Migration

• Evolutionarily, this means the movement of alleles, or gene flow between populations

• This will always be less than the movement of individuals

Migration

• When populations differ in allele frequencies, migration can be a powerful force

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Migration

• Genetic patterns due to migration tend to be temporary

• Over the longer term, migration is a homogenizing force—it makes populations more genetically similar

D.R. Robertson

Migration

• Populations in Central and Eastern Pacific (divided by 5000 km of deep ocean) are not genetically different in these two reef fish species*

D.R. Robertson

*(and 18 out of 20 species studied: Lessios and Robertson 2006)

Geographic variation in natural populations

from Futuyma (1998), p. 259

Often, morphology varies geographically

With low migration, allele frequencies vary greatly over small geographic distances

from Futuyma (1998), p. 319

As in pocket gophers...

1999 R.M. Timm

Geographic barriers to dispersal often separate

genetically different populations

Historical separations between Gulf of Mexico and SE Atlantic drainages from Avise (1994)

from Avise (1994), p. 244

The Florida peninsula is a marine biogeographic barrier

It forms a long-term historical barrier to migration and gene exchange (gene flow) within many species

Natural selection creates patterns of geographic

variation

Clinal variation

from Volpe and Rosenbaum (2000), p. 110

In D. melanogaster, the alcohol dehydrogenase (Adh) locus shows a cline—a regular change in frequency of a trait across a geographic transect

from Futuyma (2005)

The frequency of AdhF decreases towards the equator on 3 continents: these are parallel clines

from Futuyma (2005)

Clinal variation at lactate dehydrogenase-B in the mummichog Fundulus heteroclitus: Ldh-Bb increases towards the north

This allozyme has a higher catalytic efficiency at lower temperatures

Races and subspecies

Races and subspecies

• A race is a geographic population with well defined, discrete differences in one or more traits

• Taxonomically, races may be recognized and named as subspecies

Red-shafted C. auratus cafer

Yellow-shafted C. auratus auratus

Subspecies of Northern flicker (Colaptes)

Color races of Heliconius butterfliesgeographic color races of H. erato

geographic color races of H. melpomene

these races display warning (aposematic) coloration to deter bird predation

Allopatric color races

Each color race of dart-poison frogs (Dendrobaetes) lives on a different island in Bocas del Toro in western Panamá

Most animal color races are allopatric (they occupy non-overlapping ranges)

Parapatric races or subspecies share common borders

from Futuyma (2005)

from Futuyma (1998), p. 258

Parapatric subspecies sometimes interbreed where their borders meet

Hamlets: sympatric races?

• 11-12 forms of Hypoplectrus in Caribbean, Florida, Bahamas

• Morphologically identical, but strikingly different color patterns

• 6 “races” live together on the same reefs, and mate like-with-like

Racial variation

• morphological races

• host races in insects– races feed and oviposit on different host

plants– e.g. apple and hawthorn races of

Rhagoletis

• physiological races

• sex races

Physiological racesVermont: embryos survive 5- 28

New Jersey: 5- 28

South Florida : 11- 33

North Florida : 9- 33

Northern leopard frog Rana pipiens

from Futuyma (1998)

Sex races