Download Notes Chapter 16 - Spring Branch ISD

Evolution of Populations
Notes Chapter 16
16-1 Genes and Variation
I. Variation and Gene Pools
A. Variation – different individuals in a population have different
traits
B. Gene Pool – all genes in a population
C. Gene frequency – how common a gene is in a population
D. In genetic terms, evolution is defined as the change in gene
frequency in a population over time
II. Two main sources of variation that result from sexual reproduction
A. Mutations – a change in the DNA sequence
B. Gene Shuffling – genes may form new combinations during
meiosis
Example: crossing over
C. Traits
1. Single gene trait – a trait controlled by a single gene (2
alleles)
Example: “widow’s peak” (have or don’t have)
2. Polygenic trait – a trait controlled by more than one gene
(many alleles)
Example: height (many variations)
16-3 Speciation
III. Speciation – how a species forms
A. Behavioral isolation – when 2 populations are able to breed, but
have differences in reproductive behavior that prevent mating
B. Geographic isolation - when 2 populations are able to breed, but
are separated by barriers (rivers, mountains, etc)
C. Temporal isolation – when 2 populations are able to breed, but
reproduce at different times
D. Reproductive isolation - species may be able to mate & the
resulting offspring may be viable, but if those offspring are cannot
reproduce then isolation is still occurring.
Example: donkey + horse = mule which is sterile!