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.