Mad about Science: Convergent evolution

By Brenden Bobby
Reader Staff

Have you ever thought about the similarities between a cow and a triceratops?

Certainly, there are huge differences between these two creatures that have been separated by a gulf of 66 million years, but the similarities between the two are more shocking than the differences.

Both animals were/are herbivorous quadrupeds grazing low-lying shrubs and grasses for sustenance, and both developed horns to defend themselves. Despite their similarities, their last common ancestor was probably a unicellular organism. So what gives?

This is a form of convergent evolution, whereby animals adapt to their environment over many generations to develop similar traits to completely unrelated animals. 

It’s a difficult concept to wrap one’s head around, as it takes hundreds, sometimes thousands of generations to occur. It’s believed that in most cases in the natural world, female animals seek attributes in males they believe will lead to better offspring that have a higher chance of survival. It’s possible that the animals don’t completely understand this process in the way that you or I do, but that some kind of instinct may be guiding them toward producing more efficient offspring.

Think about this in humans for a moment, as it mirrors the natural world. A human female has aspirations for both herself and her future children. She may value having athletic children that are exceptionally skilled at running and gaining fame and prowess by competing against other humans at sports. Perhaps wealth and status is something she desires for her children to either obtain or to pass down and ensure the familial line continues to be wealthy and influential. Either way, she will likely seek out a mate that offers the highest chances of producing offspring that will fulfill those ambitions.

Of course, this is a gross oversimplification of the human experience, particularly in this age. However, it was on clear display throughout history, particularly among the wealthiest and most powerful families in the world. Aristocratic families often married and procreated with the intention of long-term dynastic rule or to retain a bloodline that often ended up being at odds with one another, as incestuous relationships are never healthy for the survival of a family line.

It’s likely that much of this oversimplification is what many animals seek in mates. In birds, a male with wings that are slightly longer than his rivals may be more desirable to a female, as the young could be more likely to inherit that trait to fly more effectively, therefore outcompeting other birds. This seems odd when examining a single lineage as a sample set, but really begins to make sense when examining a species as a whole across thousands or even millions of breeding pairs across hundreds or thousands of generations.

Nature likes to play numbers. The commonality between chickens and cockroaches and their incredible ability to survive has little to do with intelligence and everything to do with flinging huge numbers of eggs and offspring into the world in hopes that the line will continue. In many cases this works, and is another form of convergent evolution.

In the case of the cow and the triceratops, the appealing traits that led to their convergent evolution were almost certainly directly linked to their environment. Due to their size, cows and buffalo have few natural predators — particularly if they have plenty of room to run and trample threats to the herd. Horns allow for the cow to deliver the full energy of its charge into a singular point of contact against a threat. It’s likely the triceratops developed horns for this identical reason. 

Meanwhile, both creatures needed to grow to great sizes in order to stay safe from predators, which meant their weight needed to be dispersed across four legs. The teeth and jaw structure of a cow and a triceratops are similar, as well, which is most likely because of the grassy diets of both creatures.

Convergent evolution isn’t a trait unique to animals. It is common in plants, too. Interestingly, while animals — and particularly female animals — are capable of choosing their mates to pass on desirable traits to their offspring, plants are left to the mercy of the wind and other animals like pollinators to spread their genetics. Few plant species are strictly clonal, as species that clone themselves over long periods of time become susceptible to disease and environmental changes.

To overcome the absence of choice, plants rely on raw numerical advantage and rapid reproductive cycles in order to evolve effectively, both reflecting and directly shaping the environment in which they exist.

One of the most obvious forms of convergent evolution in plants is the annual lifecycle. Annual plants — those which exist for a single growing season before dying — are a common staple at nurseries and home and garden stores around the world, yet they only represent about 6% of all plants, yet it has been estimated that as many as 120 different species developed this trait independently from one another. This means that the annual lifecycle is a direct response to a hostile environment in which the plant cannot exist for a certain portion of the year, instead investing its full energy into the seeds to continue its genetic lineage.

A subject spanning millions of years and countless generations of life isn’t done justice in a few hundred words, but it sure is fun to think about.

Stay curious, 7B.