In Biston betularia, there are two alleles for color pattern:
M = dominant, melanic; frequency = pEach individual has two alleles, so the genotypes and phenotypes in the population are:
m = recessive, typical; frequency = q
MM = homozygous melanic (frequency in the population of p2)Assume that the proportion of typical individuals in the population is 81%. What is the value of q?
Mm = heterozygous melanic (frequency in the population of 2pq)
mm = homozygous typical (frequency in the population of q2)
q2 = 0.81 so q = 0.9 (square root of 0.81); 81% of the individuals are typical; 90% of the genes are m.
This means that p = 0.1; 10% of the genes are M.
What proportion of the individuals are homozygous melanic? q2 = (0.1)2 = 0.01 (only 1% are homozygous melanic).
What about heterozygotes?
2pq = 2 (0.1) (0.9) = 0.18, 18% of individuals are heterozygous.
In the population as a whole, 81% of individuals are typical, 19% (1% + 18%) are melanic.
When reproduction occurs, each sperm cell and each egg cell must bear either m or M, and with frequencies p (0.9) and q (0.1) respectively. Zygotes (new individuals) are formed by the fusion of eggs and sperm. Allele frequencies in the population determine phenotype frequencies after reproduction:
(p + q) x (p + q) = (p + q)2 = p2 + 2pq + q2
(p2 = homozygous melanics, 2pq
= heterozygous melanics, q2 = homozygous typicals)
| EGGS | ||
| SPERM | M (p = 0.1) | m (q = 0.9) |
| M (p = 0.1) | MM (p2 = 0.01) | Mm (pq = 0.09) |
| m (q = 0.9) | Mm (pq = 0.09) | mm (q2 = 0.81) |
The proportions of the three categories of individuals
in the population after reproduction has occurred are shown in the table.
Note that if nothing happens to change gene frequencies, they stay the
same or remain in equilibrium from one generation to the next.
We define evolution (on a population genetic basis) as a change in gene frequencies
What processes change frequencies?
Mutation: chemical changes in genes that alter the base-pair sequence and thus have the potential to alter the sequence of amino acids in proteins, which can alter their physiological and structural properties, thus the phenotypic expression of the genes. For example HbBs, caused by a change in the sixth codon, causes change from valine to glutamic acid in the sixth AA in the B protein
Gene flow (emmigration, immigration): movements of individuals into and out of populations; these individuals may carry new genes into populations, or at the very least may alter the frequency of genes in a population. Gene flow from one population to another may also occur through dispersal of gametes. Genes flow from one population to another through interbreeding at the edges of populations.
Non-random mating:
Inbreeding (mating with genetic relatives) may lead to changes in the overall population gene frequency; You know you are from Oklahoma is your family tree doesn't fork; you know you are from Oklahoma if you have to ask "If mamma and daddy get a divorce will they still be cousins?
preferential mating based on phenotypic characters can alter the frequency of genes for those characters; rare-male advantage in fruit flies for instance; I would really include this as a form of selection because it is based on choices by conspecifics (social environment)
Selection: due to the abiotic, biotic,
and social environments; this is the most important cause of evolutionary
change and the only one that can lead to adaptation
Genetic drift: change in gene frequencies in small populations due to "sampling error" - non-selective mortality can alter gene frequencies in small populations more readily than in large populations simply because of population size.
The Founder Effect refers to this type of change in gene frequency when a small sub-population is removed from the larger population - the sample is unlikely to contain exactly the same gene frequencies as the larger population.
Populations that have had reduced population size due to some non-selective event pass through a period of reduced genetic diversity and may recover their previous size but not their previous genetic diversity (Population Bottlleneck) -
See textbook p 427 - A random sample?
See fig 20.6 - an "epidemic"??