Download MacroEvolution - WordPress.com

Macroevolution
Microevolution: Changes in allele frequency in a population over time
Macroevolution: Broad patterns of evolutionary change; large scale history of life
Natural Selection as a Mechanism for Adaptive Evolution


Acts on phenotype: physical or chemical expression of an organism’s genes, not
genotype (only indirectly)
Changes allele frequencies in a way that increases adaptation
Phenotypic Variation



Results from different alleles present at locus
Morphs: contrasting phenotypes
Polymorphic: population with 2 or more morphs in noticeable frequencies
Three types of selection1. Directional Selection: favors individuals at 1 extreme end of the phenotypic range
2. Disruptive Selection: favors individuals at both extremes of the phenotypic range
3. Stabilizing Selection: favors intermediate variants and acts against extreme phenotypes
Polygenic Control
- It is rare to have alleles at a single locus controls a phenotype
- More common for interaction of alleles at several loci for expression of a single phenotype
- Characters under polygenic control result in a range of phenotypes
- A normal distribution is a bell curve
Species: Group of like organisms
Speciation: Process by which one species splits into 2+ spp
yields diversity of life (100 million species exist)
The Concept of Biological Species Based on Reproductive Isolation
Biological Species Concept: group of populations whose members can interbreed in nature and
produce viable, fertile offspring.



Share common gene pool
Gene flow between populations
Do not exchange genes with different species: reproducibility
Reproductive Isolation: Prevents 2 different species from interbreeding and producing viable,
fertile offspring. Prevents gene flow, prevent formation of hybrids.
1. Pre-zygotic Barriers: occur before zygote is formed, prevents mating attempts and
prevents fertilization after mating.
a. Habitat Isolation- same geographic area but don’t mix
b. Temporal Isolation- breed at different times
c. Mechanical Isolation- mating attempted but morphological difference in sexual
structures prevent successful mating
d. Gametic Isolation- molecular or chemical difference between species, egg and
sperm are incompatible.
2. Post-zygotic Barriers: prevent the hybrid zygote from developing into a viable, fertile
adult.
a. Reduced Hybrid Viability- genetic incompatibility… embryo dies early in
development or has impaired survival in its environment.
b. Reduced Hybrid Fertility- hybrids produced, but are sterile or not very fertile bc
meiosis does not occur normally.
c. Hybrid Breakdown- 1st generation is fertile but when they mate w/each other or
w/either parent species, offspring of the next gen are weak or sterile.
Limitation of the Biological Species Concept:
-can’t be applied to fossils, asexual organisms (prokaryotes)
-it emphasizes absence of gene flow, but gene flow can occur between distinct species
Other definitions of species: focus on unity within a species and apply to sexual/asexual species



The morphological species concept defines species by structural features, but relies on
subjective criteria
Ecological species concept views species in terms of ecological niche; emphasizes role of
disruptive selection
The phylogenetic species concept defines species as smallest group of individuals in a
phylogenic tree. Difficult to determine degree of difference to separate species
The Process of Speciation
Evolution of a new species occurs when population reproductively isolated from other
population, or gene pools begin to diverge in genetic composition.
Allopatric Speciation
 Occurs with geographic speciation
 Population becomes geographically
separated then evolves by natural
selection and/or genetic drift
 Gene flow interrupted
 Most common mode of speciation
 Geographic Barriers, Migration,
Isolation by Chance
Sympatric Speciation
 Occurs without geographic speciation
 New species arises within range of
parent population; not isolated
 Less common mode of speciation
 Polyploidy, Sexual Selection, Habitat
Differentiation
1. Polyploidy: more than 2 sets of chromosomes, occurs as mistake in cell division, can
make new species in a generation.
a. Autopolyploid: individual w/more than 2 X sets from a single species
-Doubling number of X before meiosis
-Tetraploid can then self/cross w/other tetraploid
-Can’t interbreed w/diploid parent species
b. Allopolyploid: species w/multiple sets of Xs derived from different species
-Hybrid may be able to propagate asexually
-After generations, sterile hybrid may develop into fertile polyploidy
2. Sexual Selection: barrier to gene flow
a. Evidence = mechanism for sympatric speciation
b. Females select males based on appearance
3. Habitat Differentiation
a. Sympatric speciation is the result from appearance of new ecological niches
b. Subpopulation uses a habitat/resource not used by rest of population
Rate of Speciation: Speciation is a result of changes in few/many genes


Difficult to observe directly, used the fossil record to study
Different number of genes for different organisms: genetics of speciation
Patterns in Fossil Record and Speciation Rate
- Ancestral species found in fossils, but not transitional species. 2 Different models to explain:
1. Punctuated Equilibrium
a. Suggests fossil record not incomplete, not missing transitional species
b. Specieation occurs in rapid bursts which punctuate long periods of stasis
2. Gradual Model
a. Gradual accumulation of adaptive changes in earth’s population
b. Slow/constant, rarely observed in fossil record
Speciation rates: 4000 years to 40 million years. Average 6.5 mil years
- The punctuated pattern in the fossil record + evidence from labs suggest that speciation can
also be observed.
- Speciation may require change in a single gene or many genes