BIO 402: Field Biology

BIOGEOGRAPHY AND EVOLUTION

EVOLUTIONARY CHANGES

Species change in appearance over time in response to the environment in which they live

• Evolution = genetic change in a population over generations
• Time measured in generation
• The population is the typical unit of study (not the individual)
  population = a group of individuals living in an area which can interbreed
• Changes noticed as variation across the range of distribution
  variation = difference in genotypes and phenotype between individuals and populations

THEORY OF EVOLUTION

• Developed by Charles Darwin (1809-1882)
• Gathered much data from round world trip starting in 1831
• Especially amazed with Galapagos Islands
• He was captivated by the diversity of finches, more so that on mainland Ecuador

DARWIN’S THEORY OF EVOLUTION

• Many members are present in a population and each individual is genetically unique
  (i.e., there is genetic variation in the population)
• In each generation many more individuals are born than can survive to reproductive age; only the best adapted are able to survive (survival of the fittest)
• The survivors reproduce and pass their genes on to their offspring
• As a result, the next generation is better able to survive in that particular environment

The results of evolutionary change can be seen in numerous ways:

• variation in the appearance across distributional range
• variation in physiology or ability to live from one location to another
• recognized subspecies or varieties across the overall distributional range
• closely related species

Commonly, these differences occur with non-overlapping distributions

Example: Variation in the western yarrow, Achillea lanosa

Clausen et al (1948) published paper on genetic variation in the common yarrow

• Saw differences from low elevation to high mountains in California
• Coastal forms over 1 m tall, mountain forms 30 cm
• Experimented by transplanting individuals from many elevation to three sites
     *Stanford (100 ft)
     *Mather (4,600 ft)
     *Big Horn Lake (10,000 ft)
• Grew plants for three years at new elevations & measured resultant plant heights
  Observed phenotypic changes, but point of origin important  in average size
• Recognized variation as ecotypes

GENETIC CHANGES IN POPULATION VARIATION

As one studies variation in populations over time, one notices:

• populations can increase their genetic variation
• populations may decrease their genetic variation

Means To Increase Population Variation

• Mutations – changes in DNA structure; works at three levels
  a) nucleotide changes (a single nitrogen base)
         influences only one gene and it’s product protein
         ex.: sickle-cell anemia
  b) gene level (whole gene change, gene changes function)
         ex.: white flower in population of blue flowers
  c) chromosome level (change in number)
         ex.: aneuploidy (plants & animals), polyploidy (plants only)
  
• Gene Flow – interbreeding individuals, one population to another
  immigration leads to introduction of new genes into population
  result is production of a hybrid with intermediate characteristics
  
• Recombination – new combination of traits (genes)
  commonly result of crossing over and independent assortment during meiosis
  develops after new genes are introduced into a population

Means To Decrease Population Variation

• Genetic Drift – chance variation in allele proportions over generations
  process is commonly unobservable in a typically large population
  can be quite evident in small populations
         a gene may be lost from a small population by random chance
         gene loss referred to as the Founder Effect
         idea developed from observing populations on islands
         the founders of the population had a limited gene pool
  
  Example: Greenhouse frog introduced into Florida

• Natural Selection – one phenotype is better able to survive
         the result of species living in an environment
  several types of selection, based on type of environment
  
  a) stabilizing – maintain same phenotype
         extremes of population poorly adapted
         ex. leg length of Impalas (in African savannas)
  
  b) directional – one extreme better able to survive
         commonly seen at edge of species distribution
         ex. tolerance of insects to pesticides
  
  c) disruptive – extremes selected for
         very common when human selectively breed a new variety
         ex. varieties of dogs, cats, corn

MUTATIONS AND DISTRIBUTIONS

Mutations may create a different individual that is better adapted to the environment
             typically a mutation has little or no effect on distribution
             a mutation may create a new form which is:
                           a) better able to live in the current range (stays & lives)
                           b) poorly able to live in current range (lost & dies)
                           c) better able to live in a new range (moves)

Result of the last is that a species expands its range & has a new form

              Ex. most of cultivated crops are human-modified to grow in fields
                           cotton naturally a perennial shrub that requires several years to grow (tropics)
                           can only grow as shrub in tropics
                           cultivated as an annual (subtropical)

Mayr’s Hypothesis and species distribution

• Any genetic changes that are an adaptive advantage within normal range will be incorporated into the gene pool with no change in distribution
• Any genetic changes at the range edge the are not an adaptive advantage within, but outside the current distribution can allow for range extension

Possible outcomes of Mayr’s Hypothesis

• mutation at center & of advantage within the population range:
               mutation spreads throughout population with no range extension
• mutation at edge and or advantage within the population range:
               mutation spreads throughout population with no range extension
• mutation at edge and of advantage within and outside the population range:
               mutation commonly swamped by genes within population with
               no range extension
• Mutation at edge and of advantage only outside the population range:
               mutation not incorporated into much of current population;
               but there is range extension due to the new form