Last day... outlined Darwin’s theory of natural selection, & covered some of the evidence he used to support evolution & natural selection

Ended on evidence from Geographic Distribution: - species isolated by strong barriers tend to be distinct, though physical conditions are similar - species in the same area are related, even if occupying different habitats

- islands tend to have endemic spp., but related to nearest mainland - frogs, mammals, generally absent from oceanic islands Strawberry Poison Dart Frog

Classification: - general pattern of groups within groups

- homologous organs (mouthparts of insects, bones of vertebrate limbs)

- close similarity of embryonic stages Romanes 1892, after Ernst Haeckel

- rudimentary organs (teeth in baleen whales, wings of flightless birds & beetles) Emu with chicks

These points (and more!) converted most biologists in England to a belief in evolution (if not natural selection...) in about 10 years

Problems with natural selection led to increasing skepticism about Darwin’s mechanism for evolution - confusion due in part to lack of understanding of heredity

Answer came in 1866 from Austrian monk - Gregor Mendel Studied peas, worked out the essentials of heredity, & published promptly

How does heredity work? Mendel succeeded by studying simple characters like flower color & plant height that are inherited in straight-forward manner Traits he studied did not blend Only one trait was expressed in first generation offspring Dominant

In 2nd generation, both traits show up (in 3:1 ratio) 1st generation offspring must contain info for both traits, although exhibit only one Some 2nd generation offspring inherit 2 copies of ‘hidden’ trait, & exhibit it Recessive

Most plants & animals carry 2 sets of chromosomes One set from mother (in egg), other set from father (in sperm) diploid Today, understand this occurs because hereditary info is contained in genes located on chromosomes

One gene (e.g. for flower color) has different versions called alleles (e.g. purple or white)

Gene can be thought of as segment of DNA that codes for a protein (or polypeptide) Code is written using 4 bases: Adenine Thymine Guanine Cytosine

Sequence of 3 bases codes for a particular amino acid in the protein

How does this make a flower purple or white? ‘Purple’ allele codes for a protein that makes flower purple ‘White’ allele may code for a ‘defective’ protein

One copy of dominant purple allele produces sufficient protein to turn flower purple 2 copies of same allele = homozygous 1 copy each of different alleles = heterozygous Phenotype (observable characteristics of an organism) does not always reveal the genotype (set of genes or alleles an individual has)

Not all genetic characters work the same way - some traits show incomplete dominance (heterozygotes show intermediate phenotype)

Another complication - some characters are affected by >1 gene, & each gene may have >2 alleles Quantitative characters – typically show continuous variation

Diploid organisms get one copy of gene from mother and one copy from father But parents have 2 copies of each gene - how does it work so only one copy gets passed on?

Normal cell divisions (mitosis) produce 2 ‘daughter’ cells with same set of genes as original cell - no good for producing gametes in diploid organisms

Gametes instead produced by meiosis - ‘reduction division’ that results in haploid cells - chromosomes replicate their DNA before meiosis - diploid organisms now have 4 copies of genes in these cells How many chromosomes are in this micrograph? a) 1 b) 2 c) 4

THEN 2 rounds of cell division, - 4 daughter cells, not 2 - each haploid (1 copy of genes)

Meiosis does not just reduce chromosome number, it also creates new gene combinations - homologous chromosomes come together as pairs (tetrad), process called synapsis - chiasmata form – crossing-over of chromatids