n July this year, the Seattle Times ran a story on an orca called Tahlequah – she was pregnant, again. And just yesterday, she gave birth. The story is particularly noteworthy because a couple of years ago, Tahlequah also bore a calf, which died, and she then carried the dead baby on her nose for 17 days. My friend Heather commented at the time that this orca mum belonged to a pod, and a population, on America’s Pacific Northwest that is in deep trouble (for a variety of reasons, all canvassed in that last link), with adults suffering from starvation and very few calves being born.
Heather & I were both interested in the story as this population was also the focus of a question in the 2016 Scholarship Biology exam. With the recent posts, I thought it would be good for the schol students to have a look at that question¹ – and for me to write an extended response to it.
But before we move to that, a note for students: if you’ve looked at the question paper, you’ll know there’s a lot of reading to do before you begin. Once you’ve done that reading, spend time planning your answer – it will be time well spent as you’ll be more likely to then write a concise, cogent, logically-structured response that interweaves your own biological knowledge with information from the resource material provided in the paper, and less likely to both omit relevant material and to insert irrelevant content. And with that said, here we go:
These orca populations will most likely be descended from an ancestral population in the general area, but now differ significantly in terms of appearance, what they eat, how they behave, their social systems, and the ranges that they occupy². (There’s some great information on NZ orca ecotypes in this FB post by Ingrid Visser.) Their divergence into distinct groups – which do not interbreed ie they are genetically isolated – is most probably the result of intraspecific competition for food³.
That is, the availability of different food sources acted as a selection pressure. For example, the ‘resident’ orca population hunts mainly for salmon, using echolocation to do this, in comparison to the ‘transient’ population, which hunts for seals and other whales and dolphins (cetaceans). The cetaceans would be able to detect echolocation ‘pings’ by the orca, and so this group hunts by stealth. Differences in pod sizes may also reflect the demands of different hunting styles.
Now, to address the evolutionary side of the first part of the question: As the separate resident, transient (and offshore) populations formed and their interactions dwindled away, we’d expect to see changes in their gene pools, due to selection for/against particular adaptations, &/or genetic drift. Mutations would spread through these separate gene pools if they were of some adaptive advantage – for example, the populations are recognisable on the basis of consistent, heritable differences in things like the shapes of their dorsal fins and the patterns of the ‘saddle patch’ on their backs.
While the map provided in the question paper (similar to this one) indicates that it’s possible for transient & resident orca to come into contact, they don’t interbreed, and this reproductive isolation has probably existed for a long time: up to 700,000 years for these groups. This means that there are highly likely to be reproductive isolating mechanisms in place. For example, orca from the transient group may choose to mate only with orca with the physical characteristics of that group (an example of sexual selection). Or, the distinct differences in both social behaviour and vocalisations that exist between the groups may impede interactions between them. In either case, if these differences in social and feeding behaviours have a genetic underpinning, then the result of this reproductive isolation is likely to be reflected in ongoing changes in the populations’ gene pools: aka evolution.
While the first part of the question focuses on just the resident & transient orca populations, the second part asks you to discuss the future evolution of all three (ie including the ocean population), with a justified opinion on what you think might happen ie explain why you’ve come to that opinion. Here are some points to consider, in forming an opinion.
All three populations took a big hit from whaling, and their current numbers are low (especially the southern resident group, with only around 90 animals¹¹ – this population is listed as endangered). That is, they’ve been through a population bottleneck and are likely to have relatively lower genetic variation compared to other, larger groups. Their small population size puts them at risk in the event of a major environmental disaster, while the lack of genetic variation means that alleles may be lost via genetic drift, or that harmful alleles may become widespread in the population.
On top of that, because orca don’t reach sexual maturity until around 18 years of age, and females only bear calves every 4-8 years, then population growth is very slow. This means that if deaths – due to old age but also to things like starvation (as happened to Tahlequah’s first calf and to other calves in her pod), pollution, & disease – exceed births for any length of time, all these Pacific Northwest populations are at risk of extinction.
It’s possible that if the three groups survive, and their reproductive isolation continues, they might eventually become so different in genetic terms that they’re recognised as distinct species or subspecies. An alternative would be that two or more of the three groups rejoin (for example, if the residents are forced out of their current range due to collapse of the salmon fishery), although given the existing social and behavioural differences between them that may be a fairly long shot.
The featured image is from Wikipedia’s Creative Commons & used under that licence. This image was originally posted to Flickr by * Minette at https://www.flickr.com/photos/7232133@N08/416761172
¹ Analyse the information provided in the resource material, and integrate it with your biological knowledge to:
- discuss the evolutionary AND ecological processes that may have resulted in the distinct ecological niches of the resident and transient orca
- analyse the data to discuss what may occur in the future evolution of the three populations of orca; include your justified opinion on this.
² In terms of how to answer the question, this sentence was an observation based on the resource material. In the one that follows, I’ve written a justification – if you’ve seen a marking schedule for Schol Bio then you’ll know that justifying statements (both facts, & opinions) is essential if you’re going to do well.
³However, I haven’t left it at that, and neither should you 🙂 Competition is a selection pressure, so I’ve gone on to explain how that might operate.
¹¹ Remember to give specific examples; in this particular case, the actual population size of one or more of these groups.