Speciation is the process by which new species arise. It occurs when populations become reproductively isolated from one another through mechanisms that prevent interbreeding, such as geographical isolation, differences in mating behaviors or seasons, or changes in chromosome numbers. Reproductive isolation allows populations to diverge genetically as new traits evolve in response to different environmental pressures without gene flow between them. The fossil record shows that new species tend to appear and disappear abruptly rather than through gradual change over time, fitting a pattern of punctuated equilibrium more than constant evolution.

1. SPECIATION

2. How do new species arise?
Note that mutation, recombination, and selection change the
average characteristics of a population
This provides for “evolution” as adaptation
But it does not by itself explain “speciation,” the production
of new species and increase in the number of species

3. What are species?
Taxonomic definition: the members of different species have
consistent differences in a significant number of traits
same species different species
Taxonomic definition clustered traits divergence of traits
Genetic definition: the members of different species are
reproductively isolated
same species different species
Genetic definition interbreeding reproductive isolation

4. Genetic speciation implies taxonomic speciation
For interbreeding populations, any new allele (trait),
generally small, will spread throughout the population;
clusters of new alleles will tend to disperse randomly;
differences will be gradual
For non-interbreeding-- “reproductively isolated” --
populations, alleles will not be shared; differences in
new alleles and clusters of new alleles will accumulate;
populations will diverge; the more they diverge, the
more isolated they become

5. How are populations reproductively isolated?
Prezygotic mechanisms prevent fertilization
Different habitat
Sexual maturation at different seasons
Different mating behavior
Different sexual organs
Gamete incompatibility (no sperm-egg fusion)
Postzygotic mechanisms limit hybrid survival or reproduction
Hybrids inviable
Hybrids sterile (no gonads)
Hybrids sterile (abnormal meiosis, incomplete gametes)
F2 breakdown
Every natural species distinguished from every other species by
one or more of these factors

6. But how do populations BECOME reproductively isolated?
Geographical isolation (allopatric speciation)
Mutation, recombination,
selection in differing
environments stabilize
different traits, some
producing reproductive
Isolation
Once isolated, populations
diverge further, giving
different genera, etc.

7. Examples of speciation through reproductive isolation
Fruit flies in the Hawaiian islands

8. Examples of speciation through reproductive isolation
Darwin’s finches

9. Examples of speciation through reproductive isolation
Evolution of horses
(from fossil evidence)

10. Sympatric speciation
Polyploidy: change in chromosome number gives
Immediate reproductive isolation
Diploid
Chromosome
Duplication,
Mitosis
Without
Cytokinesis
Tetraploid

11. Examples of polyploidy as speciation:
Tobacco
Nicotiana otophora x Nicotiana sylvestris
2n = 12 2n = 12
Nicotiana tabacum
2n = 24
(occurred naturally, but verified experimentally)
Rado-cabbage
radish (Raphanus sativus) x cabbage (Brassica oleracea)
2n = 18 2n = 18
“radocabbage”
2n = 36 (radish head, cabbage root)
(experimental, but shows possibility of intergeneric speciation)

12. The tempo of evolution: is evolution gradual?
Fossil evidence shows :
Species morphology is stable over long periods
Species appear and disappear abruptly (in evolutionary time)
“Punctuated equilibrium” may describe life history better than “evolution”
Morphological trait Morphological trait
time
Gradual
change
Punctuated
equilibrium

13. Flower compatibility may affect the tempo of evolution:
Self compatibility -> genetic constancy -> high speciation, high extinction -> low diversification
Self incompatibility -> genetic variability -> low speciation, lower extinction -> high diversification
Self-incompatibleSelf-compatible
Science 22 Oct 2010, p. 493
Speciation rate Diversification rate

14. Summary
Evolution of a population does not itself imply the formation of species
Speciation occurs through reproductive isolation
Reproductive isolation can occur through geographical isolation
Tetraploidy is one method of sympatric isolation
The appearance of new species, observed through study of fossils,
occurs in a “punctuated equilibrium” pattern

15. Other questions:
Are there any cases of gradual
phenotypic change (instead of
punctuated equilibrium)?
Are there alternatives to selection
that can explain the pattern
of changes in fossil samples?
(Raup: the changes are not
significantly different from random;
also consider genetic drift)
What is the best way to sample
fossils to estimate the pattern
of increase in species over geologic
time? (Right: number of marine
genera estimated from fossils in
three different ways)