It took many, many generations for the Puijila darwini to evolve from this…..
When you think about it, 25 million years is a very long time for organisms to evolve and change. The speed of change from Puijila darwini to elephant seal was extremely slow, with each tiny change taking many generations to complete. A Puijila darwini didn’t suddenly give birth to a live elephant seal!
There were several intermediate species that evolved along the way before the descendants of the Puijila darwini evolved into what we know today as the ‘elephant seal’.
An ‘intermediate’ species is a species of animal that exhibits traits common to both an ancestral group (in this instance the Puijila darwini) and its descendant group. (the elephant seal)
Over time the descendants of the Puijila darwini evolved to look less ‘Puijila-like’ and more ‘elephant seal-like’ Amongst other things, the Puijila-like ‘intermediate species’ would have had: shorter legs and webbed feet that began to look like flippers; the beginnings of a layer of blubber; ears enabling the species to hear better underwater; an ability to dive deeper; eyes that could see better in murky water.
Amongst other things the ‘elephant seal-like’ intermediate ancestor would have had: very short legs and the beginnings of flippers; a much thicker layer of blubber; an improved sense of hearing to help locate prey in the semi darkness at great ocean depths; an ability to dive even deeper and swim even faster than their ‘puijla-like’ intermediate ancestors.
Today the elephant seals can dive to phenomenal depths. You can make comparisons about how deep different organisms are able to dive at Earthguide. You will see that the elephant seal is a truly amazing diving ‘machine’!
The elephant seal has the ability to dive to great depths hunting prey since it is able to store all the oxygen it needs for long dives in its blood instead of its lungs. In fact the elephant seal, before a long dive, expells all the air from its lungs and collapses them! This ensures that the animal does not suffer decompression sickness (‘the bends’) which is a sickness that humans sometimes suffer from after diving to great depths.
The elephant seal has eyes that allow it to spot prey from a distance in the murky depths. Its whiskers enable it to sense the underwater currents (the ‘wake’) that form after prey has swum close by.
Do you remember how the Puijila darwini had a tail? The elephant seal still has a tail which it has inherited from its ancestors. However the tail of the elephant seal is merely ‘vestigal’- a leftover trait that was useful to the Puijila darwini in diving and swimming in lakes and rivers but serves no useful purpose for elephant seals diving deep in the ocean.
During the Oligocene period of geological time (33 million to 23 million years ago), when the Puijila darwini first appeared, the Canadian Arctic was much warmer than it is today. If you look at this reconstruction of the Earth 25 million years ago you can see how little snow and ice there was in the Canadian Arctic.
In the Oligocene epoch the Canadian Arctic was a landscape of wetlands, rivers and lakes in which the Puijila darwini was well equipped to hunt, catch prey and survive.
So why did the Puijila darwini start evolving into different species?
At the beginning of the Miocene epoch of geological time, (23 million to 5 million years ago) which directly followed the Oligocene epoch, the climate of the Canadian Arctic became much drier. This resulted in a loss of habitat for the Puijilas as forests died and lakes and rivers disappeared to be replaced by extensive grasslands.
A loss of habitat meant a loss of sources of food. Not only did the Canadian Arctic become drier, but the region also became much colder, making the Canadian Arctic a hostile and challenging place in which to survive.
Where would the descendants of the Puijila darwini find new sources of food?
‘Sexual dimorphism’ means that the males of a species look radically different and often behave in noticeably different ways.
This male elephant seal has a giant proboscis which it only uses on land during the breeding season. The breeding season is all about male elephant seals competing to gain control of a group of females called a ‘harem’. This male’s bulging proboscis amplifies bellowing sounds coming from its mouth to intimidate and warn away other male competitors who want to take control of his harem.
Another feature of male elephant seals is their enormous size. All elephant seals are fathered by giant males who once won control of a harem of females in fights with other smaller males. So it is always the giant males’ DNA that is passed onto the next generation and never the DNA of the smaller males.
The evolutionary pressure on male elephant seals is therefore to increase in size.
If ever the vast bulk of male elephant seals stops them from feeding effectively then the evolutionary pressures to increase in size generation by generation, will stop.
In contrast there is no evolutionary advantage for female seals to grow a massive bulk or make large grunting sounds through their proboscises; females do not have to fight with each other to try to dominate.
One of the main drivers of evolution is when organisms find themselves in a different environment in which they need to survive and find sources of food. Changing from land based to sea based hunters meant that the descendants of the Puijila elephant seals had to adapt very quickly to new conditions.
Once a species has adapted to new habitats and new conditions the pace of evolution and change tends to slow right down.
Elephant seals are efficient hunters in deep oceans and they do appear to be in perfect harmony with their environment. They are well adapting to exploiting nutritious sources of protein, in the form of squid, octopus fish and small sharks, at depths which other ocean hunters find difficult to reach.
If their sources of food remain stable then there may be little evolutionary pressure for their descents to adapt and change. Elephant seals may continue to look the way the do at the moment for the next 5 million years- so long as us humans protect their food sources and their breeding grounds!