G. RANDOM CHANGE

IN A

POPULATION

Evolution happens with populations, not individuals

We say that a population has evolved when it is different from the original population in its allele frequency or genetic make-up

This difference in genetic make-up results from specific alleles being passed on to the next generation in a non-random pattern

1. Genetic Drift

in small populations, alleles can be lost at random from the population

eg. if there are two populations each with a rare allele at a frequency of 1%

in one population of 50,000 [500 have it] and in another population of 500 [5 have it]

if some random event kills 20 % of the population it may kill the 5 who have it, but is unlikely to kill all of the 500 in the larger population

1b. Bottleneck Effect

this is a large, usually temporary, reduction in the population that usually results in a significant genetic drift

eg. there were once 100,000 elephant seals but we hunted them

by the 1890’s there were only 200 elephant seals

now there are 30,000 elephant seals and all are descended from the 200 so much genetic diversity was lost

eg. cheetahs were once captured by Romans to use as hunting pets

they killed so many cheetahs capturing them, that the population went through a bottleneck

now if we do skin transplants among these cheetahs,

all East Africa cheetahs accept foreign tissue which implies limited diversity

with South African cheetahs, we see lots of rejection implying diversity

1c. The Founder Effect

this results when a small population colonizes a new area,

again causing a limited number of alleles to be present which is a form of genetic drift

eg. in the 1971 five adult pairs of lizards were moved to a new island in the Mediterranean

Italian wall lizard

the old island had sparse vegetation and many insect species to eat

the new island has dense vegetation and many fruits species to eat

the lizards have been on the island for only 36 years but now they have larger heads with bigger chewing muscles

also an enlarged pouch in their intestine for bacteria that break down cellulose

2. Gene Flow

often animals live in isolated colonies

however, once a year, males are driven out of the home colony and move to a new colony [eg lions]

depending on the species, females may be driven out instead [eg bees]

in either case, these individuals bring their alleles with them

and will likely alter the genetic balance in the new colony

Published online: 28 April 2007; Climate change alters genes on the fly

Global warming is influencing the genetics of fruitfly populations.

Hoffman's team sampled the flies in 2006, and found that the distribution of AdhS had shifted some 400 kilometres south from where it was two decades earlier.

The 'S' version of Adh seems to encourage survival in hot, dry conditions.

Therefore we can see that global climate change can directly affect the alleles in a population.

3. Mutation

the random process of making changes in alleles themselves

most mutations appear to be neutral [no visible change] or harmful,

but a very few will be beneficial

Mutation is not random!

Genes that contains the sequence TATA in its promoter

are more likely to have evolved than that of a genes that do not have TATA in its promoter.

3b. Gene Duplication

a segment of a chromosome or a single gene is accidentally duplicated during DNA replication

this duplication has no immediate effect, positive or negative

the organism now has a spare copy of a gene

so that when a change is made, in the duplicate a working copy remains

if the change produces a less useful protein, the organism still has the original and so does not suffer

if the change produces a more useful protein, then organism directly benefits

Possible benefits of gene duplication

On to selection

Choanoflagellates are single-celled zooplankton that diverged from animals 500 milions years ago.

They have collagen, integrin and cadherin domains, though they have no skeleton or matrix binding cells together;

and proteins called tyrosine kinases that are a key part of signaling between cells, even though Monosiga is not known to communicate, or at least does not form colonies.

They have five immunoglobulin domains, though they have no immune system;