Lecture 3 Evolution

A unifying theory in Biology: ‘Nothing in biology makes sense, except in the light of evolution.’ Dobzhansky

What is: evolution? What is a theory? Why is it important for species to evolve? ‘Evolution is a population thing’: Individuals do not evolve, evolution is a process which occurs over generations in a population. Population is evolutionary unit

Evolution has a genetic basis: Genes are heritable units that determine traits Variation in genes occurs naturally: due to mutation Allele: genes may occur in variant forms; these variant forms are known as alleles Gene Pool: the total collection of gene within a population

What forces or factors may result in an evolutionary change in a population? Genetic drift: ‘chance variation in a population’ Natural selection: Selection in a genetically variable population for those with traits which favor survival and reproduction

Natural selection: drives evolution by favoring those with the best fitness Results in a gradual change in the population/gene pool Adaptation: a trait that is possessed by an individual enables an enhanced ability to survive and reproduce Fitness: ‘the ability to survive and leave reproductively viable offspring’

Examples of Evolution 1. Gartersnake-Oregon Newt interaction 2. modification of beak size in Galapagos finches – studies of Peter and Rosemary Grant No new species arose in either case Species: ‘A group of similar appearing organisms that share the same gene pool’

Fig. 5-8

Fig. 5-9

Selection pressure within a species Selection for one extreme Selection for the ‘average’ Selection for two extremes (bimodal)

Sexual selection: an aspect of natural selection A process in which (usually) females select amongst potential mates (p 674) Example - Peacocks: female selects males with large, bright, colorful tails- why? Handicap hypothesis (p 704)

Sexual selection – an aspect of natural selection: includes two aspects Intrasexual: males compete - selection leads to evolution of structures used in combat with other males Deer’s antlers or ram’s horns, bright colors, etc. selective pressure is for ability to compete successfully with members of same sex Intersexual selection- Females select amongst winners agents of selection Selection based on various cues – appearance and behavior goal (evolutionary sense) select mate with best fitness

Female peahens prefer to mate with males with greater number of eyespots in their tail feathers

Geographic Range and Evolution The geographic range of a species can result in phenotypic variation of neighboring populations A wider geographic range includes a broader range of environmental conditions In general, the greater the distance between populations, the more pronounced the phenotypic differences

Geographic Variation in Species: clines and ecotypes Ecotype: Subpopulation of a species adapted to a specific environment Cline: measurable change in a species characteristics over an environmental gradient Variation in white tailed deer – larger animals at higher latitudes in NA

Natural Selection Can Result in Genetic Differentiation Geographic isolates result when gene flow among subpopulations is prevented The isolation is rarely complete, and so these isolated subpopulations are often classified as subspecies because of a set of unique characteristics Appalachian salamanders (Plethodon jordani)

Genetic Variation of Ecotypes An ecotype is a population adapted to its unique (often abrupt) local environmental conditions Yarrow (Achillea millefolium) response to altitude How much variation observable in the field is due to genetic variation and how much is due to impact of environment (phenotypic plasticity)? Common Garden Experiment Seed collected from plants of same species growing in different environments grow in same location(s)

Evolution without natural selection - Genetic drift: Genetic drift refers to fluctuations in allele frequencies in a population arising from chance events. Most pronounced in small populations resulting from Natural disasters Founding events High levels of predation Random: population not selected based on adaptations

Figure 4 Genetic drift: Changes in allele frequencies across generations due to chance events: Random disturbances Small populations Founder effects Effect of genetic drift on a population. Because of chance events in the population, allele frequencies in later generations differ significantly from allele frequencies in the initial generation.

Several Processes Can Function to Alter Patterns of Genetic Variation Inbreeding is the mating of closely related individuals Increases homozygosity at all loci Offspring of closely related individuals are more likely to inherit rare, recessive, or deleterious genes that lower overall fitness These consequences are referred to as inbreeding depression

How do new species arise? Allopatric speciation: spatial isolation –geographic isolation Spread of species into new areas Founding population in isolated location Geologic changes – gradual process Sympatric speciation: different niches/same habitat or geographic area Genetic changes (as ploidy levels in plants) Disruptive selection: divergent genotypes favored

Species have become specialists feeding on seed providing optimal energy efficiency

Adaptive Radiation: Single ancestral from give rise to multiple ‘specialist’ forms

Summary: Evolution involves heritable differences amongst individuals in a population which ultimately impact fitness Organisms become ‘tailor made’ for their niche within an environment by processes of evolution Characteristics of individuals making up current populations are a product of natural selection in ancestral populations