Evolution & Natural Selection-- General Biology

Evolution

A change in the genetic makeup (the in the genotypes– the combination of alleles) of a population over time.

How can genetic make-up change? / How can evolution occur?

• Introduction of new alleles

• Genetic Drift

• Natural Selection

Introduction to new alleles (new traits– eye color, hair color, limb length, fur color…)

How?

• Mutation (copying error, add new alleles to a population)

• Bringing in a new individual / new genetic material

Genetic Drift

• Based purely on random chance

  • Ex) Red flower population becomes endangered by chance, now there are majority white flowers

Population gets separated– Ex) earthquake creates a canyon and individuals on either side may evolve differently

Natural Selection

• A process through which evolution occurs

• 4 fundamental components (VHDP)

  • Variation

  • Heritable

  • Differential fitness

    • Some traits may be more advantageous than other traits

    • Ex) if a bird has a nest with four babies and all four babies grow to reproduce– the parent has a fitness of two. If the babies die– the parents have 0 fitness

    • Ex) melanin based on location and climate

  • Proportion changes

    • Those with advantageous traits become more common in the population, while those with other traits become less common

→ Having differences in traits among individuals in the population that are able to be passed from one generation to the next allows for evolution to occur. Individuals with more advantageous traits will have more offspring in the next generation and bc those individuals had heritable traits that gave them the advantage, their kids are more likely to also have those traits. As the generations go on, there will be more and more individuals with those advantageous traits.

• Organisms “try” to increase fitness

  • Their traits make it seem like they are trying to increase their fitness, however they’re not actually “trying”, they are just doing what generations before them did– things that worked to increase their fitness

• Natural Selection DOES NOT EQUAL perfection

• Only acts on existing traits (cannot make new advantageous traits appear, can only work with what its already got) 

• Very difficult to go backwards 

• Conditions change (an advantageous trait can turn into a disadvantageous trait)

• Example: cane toad populations in the most extended part of the range had extended limbs due to longer limbs being able to travel further– thus getting to a place with less survival competition 

Sources of Variation

• Mutation

• Mitosis vs. Meiosis

• Crossing Over

Mitosis

Single parent cell duplicates itself and its DNA, making two identical daughter cells– both of which look identical to the original parent cell

Important for cloning and growth

Meiosis

•  Diploid (2N) (cell has two copies of each chromosome– not necessarily identical) parent cell goes through division twice– once to separate the various chromosomes (two identical copies of each chromosome in each new daughter cell)…secondly to split off…only containing half of the chromosomes the parent cell had. The end result is 4 daughter cells, 2 pairs are identical to each other, however, the pairs are different from the other pair. The daughter cells are haploid (N)

  • Importance? Sexual Reproduction.

    • Each part contributes one haploid to the baby– otherwise the baby would have four sets of chromosomes when it's only supposed to have two (diploid)

Mitosis vs. Meiosis

Mitosis is for cloning and growth

Meiosis is for sexual reproduction