Quantitative Genetics & Evolutionary Genetics

Transcription

1 Quantitative Genetics & Evolutionary Genetics (CHAPTER 24 & 26- Brooker Text) May 14, 2007 BIO 184 Dr. Tom Peavy Quantitative genetics (the study of traits that can be described numerically) is important for two reasons 1. Most of the key characteristics considered by plant and animal breeders are quantitative traits 2. Many of the traits that allow a species to adapt to its environment are quantitative traits Discontinuous vs. continuous traits (discrete states) (continuum of variation) POLYGENIC INHERITANCE Most quantitative traits are polygenic and exhibit a continuum of phenotypic variation Polygenic inheritance refers to the transmission of traits that are governed by two or more genes The locations on chromosomes that affect the outcome of quantitative traits are called quantitative trait loci (QTLs) QTLs may contain many genes Some or all of which may affect quantitative traits 1

2 QTLs Are Now Mapped by Linkage to Molecular Markers Molecular markers, such as RFLPs, are now being used as reference points along chromosomes These genetic markers have been used to construct detailed genomic maps These maps make it easier to determine the number of genes that affect a quantitative trait Detailed genomic maps have been obtained from Model organisms Organisms of agricultural importance The data are analyzed by computer programs that can statistically associate the phenotype (e.g. fruit size) with the molecular marker 2

3 Evolutionary Genetics Biological evolution A heritable change in one or more characteristics of a population or species across many generations It can be viewed on a small scale (single gene) Or large scale (formation of a new species) Microevolution Changes in the gene pool with regard to particular alleles over measurable periods of time Macroevolution Relatively large changes in form and function that are sufficient to produce new species and higher taxa Evolutionary biologists often focus on traits that fall into three broad phenotypic categories 1. Adaptation Traits that make a species survive better in its natural environment 2. Reproductive success Traits that promote fertility, the ability to find a mate, etc. 3. Reproductive isolation Traits that prevent two different species from successfully interbreeding 1. Genetic variation at the species level Genetic variation is a consistent feature of most natural populations Allelic differences in genes Changes in chromosome structure Alterations in chromosome number All of these influence the phenotype of the individual 3

4 2. Natural selection at the species level Most species produce many more offspring than survive and reproduce This creates a struggle for existence that results in the survival of the fittest species become better adapted to their environment and/or more efficient at reproduction Over time, natural selection may change the allele frequencies of genes (e.g. fixation & elimination) The biological species concept (Ernst Mayr) A species is a group of individuals whose members can interbreed to produce viable, fertile offspring The members of one species, cannot successfully interbreed with members of other species There are several different ways to achieve reproductive isolation (Table 26.1) Prezygotic mechanisms Prevent the formation of a zygote Postzygotic mechanisms Prevent the development of a viable individual after fertilization has taken place Molecular genetics has greatly facilitated our understanding of speciation and evolution Differences in nucleotide sequences are quantitative They can be analyzed using mathematical principles in conjunction with computer programs Thus, evolutionary changes at the DNA level can be objectively compared among different species Establish evolutionary relationships Comparative genomics 4

5 Homologous Genes are Derived from a Common Ancestral Gene Two genes are said to be homologous if they are derived from the same ancestral gene Genes can exhibit interspecies homology and intraspecies homology Orthologous genes or orthologs are homologous genes found in different species Paralogous genes or paralogs are homologous genes found within a single species A gene family consists of two or more copies of homologous genes within the genome of a single organism Figure 26.9 Evolution of paralogous and orthologous genes The current model links all life on our planet Figure Three main evolutionary branches 5

6 Final Thoughts Molecular Cellular Organismal Population Structure Function Evolution 6