Premises: Genetic variability within a species
Natural
selection is differential reproduction of alternative hereditary variants
determined by environmental conditions.
Selection
may be due to differences in survival,
fertility, rate of development, mating success, or other aspects of a life cycle (e.g.,
encystment).
Dynamics
of genetic change:
The
Hardy-Weinberg Law
Imagine
2 alleles, A and a. Given the frequency
distributions of the two alleles as p and q, respectively, and assuming random
recombination, the equilibrium
frequency of the genotypes will be a quadratic:
Parental
gamete maternal gametic
frequencies
frequency
p(A) q(a)
P(A) p2 (AA) pq(Aa)
Q(a) pq(Aa)
q2(aa)
The
H-W Law assumes that gene frequencies remain constant from generation to
generation. This is not true in the
real world – indeed if it were true then there would not be evolution.
Causes
of gene frequency change include:
Mutation
Migration
Genetic
drift – when populations are finite (as always) – gene frequency distributions
will not follow the H-W law. Especially
important after large culling events (such as climatic change- or impacts).
Origins
of Species
Reproductive
isolation - the prevention of mixing
genetic information across species lines.
Ecological
isolation - e.g., species living in
different habitats
Temporal
isolation – e.g., seasonal differences in mating
Behavioral
isolation - e.g., in mating
Mechanical
and gametic isolation – e.g., shape of pollen
Leading
to
Adaptive
radiation