Approaching Modern synthesis
First stage of synthesis:
Accumulation of more information by “other players”
Others provided additional information
Physiologists
R. Woltereck (1912)
Working with Daphnia (waterflea)
See: Dodson, S. 1989. Predator-induced reaction norms. Bioscience 39:
447-452
Observed cyclomorphogenesis
Seasonal changes in the phenotype of an organism across successive
generations
He noticed that if he maintained clonal lines of this species, they would
exhibit changes in body shape during the year
Headshape changes
Rounded from about midsummer to spring, and thereafter
becomes helmet-shaped, reverting to the rounded shape at midsummer
Note also that tailshape changes – makes Daphnia longer
Also, summer generations tend to be smaller and more transparent than at
other times
What do you think causes this change?
What kind of experiment would you design to figure out what causes this
change?
Important points
Showed that even traits that are environmentally influenced may be
inherited
Introduced concept of Norm of reaction
Same genotype may produce different phenotypes in different environments.
When you fertilize a plant, for example, it will grow larger and more
robust than when you don't.
Ecologists and biosystematists
Lots of debate about the nature of species
Many species observed to vary geographically
Ring species (Rassenkreis)
"race circle".
Seriously confound typological thinking about species
Larus Rassenkreis
The herring gull
(Larus argentatus) and the lesser
black-backed gull (Larus fuscus)
occur together in northern Europe, where they nest in different sites, have
different appearances, and do not interbreed.
As one goes east
or west around the North Pole, around which they form a ring, things get
confusing.
There is a
complete series of intermediate subspecies of one species or the other at each
locality between the two species representing the ends of the chain of
subspecies
Ensatina eschsholtzii Rassenkreis
There are seven subspecies in the species.
The blotched and unblotched forms at the southern end of the ring (blue
zones on map) in southern California
behave like separate species, but there are not any species boundaries between
populations to the north.
See: Moritz C., C.J. Schneider, and
D.B. Wake. 1992. Evolutionary relationships within the Ensatina eschscholtzii complex confirm the ring species
interpretation. Syst. Biol. 41(3):273-291
Subspecies
Different morphs in different geographic locations, but can interbreed
where the morphs overlap
e.g. Elaphis corn snakes in southeastern US
Different morphs of deer mice (Peromyscus
maniculatus) in different locations
e.g. in White Sands of New Mexico
Different ecotypes of plants, such as Achillea
and Potentilla in California
What was nature of species?
How could individual interbreed, yet look so different?
If they interbreed, why doesn’t blending inheritance quickly break down the
differences?
Lee R. Dice (1940)
L.R. Dice, director of the "Laboratory of Vertebrate Genetics"
and discoverer in 1927 of the dunes' "white mice."
Discovered polygenic Mendelian inheritance of continous coat color
variation in mice
Epistatic
(interacting loci) similar (not identical) to those involved in cat coat color
Important because mouse coat color varies geographically, suggesting
geographic variation could be result of traits inherited through Mendelian
factors.
Examples of natural variants
White sands, dark sands
Just as dark-colored mice occur on black lava beds, so do light-colored
mice occur on the white gypsum sands of southern New Mexico. Shown are pocket
mice from two populations of the same species (Perognathus flavescens). The top specimen is from White Sands, New
Mexico; the bottom specimen is from tan, sandy areas of southern New Mexico to
the west.
Mouse coat color genetics
Schematic figure
A schematic of the dorsal/ventral distribution of banded hair vs. solid
hair in the various mouse agouti alleles. Dorsal is the animal's back; ventral
is the animal's belly.
Melanin synthesis pathway
Biosystematists
Clausen, Keck
& Heisey
Achillea species (yarrow)
Within and among population variation in a common garden
Variation across a geographical gradient from sea level at coast to
timberline in Sierra Nevada
