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Chapter 26: Problem 15

Describe how populations with substantial genetic differences can form. What is the role of natural selection?

Short Answer

Expert verified
Answer: Populations with substantial genetic differences form through the combined forces of genetic diversity, gene flow, genetic drift, and natural selection. These genetic differences arise when populations become isolated, limiting the exchange of genetic material. Natural selection plays a crucial role in shaping these differences by favoring individuals with traits best suited for their specific environment, leading to the spread of beneficial traits in each population while decreasing the prevalence of less advantageous traits. Over time, this process amplifies genetic differences between populations and may even lead to speciation, the formation of distinct species.

Step by step solution

01

Understanding the Concepts

First, let's define the main concepts involved in genetic diversity and the formation of populations with substantial differences: - Genetic diversity: Variation in the genetic makeup of individuals within a population. - Gene flow: The transfer of genetic material between populations, often through migration or interbreeding. - Genetic drift: Random changes in the genetic makeup of a population due to chance events. - Natural selection: A process by which individuals with certain heritable traits are more likely to survive and reproduce than others, leading to changes in the genetic makeup of the population.
02

Formation of Genetic Differences Between Populations

Populations with substantial genetic differences form when the forces of genetic diversity, gene flow, and genetic drift act, and the populations become isolated from each other. Some scenarios that can lead to genetic isolation and the formation of distinct populations are: 1. Geographic barriers such as mountains, rivers, or large distances can separate populations and limit gene flow. 2. Social or mating preferences might divide a species into different populations that do not mate and exchange genes. 3. Small populations might have higher rates of genetic drift, causing them to diverge from larger populations more quickly. As populations become isolated and cease to exchange genetic material, genetic differences can accumulate, leading to the formation of populations with substantial genetic differences.
03

Role of Natural Selection

Natural selection plays a crucial role in shaping the genetic differences between populations. When populations are separated and face different environmental challenges, natural selection favors those individuals with traits best suited for their specific environment. This selective pressure leads to the spread of beneficial traits in each population while decreasing the prevalence of less advantageous traits. Over time, this process amplifies genetic differences between populations and may even lead to speciation, the formation of distinct species.
04

Summary

In conclusion, populations with substantial genetic differences can form through the combined forces of genetic diversity, gene flow, genetic drift, and natural selection. Genetic differences arise when populations become isolated, limiting the exchange of genetic material. Natural selection drives the adaptation of each population to its specific environment, further increasing genetic differences and potentially leading to the formation of distinct species.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Genetic Diversity
Genetic diversity refers to the range of genetic differences within and between populations. It is the raw material for evolution and adaptation. A diverse genetic pool ensures that a population can survive changes in the environment by providing a rich array of traits from which selection can occur. For example, within a forest, some trees might possess genes that make them resistant to certain pests. If those pests overwhelm the forest, only the resistant trees will survive and reproduce, passing on their beneficial genes to the next generation. This creates a natural reservoir of traits that can be beneficial in unforeseen circumstances, contributing to the long-term survival of a species.
Gene Flow
Gene flow is the movement of genes between populations, which can occur when organisms move from one population to another and reproduce. This process can introduce new genetic material to a population, potentially increasing its genetic diversity. For instance, imagine a bird that migrates from one island to another. If it mates with the local birds, it introduces new genes into the population. Regular gene flow between populations can prevent them from becoming too genetically distinct and may slow the process of speciation. Conversely, limited gene flow can lead to increased genetic differences.
Genetic Drift
Genetic drift is a random process that can cause significant changes in the genetic makeup of a population, particularly in small ones. Events like natural disasters can randomly remove individuals regardless of their genotype, potentially altering the gene frequencies within the remaining population. Imagine a flood wipes out a large portion of a small rabbit population. If the surviving rabbits happen to carry unique alleles, future generations will be more likely to inherit those alleles. Over time, this can lead to populations with distinct genetic makeups apart from their original gene pool.
Natural Selection
Natural selection is the filter through which genetic diversity is sifted, keeping beneficial traits and removing disadvantageous ones. It is guided by the environment and individuals' interactions with it. A simple way to envision this is through the case of moths during the Industrial Revolution in England. Moths with dark coloring were less visible on pollution-darkened trees, thus surviving predation more effectively than their light-colored counterparts. Over time, the moth population shifted to predominantly dark coloring. This example demonstrates how organisms with favorable traits have a better chance of surviving and reproducing, influencing the genetic composition of their population.
Speciation
Speciation is the process by which new species are formed. It occurs when populations become so genetically distinct that they can no longer interbreed and produce fertile offspring. Geographic isolation is a common reason for speciation; separated populations adapt to their unique environments, accumulate genetic differences through drift and selection, and eventually evolve into distinct species. An emblematic example of speciation is observed in the Galápagos finches, which have evolved into several species, each with unique beak shapes suited to their particular food sources on different islands.

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Most popular questions from this chapter

What are the two groups of reproductive isolating mechanisms? Which of these is regarded as more efficient, and why?

Under what circumstances might a lethal dominant allele persist in a population?

The use of nucleotide sequence data to measure genetic vari- ability is complicated by the fact that the genes of many eukaryotes are complex in organization and contain \(5^{\prime}\) and \(3^{\prime}\) flanking regions as well as introns. Researchers have compared the nucleotide sequence of two cloned alleles of the \(\gamma\) -globin gene from a single individual and found a variation of 1 percent. Those differences include 13 substitutions of one nucle- otide for another and three short DNA segments that have been inserted in one allele or deleted in the other. None of the changes takes place in the gene's exons (coding regions). Why do you think this is so, and should it change our concept of genetic variation?

What genetic changes take place during speciation?

Recent reconstructions of evolutionary history are often dependent on assigning divergence in terms of changes in amino acid or nucleotide sequences. For example, a comparison of cytochrome c shows 10 amino acid differences between humans and dogs, 24 differences between humans and moths, and 38 differences between humans and yeast. Such data provide no information as to the absolute times of divergence for humans, dogs, moths, and yeast. How might one calibrate the molecular clock to an absolute time clock? What problems might one encounter in such a calibration?
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