Mechanisms of evolution Natural selection acts on the

Mechanisms of evolution • Natural selection acts on the range of phenotypes in a population. • Phenotypes are largely determined by genotypes.

Populations, not individuals, evolve Individuals cannot acquire new phenotypes within their lifetimes and pass on those acquired traits to offspring

Populations, not individuals, evolve • Gene pool – all of the alleles of all the individuals in a population. • Allelic frequency – the percentage of any specific allele in the gene pool.

Populations, not individuals, evolve • Genetic equilibrium – allele frequencies remain stable over many, many generations. • Evolution occurs as a result of changes in allele frequencies within a population.

Genetic (Hardy-Weinberg) equilibrium 5 conditions must be met in order for a population to achieve equilibrium 1. Large population size 2. Random mating 3. No gene flow 4. No mutations 5. No selection Does this ever happen in nature?

Changes in genetic equilibrium • Mutations introduce new alleles. • Mutations are therefore the ultimate source of genetic variability within populations. • What are mutations and what causes them? • Are mutations always bad?

Changes in genetic equilibrium • Genetic drift – fluctuations in allele frequencies due to random events. • Effects of genetic drift are especially acute in small, isolated populations • Gene flow – the gain or loss alleles due to migration of individuals between neighboring populations.

Natural selection acts on variations Three types of natural selection: 1. Stabilizing selection 2. Directional selection 3. Disruptive selection Stabilizing selection Directional selection

Natural selection acts on variations • Stabilizing selection favors average individuals in a population. Selection for average size spiders Normal variation

Natural selection acts on variations • Directional selection favors one of the extreme variations of a trait. Normal variation Selection for longer beaks

Natural selection acts on variations • In disruptive selection, individuals with either extreme of a trait’s variation are selected for. Selection for light limpets Normal variation Selection for dark limpets

The Evolution of Species • Speciation – the evolution of new species due to significant changes in the gene pool of a population over time. • Occurs when members of similar populations no longer interbreed to produce fertile offspring. • Gradualism vs. Punctuated Equilibrium.

Physical barriers can prevent interbreeding • Geographic isolation - a physical barrier divides a population. • The formation of a river may divide a single population of frogs into two populations. • Over time, these two populations could evolve into two, distinct species.

Reproductive isolation can result in speciation • As populations become increasingly distinct, reproductive isolation can arise. • Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring.

A change in chromosome numbers and speciation • Polyploid – an individual or species with a multiple of the normal set of chromosomes. New polyploid species Abnormal gametes (2 n) Fertilization Zygote (4 n) Nondisjunction Sterile plant Fertilization Parent plant (2 n) Meiosis begins Normal meiosis Normal gametes (n) Zygote (3 n)

Diversity in new environments • Divergent evolution - populations change as they adapt to different environmental conditions, eventually resulting in new species. • Adaptive radiation - type of divergent evolution in which an ancestral species evolves into an array of species to fit a number of diverse habitats

Diversity in new environments Extinct mamo Amakihi Possible Ancestral Lasan finch Crested honeycreeper Kauai Niihau Molokai Oahu Maui Lanai Akialoa Kahoolawe Akepa Akiapolaau Akikiki Liwi Hawaii Apapane Maui parrotbill Palila Ou Grosbeak finch

Different species can look alike • Convergent evolution – distantly related organisms evolve similar traits. • Convergent evolution occurs when unrelated species occupy similar environments in different parts of the world.

PARALLEL NICHES MARSUPIALS Koala PLACENTALS MARSUPIALS PLACENTALS Banded anteater Giant anteater Tree sloth Sugar glider Wombat Woodchuck Tiger cat Marsupial mole Dunnart Flying squirrel Ocelot Common mole Shrew Tasmanian wolf Gray wolf