The Extinction Vortex
Who gets hit first by extinction?
What are background extinction
rates?
• Vary, for different groups of organisms
• Species longevities range from 2-10
million years (4 my ave.)
• Marine invertebrates: 2-5 families lost
every million years
• …but many other species not found in
fossil record must appear & disappear at
even higher rates…
Mass Extinction Events:
What makes species susceptible to
extinction?
• What is the ultimate thing that makes a
species more likely to go extinct?
• Small population size
• In other words, rare species are the most
at risk
What does it mean to be a “rare
species?”
• Describe 3 kinds of rarity
• Wilson uses 3 bird species to illustrate the
“layers” of rarity (p. 228-229)
• Each group member review & describe
what makes each of these three birds rare:
– Bachman’s warbler
– Kirtland’s warbler
– Red-cockaded woodpecker
Types of Rarity
• Show the distribution on an imaginary
“range map” for each species
• Compare among species…
• How do species get to be so rare in the
first place?
• Be prepared to report back to class
What are the main causes of rarity?
• Specialization (e.g. red-cockaded woodpecker)
• Small geographic range (endemic species) (e.g.
Kirtland’s warbler)
• Sparse population within range (e.g. Bachman’s
warbler)
• Large body size (= small population size)
• Top predators always rare – nature of food
chains & energy transfer
• Chance events can isolate a population (…this
may lead to speciation…)
What are two kinds of chance
events that can isolate a
population?
• Most of individuals die/are destroyed
somehow (disease, cataclysmic event…)
– This is called the “Bottleneck Effect”
• A small group of individuals gets
separated from the rest – e.g. birds blown
off course during migration
– This is called the “Founder Effect”
What does it do to the species,
when population size gets very
small?
• Each group get a bag of M&M’s (don’t eat
them yet!!)
• For our purposes, assume that M&M color
represents individual phenotype/genotype
• Is there “genetic” variation in your
population?
• Are all genotypes equally represented?
What happens when population
size gets very small?
• In the worldwide “population” of M&M’s,
the relative abundance of colors is:
– brown:orange:yellow:green:blue:red
–
6 :
6 : 3 : 3 : 3 : 1
• If your bag (small population) of M&M’s
was a perfect representation of this color
mix, and it had a total of 110 M&M’s, how
many of each color would you expect to
have?
What happens when population
size gets very small?
• How many M&M’s of each color would
you expect to have?
• How many do you actually have?
• Has there been a change in “gene
frequency” in your smaller population?
What happens when population
size gets very small?
• Has there been a change in “gene
frequency” in your smaller population?
• NOW – someone in your group should
close their eyes, and take a random
sample of 10 M&M’s from the bag.
• What is the color distribution now?
• Now has there been a change in gene
frequency??
What happens when population
size gets very small?
• Has there been a LOSS of “genetic”
diversity??
• Usually the color distribution in your new,
very small population, is very different than
it was in the whole population, & some
colors are missing
• This phenomenon of random change in
gene frequency & loss of diversity is called
“random genetic drift”
What happens when population
size gets very small?
• Two genetic problems arise in very small
populations:
• One is genetic drift, as we have just seen
with the M&M’s (random loss of genetic
diversity, just based on who survives &
leaves offspring)
• The other is…
• Inbreeding!
Problems at the genetic level in
small populations
• These two problems for small populations
lead to a phenomenon known as…
• “The Extinction Vortex”
The Extinction Vortex: