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Selection:
“Natural”
Natural
vs.
“Unnatural”
Unnatural
Jason Cryan, Ph.D.
Deputy Museum Director
for Research & Collections
Evolution is the pattern, Selection is the process
Postulates of Natural Selection
Individuals within species are variable
Some variation is passed to offspring
More offspring produced than can
survive
Offspring that survive and reproduce
are those with most favorable
variations
1809-1882
“The Origin of Species…” on the INTERNET
http://www.talkorigins.org/faqs/origin.html
Selection: Acts on
individual phenotypes to
produce a distinct
change in trait
distributions within a
generation
Evolution: A
population-level
change in allele
frequencies across
generations, as a
consequence of
selection
Fitness: the ability of an individual to survive and reproduce in
its environment, relative to other individuals in its species
The offspring of a ‘more fit’ individual will constitute a
greater percentage of the next generation than the offspring
of ‘less fit’ individual
Adaptive / Fitness
Landscapes
Adaptive Peaks
Adaptive Valleys
Fitness
The Plant Kingdom
“The evolution of plants is an
important chapter in the history of
life. However, it’s a pretty dull
chapter, so we’ll skip it.”
a fern
or something
Important Selective Forces
Eat!
Don’t get Eaten!
Don’t get Sick!
Reproduce!
SEXUAL
BeautySELECTION
is in the
“Differences among individuals
eye of the
beholder
in success
at getting
mates”
Sexual Dimorphism
Reproductive structures
Secondary sexual characters
Body size
Coloration
Exaggerated traits
Calling behavior
In most cases, males are more conspicuous
Asymmetries in Sexual Reproduction I
Females tend to invest more in offspring than males
Gamete size: eggs larger than sperm; embryos
generally not provisioned by males
Parental care: females usually responsible for
offspring care during maturation
Asymmetries in Sexual Reproduction II
Differential limits on reproductive success
Female reproductive success limited by egg
production (limited offspring number)
Male reproductive success limited by number
of mates (unlimited offspring number)
Prediction
1. Female reproductive success should level off
after a certain number of matings
2. Male reproductive success is proportional to the
number of mates
Behavioral Consequences of
Asymmetric Limits on Fitness
Intrasexual Selection (Competition): If access
to
females is the primary limiting factor in
reproductive success, males will compete with
other males for mating opportunities
Intersexual Selection (Female Choice): If
female reproductive success is not limited by
access to males, then females will be selective
about which males they mate with in order to
maximize the quality of the male’s reproductive
investment
Males
Usually minimal investment in offspring
Gamete size: sperm smaller than eggs
Parental care: in most non-human animal
species, males have little or nothing to do with
raising young
Male-Male Competition
Combat
Advertisement
Sperm Competition
New Male Takeover
Combat
Body size related to competitive ability
Mating males larger than non-mating males
Some male structures enhance competitive ability
Galapagos Marine Iguanas
Advertisement: Leks and Displays
Leks: male-defended territories solely for purpose of
displays and mating...no resource being protected;
1 or very few males mate with most females
Mating Center
Sperm Competition
When females participate in multiple
matings, the male sperm competes to
fertilize the eggs
Strategies for sperm competition:
1. Increased ejaculate volume
2. Physical removal of sperm from
previous matings
3. Copulatory plugs
4. Prolonged mating and female guarding
New Male Takeover: Infanticide
Males form coalitions to take
over pride
Offspring of previous coalition
killed by new males
Infanticide amount to about
10% of lion mortality
Females abort fetuses when
new males take over pride
Humans: When an unrelated stepfather lives with a family
including children under the age of 2, infanticide is
70× more likely to occur (Hrdy, S.B. 1999. Mother nature: a
history of mothers, infants, and natural selection. New York: Pantheon
Books)
It’s not just the guys….
Females
Usually a costly investment in offspring
Gametes: egg production, embryonic provisioning
Parental care: protection & provisioning
Female Choice
Females of many species mate preferentially with males
that have larger, more intense, or more exaggerated
characters such as color patterns, ornaments,
vocalizations, or display behaviors
Sexual characters & courtship displays originally thought
of as species-specific signals designed to encourage
females to mate
Darwin hypothesized that displays were used to
advertise male quality, and that females actually use
this information to choose among possible mates
Male Quality:
genetic quality
general health
parasite load
Nuptial Gifts
Females get direct benefit through resource acquisition
Runaway Selection
A self-reinforcing process of preference for a nonadaptive
(or even maladaptive) trait
Elements of Evolution
by Natural Selection
1. Individuals within species are variable
2. Some variation is passed to offspring
3. More offspring produced than can survive
4. Offspring that survive and reproduce are those with most
favorable variations
Elements of Evolution
by Sexual Selection
1. Individuals within sexes are variable
2. Some variation is passed to offspring
3. The variation leads to differential mating success among
individuals
Further Reading