population structure week 6

Question 1

Intended learning outcomes

Answer 1

• Discuss why defining populations is important and describe the different methods for determining marine mammal population structure

Question 2

Why is it important to define populations?

Answer 2

• The key questions:
  – Distribution = where do we find the animals?
  – Population structure = one or several groups?
  – Abundance (population size) = how many animals?
  – Population trends = increasing or decreasing?
• Necessary to answer in order to understand:
  – Life-history, ecology, behaviour, evolution, etc.
  – Impacts from human activities (whaling, noise, pollution, etc), as well as recovery, conservation and management

Question 3

Defining populations

Answer 3

We have defined the ocean as multiple distinct ocean basins (e.g. North Atlantic, South Pacific, Indian Ocean)… But all oceans are connected

Marine mammals live in a world very different from ours, and we know little about their barriers to dispersal …and hence little about the number of species, subspecies and populations!

Question 4

What is a population?

Answer 4

Geographical population
* Occupying a specific area at a specific time
* Traditional definition of a population
* Stocks; subject to commercial exploitation
* May no reflect a real population in biological sense…

Genetic population
* Sexual reproduction among all members
* Significant genetic differentiation from other populations
* Reflect long-term evolutionary patterns

Demographic/ecological population
* Demographic trends of population defined by local birth
and death rates, with no immigration
* Reflect contemporary ecological patterns
* Hard to estimate/define

Depends on our goal and timeframe!

Question 5

Population structure

Answer 5

A species is split up into smaller units “populations” by barriers
* Physical (e.g. land or depth)
* Oceanographic (e.g. temperature)
* Behavioural (e.g. shift to other prey)

After some time, these different units may develop differences (genetic, morphological, behavioural, acoustic, etc) either by chance or by local adaptation
…and with more time, this micro- evolutionary variation (populations) may give rise to macro-evolutionary variation (species)

Question 6

There are many ways to delineate population boundaries

Answer 6

• Draw lines on a map (not ideal)
• Morphometric analyses to find size/shape differences
• Acoustic recordings to detect different dialects
• DNA analyses to find genetic differences
• Satellite tagging to map range and contact rates
• Behaviour, life-history, stable isotopes, fatty acid profiles, contaminant loads, etc.

Multiple lines of evidence is the best!

Question 7

Lines on a map

Answer 7

• Typical in whaling and fisheries management, or when information is lacking from other studies
• May or may not represent actual populations

Question 8

Morphology

Answer 8

• Population differences in shape reflects
  – Evolutionary patterns
  – Adaptations to foraging and habitat in general – OR, just random signal caused by isolation
• Museum specimens (skulls) are relatively easy to obtain
• Often used in combination with other information (e.g. genetics)

Ringed seals differ in skull shape… ringes seal subspecies differ in skull shape

Question 9

Population genetics

Answer 9

• The most reliable and often used method for delineating marine mammal populations (and subspecies and species)!
• Rapid development of methods and lowering of costs!
• Mitochondrial DNA (mtDNA; mitogenome)
  
  • Shorter (16kb) and more abundant in the cell, so eary to sequence (also in ancient samples); only maternal population structure!
• Nuclear DNA (nuDNA; SNPs)
  
  • Genomes typically 2.4gb; reflect both male and female population structure
  • Much additional information about selection, demographic history, etc.

Question 10

Samples for DNA extraction are the key to success

Answer 10

Tissue from stranded animals
Bone powder from museum specimens
Environmental DNA
Biopsies from free-living animals

Question 11

Satellite tagging

Answer 11

• Good for detecting movement and home ranges
• Mainly reflect recent ecological patterns, and may also reflect longer term evolutionary patterns

Southern elephant seal movements reflect the existence of 3 populations (yellow, red and combined green/orange)

Question 12

Acoustics

Answer 12

• Population structure defined by groups or regions where animals ”speak in different dialects”
• Ecological patterns (or evolutionary?)
  – Spermwhales,humpbacks,orcas,etc
  – Ringedseal,harbourseal,etc
• …however, acoustic communication may change over time

Pygmy blue whale song differs across the Indian Ocean, suggesting multiple distinct populations, including one (yellow) that was previously unknown

Humpback whale song differ between regions, but change over time as they hear and learn new themes…