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.