Evolution by Natural Selection

• Struggle for Existence– Members of each species compete for food,

space, and necessities of live.– Predators that can run faster catch more prey– Prey that are better camouflaged avoid being

caught.

Evolution by Natural Selection

• Survival of the Fittest– Survival depends on how well-suited an

organism is to its environment– Fitness: ability to survive and reproduce– Adaptation: any inherited characteristic that

increases an organism’s chance for survival.– Darwin called survival of the fittest natural

selection– Underwater Astonishments

Evidence of Evolution• Fossil Record

– Diff. layers of rock form a different times in the Earth’s history.

– Fossils show gradual change over time.

Evidence of Evolution• Biogeography

– Finches found all over the world evolved different adaptation to survive in their habitat.

– Finches on the Galapagos evolved from a mainland species.

– Darwin found that animals in similar habitats evolved similar characteristics.

Evidence of Evolution• Homologous Body Structures

– Structures that develop from the same embryonic tissue but have different mature forms.

– Suggest evidence that organism descended with modification from a common ancestor.

Evidence Evolution

• Similarities in Early Development– Early embryo stages of

many vertebrates look very similar.

– The same groups of embryonic cells, develop in the same order, in similar patterns, to produce homologous structures.

Significance of biochemical differences

• Vestigial structures – structures that have no function in the living organism but may have been used by its ancestors.

• Ex: human appendix, python leg bones

•DNA evidence – the more similar the genes are, the more closely related organisms are thought to be.

•Direct observation – We’ve seen evolution occur in cases like:

•Bacteria becoming resistant to antibiotics•Insects that become resistant to pesticides

• Species - group that can interbreed and produce fertile offspring.

• Speciation – the formation of new species, occurs whenever reproductive isolation develops

17.3 The Process of Speciation

Reproductive isolation develops by:

• Behavioral Isolation

• Geographic Isolation

• Temporal Isolation

17.3 The Process of Speciation

Behavioral Isolation• Two populations are capable of interbreeding• Differences in mating rituals prevent interbreeding• Ex: Similar birds will not interbreed b/c of

different mating songs

Geographic Isolation

• 2 populations are physically separated by barriers– Rivers– Mountains– Bodies of water

Temporal Isolation• Two or more species reproduce at different

times

• Ex: One form of cicada emerges every 17 years, the other emerges every 13 years.

How do new species evolve?

1. Founders arrive – A few finches travel from South America to one of the islands. There, they survive and reproduce.

2. Geographic Isolation – Some birds from species A cross to a second island. The two populations no longer share a gene pool.

17.3 The Process of Speciation

3. Changes in the Gene Pool – Seed sizes on the second island favor birds with larger beaks. The population on the second island evolves into a population, B, with larger beaks.

4. Behavioral isolation – If a few population-B birds cross back to the first island, they will not mate with the birds of population A. The gene pools are now separate. Populations A and B are separate species.

17.3 The Process of Speciation

5. Ecological Competition – As species A and B compete for seeds on the first island, they continue to evolve. A new species, C, may evolve.

6. Continued Evolution – The process continues, leading to the formation of all 13 finch species on the Galapagos.

17.3 The Process of Speciation

Chapter 17: Evolution of Populations

Populations and Gene Pools• Population – a group of individuals of the same species

that mate and produce offspring• Gene pool – all genes and the alleles for those genes

present in a population• Allele frequency – the number of times an allele occurs in

a gene pool compared to the total number of alleles in that pool for the same gene.

17.1 Genes and Variation

Populations and Gene Pools• Take home message: Evolution, in genetic terms, involves

a change in the frequency of alleles in a population over time.

• Note: Although natural selection acts on individuals it is the population that evolves, not individuals

17.1 Genes and Variation

Genetic Drift• Genetic Drift = random change in allele frequency• Acts on small populations• These chance occurrences can cause an allele to

become more or less frequent in a population

17.2 Evolution as Genetic Change in Populations

Genetic Bottlenecks• Genetic Bottleneck = a change in allele frequency

following a dramatic reduction in the size of a population

• Could result from disease, rapid climate or environmental change

• Can reduce a populations genetic diversity

17.2 Evolution as Genetic Change in Populations

Founder Effect• Founder Effect = when allele frequencies changes as

a result of the migration of a small subgroup of a population

• Creates a new gene pool that could be different from the parent population

17.2 Evolution as Genetic Change in Populations

Single-Gene and Polygenic Traits• Single-Gene Trait

– Controlled by one gene that has two alleles

– Two distinct phenotypes

– Ex: bands or no bands on snails

• Polygenic Trait– Controlled by more

than one gene

– Many possible genotypes and phenotypes

– Ex: Human Height

17.1 Genes and Variation

Stabilizing selection

Directional selection

Disruptive selection