Download 04_Evolution - School of Life Sciences

Example of hypothesis testing:
Niko Tinbergen observed that
digger wasps could return to their
exact nest location after
spending considerable time away
(ignoring the 100s of other nests
at the same location).
Observation: Wasps can relocate their inconspicuous
nests.
Question: How do digger wasps relocate their nests?
Question: How do digger wasps relocate their nests?
Hypotheses:
The wasp returns to any nest.
The wasp uses chemical scents to relocate the nest.
The wasp uses visual landmarks to relocate the nest.
Tinbergen further observed that when wasps left
the nest, they spent time flying back and forth
around the nest entrance.
Null hypothesis: visual landmarks are not used to
relocate the nests.
Tests & Predications:
1. If I modify the visual landmarks around the
nest, the wasp will not be able to find it.
2. If I move the landmarks (keeping them
intact), the wasp will return to the wrong
location.
3. If I give the wasp landmarks (pine cones),
she will use them to relocate her nest.
Important to only manipulate one thing at a time!!
Results:
1. Wasps took much longer to find their nests if
landmarks were manipulated.
2. If landmarks were moved to a new location (intact),
the wasp searched for her nest in that new location.
3. Experimental manipulation of landmarks allowed us
to predict where the wasp would look for her nest.
Conclusions:
The wasp probably uses visual landmarks to
relocate the nest.
Why say probably?
Was this an example of a proximate or
ultimate question?
Another example of hypothesis testing:
Observation: Adult black-headed gulls remove
broken egg shells from their nests.
Question: Why do gulls remove broken egg shells?
Tinbergen was not interested in the mechanisms
of egg shell removal (how they were
recognized), but why they were removed (in an
evolutionary sense)
Question: Why do gulls remove broken eggshells?
Hypothesis:
Broken egg shells attract predators to the nest.
Tests & Predications:
The presence of broken egg shells increases the
probability of nest predation.
Results:
The presence of egg shells near the nest increased
the probability of predation.
Conclusions:
Black-headed gulls (or some ancestor) likely
evolved the behavior of removing eggs shells to
reduce the probability of their nests being located
by predators.
Why say probably?
What are some alternative testable hypotheses?
Scientific Method
Humans have been using this method in some
form for a long time.
Blurton-Jones (1976) documented Kalahari
bushmen’s (!Kung) knowledge of animal behavior
Hunter-gatherer society, similar to most of human’s
history.
- Discriminated data
from theory
- Developed
hypotheses
- Used reasoned
skepticism
“When asked whether newborn cape
buffaloes stayed with their mothers or
were kept hidden, one man replied that
he had not looked ‘since buffaloes kill
you, you don’t go after them.’
However, he then suggested that since
buffaloes are like cows, and newborn
calves stay with their mothers, the same would be true of buffaloes.
Or, that since newborn buffaloes are large and conspicuous, the
option of trying to stay hidden wouldn’t work very well.”
These are examples of comparative (behavior same as
similar species - phylogenetic) and ecological (based on
interactions with the environment) hypotheses.
Limitations of Science
- Restricted to those things that can be logically
tested and falsified.
- More than one hypothesis can predict the same
outcome of a test.
- Results can be interpreted in different ways,
leading to different conclusions.
- Hypotheses constantly being reevaluated and
modified as more results and information are
gained.
What is Animal Behavior?
The study of how and why animals interact
with each other (both within and among
species) and their environment.
Proximate questions - how
mechanisms responsible for interactions
Ultimate questions - why
how these interactions influence an individual's
survival and reproduction.
Ultimate reasons
Proximate reasons
What is a species?
Species are groups of interbreeding organisms.
(organisms potentially capable of reproducing naturally among themselves, and
producing viable offspring)
Biological Species Concept (E. Mayr)
Genetic diversity - Amount of genetic variation within or
among populations of a given species.
Population—All organisms of the same kind found within a
specific geographic region. (have the potential to interact)
Genetic diversity, along with environmental variation (gene
by environment interaction), determines phenotypic
diversity.
Evolution
The process of change in the traits of populations over time.
Traits must have genetic basis.
Evolution does not occur within an individual.
Evolution does not occur within a generation.
Evolution is the change in allele frequencies in a
population over time
Mechanisms of Evolution:
Mutation
Genetic Drift
Migration
Natural Selection
Sexual Selection
Evolution by Natural Selection
First proposed by Charles Darwin in On the Origin
of Species, 1859
1. There must be variation
among individuals of a
species.
2. This variation must be
heritable.
3. This variation must lead to
differential reproduction.
Adaptive evolution occurs primarily through
natural selection
Natural Selection is the process that determines
which individuals within a species will reproduce
and pass their genes to the next generation.
Evolution in action
Pepper Moths in England
Increasing pollution led to sootcovered trees without lichens
Light colored moths
easy for birds to see
on soot-covered tree,
dark colored moths
harder to see
By 1950, most moths black
•Evolution does not just happen on long time scales
•Evolution is important for real-word issues:
agricultural, conservation, health
* Disease dynamics
* Invasive species issues
* Antibiotic and pesticide/herbicide resistance
Coevolution:
When two or more species interact closely they can influence
each other’s evolutionary direction.
In tightly coevolved interactions, evolutionary change in one
species will lead to evolutionary change in other or the second
species may go extinct.
Red Queen Hypothesis
Lewis Carroll's Through the Looking
Glass: “in this place it takes all the
running you can do, to keep in the
same place."
Examples: host / parasite coevolution
predator / prey dynamics
How does evolution by natural selection affect
behavior?
Although not physical traits, behaviors do have
heritable, genetic variation that leads to
differential reproductive success.
Therefore, natural selection has shaped
behaviors that we see today.
Hanuman langurs
lions
Why do new males commit infanticide?
Hypothesis 1: infanticide is a social pathology that is
caused by overcrowding and misplaced aggression.
Hypothesis 2: infanticide leads to increased reproductive
success for the male.
Hypothesis 3: infanticide is a population regulatory
mechanism that prevents overpopulation
Hypothesis 1: infanticide is a social pathology that is caused
by overcrowding and misplaced aggression.
Predictions Males should kill females and juveniles as well as infants
Infanticide should occur more frequently when densities are
high.
Not supported by data.
Hypothesis 2: infanticide leads to increased reproductive
success for the male.
Predictions -
Males should kill infants that are not related to him.
Killing unrelated infants should increase a males reproductive
success. (fitness)
Common estimates of reproductive success:
number of surviving offspring
number of matings obtained
survival rates
feeding rates
Data:
Males kill unrelated infants (present when they take over a
group).
Females go into estrous more quickly.
More food available for male’s offspring.
Hypothesis 3: infanticide is a population regulatory
mechanism that prevents overpopulation
“for the good of the species”
Predictions Infanticide should occur more frequently when densities are
high.
Group Selection
V.C. Wynne-Edwards
Animal Dispersion in Relation to Social
Behavior (1962)
Populations of self-sacrificing individuals
would survive and outcompete populations
of selfish individuals
Can natural selection act on groups or
populations rather than individuals?
Individual vs. Group Selection
G. C. Williams
Adaptation and Natural Selection (1966)
Natural selection will act more strongly on the
individual at the expense of the group.
Group selection is therefore not evolutionary
stable. Open to selfish individuals and
cheating.
Conditions necessary for group selection:
very low migration between groups
founding groups small or related
group extinction rates near individual death rates.
Alcock’s book takes an adaptationist approach.
Does all behavior need to be explained in terms of
natural selection?
Other explanations:
drift, founder events, learning/cultural transmission,
ecological or genetic constraints, by-product other
processes.
(note - these other processes still
based on evolutionary principles)