Keystone Evolution Evolution by Natural Selection Struggle for
Keystone Evolution
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
The Process of Speciation • Species - group that can interbreed and produce fertile offspring. • Speciation – the formation of new species, occurs whenever reproductive isolation develops
The Process of Speciation Reproductive isolation develops by: • Behavioral Isolation • Geographic Isolation • Temporal Isolation
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.
The Process of Speciation 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.
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.
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.
Evolution of Populations
Genes and Variation 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.
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
Evolution as Genetic Change in Populations 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
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
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
Genes and Variation 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
Stabilizing selection
Directional selection
Disruptive selection
- Slides: 28