Natural Selection and Evolution of Species
Variation
- needed for natural selection & evolution
- exists between members of same species
- some variations are inherited
- if there was no variation, all individuals of a population would have
exactly the same genes & any change in environment would kill off ALL
individuals (no opportunity for natural selection to occur)
- variation can occur during sexual reproduction due to rearrangement of
genetic material into new combinations through
o independent assortment of chromosomes
all of one set of homologous chromosomes don’t line up on one
side of cell during homologous pairing in first division of meiosis
o crossing over during meiosis
parts of chromatid of one homologous chromosome (mums) swaps
with corresponding part of the paired homologous chromosome
(dads) during first division of meiosis.
o random mating
each egg & sperm has ability to reach maturity & then fertilise
opposite gamete- purely random
- variation can also be caused by mutations
- What causes mutations?
o Can be caused by exogenous (external) or endogenous (native)
factors
o Or may be caused by errors in cellular machinery
o Physical/chemical agents that induce mutations in DNA are called
mutagens & said to be mutagenic
o Exogenous factors: environmental factors like sunlight, radiation,
smoking
o Endogenous factors: errors during DNA replication, toxic byproducts of cellular metabolism (Substitution, Insertion, Deletion,
Frameshift)
Selective pressures in environment which lead to evolutionary change
- predation
- climate change (increases/decreases environmental temperature)
- resource shortage/competition
- disease
- isolating mechanisms:
o geographical
o behavioural
o temporal (time)
o seasonal
o reproductive
o religious (humans)
Selective pressures leading to extinction
- all species extinctions due to species being unable to adapt to changes in
environment
-
these changes are usually rapid and/or long-lasting so natural selection is
too slow to allow individuals to survive & pass on their beneficial genes
increased human population
land clearing/ habitat destruction/ fragmentation
pollution/ acid rain
climate change/ global warming (anthropogenic climate change)
introduced/ invasive species
disease/epidemic
asteroids/ cosmic radiation
Natural Selection
- process where organisms best adapted to environment survive & pass on
beneficial characteristics to offspring
- over time the frequency of favourable phenotypes will increase in
population
- Steps:
o Mutation (most lethal/some beneficial)
o Variation (random union of gametes)
o Change in the environment
o Some individuals have beneficial genes and survive to reproducing
age (most of population is less suited and die out)
o Offspring produced- pass on the beneficial genes
o More of offspring in successive populations contain beneficial
genes
o With same change in environment more of the population survives
- Natural selection leads to change in characteristics of a population e.g.,
pesticide resistance, bird beaks, flowers & pollinators
Artificial Selection & Selective Breeding
- knowing about genes/alleles (through phenotypes) allows you to decide
which individuals to breed together
- you can then use this theory to help improve plants/animals (mainly for
increased yields)
- Steps:
o Choose best plants/animals and breed them together
o Then choose the best animals in their offspring (F1 generation)
o Breed these ones again to give an F2 generation
o Carry this on over many generations until you have the
plant/animal with the best desired characteristics
Speciation/Evolution
- over many generations the allele frequencies in a gene pool can change
- this can lead to the changing population evolving into a separate species
- Steps:
o Separate 1 population into 2 or more populations
o Prevent the genes from one population mixing with the other
o Different environmental changes/selective pressures in each
isolated population
o Natural selection occurs in different direction in each population
(gene pools affected in diff ways)
o Given enough time & continued same changes in environments the
populations continue to experience differentiation in their gene
pools
o Eventually the two gene pools are so different that if they were in
same area they would not be able to interbreed & produce fertile
offspring different species
- Different species- the homologous chromosomes don’t match & cannot
pair during fertilization & no further development can occur (at best an
infertile offspring is produced)
Factors affecting population gene frequencies:
- mutation- sudden & permanent changes in an individuals genome
- natural selection
- migration- new genes/individuals into a population
- random genetic drift
How can natural selection & isolation of a population contribute to the
development of a new species? (SPECIATION)
- Starts off with ancestral population (interbreeding between individuals &
gene flow throughout population, every gene has a homologous matchfertile offspring)
- Change in the environment separating ancestral population into 2 or
more populations
o Isolating mechanisms: geographical, behavioural, temporal,
reproductive, seasonal
- A barrier to gene flow/ no mixing of genes between populations
- In each population the individuals experience different environmental
influences/changes
- Natural selection runs along different paths in each population over
successive years
- Due to natural selection in each of the populations being different each of
populations gene pools change in diff ways
- Genetic differences become so great that if some of the populations came
back together again they would not be able to interbreed to produce a
fertile offspring (no matching of homologous chromosomes)
Evolution
- the change in inherited characteristics of biological populations over
successive generations
- cause diversity at every level of biological organization including species,
individual organisms & molecules like DNA and proteins
- Micro-evolution Natural selection
- Macro-evolution speciation/ totally new looking individuals
Evidence for evolution:
- evolutionary relationships between groups using physiological, molecular
& evidence in phylogenetic trees
- fossils
o superposition
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-
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o complexity of fossils- more advanced at top (newer)
o number of fossils greater at top
homologous structures
o a structural feature that appears in different animals, underlining
anatomical commonalities showing descent from a common
ancestor
pentadactyl limbs
o 5 digits on limbs
vestigal organs
o remnants of organs that were in use in the ancestor but not needed
anymore e.g., tailbone, wisdom teeth
o implies ancestor & evolution
comparative anatomy
o study of anatomical structures of closely related groups of
organisms (shows similarities & changes over time)
comparative embryology of vertebrates
o look same due to common ancestor
comparative biochemistry & genetics
o sequence of bases in common genes in DNA (e.g., less change
between humans & chimps- more closely related)
o hybridization (higher temp of separation of hybridized DNA- more
closely related)
o sequence of amino acids in a protein (similar sequences- closely
related)