Inbreeding and Inbreeding Depression in Southern Resident Killer Whales

What is inbreeding? 

Inbreeding occurs when two individuals with a recent common ancestor reproduce. The offspring inherit some identical segments of their genetic material from each parent because those parents inherited the same segments through their shared ancestor. An example would be mating between cousins with a common pair of grandparents. Inbreeding typically increases over subsequent generations in small and isolated populations.

What is inbreeding depression?

Inbreeding depression refers to the reduced ability to survive and reproduce that can result from inbreeding. Inbreeding depression occurs when an individual inherits two copies of harmful versions of genes from each parent because of their shared ancestry. These harmful versions, called deleterious variants, are usually rare. In non-inbred individuals, they are usually benign because they are typically masked by a normal variant that expresses the normal gene. Deleterious variants can cause inbred individuals to have higher disease risk, shorter lifespans, or reduced fertility.  

What are the consequences of inbreeding depression?

Inbreeding depression reduces the survival and reproduction ability of inbred individuals. For example, inbred females may not live long enough to produce enough offspring to replace their parents. Their shorter lifespan can lead to further population decline and eventual extinction. Inbreeding can also decrease an individual’s ability to adapt to stressful conditions, such as disease.

Why is inbreeding bad in some cases and not in others?

The impact of inbreeding depends on several factors, including:

• Historical population size over hundreds of generations
• Speed of the population decline
• Amount of interbreeding with other populations
• Affected fitness components (survival, reproduction, or both) and age classes (juveniles, adults, or both) 
• Breeding and social structure 

For example, endangered Southern Resident killer whales are known to breed only within their population. There are no known cases of Southern Residents mating with individuals from other populations, despite occasional close proximity to other killer whale populations such as the Northern Residents and Bigg’s (transient) killer whales. Our earlier genetic work suggests that only a few adult males sired most of the Southern Resident calves. 

Large populations (or populations that were large in the recent past) typically have more rare deleterious variants. These can reduce the survival and reproduction of inbred individuals when a population becomes small. 

Alternatively, a population that has been small for a very long time may have fewer deleterious variants. In that case, the degree of inbreeding depression may be less significant in inbred individuals. Low levels of inbreeding in a small population allow natural selection to purge the highly deleterious variants. This purging leaves the population with less genetic variation and fewer highly deleterious variants.

Does inbreeding always cause inbreeding depression?

Genetic mutations provide a constant input of deleterious variants. That means all populations and individuals carry a “genetic load” of deleterious variants. And all populations are expected to exhibit inbreeding depression. However, the strength of inbreeding depression and the consequences of inbreeding depression for conservation depend on several factors. 

Populations that have been small for a long time are expected to have a lower genetic load than historically larger populations. Generations of inbreeding in a small, isolated population will expose deleterious variants, which may then be purged through natural selection. Purging and loss of deleterious variants by chance is expected in all small populations, but no population is expected to be free from inbreeding depression. 

The effects on population viability depend on the following:

• Strength of inbreeding depression (affected by historical population size as described above)
• Environmental conditions
• Which demographic characteristics (age- and sex-specific survival and reproduction) are most affected by inbreeding 

Some species show inbreeding depression only in young animals or on mainly one sex. However, in other populations, inbreeding has been shown to depress survival and reproduction in both young and old individuals of both sexes.  

Some populations only display strong inbreeding depression in difficult environmental conditions. Conversely, other populations appear to suffer inbreeding depression across a broad range of environments. 

Can the risk of inbreeding depression be reduced?

Wildlife managers sometimes move individual animals between isolated populations to add genetic diversity. That reduces the risk of an individual inheriting harmful variants from both parents. However, that is easier with terrestrial wildlife than marine mammals and depends on the populations to reproduce with the introduced animal. In other cases (e.g., California condor), remaining wild animals are bred in captivity to help increase the number of offspring and increase chances of survival. Breeding interventions are unfeasible in wild whale and dolphin populations. Mitigating risks associated with other stressors (e.g., fluctuations in salmon prey, toxic pollutants, disturbance, and noise from ships and other vessels) is vital for supporting the Southern Residents' population.