Approaching Modern synthesis

First stage of synthesis: 

Accumulation of more information by “other players”

Others provided additional information


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


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


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

Variation among clones from a population (San Gregorio, coast) single population being grown in gardens at three locations

Variation among clones from a population (Groveland, foothills) single population being grown in gardens at three locations
Tenaya Lake, Yosemite, population


Beliefs of “other players”

Only one kind of variation

Large mutations and slight individual variants are extremes of a continuum

Not all mutations deleterious

Some neutral, some beneficial

Genetic material invariant

No “soft” inheritance


Single gene can influence multiple characters

Recombination the most important source of genetic variance in populations

Continuous phenotypic variation does not conflict with particulate inheritance

Can be explained as result of multiple factors (polygenes) together with epistasis

Selection is effective force for evolutionary change

Natural selection was easy extension from artificial selection

These ideas paved way for first wave of Evolutionary Synthesis

But before going there, we will explore what was happening prior to the synthesis regarding the study of discretely distributed (Mendelian) traits