BIO220-Lecture 02 - The Evolution of Sexual Differences.pdf

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13 Mar 2014
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How different are the sexes, and why?
1
About 90% of genes are shared between
males and females, but .
50-90% of all genes in the
fly genome (~13,000) are
expressed differently in
the sexes
Gene expression of shared genes
in most tissues differs between the
sexes
This is most evident in the gonads
But also in most other tissues (e.g.,
liver, brain)
2
The Evolution of
Sexual Differences (Dimorphisms)
The puzzle of conspicuous ornaments
Darwin’s answer (hypothesis)
Origin of sexual differences
Male-male competition
Sexual dimorphism:
conspicuous ornaments
Frigate Bird
Widow Birds
Sexual dimorphism:
conspicuous behaviours
Greater Sage-Grouse
http://www.youtube.com/user/PatricelliLab
Sexual dimorphism: armaments
the transcripiton process differs
Much diversity rises b/c differences bt wn males and females
there isnt much diversity bt wn females, but there is a huge diff
in males, thus this is the resultant of diversity
Males display , have huge sacs, motive to attract females
1 or 2 males usually attract all the females
used in competition between males, competing for mates
use as a display compare the distance btwn the stocks,
longer stock males win
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2
Peacock Displays
Survival of the Fittest?
7
Peacock Displays:
Survival of the Fittest?
Mating
Success
+
Survival
(predation)
-
8
Sexual
Selection
9
Sense of
Beauty
Sexual Selection
depends on the advantage which certain
individuals have over other individuals of the
same sex and species, in exclusive relation to
reproduction. Charles Darwin
Sexually selected traits function to
enhance mating success
It is a subset of natural selection but
it can be useful to consider them
separately
11
Natural selection
FITNESS
+ +
Survival
Reproductive
rate
Sexual selection
+
Number of
mates
this trait has nothing for fitness, the color attract predators,
this puzzled Darwin b/c this decreased survival
Darwin concluded : the negative affects of this for survival
may have a compensation through mating success. A tradeoff,
bigger tail more mating success , decrease survival. Females have
tails that are optimal for survival.
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3
12
Fitness in Males & Females
FITNESS
Survival
Reproductive
rate
Mating
success
usually males
usually females
Why sexual selection?
Why two sexes?
Unequal-sized gametes
Anisogamy!
The origin of sexual differences
14
EGG
Resource-rich
Few
Large
SPERM
Inexpensive
Many
Small
Anisogamy
15
Parental Investment Theory
Robert Trivers (1972):
Sexes differ in their reproductive investment
Members of the sex that invest little in
offspring will compete among themselves to
mate with members of the sex that invest
more in offspring
16
The Difference Between the Sexes
Tiny, mobile gametes
(sperm)
Fitness limited by
access to fertilizable
gametes
Large, resource-rich
gametes (egg)
Fitness limited by
access to resources
Competition
among males for mates
Choosing
among males for mates
MALES FEMALES
17
Number of mates
Number of offspring
Males
Females
Males Limited by Access to Females
Females = reproductive rate ( # of eggs ) and survival ( the amount
time eggs are produced)
Males = access to eggs
The key difference is the gametes between the two sexes,
sperm and eggs. to determine the sex of the species you would
decided which one was male or female, look at the size of the
gamete , the smaller gamete = male
Females invest ALOT in gametes, every offspring has a mother
and father , limiting factor the # of offspring a female
can produce.
In some species males invest more than females after offspring
in these cases there maybe that the males will be “ picky
so the sex roles will be reversed. The roles of females follow to
nurture because they are investing more
Males high fitness proportional the amount of mating
strong sexual selection in males, weak in females
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