Part 2

 Evolution does not occur in individuals but in populations.  A population is an interbreeding group of individuals of one species in a given geographic area.  A population evolves because the population contains the collection of genes called the gene pool. As changes in the gene pool occur, a population evolves.

 Evolution requires changes of allele frequencies in a population to adapt and survive within the environment.  Allele frequency, or gene frequency, is the proportion of a particular allele (variant of a gene) among all allele copies being considered.  Types of Evolutionary Mechanism Mutation Gene Flow Genetic Drift Natural Selection Environmental Fitness Speciation

 A random change in a population's gene pool. It is a change in the nature of the DNA in one or more chromosomes.  Mutations give rise to new alleles; therefore, they are the source of variation in a population, changing the allele frequencies  Mutations can be harmful or beneficial to the individual or population.

Gradual natural process by which biological traits (alleles) become more or less common in a population through reproduction and environment.

Different environmental changes can affect the expression of traits (allele frequency) in a population. These changes can eliminate traits that are rare in the population or develop new species within a population.

Speciation  Process by which new biological species arise. Mom, Dad… There’s something you need to know… I’m a MAMMAL!

A population whose members can interbreed & produce viable, fertile offspring. Being reproductively compatible is a key component Western Meadowlark Sturnella magna Eastern Meadowlark Sturnella neglecta Distinct species: songs & behaviors are different enough to prevent interbreeding

New species are formed by a series of evolutionary processes which cause reproductive isolation.  Reproductive Isolating Mechanisms  Pre-zygotic barriers are various forms of isolation  Post-zygotic barriers are generally related to fertility & viability of offspring

 This means there is an obstacle to mating or to fertilization if mating occurs behavioral isolation geographic isolation ecological isolation temporal isolation mechanical isolation gametic isolation

 New species occur in different areas physical barrier separated ancestor species Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel Ammospermophilus spp

 Species occur in same region, but occupy different habitats so they rarely encounter each other 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest

 Species that breed during different times of day, different seasons, or different years cannot mix gametes. Eastern spotted skunk (L) & Western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

 Unique behavioral patterns & rituals isolate species identifies members of species attract mates of same species  courtship rituals, mating calls Blue footed boobies mate only after a courtship display unique to their species That’s hot…

 Morphological differences can prevent successful mating Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen. Plants

 For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals! Damsel fly penises Animals Get that thing away from me!

 Sperm of one species may not be able to fertilize eggs of another species  biochemical barrier  sperm cannot penetrate egg  chemical incompatibility  sperm cannot survive in female reproductive tract Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species are unable to fuse.

 Prevent hybrid offspring from developing into a viable, fertile adult

 Genes of different parent species may interact & impair the hybrid’s development Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail.

Mules are vigorous, but sterile  Even if hybrids are vigorous they may be sterile chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes Donkeys have 62 chromosomes (31 pairs) Horses have 64 chromosomes (32 pairs) Mules have 63 chromosomes!

 Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile.

 Current debate: Does speciation happen gradually or rapidly? Or both? Gradualism Punctuated equilibrium

 Gradual divergence over long spans of time  assume that big changes occur as the accumulation of many small ones

 Rate of speciation is not constant rapid bursts of change long periods of little or no change species undergo rapid change when they 1 st bud from parent population

 Gene flow (aka gene migration) is the transfer of alleles or genes from one population to another.  Migration into or out of a population may be responsible for a marked change in allele frequencies (the proportion of members carrying a particular variant of a gene).  Immigration may also result in the addition of new genetic variants to the established gene pool of a particular species or population.