Download Evolution of Populations

Genes and Variation
 Before we continue, let’s review some genetics vocabulary:
 Genotype
 Genetic make-up of an organism
 Ex: TT , Tt or tt
 Phenotype
 Physical characteristics of an organism
 Ex: tall plants or short plants
 Allele
 One of a number of different forms of a gene
 Ex: (T) for tall plants or (t) for short plants
Genes and Variation
 Genotype and Phenotype in Evolution
 Natural Selection acts on PHENOTYPES
 It does NOT directly act on genes
 In any population, some individuals some individuals
have phenotypes (traits) that are better fit for their
environment
 Those individuals have higher FITNESS.
 Higher fitness = more offspring & more of
their genes are passed to the next generation
 As a result, will the genes in a population
change?
Genes and Variation
 Sources of Genetic Variation
 Mutations
 Sexual Reproduction
 1. meiosis
 Law of segregation
 Law of independent assortment
 Crossing over
 2. random fertilization
Evolution of Populations
Natural Selection on Polygenic Traits
Natural Selection on Polygenic Traits
 Traits that are controlled
by more than one gene
produce a range of
phenotypes. This range can
often be shown by a bell
curve.
 Natural selection can act
on polygenic traits in one
of three ways:
 Directional Selection
 Stabilizing Selection
 Disruptive Selection
Natural Selection on Polygenic Traits
 Directional Selection
 Individuals at one end of a distribution curve have
higher fitness than individuals in the middle or at the
other end of the curve
Natural Selection on Polygenic Traits
 Imagine a population of birds that live on an island. The
island has many large seeds with thick shells, but few small
seeds. Birds with bigger beaks would have more to eat than
birds with smaller beaks. Birds with bigger beaks would live
longer and have more offspring.
Natural Selection on Polygenic Traits
 What would happen to this population over time?
Natural Selection on Polygenic Traits
 Natural selection would move in one direction—toward
larger beaks.
Natural Selection on Polygenic Traits
 Stabilizing Selection
 Individuals near the center of the distribution curve
have higher fitness than individuals at either end of
the curve.
Natural Selection on Polygenic Traits
 Large spiders are easily seen by predators. Small spiders have
difficulty eating large insects they catch. Spiders of average
size are not easily seen and can easily eat insects they catch.
 Another scenario: Human babies must be able to exit the
birth canal. Babies that are too big have more problems
exiting. On the other hand, babies born too small may not
have the fat reserves to survive.
Natural Selection on Polygenic Traits
 What would happen to this population over time?
Natural Selection on Polygenic Traits
 In this example, the fitness of the extremes is lower than that
of the average.
Natural Selection on Polygenic Traits
 Disruptive Selection
 Individuals at the upper and lower ends of
distribution curve have higher fitness than individuals
near the middle of the curve.
Natural Selection on Polygenic Traits
 A type of crab ranges in color from almost white to dark
brown. Light-colored crabs are not easily seen on sand.
Dark brown crabs are not easily seen on dark rocks. Tan
crabs can be seen on either dark or light surfaces.
Natural Selection on Polygenic Traits
 What would happen to this population over time?
Natural Selection on Polygenic Traits
 The population might split into two groups: one with lighter
shells and one with darker shells.