Phylogeny And Evolution Of Cactus

Phylogeny And Evolution Of Cactus

Without a steady trend of genera or any fossil records, it’s become difficult for biological evolutionists to string the phylogeny and evolution of cactus together.

However, we can still find the early origins of the cactus by learning about its sub-families and working backward.

In brief, the earliest branches of the cactus family tree are Cactoideae, Maihuenioideae, Opuntioideae, and Pereskioideae. These, alongside gradual and adaptive evolution, have given rise to more than thousands of species of cacti.

But before we take a deep dive into the world of cactus phylogeny, let’s cover the basics.

Understanding Cactus Phylogeny

To extract the phylogeny of a plant family such as the cactus, experts need to first gather an accurate list of characteristics that are common to the cacti.

Understanding Cactus Phylogeny

Then, they will have to produce and follow a line of a series of adaptations and developments which will reveal the overall evolutionary history.

For the cacti family, evolutionary experts have found it particularly difficult to develop accurate phylogenetic schemes as they rarely point out definable clades.

This is mainly due to the details of many species and genera being incomplete or without sufficient information. An example of this is the poor fossil record.

No fossil cacti were found, which led to the descriptions of cactus variation and geographical distributions remaining untouched. As a result, sub-families within the cactus family are constantly split, re-grouped, and modified, even to this day.

To be able to make any statements about the evolutionary past of cacti, experts rely on physical descriptions (appearance) and biochemical (chromosomal and DNA-related) data.

How Did Cacti Originate?

In the past few decades, there has been a lot of debate surrounding the basal origin of cacti in the field of phylogenetics.

How Did Cacti Originate

Around 2005, it was believed that a group of 4 species of small with the genus name ‘Pereskia’ was the possible ancestor of cacti. However, it was soon discovered that this genus did not include all the common ancestors of a cacti’s phylogenetic chain.

Later in 2011, it was confirmed that Pereskia did not show a link between all the members of the common ancestral groups of the ‘Core Cacti’. Meaning that the clades did not connect to the main species found in all cacti varieties.

However, it was soon theorized that the most reliable common ancestor of all cacti species is possibly Rhodocactus Grandifolius.

It emerged from the Pereskia s.l.Clade and thus shares similarities with it, such as pores for photosynthesis on their stems (stomata) and a late formation of bark.

This cladogram shows the relations between the early ancestors of cacti –

This cladogram shows the relations between the early ancestors of cacti

Subfamilies of Cactus

There are many branches of the genealogical tree of the cactus, with a possibility of nearly 2000 different species. Branches of the cacti family tree have given scientists a deeper look into the adaptive evolutionary process of cacti.

However, the four main sub-families, or branches, are the following-

1. Cactoideae

This is considered the most diverse sub-family in the cactus family, from which 90% of all cacti’s genetic variations emerge from. It has given rise to over 1000 species, which are all either epiphytic or terrestrial.

Subfamilies of Cactus : Cactoideae

These cacti have no glochids or leaves with funnel-shaped flowers. They have visible spines which come in various hues, densities, and structures. The cotyledons of the members of this branch are tiny in the seed and do not grow in direct proportion to the expanding embryo.

2. Maihuenioideae

This is the smallest subfamily of the entire cactus family and includes two known species of cactus – Maihuenia Patagonica and Maihuenia Poeppigii. These are both cushiony and produce a viscous, mucus-like substance.

Subfamilies of Cactus : Maihuenioideae

Cacti from this subfamily are mostly found in high-elevation habitats.

3. Opuntioideae

Cacti in this sub-family can appear to be small shrubs to high-reaching and branchy plants. There are about 9 different and known genera with nearly 300 species under this sub-family.

Subfamilies of Cactus : Opuntioideae

Members in this subfamily are known for their quick-drying leaves, glochids, and seedlings with plump cotyledons.

4. Pereskioideae

Native to the West Indies and Central America, the Pereskioideae sub-family gives rise to over 20 types of wild trees, shrubs, and vines.

Subfamilies of Cactus : Pereskioideae

These plants are branchy, woody, and barely succulent. Almost all of their members have leaves and glossy black seeds with cotyledons resembling flesh.

Beginning of Appearance of Cacti

Before cacti became prevalent in dry climates, they were frequently prominent in tropical lands which experienced drought and prolonged periods of no rain/humidity.

Experts believe that the first cacti species were small shrubs and succulents, which had a number of leaves to aid photosynthesis. This is because early variations of new species of plants tend to look like their most recent ancestor.

Evolutionary biologists who held the idea that Leuenbergeria is the most accurate ancestor also believed that- The initial cacti had evolved to store water through quickly reacting to rainfall, using minimal levels of water during photosynthesis and transpiring as less moisture out of the plant as possible (closing stomata).

Others who were on the contrary belief of Pereskia being a more suitable common ancestor believed that the early cacti evolved to switch their gas exchange systems. The system supposedly changed from begging the usual C3 mechanism to CAM cycling.

Did Cacti Evolve?

Discussion around the true ‘Core Cacti’ species eventually led to the belief that cacti did, in fact, evolve from a clade, including the genus Rhodocactus, Pereskia, or both.

Did Cacti Evolve

Like most plants, cacti have evolved according to how they are eventually distributed.

Being affected by the change of climates, weather patterns, animals inhabiting the area, water supply, and more, cacti evolved and developed characteristics in order to thrive in their conditions.

For evolutionary change to occur, cacti first needed to undergo adaptations. First, the initial cacti species experienced selection pressure from their environment, whether it be through water availability, predation, climate changes, etc.

Among the species, there might have been a few mutated cacti with the ability to withstand the selection pressure, giving them a selection advantage.

Through time, the unaffected cacti died out while giving into natural selection, while the mutated cacti survived and populated the area.

Their mutated alleles took up the gene pool, which then gave way to other species of cacti to inherit. In this way, newer species of cacti formed and were distributed.

A Brief History of the Evolution of Cacti

Unlike animals and other plants, cacti did not leave fossils behind for biologists to determine their original ancestors. However, by understanding how natural selection affects speciation and distribution, experts were able to recognize the patterns of the global spread of cacti.

Most evolutionary biologists believe that cacti emerged from South America and the southern regions of North America. For these initial species to spread, the distribution would have occurred after the Gondwana split nearly over 140 million years ago.

Experts also believe that the Leuenbergeria group might have first emerged in Central America before moving downwards into South America. Then there are the core cacti species, which may have evolved during the past 25 million years.

The current and most prevalent species of cacti are thought to be only 10 to 5 million years frequent, while succulent plants (similar to cacti) have diversified in areas like South Africa, Madagascar, and North and South America.

These emerging prevalence of different cacti species, alongside the diversity of cacti and similar succulents, presumably played a huge role in the evolutionary history of the cactus.

Conclusion

In summary, the phylogeny and evolution of cactus is a field of study that is being modified to this day. Its gray areas of solid evidence and genealogical history have opened the floodgates of up-and-coming research.

Therefore, new chains of the cactus’ ancestral and evolutionary background are routinely being theorized. Hopefully, our information has lit most of these dim areas for you.

References:

  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8952820/
  • https://www.jstor.org/stable/1224092

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