Modern evolutionary theory, also known as neo-Darwinism, explains how evolution occurs through natural selection acting on genetic variations in populations over multiple generations. Natural selection favors individuals whose genetic variations increase their chance of survival and reproduction, causing gene frequencies in populations to change over time. With enough genetic change over long periods, new species may form. Evidence for evolution comes from fossils that can be carbon dated, showing lineages of organisms changing over time, as well as similarities in cell biology and genetics between related species.

1. MODERN THEORY
OF EVOLUTION

2. MODERN THEORY OF
EVOLUTION (NEO-
DARWINISM)
 Includes scientific evidence from genetics
 When the allele frequency of a population
changes over successive generations
 When the changes are great enough, a new
species may be formed
 A species is a group of individuals capable of
interbreeding and producing fertile offspring

3. CHANGES IN EVOLUTION DUE
TO NATURAL SELECTION
 A great many offspring are produced but not all of
them survive – population sizes stay fairly constant
 ‘Survival of the fittest’ – struggle for life for limited
resources e.g. food, avoiding predators and disease,
tolerance to changes in environmental conditions e.g.
temp
 Individuals best adapted to their environment have a
selective advantage – they are more likely to survive and
produce offspring

4. EVIDENCE FOR THE THEORY
OF EVOLUTION – DARWIN
SAID……
 Each species living today arose from a pre-
existing species
 All species have evolved from one ancestral type
 Natural selection provides a mechanism for one
species to change into another

5. FOSSILS
 K-Ar dating (potassium is found in many rocks and it
decays to argon very slowly)
 Can date back 3000 million years
 Carbon dating is used on younger fossils (up to 50 000
years old) because it decays faster than potassium
 After dating, fossils are put into chronological order
 This gives an idea of how one group of organisms
evolved into another

6. PROBLEMS FROM FOSSIL
EVIDENCE
 Fossilisation is rare because most organisms
decay
 Better evidence comes form cell biology and
biochemistry - by looking at the genetic code
and physiological processes e.g. respiration
 These are similar in closely-related organisms

7. NATURAL SELECTION
 ‘Fitness’ is the ability of an organism to pass on
its alleles to subsequent generations, compared
to other individuals of the same species
 The ‘fittest’ individual is one that produces large
numbers of offspring that survive to reproduce
themselves
 The genetic characteristics of a population
gradually changes from generation to generation
in response to changes in the environment

8. NATURAL SELECTION
AFFECTS THE GENE POOL
 Gene pool is all of the genes and their alleles
present in a species
 There is an increase in the frequency of alleles
that give an advantage, and a decrease in the
frequency of alleles that do not give an
advantage

9. GENE MUTATIONS AND
EVOLUTION
 Gene mutations are random events that result in a new
allele
 Most are harmful
 Occasionally, one provides a new phenotype that has a
selective advantage over other individuals in a
population
 Over generations, the frequency of mutant alleles may
increase (= micro-evolution) e.g. antibiotic resistance in
bacteria; pesticide intolerance in insects

10. GENE MUTATIONS AND
EVOLUTION continued
 Changes at a higher level than species level is
called macro-evolution
 This results in the formation of higher
taxonomic groups e.g. new genera or classes.