Transcript Biology 4.24 Evolution Within a Species

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.
Activity
•Answer the questions from pages 309 to 310
and 319 to 320 in the Biozone books.
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.