Chapter 5Evolution 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 area2.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) = indextotal 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. Microevolution – below the species level –

example: different breeds of dogs2. Macroevolution – at the species level (speciation) –

example: domestic dogs versus African wild dogs

Why does evolution happen?

Genetic diversity leads to evolutionGenes (genotype) control physical traits (phenotype)There is genetic diversity within a population. This is due to two factors working together:1.Mutations2.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 selection2.Natural selection3.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. Individuals produce an excess of offspring (overproduction)2. Not all offspring can survive (competition)3. Individuals differ in their traits (genetic diversity)4. Differences in traits can be passed on from parents to offspring5. 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 slow2. The population has high genetic variation for new traits to

be selected3. The population is relatively small so that a beneficial

mutation can spread quickly4. 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