Sylfae Bio 100
Chap 12
Evolution – heritable change in a line of descent; occurs by mutation, natural selection, genetic drift and
gene flow.
Microevolution - A change in gene frequency in a population.
Individuals do not evolve , populations do. A population is defined as any group that can
successfully interbreed.
Individuals in a population share a gene pool. All of the genes in the entire population constitute the gene
pool.
Mutation – heritable change in the sequence of DNA; the only source of new alleles
Lethal
Neutral
Beneficial – bestow survival advantages
Genetic drift – Change in allele frequencies in a population over time due to chance alone. Effects are
most pronounced in small populations.
In each generation, some individuals may, just by chance, leave behind a few more
descendents (and genes, of course!) than other individuals. The genes of the next generation will be the
genes of the "lucky" individuals, not necessarily the healthier or "better" individuals. That, in a nutshell, is
genetic drift. It happens to ALL populations — there's no avoiding the vagaries of chance.
Natural selection – differences in survival and reproduction among individuals of a population
Misconceptions about natural selection
Because natural selection can produce amazing adaptations, it's tempting to think of it as an all-powerful
force, urging organisms on, constantly pushing them in the direction of progress — but this is not what
natural selection is like at all.
First, natural selection is not all-powerful; it does not produce perfection. If your genes are "good enough,"
you'll get some offspring into the next generation — you don't have to be perfect. This should be pretty
clear just by looking at the populations around us: people may have genes for genetic diseases, plants
may not have the genes to survive a drought, a predator may not be quite fast enough to catch her prey
every time she is hungry. No population or organism is perfectly adapted.
Second, it's more accurate to think of natural selection as a process rather than as a guiding hand. Natural
selection is the simple result of variation, differential reproduction, and heredity — it is mindless and
mechanistic. It has no goals; it's not striving to produce "progress" or a balanced ecosystem.
This is why "need," "try," and "want" are not very accurate words when it comes to explaining evolution.
The population or individual does not "want" or "try" to evolve, and natural selection cannot try to supply
what an organism "needs." Natural selection just selects among whatever variations exist in the
population. The result is evolution. At the opposite end scale, natural selection is sometimes interpreted as
