Principles of Biology - Biology 102 Lake Tahoe Community College
Spring Quarter Instructor: Sue Kloss
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Ch. 24 - Origin of Species
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Introduction
A. speciation - origin of species; appearance of new species is the source of biological diversity
B. microevolution - adaptations that evolve in a single gene pool, or population
C. macroevolution - evolutionary change above the species level - appearance of feathers, for example,
that defines higher taxa; broad patterns of evolution
I. Biological species concept emphasizes reproductive isolation
A. species - latin for kind or appearance
B. biological species concept - potential to interbreed and produce viable fertile offspring; reproductive
compatibility (Fig. 3)
1. reproductive isolation - biological factors that impede 2 members of a species from
interbreeding (fig.4)
a. prezygotic barriers impede mating or hinder fertilization of the egg
1. habitat isolation- diff habitats
2. temporal isolation-breed difft times of year or day
3. behavioral isolation- diff courtship rituals
4. mechanical isolation-diff morphology prevents mating
5. gametic isolation- receptors on eggs for sperm
b. postzygotic barriers impede a hybrid zygote from developing into a fertile adult
1. reduced hybrid viability – genes of parents interact in ways that impair devel/surv to healthy viable adult
2. reduced hybrid fertility – mules eg cant repro
3. hybrid breakdown – rice strains – deleterious alleles – F2 gen. happens after F2
Occurs because of incompatible genes of hybrids, but first gen is okay
2. limitations of biological species concept -
a. can’t evaluate fossils or asexual organisms for these traits
b. some species do hybridize, still not the same species
C. other definitions of species
1. morphological species concept - characterizes a species by body shape, size, etc.
a. in practice, this is how most scientists classify species
b. disadvantage - relies on subjective criteria
2. paleontological species concept - focuses on morphologically discrete species known only
in fossil record
a. forced to use this bc there is no way to get more info
3. ecological species concept - if similar in appearance, distinguish by ecological niche
a. two spp. of Galapagos finch similar in appearance but not in niche, what they eat eg.
Accommodates asexual as well as sexually reproducing orgs
4. phylogenetic species concept - set of organisms with a unique genetic history-smallest group forming a branch on tree of life with a common ancestor
a. can use physical characteristics or molecular sequences
b. difficulty may be in determining just how much difference indicates a new species
II. Speciation can take place with or without geographic separation (Fig. 5)
A. allopatric speciation - gene flow is interrupted when a population is subdivided into 2 geographically
distinct subpops (Fig. 6)
1. rivers, canyons, roads may all be formidable enough barriers depending on the species
mobility
2. geographic isolation is NOT by itself a biological isolating mechanism; these must be intrinsic
to the species
B. Sympatric speciation - speciation takes place in geographically overlapping populations; members are
not in isolation from others
1. mechanisms include chromosomal changes and nonrandom mating that reduces gene flow
a subset of the pop. is reproductively isolated bc of switch to a habitat, food source or other
resource not used by parent population eg mosquitoes in bomb shelters, London, WWII
2. polyploidy - sometimes mistakes in cell division result in polyploidy - extra sets of chromosomes
a. if diploid cells become tetraploid, reproductive isolation can occur in one generation;
plants that are tetraploid can either fertilize themselves or with another tetraploid individual
b. when 2 different species mate, chromosomes from one species can not pair correctly
with chromosomes from a different species (homologs)
1. plants may propagate themselves asexually
2. various mechanisms can change subsequent generations to fertile polyploids
this is called an allopolyploid - they can be fertile with each other but not parental
species (Fig. 9) - represent a new biological species
3. many crop plants are allopolyploids
3. Habitat differentiation and sexual selection
a. polyploid speciation also occurs in animals, although it is less common than in plants
b. other mechanism of sympatric speciation - Lake Victoria in E. Africa - fills and dries over
evolutionary time
1. the current lake is only 12000 yrs old, home to more than 500 spp of cichlids
2. all very similar genetically (fig. 10)
a. different fish exploit different food sources
b. sexual selection may be occurring - based on color, breached in lab
setting. species began to diverge only recently. genetic drift may have
played a role; sexual selection reinforces the color difference
4. genetics of speciation - we can pinpoint genes that account for species differences - in mimulus
2 gene loci have been identified as significant in 2 species - one pollinated by bee, one by hummer
one locus influences flower color, one amt of nectar produced-
a. no postzygotic barriers - can be cross-pollinated in greenhouse
b. allelic diversity has resulted in speciation
5. Tempo of Speciation (fig. 13)
a. gradulism model - Darwin - natural selection
b. punctuated equilibrium- apparent stasis punctuated by sudden change
1. may not necessarily be able to discern changes at the molecular level in fossil
record, so it may appear sudden when actually it may have taken 50,000 yrs.
2. also, changes to behavior, physiology or internal anatomy may not be
preserved in the fossil record.
III. large scale changes can accumulate thru many speciation events
A. speciation can occur from changes as small as the color on a cichlid’s back
1. as species diverge, these changes can become more pronounced as they accumulate
2. changes accumulate in macroevolution just as they did in microevolution
a. natural selection,
b. mutation
c. genetic drift
d. gene flow
3. cumulative small evolutionary changes may account for macroevolutionary change
Obj. ch 24
1. Describe eventual outcomes in hybrid zones
2. Define biological species concept.
3. Distinguish between prezygotic and postzygotic isolating mechanisms.
4. Describe five prezygotic and 2 postzygotic isolating mechanisms and give an example of each.
5. Explain a possible cause for reduced hybrid viability.
6. Explain how hybrid breakdown maintains separate species even if fertilization occurs.
7. Describe some limitations of the biological species concept.
8. Define and distinguish among the following: ecological species concept, paleontological species concept, phylogenetic species concept, and morphological species concept.
9. Distinguish between allopatric and sympatric speciation.
10. Explain the allopatric speciation model and describe the mechanisms that may lead to divergence of isolated gene pools.
11. Define sympatric speciation and explain how polyploidy can cause reproductive isolation.
12. distinguish between autopolyploidy and allopolyploidy. Detail an example of how each type could occur.
13. Describe how cichlid fishes may have speciated in sympatry in Lake Victoria.
14. describe 2 models related to tempo of speciation.