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 - 500 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 - 15-58% of genes

exhibit variation Within individuals - 3-17% 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