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Unit 1: Evolution
chapter 22: descent with modification
chapter 25.2 & 3 & 4 : history of life on earth
chapter 23: evolution of populations
chapter 24 and 25.6 : origin of species
chapter 26.1 and 26.3: phylogeny
Chapter 24: Origin of Species
How do we know that certain
organisms belong to the same or
different species?
• microevolution = change in allele frequencies in a population (caused by
mutation, natural selection, genetic drift and flow, etc.)
• macroevolution = broad pattern of evolution above species level
– Results in origin of new groups (e.g., mammals, flowering plants)
through a series of speciation events
• speciation =
– process by which one species splits into two or more species
– bridge between micro- and macroevolution
What is a species?
Biological species concept:
A species is a group of populations whose members have the potential to
interbreed in nature and produce viable, fertile offspring. The focus is on
reproductive compatibility.
There are other species concepts (morphological, ecological, phylogenetic,
etc.) that emphasize other aspects of species identity. (pg. 504)
How are new species formed or maintained?
• With gene flow…
– populations interbreed, share alleles
– maintains a single species
• Without gene flow...
– populations no longer exchange alleles
– genetic changes occur independently (genetic isolation)
• Speciation requires reproductive isolation
(according to biological species concept).
– barriers to interbreeding (prezygotic and postzygotic)
– blocks gene flow
– maintains separate species
Reproductive barriers (Fig. 24.3)
prezygotic barriers
• habitat isolation
Prevent fertilization by:
- Impeding the act of mating
- Preventing attempted mating from
being completed successfully
- Specifically hindering the occurrence
of fertilization if mating is completed
– same geographic region, different habitat
water-bound
terrestrial
• temporal isolation
– breed at different times
- Impede the act of mating
- Prevent attempted mating from
being completed successfully
- Specifically hinder the occurrence of
fertilization if mating is completed
• behavioral isolation
– different courtship rituals
• mechanical isolation
– mating is attempted but not completed
- Impede the act of mating
- Prevent attempted mating from
being completed successfully
- Specifically hinder the occurrence of
fertilization it mating is completed
• gametic isolation
– sperm cannot fertilize egg
- Impede the act of mating
- Prevent attempted mating from
being completed successfully
- Specifically hinder the occurrence of
fertilization it mating is completed
postzygotic barriers
Mating attempt completed successfully
and fertilization achieved; prevent
hybrid from surviving/thriving and/or
reproducing
• reduced hybrid viability
– offspring of members of different species have
lower survival
• reduced hybrid fertility
– offspring of members of different species are
sterile
male
donkey
female
horse
sterile mule
• hybrid breakdown
– first generation between two species is viable
and fertile
– second generation is not viable and fertile
“Biological species concept” has limitations…
• fossils
• asexual organisms
• Some species are distinct despite (very limited) gene flow.
More than 20 species definitions exist, including…
Morphological species concept
• distinguishes species by body structure
• works for asexual and sexual species
• actually used more in practice
• very subjective
Ecological species concept
• distinguishes species by organismal niche in environment
• works for asexual and sexual species
Phylogenetic species concept
• distinguishes species by the smallest group of individuals which
share a common ancestor
• a species represents one branch of the tree
• determining degree of difference can be complicated
Speciation can happen with geographic separation…
• allopatric speciation
– a population is divided into geographically isolated populations
– gene flow is limited or eliminated
What is the process?
• Start with one species, one population
• Geographic separation occurs.
• Gene flow stops.
• One species, two populations
• Genetic change occurs independently in each population.
– different mutations
– natural selection
– genetic drift
• If the genetic changes lead to the establishment of reproductive isolation
mechanisms in these populations, a speciation event can occur.
• Note: Geographic isolation alone is not enough for speciation. Genetic
changes in the isolated populations must give rise to reproductive isolation in
order for speciation to occur.
Example of allopatric
speciation following the
rise of the Panamanian
land bridge
Speciation can happen without geographic separation…
• sympatric speciation
– occurs in populations that live in the
same area
– less common
– gene flow is reduced by:
• polyploidy
– cell division accident results in
additional set of chromosomes
– more common in plants
– oats, cotton, potatoes, tobacco and wheat
– can be created in labs
• habitat differentiation
– a subpopulation is able to exploit
a new habitat or resource
• sexual selection
Fig. 24.14: What happens if species with incomplete
reproductive barriers come into contact with one another?
How quickly does speciation occur?
A survey of data from 84 groups of plants and animals gives us
some insight into the total time between speciation events:
– minimum: 4,000 yrs (cichlids)
– maximum: 40 million yrs (some beetles)
– average: 6.5 million yrs
and rarely less than 500,000 years
This has implications for mass extinction recovery times…
Evolution is not goal oriented and
cannot create perfect organisms.
• Natural selection cannot create “perfect” organisms!
– Adaptive evolution ≠ perfectly “engineered” match between environment
and organism
– Adaptive evolution = good (not perfect) fit; dynamic process!
• Evolution “tinkers” via gradual modifications of existing structures
and variations, and given enough time, these gradual
modifications can produce significant changes (i.e., new species)!
• Natural selection and mutation are not predictive.
– can only improve a structure in current environment
– no anticipation of future advantageous adaptations
– no “on demand” system of generating novel, useful alleles
(must work with what’s available; limited by historical
constraints)
• Adaptations are often compromises.
• Chance, natural selection, and the environment interact.
Complex structures can arise (via evolution) from
simpler forms.
Limpet (simple mollusc)
mollusc Pleurotomaria
Nautilus
marine snail
squid