Lecture 20- Evolution and natural resources I: Application of evolutionary theory to exploited wild populations Flashcards
What is meant by unnatural selection?
-Agriculture and aquaculture choose the most desirable animals to breed from with the specific goal of increasing the frequency of desirable phenotypes (i.e. selective breeding)
-Hunting and fishing go in the opposite direction – harvest the most phenotypically desirable animals from wild populations. -This imposes selection that can reduce the frequencies of desirable phenotypes
The concept of ‘unnatural selection’
-Undesirable changes occur over time in exploited populations (e.g., reduced body size, earlier sexual maturity, reduced antler size, reduced boldness etc.) due to selection against desirable phenotypes - a process called ‘‘unnatural’’ selection
-Unnatural selection generally acts at cross purposes to the long- term goal of sustainable harvest and can reduce the frequency of phenotypes valued by humans
-maturity occurs earlier, disappearance of the traits the hunter desire
How is hunting not always unnatural?
-Darwin on Inca hunting practices:
‘The various kinds of deer were examined; the old males and females were likewise killed, “but young females, with a certain number of males, selected from the most beautiful and strong”, were given their freedom.
‘Here, then, we have selection by man aiding natural selection.
‘So that the Incas followed exactly the reverse system of that which our Scottish sportsman are accused of following, namely, of steadily killing the finest stags, thus causing the whole race to degenerate.’
-what we do now is the reverse
What are the genetic effects of harvest on wild population?
-Genetic effects of harvest on wild populations
-Harvesting need not only be selective to cause genetic change:
intensity of harvesting (how many organisms harvested) also important! Because increased mortality selects for early maturation irrespective of phenotype
Harvesting can cause three types of genetic change:
-1. Alteration of population subdivision
-2. Loss of genetic variation
-3. Selective genetic changes
What is the alteration of population subdivision?
- Virtually all species have separate local breeding groups (sub-populations)
- Harvesting group of individuals made up of multiple subpopulations can result in:
1. Extinction of a subpopulation
2. Reduction in gene flow as fewer migrants- increased genetic drift
3. Reduce resource competition in certain areas, promoting immigration and swamping of local adaptations - if you fish a large amount from one area instead of spreading out the effects (2)
What is loss of genetic variation?
- can result loss heterozygosity and allelic diversity
- can result in a smaller effective population
- Reduced population size due to harvesting can reduce number of migrants and cause loss of genetic variation within subpopulations
What is the effective population size?
-the number of individuals in a population that are capable of reproducing
What is census population size?
N(e) related to census population size (Nc), subpopulation sex ratio, and number of lifetime progeny produced by males and females
What are some other causes of loss of genetic variation?
- 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), the ones that survive hunting season will be more and more genetically related through time thus reducing the genetic diversity in the population
What is selective genetic changes?
- Selective removal of phenotypes (e.g. large body size)
- Exploitation also increases overall mortality rates, which in turn selects for earlier maturation even if harvest is independent of phenotype
- not just about selection but about increased mortality, this selects for earlier maturation
Notes case study: Bighorn sheep
- big horns are correlated with body size
- people hunt males with big horns
- males with big horns have better fitness in terms of offspring
- genetically passed down (the horns)
- the big horned males shot much younger (4-6) as opposed to 8 years so don’t have as good reproductive successive
- the small horned males now dominate
- Selection against high-breeding-value rams imposed by trophy hunting
Notes case study? Foxes (selective hunting) or unnatural selection
- reduction in frequency of the silver morph of the fox in eastern Canada resulting from the preferential harvest by hunters of the more valuable silver morph.
- red and silver fur foxes, silver more valuable, the silver declines due to the hunting actions
What are some of the evolutionary effects of fishing?
-fishing is almost always non-random
-If phenotypic variation results from genetic variation, 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 – many large fisheries target spawning-ready fish
How much fishing now occurs?
All oceans now fished – few unutilized resources remain
About 75% important commercial species fully or over-exploited Rise of aquaculture
Why is fishing non random?
- you fish at certain times, gear, what is comfortable for you etc.
- select for big fish
What is the selection generated by fishing?
-Selection on length-at-age due to fishing gear (e.g. nets)
Selection on age-at-maturation due to:
− age at entry into a fishery
− location of a fishery
-the holes in the nets, select for big fish, as long as you are small can escape so this selects for slow growers since they last longer in the non vulnerable size
