Upload
lynette-park
View
215
Download
1
Tags:
Embed Size (px)
Citation preview
Chapter 24:The Origin of Species
Question? What is a species? Comment - Evolution theory must also
explain how species originate.
Question How many species of African Violets are
here?
Two Concepts of Species1. Morphospecies
2. Biological Species
Morphospecies Organisms with very similar morphology or
physical form.
Problem Where does extensive phenotype variation
fit?
Two Schools1. Splitters - Break apart species into new ones
on the basis of small phenotype changes.2. Lumpers - Group many phenotype variants
into one species.
Biological Species A group of organisms that could interbreed in
nature and produce fertile offspring.
Key Points Could interbreed. Fertile offspring.
Morphospecies & Biological Species
Often overlap. Serve different purposes.
African Violets 15+ species 5+ varieties (natural hybrids) 50,000 cultivars
Example Problem S. magungensis Cluster:
S. Magungensis S. magungensis var. minima S. magungensis var. occidentalis
Current Placement One species and two varieties. However :
Not all live in the same habitat. Not all flower under the same temperatures.
Possible New Placement
Two species and one variety. S. magungensis var. occidentalis may be a new
species.
Speciation Requires:1. Variation in the population.
2. Selection.
3. Isolation.
Reproductive Barriers Serve to isolate a populations from other gene
pools. Create and maintain “species”.
Main Types of BarriersPrezygotic - Prevent mating or fertilization.
Postzygotic - Prevent viable, fertile offspring.
Prezygotic - Types1. Habitat Isolation
2. Behavioral Isolation
3. Temporal Isolation
4. Mechanical Isolation
5. Gametic Isolation
Habitat Isolation Populations live in different habitats or
ecological niches. Ex – mountains vs lowlands.
Behavioral Isolation Mating or courtship behaviors
different. Different sexual attractions
operating. Ex – songs and dances in birds.
Temporal Isolation Breeding seasons or time
of day different. Ex – flowers open in
morning or evening.
Mechanical Isolation Structural differences that prevent gamete
transfer. Ex – anthers not positioned to put pollen
on a bee, but will put pollen on a bird.
Gametic Isolation Gametes fail to attract each other and fuse. Ex – chemical markers on egg and sperm fail
to match.
Postzygotic Types1. Reduced Hybrid Viability
2. Reduced Hybrid Fertility
3. Hybrid Breakdown
Reduced Hybrid Viability Zygote fails to develop or mature. Ex – when different species of frogs
hybridize.
Reduced Hybrid Fertility Hybrids are viable, but can't reproduce
sexually. Chromosome count often “odd” so meiosis
won’t work. Ex - mules
Hybrid Breakdown Offspring are fertile, but can't compete
successfully with the “pure breeds”. Ex – many plant hybrids
Introgression Transfer of alleles between two species, but
only a partial gene flow. Result – Some intermixing of genes between
two species.
Importance A possible mechanism for gene flow between
similar species.
Question Actively evolving species like Quercus. Good isolation mechanisms or poor ones? Isolation mechanisms may not have fully
developed yet.
Other Concepts of Species
1. Recognition Species Concept
2. Cohesion Species Concept
3. Ecological Species Concept
4. Evolutionary Species Concept
Recognition Species Concept Species are defined by the ability of the
individuals in the population to recognize certain characteristics in each other.
Ex – mate recognition
Cohesion Species Concept
Emphasizes cohesion of phenotypes (complex of genes and set of adaptations).
Ecological Species Concept Emphasizes a species role or function in the
environment.
Evolutionary Species Concept Emphasizes evolutionary lineages and
ecological roles.
Comment AP exam will focus on morphological and
biological species concepts.
Modes of Speciation1. Allopatric Speciation
2. Sympatric Speciation
Both work through a block of gene flow between two populations.
Allopatric Speciation Allopatric = other homeland Ancestral population split by a geographical
feature. Comment – the size of the geographical
feature may be very large or small.
Example Pupfish populations in Death Valley. Generally happens when a specie’s range
shrinks for some reason.
Another Example
Conditions Favoring Allopatric Speciation
1. Founder's Effect - with the peripheral isolate.
2. Genetic Drift – gives the isolate population variation as compared to the original population.
Conditions Favoring Allopatric Speciation
3. Selection pressure on the isolate differs from the parent population.
Result Gene pool of isolate changes from the parent
population. New Species can form.
Comment Populations separated by geographical
barriers may not evolve much. Ex - Pacific and Atlantic Ocean populations
separated by the Panama Isthmus.
Examples Fish - 72 identical kinds. Crabs - 25 identical kinds. Echinoderms - 25 identical kinds.
Adaptive Radiation Rapid emergence of several species from a common
ancestor. Common in island and mountain top populations or
other “empty” environments. Ex – Galapagos Finches
Mechanism Resources are temporarily infinite. Most offspring survive. Result - little Natural Selection and the gene
pool can become very diverse.
When the Environment Saturates Natural Selection resumes. New species form rapidly if isolation
mechanisms work.
Sympatric Speciation Sympatric = same homeland New species arise within the range of parent
populations. Can occur In a single generation.
Plants Polyploids may cause new species because
the change in chromosome number creates postzygotic barriers.
Polyploid Types
1. Autopolyploid - when a species doubles its chromosome number from 2N to 4N.
2. Allopolyploid - formed as a polyploid hybrid between two species. Ex: wheat
Autopolyploid
Allopolyploid
Animals Don't form polyploids and will use other
mechanisms.
Gradualism Evolution Darwinian style evolution. Small gradual changes over long periods time.
Gradualism Predicts: Long periods of time are needed for
evolution. Fossils should show continuous links.
Problem Gradualism doesn’t fit the fossil record very
well. (too many “gaps”).
Punctuated Evolution New theory on the “pacing” of evolution. Elridge and Gould – 1972.
Punctuated Equilibrium Evolution has two speeds of change:
Gradualism or slow change Rapid bursts of speciation
Predictions Speciation can occur over a very short period
of time (1 to 1000 generations). Fossil record will have gaps or missing links.
Predictions New species will appear in the fossil record
without connecting links or intermediate forms.
Established species will show gradual changes over long periods of time.
Possible Mechanism Adaptive Radiation, especially after mass
extinction events allow new species to originate.
Saturated environments favor gradual changes in the current species.
Comment Punctuated Equilibrium is the newest
”Evolution Theory”. Best explanation of fossil record evidence to
date.
Origin of Evolutionary Novelty How do macroevolution changes originate? Several ideas discussed in textbook.
Exaptation Heterochrony Homeosis
Exaptation
When a structure that was adapted for one context is co-opted for another function.
Ex. – feathers and flying
Heterochrony Changes in the timing or rate of development.
Allometric Growth Paedomorphsis
1. Allometric Growth – changes in the relative rates of growth of various parts of the body.
Ex. – skull growth in primates
2. Paedomorphosis – when an adult retains features that are present in the juvenile form.
Ex. – gills in adult salamanders
HomeosisChanges in the basic body design or arrangement
of body parts.
Ex. – Hox gene clusters that gave rise to vertebrates from invertebrates.
Evolutionary Trends Evolution is not goal oriented. It does not
produce “perfect” species.
Future of Evolution ? Look for new theories and ideas to be
developed, especially from new fossil finds and from molecular (DNA) evidence.
Summary Be able to discuss the main theories of what is
a “species”. Know various reproductive barriers and
examples.
Summary Know allopatric and sympatric speciation. Be able to discuss gradualism and punctuated
equilibrium theories.
Summary Recognize various ideas about the origin of
evolutionary novelties.