A primer of ecological genetics
Ecologicalgeneticsis concerned with the genetics of ecologically important traits, that is,
those traits related to fitness such a survival and reproduction. The study of the process
of phenotypic evolution occurring in present-day natural populations.
Ecologicallyimportanttraits: those traits that are closely tied to fitness or, in other words,
are important in determining an organisms adaptation to its natural environment, both
biotic and abiotic.
Phenotypic evolution can be defined as a change in the mean or variance of a trait
across generations due to changes in allele frequencies.
The four processes that cause evolution are mutation,geneticdrift,migration,andnatural
Adaptationis the central theme of ecological genetics. An Adaptationis a phenotypic trait
that has evolved to help an organism deal with something in its environment.
Natural selection is the only evolutionary process which leads to adaptation.
The genetics of a population can also be affected by gene flow from other populations
with different genetic composition, or by genetic drift if the population size is small.
An alleleis a particular type of a given gene.
Populationgeneticsis the study of genetic variation within and among populations,
focusing on the processes that affect genotypic and allele frequencies at one or a few
gene loci. These processes include inbreeding, mutation, migration, drift, and selection.
Population genetics does not focus on phenotypes because they are complex and are
usually affected by several gene loci and the environment.
Quantitativegeneticsfocuses on phenotypes, usually without knowing the genotypes
underlying the traits. Quantitative genetics uses statistical methods such as variance,
correlation, and heritability to help understand the genetics of complex phenotypes.
A geneis a stretch of DNAcoding for a polypeptide chain; one or more polypeptides
make up a protein. The genetic information in DNA is coded in the sequence of four
nucleotides abbreviated according to the identity of the nitrogenousbasesA, G, T, or C.
The process of creating proteins from the genetic code in DNA is called geneexpression.
The first step is transcription, in which the sequence of nucleotides present in one DNA strand of a gene
copied into the
synthesized, each of base in the DNA undergoes pairing with the base in an RNA
nucleotide, which is then added to the growing RNA strand. The second step of gene
expression is RNAprocessing, in which intervening sequences or intronsare removed
from the RNA transcript by splicing and the ends of the transcript are modified. The
regions between the introns that remain in the fully processed RNA are known as exons;
these are the sequences actually code for proteins. The fully processed RNA constitutes
The messenger RNA undergoes translationon ribosomes in the cytoplasm to produce
the polypeptide that is coded in the sequence of nucleotides. In the translated part of
the messenger RNA, each adjacent group of three nucleotide constitutes a coding
group or codon, which specifies the corresponding aminoacidsubunit in the polypeptide
chain. The codon AUG specifies methionine and also serves as the startcodonfor
polypeptide synthesis. Any of the three codons UAA, UAG, or UGA specify the
termination of polypeptide synthesis. All the DNA in a cell is collectively called the genome. Genome size is typically
expressed as the amount of DNA in a reproductive cell (sperm or egg), and it differs
greatly among species.
Genes are arranged in linear order along microscopic threadlike bodies called
chromosomes. Each human gamete (sperm or egg) contains one complete set of 23
chromosomes, this is the haploidchromosome number, designated as n.
The position of a gene along a chromosome is called the locusof the gene.
Recombinationbetween loci can occur during meiosis, which creates new combinations
of alleles at different loci. Recombination is rarer between loci that are close together on
a chromosome; these loci are said to be genetically linked.
Each individual cell contains two copies of each type of chromosome, one inherited from
its mother through the egg and one inherited from its father through the sperm(so that
diploidchromosome number, 2n is 46 in humans).
At any locus, therefore, every diploid individual contains two copies of the gene– one at
each corresponding (homologous) position in the maternal and paternal chromosome.
These two copies are the alleles of the gene in that individual.
If the two alleles at the locus are indistinguishable then the individual is homozygousat
the locus under consideration. If the two alleles at a locus are distinguishable then the
individual is heterozygousat the locus.
The genotypeof an individual is the diploid pair of alleles present at a given locus.
Therefore, homozygous and heterozygous are the two major categories of genotypes.