If a butterfly flaps its wings, this tree will become useless?

Produced by: Science Popularization China

Author: Niu Jingwei

Producer: China Science Expo

Butterflies are beautiful elves that often surround plants. When we appreciate the flowers and leaves, we can often see various beautiful butterflies flying up and down among the flowers.

When describing the beauty of butterflies, people often use the term "flowery butterfly". But when scientists studied the evolutionary relationships of butterflies, they found that butterflies are not just beautiful in appearance; their evolutionary relationships are even more "flowery".

The Flower of Evolution

When you think of evolution, does this picture come to your mind?

(Photo source: Veer Gallery)

This picture shows the evolution process of human beings.

So what is evolution? The answer given to us in biology textbooks is: evolution refers to the changes in the genetic composition of a biological group in the long-term natural evolution. The result of evolution is the production of more species and more biological offspring, and enables these organisms to better adapt to the changing environment.

This means that during the process of evolution, organisms will not evolve in only one direction, just like ancient apes will not evolve only into the human species, but will also evolve into other species similar to humans.

In scientific research, scientists usually use binary evolutionary trees to describe this evolutionary relationship, just like the family tree. When you mention binary trees, does your mind drift from the biological field to the computer field? Don't drift away, this binary tree is not that binary tree.

Binary Branching Diagram Example

(Image source: hand-drawn by the author)

The only starting point for all branches at the top of a binary tree is called the root. Starting from the root, each point (also called a node) branches out into two branches (also called subtrees clade), and the end of the tree is called a leaf node.

If there are only these structures, it can only be called a binary tree, not a phylogenetic tree. Only when we give the structure biological meaning can it be called an phylogenetic tree: each leaf node on the tree represents a biological group, such as humans; an internal node represents a hypothetical ancestor, which existed in history but is often extinct; the root node represents the common ancestor of all groups.

In the long history, the differentiation of organisms is reflected in a node on the binary tree. Different differentiation directions are like branches at the node, forming new traits and slowly accumulating mutations until new strains or even species are formed.

But sometimes, evolution isn't as simple as binary branching.

Simple evolutionary branching methods such as binary trees imply the immutability of the species' genetic material itself - unless a differentiation event occurs, the genetic material remains unchanged. This is also consistent with the general connotation of Darwin's theory of evolution - unless genetic variation occurs and the fittest survive, the results of evolution will not be manifested.

But the idea of ​​challenging this argument was proposed three centuries ago. Carl Linnaeus once proposed the positive idea that natural hybridization promotes biological evolution (yes, the father of binomial nomenclature, the ancestor of plant classification, the Swedish botanist who lived in the 18th century, Carl Linnaeus). However, compared with the name of "the father of binomial nomenclature", Linnaeus's thinking on evolution did not receive the same recognition at the time.

Linnaeus

(Image source: Wikipedia)

Darwin, who was born in the 19th century, also held a neutral attitude towards Linnaeus's views. This is not difficult to understand: in the past, biologists and the public were not optimistic about hybridization. It was generally believed that these hybrid individuals would be eliminated in competition because they were "incompatible" with the previous species. However, another view is that hybridization is far more common than we think and can effectively provide raw materials for natural selection.

It is important to add that the role of hybridization in biological evolution and population differentiation has always been a focus of debate among biologists. Early cognition also believed that hybridization would inhibit population differentiation and adaptation to the environment. It was not until the middle of the last century that population genetics theory gradually became the core theory of evolutionary biology. With the development of technology, more and more research results have supported the argument that hybridization will increase the degree of population differentiation and promote biological evolution .

An article that appeared on the cover of the top scientific magazine Science in 2019 dealt a heavy blow to the traditional binary evolutionary tree view: when the researchers constructed the evolutionary tree among species of the genus Heliconius, they tried many different methods, but could not fit a suitable binary tree and could not obtain an effective binary tree relationship.

Spheniscus

(Image source: Wikipedia)

The answer was not found until the author broke away from the pure binary tree framework and tried to construct a network of systematic evolutionary relationships.

What is going on?

When the authors divided the representative samples of several key groups in the genus Spheniscus into groups of three, mixed and matched them, and tested all of them to see if there was any "relationship" between them, they finally identified 13 introgression signals. (Introgression refers to the flow of genes between two types of butterflies. The presence of introgression signals means that there is gene flow, and the most common way of gene flow is of course hybridization.)

An example of translating this passage into layman's terms is this:

Butterfly group A and group B have hybridized, and the hybrid genetic material contains some genes that neither of them originally had. If these genes are not conducive to the survival of butterflies, then this hybrid will soon be eliminated in natural selection, and there will be no gene exchange.

But there are also cases where the hybrid offspring C of A and B has better adaptability than its parents, or by chance finds a habitat that is just suitable for C to survive and there is no competition from other butterflies, then a new butterfly group will be produced.

Type C butterflies themselves are not so "conventional". They can also hybridize with other butterfly groups (including their parent groups A and B). When C butterflies and their parent groups begin to hybridize and produce new butterfly groups, their evolutionary relationships on the evolutionary tree become a network structure like the one shown below.

Examples of hybridization relationships in the genus Spheniscus

(Image source: self-made by the author)

Now, the traditional binary evolutionary tree can no longer contain this new type of relationship. When other groups saw this, they thought, "There is such a good thing? We can't be left behind!" As a result, not only did the fate of the hybrid offspring become more complicated, but the evolutionary tree also began to become more complicated.

Reticulate evolution diagram of the genus Sphenodon

(Image source: Reference [2])

As shown in the figure above, in the final test results of the reticulate branches of the genus Spheniscus, the black solid line represents the simulation of the binary evolutionary relationship, and the colored dotted line represents the introgression signal, which can be simply understood as the situation where hybridization occurs. We can clearly observe from the figure that in the genus Spheniscus, obvious crossovers have occurred, which is the reticulated hybridization trace of the genus Spheniscus.

In fact, this kind of "hybridization relationship" can occur at any stage of biological evolution. After getting rid of the traditional limitation of one-way differentiation, we found that in fact, at every node of differentiation, butterflies can hybridize with butterfly groups that have not yet produced reproductive isolation. This evolutionary relationship in which each point can radiate outward is called "reticular evolution."

Shocking! Species relationships are so diverse

Reticulate evolution, together with many other differentiation events, has led to the evolution of butterflies into the tens of thousands of different species today, with different mimicry, patterns, body postures and shapes. The slender and fragile butterflies also have a stronger ability to adapt to the environment among flowers and forests, and are known as "flying flowers."

In fact, this kind of network hybrid relationship does not only exist in the genus Spheniscus. It also exists in the range of plants we are familiar with. We can find many examples of such network evolution:

(1) Actinidia, Actinidiaceae

Figure 4: Evolutionary relationships within the genus Actinidia

(Image source: Reference [3])

Researchers from the South China Botanical Garden of the Chinese Academy of Sciences and the Wuhan Botanical Garden have jointly conducted research on 40 research samples of 25 representative kiwifruit species. The results show that there is extensive reticular hybrid gene flow in this genus. Even between some kiwifruit groups, there are more complex repeated reticular hybridization events, showing a unique two-level reticular evolution mode.

This complex network of hybridization relationships has promoted the rapid formation of the current rich species phenotypes of plants in this genus, which is beneficial to the genetic diversity of the species.

(2) Bambusoideae

Figure 5: Herbaceous bamboo and woody bamboo

(Image source: Reference [4])

The evolution of bamboo is even more amazing. In the history of thousands of years of evolution, plants of the subfamily Bambusoideae have not only undergone several times of reticular evolution, but also experienced multiple polyploidization of chromosomes! As a result, bamboo has evolved from its ancestral "cute" herbaceous bamboo to today's strong, woody, tall bamboo poles, and from the previous annual flowering of herbaceous plants to a new "mysterious pattern" that can take up to a hundred years to bloom.

Compared to butterflies, reticular evolution plays a more important role in plant evolution. After all, plants are not like animals, which can just run away when they encounter an environment they don’t like. These events of reticular hybridization make the evolutionary relationships of plants particularly complex. When faced with different environments, they can obtain new evolutionary possibilities through "split legs" and evolve into colorful species.

Binary tree - not comprehensive, but useful

We mentioned earlier that most species will experience hybridization during their evolution, which is called reticular evolution. However, the evolutionary relationship diagrams currently used in scientific research are still binary evolutionary trees.

Why is this? Why do scientists seem to selectively ignore the consideration of species hybridization?

In fact, it is not that people ignore the possibility of species hybridization, but because the binary evolutionary tree is actually the most easily available method of fitting evolutionary relationships.

There are methods for fitting network evolutionary trees, and there are also methods such as PhyloNetworks that can be used to construct network evolutionary trees. However, there are still many problems with the current methods for fitting network evolutionary trees (mainly too much calculation and insufficient accuracy), so the use of network evolutionary trees is not too widespread.

For a binary tree, we only need to consider the relationship between species and the time of differentiation to build it. However, if we consider gene flow, the situation becomes complicated. We need to consider which species have gene flow, how strong it is, and possibly the time period when it occurs, the changes in different time periods, etc., which makes the model very complicated.

In addition, for most species that have diverged a long time ago, there is generally no gene flow at present, but there was gene flow in the past (there was hybridization between the ancestors of each genera). However, due to a long period of evolution, genetic drift and new mutations will eliminate the signals of earlier hybridization, so the relationship between these genera basically conforms to the binary differentiation model, which is why scientists still use binary evolutionary trees.

Conclusion

Now we know that the evolution of species is actually a very complex process. It is not easy to perform fitting simulations. Sometimes, despite considering the influence of various factors, we have to use other methods due to operational difficulties. Although the binary evolutionary tree method is not accurate enough, it is simple enough and in most cases it is consistent with the evolutionary history of the target we are studying, so the binary evolutionary tree still has the value of wide use.

Therefore, a good method does not lie in being comprehensive but in being easy to use. This applies not only to scientific research, but also to many other things.

References:

[1]MCLENNAN D A. How to Read a Phylogenetic Tree [J]. Evolution: Education and Outreach, 2010, 3(4): 506-19.

[2] EDELMAN NB, FRANDSEN PB, MIYAGI M, et al. Genomic architecture and introgression shape a butterfly radiation [J]. Science, 2019, 366(6465): 594-9.

[3] LIU Y, LI D, ZHANG Q, et al. Rapid radiations of both kiwifruit hybrid lineages and their parents shed light on a two-layer mode of species diversification [J]. New Phytologist, 2017, 215(2): 877-90.

[4] GUO ZH, MA PF, YANG GQ, et al. Genome Sequences Provide Insights into the Reticulate Origin and Unique Traits of Woody Bamboos [J]. Mol Plant, 2019, 12(10): 1353-65.

Editor: Guo Yaxin

(Note: Latin text should be italicized.)