Transcript Chapter 5 - Evolution of Biodiversity

Chapter 5
Evolution of Biodiversity
What is biodiversity?
Three different scales – all three contribute to the overall
biodiversity of Earth
1. Ecosystem diversity – the variety of ecosystems
within an area
2. Species diversity – the variety of species within a
particular ecosystem
3. Genetic diversity – the variety of genes within a
particular species
Calculating biodiversity
• Species – a group of organisms that is distinct from other
groups in terms of size, shape, behavior, and biochemical
properties, and that can interbreed and produce viable
offspring
• The number of species on Earth is difficult to estimate
• Species are not evenly distributed on Earth
Calculating biodiversity…
For local or regional ecosystems we use two measures:
1. Species richness – the number of species in a given
area
2. Species evenness – the relative proportions of
individuals within the different species
Species richness and evenness often decline after a human
disturbance – it is helpful to know the baseline
Higher numbers mean more diversity
In most cases higher diversity = greater stability
Calculating biodiversity…
A simple biodiversity index works this way:
number of species in the area (numerator)
= index
total number of individuals in the area ( denominator)
Examples:
1. A 4x4 meter square area in a carrot patch has 300 carrot
plants, all the same species. Biodiversity index = ?
2. A 4x4 meter square area in the forest has 1 pine tree, 1 fern,
1 oak tree, 1 moss, and 1 lichen, for a total of 5 different
species and 5 individuals. Biodiversity index = ?
What causes biodiversity?
Evolution
•
A change in the genetic composition of a population over time
Levels of evolution:
1.
2.
Microevolution – below the species level –
example: different breeds of dogs
Macroevolution – at the species level (speciation) –
example: domestic dogs versus African wild dogs
Why does evolution happen?
Genetic diversity leads to evolution
Genes (genotype) control physical traits (phenotype)
There is genetic diversity within a population. This is due to two
factors working together:
1. Mutations
2. Genetic recombination
Mutation
• Mistakes in copying of a gene
• Can be caused by environmental factors (example: UV
light, some environmental chemicals - carcinogens)
• If it occurs in a body cell, it will only affect that organism;
only mutations that occur in a sperm or egg cell can be
passed on to offspring
• Most mutations are harmful – they may cause sever
illness or death. Even if they do not cause direct death
they may make the organism less likely to survive (ex:
color change that makes them stand out to predators)
• A mutation that increases the organism’s chances of
survival may lead to evolution (‘survival of the fittest’)
Genetic recombination
• During meiosis, chromosomes ‘trade’ sections in a process
called crossing over. This creates new combinations of genes.
• During sexual reproduction, new combinations of
chromosomes passed to the offspring from the parents
So how does evolution happen?
Three ways:
1. Artificial selection
2. Natural selection
3. Random processes
Artificial Selection
• Humans determine which individuals breed, typically with a
preconceived set of traits in mind
• Examples:
• Breeds of dogs, horses, cattle
• Unintended consequences:
• Antibiotic resistant bacteria
• Pesticide resistant insects
• Modern science:
• Genetic engineering techniques
• We now have the ability to take DNA from totally different species
and create GMOs – genetically modified organisms
• Inserting a bacterial gene that is a natural insecticide into crop plants
Natural Selection
•
•
Charles Darwin: On the Origin of Species by Means of
Natural Selection (1859)
Key ideas:
1.
2.
3.
4.
5.
•
•
Individuals produce an excess of offspring (overproduction)
Not all offspring can survive (competition)
Individuals differ in their traits (genetic diversity)
Differences in traits can be passed on from parents to offspring
Differences in traits are associated with differences in the ability
to survive and reproduce
Natural selection does NOT select for specific traits that tend
toward a predetermined goal
Natural selection favors any combination of traits that
improves an individual’s fitness – the traits are adaptations
Nonadaptive (random) evolution
• Genetic composition changes over time, but it is not related to
differences in fitness
• Mutation – random changes can lead to different composition
over time
• Genetic drift – in small populations, random mating and
mutation can cause the composition to shift in one direction,
making it different from the original population.
• Bottleneck effect – a drastic reduction in the size of a
population can cause following generations to reflect a
different genetic makeup than the original population.
• Founder effect – a small subset of an original population
becomes isolated and the subsequent population is more like
those founders and less like the original, more diverse,
population.
Speciation
• Once ‘enough’ genetic variation exists, a population becomes
a different species – this is macroevolution
• Geographic isolation – two populations become separated by
some sort of physical barrier. Over time, genetic drift
combined with the founder effect can lead to …
• Reproductive isolation – two populations can no longer
interbreed, which means they are now two different species
The pace of evolution
• A species can adapt better to an environmental change if:
1. The rate of the environmental change is relatively slow
2. The population has high genetic variation for new traits to
be selected
3. The population is relatively small so that a beneficial
mutation can spread quickly
4. The population’s generation time is short
Evolve or die
• Species that cannot adapt to changing environmental
conditions will eventually go extinct
• 99% of all species that have ever lived on Earth are now
extinct
• 5 global mass extinctions have occurred in Earth’s history,
mainly caused by drastic changes in the environment
• Currently we are in the middle of the 6th global mass
extinction – this one caused by human activity