Speciation

species

• a group of individuals capable of interbreeding, and reproductively isolated from other groups

• may consist of several populations, all capable of interbreeding, but rarely having the opportunity

• “biological species concept” - uses a biological trait (e.g, ability to interbreed) to define “species”

Problem 1: can you tell members of the same species from appearance alone?

• it can be hard. some species are sexually dimorphic (males and females look different)

• morphology can also be polymorphic (many forms or patterns) all of these are individuals of the same ladybug battle species

Problem 2: Who actually interbreeds?

• we rarely watch. we use clues from morphology but clues can be deceive

• what do paleontologist do? compare differences in fossil records, use aging of fossil,

• people can force some species to interbreed that don’t in nature

• some organisms never reproduce sexually to begin with (e.g, some bacteria, protists, fungi, and insects) these wasp all have a bacterium that causes every member of the species to be female no breeding.

• the more we know (e.g, species range, morphology, genetic information, behavior, etc.) the more confident we can be

• for living organisms, DNA sequences are becoming the tool of choice

How do new species form?

1. isolation/seperation

2. genetic divergence

3. reproductive isolation

Isolation followed by genetic divergence:

1. one population becomes separated into two populations (often by a physical barrier but not always)

2. Each population independently experiences natural selection (may also experience mutation and genetic drift)

3. when/ if the population overlap again, they no longer interbreed

gene flow: usually reduces differences between populations

• think about our two populations separated by a river

• if several individuals were able to fly from one to the other each generation and interbreed, would speciation occur? no

How does the isolation of the population occur?

1. allopatric speciation: a physical barrier arises that prevent interbreeding. allopatric means “different countries”, considered the usual start to speciation

2. Sympatric speciation: no physical barrier arises; interbreeding between groups stops for another reason, sympatric means “same countries”, somewhat controversial but largely accepted

allopatric speciation

allopatric speciation example

Harris antelope squirrel (south rim) and white tailed antelope squirrel (north rim) due to the Colorado river

Sympatric Speciation Temporal isolation: separation in time

example: hawthorn fruit fly

• native to US

• attracted to fruit smell

• lay eggs in (small) hawthorn fruit

• than apples introduced to USA in 1623

• mutated led some lies to be attracted to apples and lay eggs on apples instead

• temporal isolation: separation in time (apples in early summer where hawthorn fruit later in summer)

• 

genetic divergence: thought experiment

1. background: the bee battle is a mimic of bumblebees that feeds on pollen as an adult

2. their larvae live underground and feed on the plant roots

   1. imagine that there is a wide, deep valley in the home range where this battle resides

   2. in the valley between the two populations, a river forms, dividing the beetles into two populations

   3. one population is now in a deeply shaded and very moist forest, while the other is in a dry, exposed, rocky area

Barriers to speciation

1. pre zygotic barriers

   • pre- before, zygotic: relating to the zygote

   • barriers that stop the zygote from forming

   • zygote: an egg cell after fusion of the egg and sperm nucleus (i.e., the first cell of a new organism)

2. post zygotic barriers

   • post: after, zygotic: relating to the zygote

   • occurs after zygote formation

examples for pre : barriers in space or time: individuals never meet, behavioral isolation: females reject their potential mates from the other population/species

examples of post: hybrid inviablilty: mating occurs, but hybrid (2 kinds of parents) and zygotes either don’t develop or die before birth. hybrid sterility: hybrids survive, they are sterile e.g, mule =sterile offspring of horse + donkey

Summary of Speciation

1. one population becomes divided into two and there’s no longer any gene flow

2. the populations become more and more genetically different over time

3. if they come back in contact with each other, they can no longer successfully mate and produce healthy offspring. barriers are a) pre b)post