DIVERSITY AND EVOLUTION Chapter 2

Diversity of life Approximately 1.5 million living species described Likely at least 10 million species today May represent only 1% of species ever to have lived on earth 1 billion species presumed to have lived

Diversity of body form Tremendous diversity within each group of plants, animals, fungi, protistans, bacteria Structural complexity - apparently purposeful adaptation of many characteristics to the environment

Reason for this diversity? Natural selection Physical environment acts on various characteristics of organisms (variation among individuals of some species) Sorts out “harmful” ones, leaving individuals with “beneficial” or “neutral” characteristics to produce next generation Keeps organisms well-suited for survival in their environment

Natural selection drives evolution Broad scale Development of various “forms” or species to best match the environment Can best take advantage of variations within that environment

History of concept of evolution by natural selection Lamarck - inheritance of acquired characteristics Darwin, Wallace - natural selection, but mechanism really unknown Mendel - genetic understanding of the acquisition of inherited traits

Evolution by natural selection - established truths 1) individuals that form a population of a species are not identical

Evolution by natural selection - established truths 2) some of the variation between individuals is heritable

Evolution by natural selection - established truths 3) all populations are capable of exponential growth, but most individuals die before reproducing, and most others reproduce at less than their maximum rate

Evolution by natural selection - established truths 4) different ancestors leave different numbers of descendents; they do not all contribute equally to subsequent generations

SPECIATION Interaction of heritable variation, natural selection, barriers to gene flow

Allopatric (Geographic) Speciation Separating single, interbreeding population into two or more spatially isolated populations Geographic barrier, remains long enough for speciation Founder effect, genetic drift (random mutations)

Parapatric Speciation No spatial isolation Portion of population invades new, adjacent habitat Little to no movement/interbreeding Differing natural selection in differing habitats

Sympatric Speciation No spatial isolation Production of new species within a population Rare Most likely to occur in insect parasites of plants, animals Requires stable polymorphism and under- or unused resource

Polyploidy Abrupt speciation by doubling the number of chromosomes Most common in plants Agricultural-wheat, alfalfa, potatoes Native-birches, willows

PATTERNS OF SPECIATION

Anagenesis One species changes into another species over time Original species “evolves” out of existence and is replaced by new species Evolutionary extinction

Cladogenesis One species gives rise to one or more additional species while still remaining Clade-set of species descended from a particular ancestral species (e.g., Darwin’s finches)

TEMPO OF SPECIATION

Gradualism Steady change in character(s) resulting in many intermediate forms exhibiting “gradual” shift

Punctuated equilibrium Rapid, abrupt changes that produce quick shifts in character No intermediate forms

REDUCTION IN VARIATION

Inbreeding depression Mating among close relatives produces an increase in expression of recessive traits, many of which are deleterious Often results from small population size Mortality may be increased “Tighter” inbreeding results in more rapid loss of genetic variation within population

But…. Not all populations are harmed by inbreeding Long-term, small populations (e.g., on islands) may be adapted to inbreeding and survive well even in face of it

Outbreeding Some degree of outbreeding usually beneficial in maintaining genetic diversity But too much can also be harmful Too many differences may lead to problems

Smaller populations Genetic variation declines faster in smaller population because of inbreeding Rule of thumb-50 individuals needed to prevent inbreeding Problem for saving California condor Only 26 individuals in 1986

Genetic drift Larger population not subject to inbreeding can lose genetic variation at rates similar to small populations via genetic drift Some individuals do not mate, not represented genetically in next generation

Genetic drift-cont. Rule of thumb-happens only in populations <500 in size Genetic drift can be counteracted by minimal levels of immigration into the population

Neighborhoods Even big populations may run into problems if individuals don’t move around much to mate Some also just don’t reproduce Effective population size may then be quite small E.g., grizzly bear in Yellowstone Actual population ~200 Effective population ~50 (25%) Subject to loss of variation

Bottlenecks Can also reduce genetic variation Bottlenecks - periodic reductions in population size can reduce genetic variation greatly even if average population size is much larger

Founder Effects Can also reduce genetic variation Founder effects - developing gene pool of growing population is limited by what variation founders had, plus mutation Pair of founders at most have 4 variations in a gene

ORIGIN OF VARIATION

Genetic Increase or decrease variability within a population DNA - mistakes or mutations during copying of genetic code Gene or point mutation - most important for enriching the gene pool Chromosome mutation - most important for rearranging the gene pool

Point Mutations Change in nucleotide base at single location Change in single amino acid within protein, or entirely different protein Frameshift mutation - insertion or deletion of single base pair

Mutagens and mutations Mutations usually produced by mutagens (e.g., weak cosmic rays) 1 mutation per gene in every 100,000 sex cells Higher organisms have ~10,000 genes 1 in 10 individuals has newly created mutation

Most mutations are harmful, but.. 1 in 1000 mutations may be beneficial 1 in 10,000 individuals per generation has a useful mutation Most individuals have at least one mutant gene (original, or passed down from ancestors)

Mutations and Speciation Estimate mutations necessary to produce new species from existing one Rate of new mutations ~1 million times greater than needed to account for known rate of evolution

Chromosomal Mutations No change to variability Rearrange what is there Deletions, duplications, inversions, translocations

Other changes Polyploidy - e.g., tetraploid Failure of gametes to reduce to haploid state during meiosis 2N + 2N = 4N

So… Mutations produce the variation, and natural selection acts upon the changes Add in: nonrandom mating, changing environment End product = EVOLUTION

Amount of Variation Results from protein analyses (electrophoresis) Within a population % of genes exhibit variation Within individuals % of genes exhibit variation

Applying this information: 1) Separate populations of organisms with movement of individuals among populations generally exhibit most variation within each population, and very little between or among populations

Applying this information: 2) Reduced movement of individuals among populations produces more variation between or among populations Populations diverge genetically

Applying this information: 3) Conservation of endangered species which move around very little will require protection of many populations in many different habitats to conserve genetic diversity within the species