SPECIATION

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 productionof new species and increase in the number of species

What are species?

Taxonomic definition: the members of different species haveconsistent differences in a significant number of traits

same species different speciesTaxonomic definition clustered traits divergence of traits

Genetic definition: the members of different species arereproductively isolated

same species different speciesGenetic definition interbreeding reproductive isolation

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, themore isolated they become

How are populations reproductively isolated?

Prezygotic mechanisms prevent fertilization

Different habitatSexual maturation at different seasonsDifferent mating behaviorDifferent sexual organsGamete incompatibility (no sperm-egg fusion)

Postzygotic mechanisms limit hybrid survival or reproduction

Hybrids inviableHybrids sterile (no gonads)Hybrids sterile (abnormal meiosis, incomplete gametes)F2 breakdown

Every natural species distinguished from every other species byone or more of these factors

But how do populations BECOME reproductively isolated?

Geographical isolation (allopatric speciation)

Mutation, recombination,selection in differing environments stabilizedifferent traits, someproducing reproductiveIsolation

Once isolated, populationsdiverge further, givingdifferent genera, etc.

Examples of speciation through reproductive isolation

Fruit flies in the Hawaiian islands

Examples of speciation through reproductive isolation

Darwin’s finches

Examples of speciation through reproductive isolation

Evolution of horses(from fossil evidence)

Sympatric speciation

Polyploidy: change in chromosome number givesImmediate reproductive isolation

Diploid

ChromosomeDuplication,MitosisWithoutCytokinesis

Tetraploid

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)

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

Gradualchange

Punctuatedequilibrium

Flower compatibility may affect the tempo of evolution:

Self compatibility -> genetic constancy -> high speciation, high extinction -> low diversificationSelf incompatibility -> genetic variability -> low speciation, lower extinction -> high diversification

Self-incompatibleSelf-compatible

Science 22 Oct 2010, p. 493

Speciation rate Diversification rate

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

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 patternof increase in species over geologic time? (Right: number of marine genera estimated from fossils in three different ways)