BIO2 MIDTERM: Microevolution and Macroevolution

Causes of Microevolution – Genetic Mutations

  • The raw material for evolutionary change → source of genetic variability

  • Source of new alleles → leads to new combinations of alleles

  • Not goal-directed → not a result of environmental necessity

  • Random events → can be good, bad, or neutral (depending on environmental conidtions)

  • Other forces act to either maintain the variation or remove it from the population


Causes of Microevolution – Gene Flow/Gene Migration

  • Movement of alleles between populations when:

    • Gametes or seeds (in plants) are carried into another population

    • Breeding individuals migrate into or out of population

  • Continual gene flow reduces genetic divergence between populations

    • Typically increases genetic diversity within the population

  • Populations of relatively sedentary organisms are more isolated from one another than populations of very mobile organisms (subspecies)


Causes of Microevolution – Non-random Mating

  • Non-random Mating = when individuals do not choose mates randomly

  • Assortative Mating

    • Individuals select mates with their phenotype and reject opposites

    • Increases the number of homozygotes

  • Disassortative Mating

    • Dissimilar phenotypes mate preferentially

    • Increases the number of heterozygotes

  • Inbreeding

    • Mating of 2 genetically related individuals

    • Chose a mate from same genetic lineage


Causes of Microevolution – Genetic Drift

  • Changes to allele frequency due to random chance

    • Can cause the gene pools of two isolated populations to become dissimilar

    • Some alleles are lost (0%) and others become fixed (100%)

  • Likely to occur:

    • After a bottleneck

    • With severe inbreeding

    • When founders start a new population

  • A random event prevents a majority of individuals from entering the next generation → next generation composed of alleles that just happened to make it

  • Stronger effect in small populations 


Bottleneck Effect

  • African Cheetah

    • Fastest living land animals (70+mph)

    • Lost nearly all genetic variability (monomorphic for almost all genes)

    • Prolonged inbreeding following a Bottleneck (10-20,000 years ago)

    • Very low sperm count, motility, deformed flagella

  • Northern Elephant Seals 

    • Low genetic variability

    • Human inflicted (1890’s)

    • Hunted to 20 individuals → now 100,000

    • May be susceptible to pollution/disease

Founder Effect

  • When a new population is started from just a few individuals

  • The alleles carried by population founders are dictated by chance

  • Formerly rare alleles will either:

    • Occur at a higher frequency in the new population

    • Be absent in the new population

  • Ex. Amish Ellis-van Creveld syndrome


Causes of Microevolution – Natural Selection

  • Adaption of a population to the biotic and abiotic environment

    • Biotic = competition, predation, sexual selection

    • Abiotic = climate, water availability, minerals

  • Requires:

    • Variation = the members of a population differ from one another

    • Inheritance = many differences are heritable genetic differences

    • Differential Adaptiveness = some differences affect survivability

    • Differential Reproduction = some differences affect likelihood of successful reproduction

  • Results in:

    • A change in allele frequencies in the gene pool

    • Improved fitness of the population

  • Major cause of microevolution



Types of Selection

  • Directional Selection

    • Individuals at one extreme of a phenotypic range have greater reproductive success in a particular environment

    • Curve shifts in that direction

    • Ex. size of modern horse, industrial/adaptive melanism, DDT-resistant mosquitos

  • Stabilizing Selection

    • Intermediate phenotype is favored

    • The peak of the curve increases and tails decrease

    • Ex. human babies with low or high birth weight = less likely to survive

  • Disruptive (Diversifying) Selection

    • Two or more extreme phenotypes are favored over intermediates → bimodal distribution

    • Ex. Capeta snails vary because a wide geographic range causes selection to vary




Types of Selection (cont.)

  • Balancing Selection

    • Maintains genetic diversity

    • Balanced polymorphism

      • Two or more alleles are kept in balance and therefore are maintained in a population over the course of many generations

    • Two common ways:

      • For a single gene, heterozygote favored

      • Negative frequency-dependent selection – rare individuals have a higher fitness (predator-prey)



Sexual Selection → a special case of Natural Selection

  • Directed at certain traits of sexually reproducing species that make it more likely for individuals to find or choose a mate and/or engage in successful mating

  • In many species, male characteristics affected more intensely than female (secondary sex characteristics, sexual dimorphism)

  • Intrasexual – same sex

    • Males directly compete for mating opportunities or territories

  • Intersexual – opposite sex 

    • Females choose with males possessing a particular phenotype

  • Ex. birds of paradise


Maintenance of Variations

  • Genetic variability 

    • Populations with limited variation may not be able to adapt to new conditions

    • Maintenance of variability is advantageous to population

    • Only exposed alleles are subject to natural selection

    • Natural selection does not cause genetic changes

    • Natural selection acts on individuals

    • Population evolves as gene frequencies change



Macroevolution = evolutionary changes that create new species and groups of species, accumulation of microevolutionary changes over long periods of time

  • Speciation:

    • The splitting of one species into two = Cladogenesis

    • The transformation of one species into a new species over time = Anagenesis



What is a species?

  • Typographical (Morphological) Species Concept

    • Species is defined by fixed, essential features

    • Each species has a unique structure that makes it distinct

  • Biological Species Concept

    • A species is a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature → interbreeding with common gene pool to produce viable, fertile offspring

    • Drawbacks = species have dimensions in space and time, sexual and asexual reproduction, unit of evolution and taxonomic category

  • Ecological Species Concept

    • Using the ability of organisms to successfully occupy their own ecological niche or habitat, including their use of resources and impact on the environment → to distinguish species

  • Phylogenetic (Evolutionary) Species Concept

    • A species is an irreducible group of organisms diagnosably distinct from other such groupings and within which there is parental pattern of ancestry and descent

    • Morphological, chromosomal, molecular characters used


Reproductive Isolating Mechanisms

  • Reproductive isolating mechanisms inhibit gene flow between species and maintain distinctiveness of species

  • Prezygotic Mechanisms

    • Discourage attempts to mate

      • Habitat isolation

      • Temporal isolation

      • Behavioral isolation

      • Mechanical isolation

      • Gamete isolation

  • Postzygotic Mechanisms

    • Prevent hybrid offspring from developing or breeding

      • Hybrid inviability (zygote mortality)

      • Hybrid sterility

      • Hybrid breakdown


Modes of Speciation

  • Allopatric Speciation

    • Two geographically isolated populations of one species

    • Become different species over time – gene flow interrupted

    • Can be due to differing selection pressures in differing environments

    • Ex. Kaibab and Abert squirrels on North vs. South rim of Grand Canyon = two different species now, Grunts of the sea = Panamic vs. Atlantic Porkfish