Transcript Outcomes of Natural Selection (Chapter 19)

Outcomes of Natural Selection and
Genetic Drift
(Chapter 19 and part of 20)
Small Steps or Great Leaps? See Fig. 20.20 in text
• Gradualism is the slow change from one form to
another
• punctuated equilibrium- instead of a slow,
continuous movement, evolution tends to be
characterized by long periods of virtual standstill
(equilibrium or stasis), "punctuated" by episodes
of very fast development of new forms.
punctuated equilibrium site
• Microevolution (IB) – adaptations that develop in
populations as a result of changes in allele
frequencies in the gene pool
• Macroevolution (IB) – over long periods of time,
many advantageous “traits” will appear and
spread through the species. This may/will lead
to a new species
The Effects of Selection on Populations
(Fig. 19-3 in text)
1.Stabilizing selection
• “weeds-out” individuals at both extremes of a
range of phenotypes resulting in the
reproductive success of those near the mean
• associated with a population that is well
adapted to its surroundings
• ex) infant mortality is higher for very heavy as
well as for very light babies
2. Directional Selection
• individuals occupying one extreme in the
range of phenotypes are favored over the
others
• ex) Peppered moth, Galapagos finches,
giraffes, cows that give the most milk.
3.Disruptive Selection
• individuals at both extremes of a range of
phenotypes are favored over those in the
middle.
• ex) grasses growing either on copper
contaminated soil or non-contaminated
soil do the best while cross pollinated
grasses don’t do well on either type of soil
Animation of Selections
Random Changes in Frequencies
• a change in allele frequency in a population due
simply to chance is called genetic drift
• Processes leading to genetic drift:
1. population bottleneck – a population becomes
reduced quickly and produces a random
change in gene frequencies (Fig. 19.6 in text)
• the allele frequency of the “new population”
would depend the surviving organisms’
alleles
• examples
– tule elk of California in text
– zoos?
2. founder effect – few individuals stray and
establish a new colony, in a new habitat, and
therefore a new population with diff. freq.
than the original pop. (Fig. 19.8 in text)
• Afrikaaners of South Africa in text
– In the 1680s Ariaantje and Gerrit Jansz emigrated from Holland to
South Africa, one of them bringing along an allele for the mild
metabolic disease porphyria. Today more than 30000 South Africans
carry this allele and, in every case examined, can trace it back to this
couple — a remarkable example of the founder effect.
• Amish in Pennsylvania
– Eastern Pennsylvania is home to beautiful farmlands and countryside,
but it's also a gold mine of information for geneticists, who have
studied the region's Amish culture for decades. Because of their closed
population stemming from a small number of German immigrants -about 200 individuals -- the Amish carry unusual concentrations of
gene mutations that cause a number of otherwise rare inherited
disorders, including forms of dwarfism and plolydactyly.