Natural Selection and the Origin of Species Chapter 5

On the Origin of Species Literature published by Charles Darwin on 24 November, 1859 Full title: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life Most likely the most important piece of scientific biological literature ever written Designed to be understood by those not fully versed in biology or naturalism. Popularized Darwin’s notions on the diversity of life through patterns of descent Outlaid evolutionary adaptation through natural selection where individuals less suited to the environment are less likely to survive and less likely to reproduce

Ecology and Adaptation Adaptation is always relative to the environment Variation Natural selection changes allele frequency There are several mechanisms of variation Mutation Independent assortment Recombination Crossing over during meiosis

Habitat Anything and everything external to an organism is its environment Physical or cultural Habitat - The specific place where an animal lives The specific habitat in which an animal lives along with how it exploits this habitat is its ecological niche

Mechanisms of Natural Selection Any factor that brings about a difference in fertility among members of a population is termed a selective agent Places a selective pressure on certain individuals in the population Results in a change in allele frequencies in the next generation Individuals, or populations, that have higher fertility rates in a particular ecological niche are said to display a greater fitness to that niche Fitness is a measure of how well-adapted a particular individual or group is to the requirements of the niche

Types of Natural Selection Directional –– long-term selection of an extreme, most adaptive, trait Advantageous allele frequency increases at expense of others Stabilizing – two or more forces favor different traits Favors balanced polymorphism – alleles maintain a constant frequency in a population over time Decreases diversity Disruptive - natural selection favors both directional and stabilizing extremes

Sexual Selection Male Reproduction Success is limited by his access to females Sexual Selection - Characteristics that increase males’ success will increase in frequency over time Intersexual selection is selection for traits that make males more attractive to females Intrasexual selection occurs in cases where males directly compete with one another Sexual selection will favor traits that make the male a stronger fighter Thus larger size is favored in males, resulting in sexual dimorphism

Kin Selection Relatives share many of the same genes Kin Selection - an individual assists a relative in reproducing and passing on its genes Also assisting in passing on its own genes Explains altruistic behavior that decreases the reproductive success of the individual Inclusive fitness - number of offspring produced plus the offspring produced by relatives who share some of the same genes

Evolution and Time Microevolution vs. Macroevolution Evolution takes time Appearance of a new species is rarely, if ever, “witnessed” Demes A local population of a species where individuals tend to resemble one another closely As a result of gene flow, nearby demes resemble one another more than demes that are further away Subspecies occur when something is present that hinders or prevents gene flow

Geographic Isolation Allopatric Speciation - new species evolves in geographic isolation from its ancestor Geographic isolation is a primary initiator of evolution Sympatric Species – new species evolve from a single ancestral species while inhabiting the same geographic location There are mechanisms that serve to separate closely related species living side by side: seasonal isolation - reproductively active at different times sexual isolation – prevents certain individuals form reproducing mechanical isolation – individuals have incompatible sex organs gametic mortality – destroyed before fertilization hybrid inviability - low health of these reduces fertility hybrid sterility - suppression of reproductive capacity

Species and Adaptation Preadaptation - the potential to adapt to a new niche Organisms do not adapt due to necessity, but because by chance they have the potential to adapt Specialized species - exhibits little variation and can tolerate minimal change within its niche Generalized species - exhibits greater diversity and therefore can survive in a variety of niches Specialized and generalized can also refer to specific traits for an organism Specialized - Hand or foot suited for specialized function of stable weight support in an upright posture Generalized - A mammalian limb with five flexible digits, adapted for many possible functions

Adaptive Radiation and Extinction Adaptive Radiation - when a species enters an uninhabited environment or one in which competition does not exist A species, or group of species, will diverge into as many variations as two factors allow: Its adaptive potential The adaptive opportunities of the available niches In many cases an organisms is too specialized to adapt to changing situations and as a result, they become extinct