Transcript Darwin meets DNA

Darwin meets DNA
How does variation arise?
• Darwin knew nothing about how traits are
passed from one generation to the next
• Even though his hypothesis of Natural
Selection depended on this
• Mendel’s work was rediscovered in the
early 1900’s
2+2=4
• Genes are the carriers of traits
• Genes are the source of inheritable variation
• Inheritable variation is the raw material for
natural selection
• Genes became the focus for understanding
the mechanisms of evolution
Inheritable variation (1)
• Comes from two sources:
• 1. Mutations in the organism’s genetic
material
• 2. Gene shuffling
Mutations
• Remember this: point mutation, frameshift
mutation
• Mutations can have positive, negative or no
effect on organisms
• Rate of mutation varies: bacteria have high
rates, humans have low rates
Gene shuffling
• Remember meiosis (especially the crossing
over phase)
• Extensive shuffling of genes occurs when
eggs and sperm are produced
• Sexual reproduction is an important source
of variation in nature
• Many genes have two alleles
Inheritable variation (2)
• Acts on the phenotype, not the genotype
• Traits can be controlled by one gene, two
genes or polygenic
• Remember crossing?
• Look at the figures on pages 485 and 486
• A polygenic trait with multiple alleles can
have a large number of phenotypes
Genes, Fitness and Adaptation
• Evolutionary fitness:
– The success an organism has in passing its
genes to the next generation
• Adaptation:
– A genetically controlled trait that increases the
organism’s ability to pass along copies of its
genes
• Ponder the bird example (a DK story).
Species (1)
• Darwin though species were organisms that
looked alike
• Today the definition is genetic:
– A group of similar looking organisms that can
breed with one another and produce fertile
offspring
• This is called reproductive isolation
Species (2)
• Individuals within a species share a gene
pool
• Different species do not share genes, so they
evolve separately
• Within a species each allele occurs with a
steady frequency when there is no selective
pressure