Lecture 32

Question 1

What is natural selection?

Answer 1

In the struggle for survival, the fittest win out at the expense of their rivals because they succeed in adapting themselves best to their environment.

Question 2

Describe the natural selection with the finches:

Answer 2

-different finches different beak sizes
-this is related to their niche, and what kind of food they eat (seed cracking species= robust beaks, grub/insect eating species= thin and probing)
-Geospiza fortis= medium ground finch, found on Daphne Major, small island so entire pop, measured since 1973, seed eating with varation in beak size, field observations show that beak size connected to the food eaten,
-beak size is heritable= strong genetic component
-79% variation in offspring beak size is predicted by parents’ beak size
-natural selection should be highly effective at bringing about evolutionary change
-1977 big drought= the seeds left= bigger
=consequence=next generation beak size was 5% bigger that the one before= adaptive evolutionary change

Question 3

How does natural selection change population size?

Answer 3

-pop size depends on what there is to eat
-also on what are the predators
–if many predators= pop of prey decreases
-as prey declines the predator pop will also decrease
-fewer predators= more prey!
= PREDATOR-PREY CYCLE

Question 4

Give an example of a predator-prey cycle:

Answer 4

-Canada lynxes and snowshoe hares
-excellent population numbers for many years (trapping record)
-lynxes soley prey on hares
=oscillations= but works only when predator focuses on one prey= rare

Question 5

What is positive density dependence?

Answer 5

• a certain level of survival of individuals is needed for a population to grow
• OK at high density= pop will decline until critical level
• but at low density = problem- pop will decline and go extinct

Question 6

What is the Allee effect and its causes?

Answer 6

-very small populations have reduced reproductive rates or survival and are in danger of a further decline in population size
causes:
1. Inbreeding
2. Strange sex ratios
3. Lack of encounters

Question 7

How does inbreeding cause the Allee effect?

Answer 7

-inbreeding is likely to be higher in smaller populations and leads to reduced fitness and survival

Question 8

How do strange sex ratios cause the Allee effect?

Answer 8

• with small populations you are more likely to encounter very unbalanced sex ratios
• finding mates= more difficult, accentuates inbreeding and reduces reproductive output if male-biased population
  eg. Kakapo breeding program
• initially produced large numbers of male chicks but not females

Question 9

How does the lack of encounters cause the Allee effect?

Answer 9

• low population density and patchiness= lower chance of encountering a mate
  eg. african wild dog (Lycaon pictus)
• now sparsely distributed and low numbers (below 60, Serengeti)
• large social groups necessary for hunting and caring for young
• numbers declining (few new individuals are encountered to join the groups and reproduce)

Question 10

How does body size influence the threat of extinction?

Answer 10

• larger animals are more prone to extinction
• need more resources
• live at lower density
• reproduce less often
• produce fewer offspring

Question 11

What is variation in populations?

Answer 11

• esential component of evolution by natural selection
• can be continuous= body height in humans
• or can be discrete= snail shell patterns
• where there are multiple forms= polymorphism

Question 12

If natural selection favours the fittest why is there still variation?

Answer 12

• mutation, chromosomal recombination
• migration of populations (=mixing of forms)
• sometimes there’s more than one way to make a living= multiple adaptive solutions

Question 13

How is variation maintained by predation?

Answer 13

• polymorphisms can be maintained because the “fittest” form may change over time
• predators exert selection on a prey population
• predators tend to prefer common forms of prey(maybe because they have a search image)
• rare forms have better survival and become more common= APOSTATIC SELECTION
• sometimes the rare form can also be a disadvantage eg. at high population density

Question 14

What is frequency-dependent selection?

Answer 14

• apostatic selection is a form of frequency-dependent selection
• the fitness of a particular phenotype (or genotype) is dependent on its frequency in the population
• apostatic selection is negative frequency-dependent selection fitness declines as frequency of the phenotype decreases

Question 15

What is sexual selection?

Answer 15

struggle between individuals of one sex, generally the males for possession of the other sex
-the result is not death to the unsuccessful competitor, but few offspring

Question 16

What are examples of sexual selection?

Answer 16

male-male competition= males compete and the strongest or most competitive gets to mate with the most females
female choice= females prefer the most ornamented males as have proven their ability to survive despite bearing the trait

Question 17

What is the sexual selection in elephant seal?

Answer 17

-elephant seal males are usually enormous
-rival males fight each for access to harems of females
-fighting and keeping harems is very costly, but if successful the rewards are great (lots of mating and offspring)
-but some males are much smaller, sneak into harems and copulate with females
=fewer mating but less risky strategy, better than fighting and losing

Question 18

What is the example of a complex alternative mating strategies?

Answer 18

• three male types (or morphs) each with a different colour patches- orange, yellow or blue
• each male morph has a different mating strategy
• blue defends against yellow sneaks in orange beats up blue