to support his theory of species variability and the co-ancestor of all things, such as "the variation of animals and plants under domestication".

These studies were not fully understood and generally valued by botanists until the rise of the modern integrated system school in biology from the 1930s to the 1940s. The masterpiece variation and Evolution of plants, written by American plant geneticist G.L. Stebbins (1906-2000), made many of the problems that plagued Darwin easily solved in the framework of Neo-Darwinism.

Then, in the second half of the 20th century, new discipline evolutionary botany was born, which brought new vitality to Darwin's botanical research 100 years ago. However, his three books on plant reproductive biology will not be revived until the establishment of plant ecology. In other words, Darwin was so prescient that his botanist crown would not be possible until his contribution was fully understood.

Darwin's botanical works have been the most widely read in recent decades, with the exception of the Origin of species. In particular, his three works on plant reproductive biology have inspired many doctoral thesis topics and new research projects, and become classic literature in the field of evolutionary ecology or plant ecology. All these not only show that Darwin's research work is solid and thorough and stand the test of time, but also show the foresight of his academic thought. The effect of cross-flower and self-fertilization in the plant kingdom can be called a classic among the classics. it has not only become the cornerstone of modern plant reproductive biology, but also provides a lot of inspiration and examples for the study of co-evolution in evolutionary ecology.

Darwin claimed that "the effect of cross-flower and self-fertilization in the plant world" was a subject of great interest to him in the past 37 years. To this end, he collected a large number of observation materials and designed many simple but ingenious experiments. During the 11-year experiment, he used several of his children as his "research assistant", using more than 60 species of experimental plants.

Using artificially controlled pollination, he observed and compared the differences in growth and traits between different offspring produced by cross-fertilization and self-fertilization among many plant species, and pointed out that inbreeding caused by self-fertilization would have many adverse effects on offspring [the so-called inbreeding decline (inbreeding depression) phenomenon]. The book was published in 1876 by his publisher, J. Murray, and was regarded by Darwin as a companion to his 1862 "on British and Foreign Orchid plants using insect fertilization techniques" (also translated as "fertilization of Orchidaceae").

The transition from crossing to self-crossing in plants is one of the most common evolutionary phenomena. In spite of this, only about 10% to 15% of the species in angiosperms are self-bred. Darwin has long noticed this phenomenon in the effects of cross-fertilization and self-fertilization in plants, and the experimental results show that self-fertilization is harmful, while cross-fertilization is beneficial.

The latter has strong advantages over the former in plant size, vitality, seed germination rate and plant fruiting strength. In addition, he notes that many species have mechanisms to prevent self-fertilization, and the easiest way is to dioecious and separate them from each other. Even if monoecious, the maturation of unisexual male flowers and female flowers on some plants is staggered, so that the "Cowherd and Weaver Girl" can not meet.

We now know that plants have other tricks to block the success of selfing, such as pollen on the same plant that contains a chemical barrier that prevents its ovule from being fertilized. Darwin called it "the cause of self-incompatibility", which is now called "self-incompatibility".

On the one hand, Darwin understood that the cost of self-fertilization is relatively low, and the self-breeding strategies of "getting the moon first" and "fertile water do not flow to outsiders" have a certain adaptive significance in the case of a relative shortage of mating objects. It is relatively easy to ensure successful mating, so that species can quickly occupy suitable habitats and achieve population expansion. In the short term, self-fertilization has obvious advantages.

On the other hand, he also clearly pointed out that in the long run, cross-fertilization has the advantage of increasing individual heterozygosity, making it easier to adapt to the ever-changing natural environment and reducing the probability of death and extinction. And the vitality of hybrid offspring is also stronger than that of inbred offspring. However, due to Darwin's lack of knowledge of genetics, he could not really explain the underlying causes of these phenomena in theory.

So why are there still so many types of self-fertilization in the plant world? First of all, self-fertilized plants have a 50% advantage over cross-fertilized plants in pollination rate. they can pollinate not only themselves but also cross-fertilized plants.

Second, self-pollination does not need to rely on the media as cross-pollination. From the point of view of population geneticists, self-fertilization has a 3x2 advantage over cross-fertilization in gene transmission: self-fertilized plants transmit two genes in their seeds, while cross-fertilized plants transmit only one. This is the simplest arithmetic problem.

Darwin is in the greenhouse.

Yuan Longping, the father of hybrid rice in China, not only firmly believed Darwin's view of heterosis, but also carried it out in the whole process of hybrid rice research. on the other hand, Darwin's conclusion that cross-fertilized plants have long-term advantages has also been supported by Professor Stebbins, a famous American plant geneticist who is one of the representatives of the modern systematic school. I remember that when I was studying at the University of California, Berkeley in the early 1980s, I had the privilege of listening to Professor Stebbins, who once gave a lecture on "is hybridization necessary?" He pointed out that although hybridization is not necessary to carry on the family line, it is wonderful for biodiversity. He pointed out that in terms of biological evolution, selfing is a "dead end".

In the plant kingdom, the transition from hybridization to self-crossing is irreversible, and the adaptability of self-crossing groups to environmental changes is poor, so the probability of extinction is extremely high. Therefore, although inbred groups have repeatedly appeared in the history of plant evolution, they are always "short-lived" (because of their high extinction rate), which is why the clans of hybrid groups are absolutely dominant in nature.

Of course, the most pleasant thing about listening to these lectures is the opportunity to hear some interesting gossip and "jokes". Professor Stebbins pointed out in particular in his class that Darwin's obsession with plant reproductive biology, especially his interest in hybridization, was probably due to the pain of his life. Darwin suffered from the "inbreeding recession": he married his uncle's daughter, Emma, and gave birth to 10 children, three of whom died young and three were infertile for life. After he studied the mechanism of self-inbreeding infertility in plants, he said with emotion that this is one of the most amazing biological phenomena. Plants are so smart.

(Miao Dezheng is a research fellow at the Museum of Natural History and Institute of Biological Diversity, University of Kansas, USA)

This article is excerpted from "the effect of different flowers and self-fertilization in the plant kingdom". By Darwin, translated by Xiao Fu, Ji Daofan, Liu Zudong, Liu Zudong, Chen Xinqi. Published by Peking University Press in September 2022.

This article comes from the official account of Wechat: science Classics (ID:kexueyuandian). Author: Miao Dexian.

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