Download A very different form of selection

Last day… talked about 5 causes of evolution
- ended up on topic of natural selection…
Selection acts on pops. in 3 different ways:
1) Stabilizing selection – average individuals favored,
extremes selected against
- tends to maintain status quo, may reduce variation
- most common form of natural selection, maintains
stability of spp.
2) Directional Selection – favors individuals at one
extreme, acts against those at other extreme,
shifts curve
- often when environment changes
- probably most important for
actually producing evolution
3) Disruptive Selection
– favors individuals at both extremes, selects against
intermediates
- probably rare – unusual to have 2 phenotypes favored,
and hard to maintain 2 forms when inter-breeding
- e.g. Black-bellied Seedcracker
A very different form of selection...
Skeptics asked Darwin how Nat. Select. could account
for bright colors, beautiful songs, etc.
Darwin came up with another
form of selection:
Sexual Selection – process
which favors individuals that
have advantage in
competition for mating
opportunities
Temminck’s
Tragopan
Two main forms:
1) Mate choice – favors traits that are preferred by
members of opposite sex when choosing mates
- e.g. song, bright colors of male birds
Blue Bird-of-Paradise
2) Intra-sexual competition – favors traits that give
advantage in contests between members of same
sex (usually males)
- e.g. large size, horns
Moose
Darwin used sexual selection to explain sexual
dimorphism (differences in form between males
& females), but sex. select. can occur in both sexes
Red-necked
Phalaropes
Selection & Variation
Genetic variation is essential for evolution, but natural
selection tends to remove it – why is there still any?
Variation is present – e.g. 30% of Drosophila loci show
variation (in electrophoresis)
In 2007, diploid human genome sequence for one individual
(J. Craig Venter) published
- comparing genes on 2 chromosomes revealed 44% of
genes were different in some way (heterozygous)
Hidden Recessives– in diploid spp. recessive alleles often
not expressed (esp. if rare) because in heterozygotes,
thus immune to selection
Heterozygote advantage
– sickle-cell anemia,
resistance to malaria
- other examples scarce?
Frequency-dependent selection – rare allele is more fit
(until it becomes common!)
- scale-eating cichlids
- still pretty rare?
Perissodotus microlepis
Variation in time & space – if selection favors different
alleles at diff. times or places, may at least slow
down selection
- e.g. Medium Ground Finch (Galapagos)
- prob. common, but unclear if this is enough to maintain
variation
Maintenance of variation still something of a mystery...
Macroevolution
Evolution above the species level
Implies longer time scale, not studied as directly as
microevolution
Important source of data: fossil record
Fossils provide not only evidence for
evolution, but data to test ideas
about processes
Archaeopteryx
How Fast is Macroevolution?
Darwin emphasized gradual change over long periods of
time, eventually producing major change
Some paleontologists claim pattern is usually different
– species remain same (stasis) for very long time,
then abrupt appearance of new sp.
Punctuated Equilibrium (Gould & Eldredge 1972)
vs. traditional Gradualism
Stephen Jay Gould
Niles Eldredge
Incomplete fossil record can produce apparent
punctuations because no fossils preserved of
intermediates
But, examples exist that appear to show clear cases of
‘Punk Eek’ & Gradualism
- e.g. some trilobites: gradual
but some bryozoans: P.E. …
Two ‘theories’ represent extremes; actual patterns
may fall in between sharp punctuations or
complete gradualism
What appears abrupt may be gradual, but relatively fast
(10’s or 100s of thousands of years for change)