W11L2 anthropogenic selection: body size and ornament Flashcards
The concept of unnatural selection
Agriculture and aquaculture choose the most desirable animals
* specific goal of increasing phenotype frequency (i.e. selective breeding)
Hunting and fishing harvest the most desirable animals from wild populations
* imposes selection that reduces phenotype frequency
* reduced body size, earlier sexual maturity, reduced antler size, reduced boldness
-Hunting and fishing reduces the frequency of phenotypes valued by humans
Undesirable trait change across time
-unnatural selection works against trait have higher fitness in natural selection
-generally acts at cross purpose to the long term goal of sustainable harvest
Genetic effects of harvest on wild populations
-Harvesting need not be selective to cause genetic change
-Intensity of harvesting (how many organisms harvested) is also important!
-Increased mortality selects for early maturation irrespective of phenotype
-If probability of survival is low, better to mature earlier: selection for faster pace of life
Harvesting can cause three types of genetic change
- Alteration of population subdivision
- Loss of genetic variation
- Selective genetic changes
Harvesting group of individuals made up of multiple subpopulations can result in:
Virtually all species have separate local breeding group
1. Extinction of a subpopulation due to spatially or temporally biased harvest
2. Reduction in gene flow as fewer migrants- increased genetic drift
3. Reduced resource competition in certain areas, promoting immigration and swamping of local adaptations
Loss of genetic variation as the result of unnatural selection
-Reduced population size due to harvesting can reduce number of migrants and cause loss of genetic variation within subpopulations
-Related to effective population size (Ne)
-Effective population size is the number of individuals in a population who contribute offspring to the next generation
Example of loss in genetic variation due to unnatural selection
-Historically large harvests can reduce -genetic variation (e.g. NZ snapper)
-Male biased harvesting can severely skew sex ratio and reduced effective population size (e.g. lobster)
-Protection of mothers with young results in individuals surviving hunting being more closely related (e.g. moose)
Lost genetic variation increases species’ vulnerability to environmental change
What is Selective genetic change
Selective removal of phenotypes
Case study: Bighorn sheep on Ram Mountain:
Effects of 30 years hunting
-Body weight and horn size highly - - - heritable
-Trophy rams shot at earlier age (~6 yo; most horn growth occurs by 2-4 yo)
-Trophy rams have lower reproductive success (max. potential not reached until >8 years) despite having trait favoured by natural selection
-Population is now dominated by small-horned, light rams that breed more often
What is breeding value
-worth of an individual’s genotype as measured by the performance of its offspring
-in ram, the individual with small horn and body size have a lower breeding value-> population size lowerd
Evolution effects of fishing
-non-random fishing results in evolutionary change
-Fishing can lead to changes in “size-at-age” and “age-at-maturation” traits- these are correlated traits
-Evolution more likely in heavily exploited stocks- increased mortality & more intense selection
-When and where fishing occurs is important (e.g. targeting spawning-ready fish)
Selection generated by fishing
-selection on length at age due to fishing gear
- selection on age at maturation due to : age at entry into a fishery, location of a fishery
example on earlier sexual maturation of arctic grayling following different fishing net
Example on Selection on age-at-maturation due to location of fishing
Neither fish stocks or fishing fleets randomly distributed in space, e.g. northeast Arctic cod
-Main feeding grounds in Barents Sea, main spawning grounds near the Norwegian coast
-Annual spawning migrations (historical fishing grounds)
-Barents sea population exploited only more recently (1950s onwards)
-Fishing confined to spawning grounds - advantage to late maturation (more eggs per spawning)
-Fishing in feeding grounds added - advantage to early maturation - risky to stay on feeding grounds
Fitness implication of harvest: big mother matter
Traditional view is that reproductive output scales isometrically with female body size
Isometric scaling: for every increase in female somatic mass, there is a
constant proportional increase in reproductive output
One 2kg fish is equal to two 1kg fish
This view is wrong! Reproductive output (number and size of eggs)
scales hyper-allometrically with female body size
Hyper-allometric scaling: reproductive output scales disproportionately with body mass
Two 1kg fish have less reproductive output than one 2kg fish
