Evolution within a species

•Aims:

•Must be able to state the observations and

subsequent deductions that Darwin and Wallace

based their theories on.

•Should be able to outline the processes involved

in evolution within species.

•Could be able to explain the use of mitochondrial

DNA in tracing species evolution.

Darwin and Wallace

•Charles Darwin (1809 – 1882)

•Alfred Russel Wallace (1823 – 1913)

•Proposed a theory of evolution

based upon natural selection in

1858

•The theory was based on 3 Observations

and Deductions:

Observation 1:

•In the wild, the number of offspring

produced by plants and animals over

their lifetime is greater than the number

of parents.

•Deduction: A struggle for survival occurs.

Observation 2:

•Over time, the size of the natural population tends to

remain fairly constant. (Fluctuations may occur from

time to time due to drought, disease, food supply

etc)

•Deduction: In the struggle to survive, some

organisms have a greater chance of survival than

others. These variations between organisms are

favoured under the conditions in a particular

environment and are reproductively more successful.

Observation 3:

•Variation exists in populations of plants

and animals; that is no two organisms in

a population are identical and some

variations are inherited.

•Deduction: Inherited traits preset in

surviving parents are passed on to their

offspring so that the genetic composition

of populations can change over time

Evolution within a species (speciation)

•Members of one population may be

separated into a number of isolated

populations. Over time:

• The separate populations may be exposed to different

selecting pressures,

•Genetic drift may produce different changes in each population

•Mutation may result in new alleles

Species•A biological species is:

a grouping of organisms that can interbreed and are reproductively isolated from other such groups.

•Species are recognized on the basis of their morphology (size, shape, and appearance) and, more recently, by genetic analysis.

•For example, there are up to 20 000 species of butterfly; they are often very different in appearance and do not interbreed.

•Answer the questions from pages 309

to 310 and 319 to 320 in the Biozone

books.

Activity

Studying populations - mtDNA

•Mitochondrial DNA (mtDNA) is

an important tool for tracing

the evolutionary history of a

species – including humans…

•Structurally identical to

nuclear DNA (double stranded

with complementary base

pairing)

•Why mtDNA?

Reasons for mtDNA:

1. mtDNA

descents via the

maternal line:

• Inherited from the

mother only. All

offspring receive just

one kind of mtDNA

exclusively from the

mother.

Three reasons for mtDNA:

2. Lack of recombination

• mtDNA passes unchanged from a female parent to all of her offspring (i.e.: no recombination as can occur during meiosis).

3. High copy number

• Each mitochondrion contains 2 to 10 mtDNA molecules and each cell has several hundred mitochondria… so many copies of mtDNA (and genes that it carries) are present in each cell. In contrast to only two copies of each autosomal chromosome in each somatic cell.

mtDNA - population differences

•Over time, populations that are geographically isolated accumulate mutations in their mtDNA.

• In the past, human population sizes were small and isolated – and mtDNA mutations began to become established in these groups.

•Members of an indigenous population in one region are characterized by closely related mtDNA sequences – these sequences differ from those present in members of indigenous populations in other regions.

•The longer that two populations have been separated the greater the differences in their mtDNA.

•The distinctive mtDNA sequences found in different populations are known as haplogroups and each designated a capital letter.