Ecology

1
Ecology

• Lecture 2, Evolution and Natural Selection

2
Evolution

• Understanding ecology requires an understanding
  of evolution and natural selection
• The theory of evolution is often attributed
  exclusively to Charles Darwin but Alfred Russell
  Wallace came to the same conclusion independently

3
Ecological explanations

• We can explain ecological phenomena in terms of
  proximate or ultimate causes

4
Proximate and Ultimate Causes

• Proximatethe immediate cause
• UltimateEvolutionary reason
• ExampleCricket chirping
• Proximate file and scraper
• Ultimate evolved complex communication system
  for attracting mates

5
Proximate and Ultimate Causes

• ProximateHow questions
• UltimateWhy questions
• Cricket chirping
• How? file and scraper
• Why? evolved complex communication system for
  attracting mates

6
Disputes can often be avoided

• By being clear about whether we are seeking
  proximate or ultimate explanations for ecological
  phenomena
• To really understand ecological systems we need
  to understand the evolutionary (ultimate) causes

7
An Evolutionary Framework

• 1859 Darwin published the origin of species by
  means of natural selection
• One could argue that all that biologists have
  done since 1859 is elaborate on Darwins Theory
  of Evolution

8
Prevailing view at the time (before Darwin and
Wallace)

• Species were immutable and unchanging
• This view was shared most naturalists of the time
• And initially Darwin probably held this view as
  well

9
Darwins voyage on the Beagle changed his view
and the course of the natural sciences

• At the age of 22 he was hired as the ships
  Naturalist
• Collected specimens of plants and animals
• Recorded observations on a journey around the
  world

10
As he travelled...

• Darwin observed struggles for survival and the
  great lengths to which animals seem to go to
  ensure survival
• He also new that all species have tremendous
  reproductive potential
• Most of this potential is not realized

11
Darwin began to doubt the prevailing view

• His observations on the Galapagos Islands were
  especially influential

12
Darwins Finches
13
Darwins Finches

• Darwin noticed that Finches occurred on each of
  the islands
• But each island had a different species of finch
• They differed from one another often resembled
  non-finch species from the mainland that were
  absent from the islands

14
Darwins Finches

• It seemed as though the finches were occupying
  vacant ecological spaces that were empty on the
  islands

15
Proposed the idea ofEvolution by means of
natural selection

• He suggested that the natural environment places
  a great deal of pressure on animals as they
  struggle to survive
• He suggested that those individuals that were
  best suited to meet these pressures would be the
  individuals that make up the next generation of a
  particular species

16
Darwins Theory

• Variation in traits exist among individuals
• Some variation is heritable
• Reproductive potential is staggering
• Certain heritable traits will confer survival
  advantages
• This will allow certain individuals to survive
  longer and leave more offspring
• May lead to changes in a species over time

17
Darwins Theory

• Darwin called this evolution by means of natural
  selection

18
Artificial Selection

• Domestic animal breeding

19
The genetic bases of evolution

• Mendels discovery of the genetic basis of
  inheritance came long after Darwins theory
• Genes come in different forms called alleles
• Different alleles code for different forms of
  proteins
• Result in different physical traits

20
Some Key Points

• Variation is random
• Genetic mutation arise randomly
• Some mutations may confer survival advantage
• Natural selection acts on whatever variation is
  present
• Unlike artificial selection, natural selection
  has no goal or direction in mind

21
Some Key Points

• Selection acts on the individual
• Adaptive traits are selected for
• Maladaptive traits are eliminated
• This happens at the level of the individual
  organism

22
Group selection hypothesis Wynne-Edwards

• Proposed to explain apparently altruistic
  behaviourLemmings that supposedly ran off cliffs
  in mass suicides to control their populations for
  the good of the species.
• Called group selection because the behaviour
  would be selected for because it is good for the
  group (i.e., the species)

23
Group selection hypothesis Wynne-Edwards

• But group selection cannot work because selection
  acts on individuals and those with that
  altruistic trait will be selected against.
• Those with the genetic predisposition to jump off
  cliffs will not be able to reproduce therefore
  this trait cannot be passed on to the next
  generation

24
Group selection hypothesis -Wynne-Edwards

• Therefore group selection cannot work
• Traits can only be selected for if they are good
  for the individual that possesses the trait

25
Some Key Points

• Natural selection results in evolutionary change
  because some individuals in a population
  contribute a disproportionate number of offspring
  (genes) to the next generation
• Reproductive behaviour therefore plays a key role
  in the evolution of a species

26
Some Key Points

• Natural selection results in evolutionary change
  because some individuals in a population
  contribute a disproportionate number of offspring
  (genes) to the next generation
• Reproductive behaviour therefore plays a key role
  in the evolution of a species

27
Some Key Points

• Traits that increase reproductive success should
  be favoured
• We call an individuals total lifetime
  contribution of offspring to future generations
  its direct fitness

28
There are different types of selection

• Directional selection
• Stabilizing selection
• Disruptive selection
• Sexual selection
• Runaway selection

29
There are different types of selection

• Directional selection

30
There are different types of selection

• Stabilizing selection

31
There are different types of selection

• Disruptive selection

32
Sexual selection

• Darwin recognized that some traits seem
  maladaptive
• e.g., Large tail feathers or bright colours
• Make birds conspicuous to predators and impair
  flight ability
• But seem to be used for
• attracting mates or
• Intra-specific combat

33
Sexual selection

• These traits, Darwin suggested, could evolve by
  sexual selection through two mechanisms
• male-male competition
• Mate choice

34
There are different types of selection

• Runaway selection
• Sexual selection that results in extreme traits
• Antlers of the extinct Irish Elk
• Antlers became so large that they may have
  contributed to the elks extinction

35
The challenge of obtaining evidence for natural
selection

• The challenge in studying evolutionary ecology is
  to determine which traits are being selected for
• Understanding the selective forces

36
The challenge of obtaining evidence for natural
selection

• The traits we see in animals or plants are often
  evolutionary trade-offs among conflicting
  selection pressures
• Studying natural selection can give us clues as
  to what ecological attributes are important to a
  particular species

37
Examples of ecological tradeoffs

• Having large stores of body fat may be adaptive
  for a bat trying to survive hibernation
• The same feature may be detrimental during the
  summer when it is trying to hunt evasive prey

38
What we observe

• May be the compromise among competing selective
  forces

39
Adding to the challenge

• Not all traits are adaptive
• Some are left over traits that were adaptive
  under previous conditions
• Maladaptive traits may persist if they are
  genetically linked to other traits that are
  adaptive.

40
Peppered moths

• A text book case of evolution in action?

41
Peppered moths

• Occurs in two forms
• Light Dark
• Pre-industrial Britain
• Light form predominated

42
Peppered moths

• Industrial pollution killed lichens on trees and
  soot darkened tree trunks
• Corresponded to a dramatic increase in the dark
  form

43
Peppered Moths - Explanation

• Pre-industrialization
• light forms were cryptic better able to avoid
  predation by birds
• Post-Industrialization
• pollution killed lichens and darkened tree trunks
  the black forms were more cryptic

44
Peppered Moths - Explanation

• Bird Predation was thought to be the selective
  force responsible for the shift in colour
  frequency

45
Peppered Moths - Explanation

• Experiments with birds confirmed that the cryptic
  forms were more likely to avoid bird predation

46
Peppered Moths Environmental Clean-up

• Environmental cleanup reduced the soot deposition
• Corresponding increase in the light form of the
  moth

47
Peppered Moths a critical look

• A closer look revealed the story is a little more
  complicated than we first thought

48
Peppered Moths a critical look

• Some populations in relatively pristine forests
  are dominated by the dark form
• In Europe the light form reappeared before
  lichens reappeared on trees

49
Peppered Moths a critical look

• The moths dont naturally rest on tree trunks
• Bird predation experiments were done in a highly
  unnatural setting

50
Peppered Moths conclusion

• What appeared to be a text book case is not quite
  so simple
• Tests of natural selection must be carefully
  designed to mimic natural situations

51
Speciation

• Examples like
• the peppered moth
• Domestic animals
• Show that species can change over time
• But this is still a long way from the evolution
  of new species

52
Speciation

• New species evolve when
• Species diverge sufficiently to result in
  reproductive isolation
• Species are usually defined based on reproductive
  isolation
• If individuals can naturally interbreed and
  produce viable fertile offspring they belong to
  the same species

53
Speciation

• New species evolve overtime
• In practice it is difficult to decide exactly
  when populations of a species have diverged
  sufficiently to be considered two species
• Species that have distinctly different
  sub-populations are often given subspecies
  designations

54
Speciation

• Sympatric speciation
• occurs when populations that remain in
  geographic contact split into distinct species
• Allopatric speciation
• sub-populations become geographically isolated
  and diverge ultimately splitting into distinct
  species e.g., Darwins Finches

55
Allopatric speciation

• Allopatric speciation may occur because
  conditions are different in the two
  geographically isolated areas
• Or due to founder effects
• By random chance a small founder population may
  be genetically different than the majority of the
  founder population
• They may be predisposed to certain evolutionary
  changes

56
(No Transcript)
57
Sympatric speciation

• Few clear example of sympatric speciation
• But could occur through disruptive selection

58
Evolutionary diversification

• As floras and faunas evolve and diversify
• It can be difficult to determine whether similar
  species share a common origin or represent
  unrelated groups that have converged to solve the
  same evolutionary challenges.

59
Convergent and parallel evolution
60
Convergent evolution

• Unrelated ancestral forms converge on a similar
  form to solve the same evolutionary problem.in
  this case flight

61
Convergent evolution

• When trails are similar as the result of
  convergent evolution we say they are analogous
  traits.

62
Parallel evolution

• Ancestral mammals became separated when Australia
  split off from the other continents
• Followed different evolutionary paths but filled
  similar niches

63
Parallel evolution

• When traits share a common origin but have
  diverged over timewe say the are homologous
• e.g., the hand of a human and the wing of a bat.