Lecture 18 Hybrid Zones, Character Displacement

What are hybrid zones?

Geographic areas where two distinct species or populations meet and interbreed producing hybrids

What can hybrid zones indicate?

Partially reproductively isolated populations that have come back into contact
Speciation in progress ("parapatric speciation")

What is a cline?

A gradual change in a particular trait or allele frequency across a geographic gradient

Why do we care about the shape of the cline in allele frequencies?

It represents a balance between gene flow into the hybrid zone and selection against hybrids and gives us a way to estimate the strength of both these evolutionary forces
Clines can differ between different parts of the genome. Parts of the genome with narrower clines are likely responsible for reproductive barriers.

What is character displacement?

An evolutionary phenomenon where differences among similar species are accentuated in regions where they coexist (sympatry) but are minimized or absent in regions where they do not (allopatry).

What is a ring species?

A connected series of populations that live around a geographic barrier. Adjacent populations can interbreed and exchange genes but the populations at the ends of the ring are so different that they cannot interbreed.

What is geographic variation?

Within species variation in the phenotype or genotype that shows geographic pattern

What are subspecies?

Populations within a species that are geographically separated and exhibit distinct morphological or genetic traits. These differences are consistent enough to distinguish one subspecies from another but the populations can still interbreed where their ranges overlap.

"What is Bergmann's Rule?"

It states that within a broadly distributed taxonomic group populations and species of larger size are found in colder environments while smaller sizes are found in warmer regions

"What is Allen's Rule?"

It states that endothermic animals (warm-blooded animals) living in colder climates tend to have shorter extremities (such as ears; tails; and limbs) while those in warmer climates have longer extremities.

What is something to be cautious about when observing clines?

Direct environmental effects can lead to changes in body size; shape; and coloration as organisms adapt to their surroundings.

"What is Gloger's Rule?"

It states that within a species of animals individuals in humid warm environments tend to have darker pigmentation than those in cooler drier areas.

"How does Gloger's Rules apply to human skin color?"

Variation is presumably adaptive as protection against ultraviolet rays and melanoma and/or prevention of excess production of vitamin D.

What is the take-home message from the classic common garden example?

Populations of this species do show clear evidence of genetic (evolutionary) adaptation to local conditions ("local adaptation")
Each population does best when grown where it came from